A review of the use of live willow poles forstabilising highway slopes

Prepared for Quality Services, Civil Engineering, Highways

Agency

D M Hiller and D J MacNeil

TRL Report 508

First Published 2001ISSN 0968-4107Copyright TRL Limited 2001.

This report has been produced by TRL Limited, under/as partof a Contract placed by the Highways Agency. Any viewsexpressed are not necessarily those of the Agency.

TRL is committed to optimising energy efficiency, reducingwaste and promoting recycling and re-use. In support of theseenvironmental goals, this report has been printed on recycledpaper, comprising 100% post-consumer waste, manufacturedusing a TCF (totally chlorine free) process.

CONTENTS

Page

Executive Summary 1

1 Introduction 3

2 Bioengineering 3

2.1 Applications of bioengineering 3

2.2 The role of plants in stabilising slopes 3

2.2.1 Prevention of soil erosion and mass movement 5

2.2.2 Root reinforcement 5

2.2.3 Slope buttressing and arching 6

2.2.4 Soil moisture modification 6

2.2.5 Surcharge from weight of vegetation 7

3 Bioengineering using live willow poles 8

3.1 The potential benefits of using live willow poles 8

3.1.1 Slope stabilisation 8

3.1.2 Riverbank erosion control 9

3.2 Reconciling the use of willow poles withthe objectives of DETR 9

3.3 Potential problems and their mitigation 11

3.3.1 Difficulties in establishing plants 11

3.3.2 Effects on adjacent property and services 12

3.3.3 Other difficulties 13

4 Case histories 13

4.1 M20, Longham Wood cutting 13

4.2 A249, Iwade 14

4.3 Great Lakes shoreline stabilisation 15

4.4 Revegetation of disturbed slopes at Lake Tahoe 15

4.5 Rock Lake stabilisation 15

5 Construction methods 16

5.1 Selection of suitable species 16

5.2 Pole length and diameter 17

5.3 Harvesting, handling and transportation 18

5.4 Planting method 18

5.4.1 Vertical or slope-normal planting 18

5.4.2 Length of pole remaining above ground 18

5.4.3 Spacing and planting pattern 18

5.4.4 Use of tree guards 19

5.4.5 Equipment and working method 19

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6 Instrumentation for performance assessment 19

6.1 Instrumentation for soil moisture monitoring 20

6.2 Monitoring of root growth 20

7 Maintenance requirements 21

8 Conclusions 21

9 References 22

Appendix A: Draft specification: Installation of live willowpoles for stabilising highway slopes 26

Abstract 33

Related publications 33

1

Executive Summary

Shallow slope failures are a widespread and costlymaintenance problem which affects the highwayearthworks, particularly slopes in overconsolidated clays.The extent of the problem was established by Perry (1989)and Andrews (1990). Perry predicted that many moreslopes would fail in future if preventative measures werenot taken.

Stabilisation of highway slopes may be undertakenusing a variety of proven hard engineering approaches(Johnson, 1985). An alternative technique makes use oflive willow poles driven into the slope. The poles providean immediate reinforcing action and subsequently grow toprovide the longer term benefits associated withestablished trees. While this technique appears to offersignificant benefits in terms of ecology, aesthetics,sustainability and finance, it requires validation for use onUK highway slopes.

The objective of this literature review was to collate andadapt previous experience of the use of live willow polesto enable a method to be established for their use inhighway slope stabilisation. The review focuses mainly onthe potential engineering benefits, but also the aestheticand ecological consequences of slope stabilisation usingwillows are also considered, particularly with regard tocompliance with existing guidance in DMRB.

The evidence from the literature has shown thatbioengineering in general, and in particular the use of livewillow poles to stabilise slopes, is potentially a versatileand cost effective alternative to more traditionalengineering methods. To ensure the maximum success ofthe live willow pole technique, care is required in theselection of species appropriate to the local conditions; inthe harvesting, handling and transportation of the poles;and in the installation procedure.

The information gained during this review has beenused to establish a draft specification for carrying out afield trial of the live pole technique. This specification ispresented in Appendix A. The results of the field trial willenable the preparation of a revised procedure for using thelive willow technique as a routine method for preventionof shallow slope failures.

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3

1 Introduction

Prevalence of shallow slope failures on highway earthworkshas been appreciated for some years. The extent of theproblem was established by a walk over survey reported byPerry (1989) and subsequently through studying aerialphotographs by Andrews (1990). Perry reported that forsome combinations of geology, slope geometry and age,more than 50 per cent of earthworks slopes had failed.Furthermore, Perry made a conservative estimate that threetimes as many slopes as had failed thus far would fail in thefuture if preventative measures were not taken.

Johnson (1985) reviewed a number of options for therepair of failed slopes and for preventative measures.These techniques all involved traditional engineeringsolutions and did not consider the potential for vegetationto provide the primary support mechanism. An alternativemeans of stabilising highway slopes is by the use of livepoles driven into the slope to provide an immediatereinforcing action and to subsequently grow to provide thelonger term benefits associated with established trees.

The objective of this literature review is to collate anduse previous experience to optimise the procedure for theinstallation of live willow poles. The information gainedfrom the literature has been used to establish a draftspecification for carrying out field trials, which ispresented in Appendix A. The proposed trials are intendedto establish the potential engineering benefits which mayaccrue through the use of live willow poles to stabiliseslopes. It is acknowledged that there may be aesthetic andecological consequences of slope stabilisation by amonoculture of willow (or any other) species but, for thepurpose of this review, these aspects are considered to besecondary to the engineering benefits. It is further notedthat, when compared with the ‘hard’ reinstatement optionsavailable (for example, Johnson, 1985), all bioengineeredoptions have the potential to offer considerable ecological,aesthetic, and financial benefits. However, to implementthe technique of stabilisation by live willow poles morewidely than in the proposed trial, consideration may needto be given to the wider requirements of the HighwaysAgency to ensure integration within the local landscape.

In Sections 2 and 3 of this report the role of plants instabilising slopes and the particular philosophy behind theuse of live willow poles are described. Potential difficultiesand adverse impacts are also discussed. Case histories fromthe UK and abroad are described in Section 4. Section 5provides details of how stabilisation of highway slopesusing live willow poles should be undertaken, includingselection of species, harvesting, handling and installationprocedures. Section 6 provides guidance on theinstrumentation required for validation of the willow poletechnique and subsequent maintenance requirements areoutlined in Section 7. The draft specification for theinstallation of live willow poles is presented in Appendix A.

2 Bioengineering

Bioengineering was defined by Barker (1996a) as ‘plantingfor engineering, ecological and aesthetic purposes’. A more

complete definition cited by Schiechtl and Stern (1996) is‘the use of plants and plant materials employed either ontheir own or in conjunction with inert or hard buildingmaterials, often locally sourced, to bring about thestabilisation of earthworks and waterways’. Although theterm bioengineering is commonly used in this context, manypractitioners prefer to use the term ecological engineering toavoid confusion by the general public with geneticbiotechnology (Barker, 1996a). With the current media andpublic interest in genetically modified organisms, carefuluse of appropriate terminology may be prudent.

Thomson (1988) used the term ‘biotechnicalengineering’ to describe the use of engineering, biologicaland ecological concepts to construct more permanent,aesthetic and environmentally acceptable solutions thanthose achieved by traditional engineering approaches.

2.1 Applications of bioengineering

Vegetation is commonly used in civil engineering as a wayof reducing the adverse environmental impact of theworks, in particular for reducing visual intrusion.However, vegetation can also contribute an engineeringfunction, such as through affecting the soil moisture,protecting and restraining soil near the ground surface andby increasing the strength and competence of the soil atgreater depths (Coppin and Richards, 1990). The effects ofvegetation may be beneficial or adverse, depending ontheir application. Schiechtl (1980) described somecommon mistakes made in the use of bioengineering inearthworks and waterway construction which renderbioengineering methods ineffective or, in extreme cases,detrimental to the works or the wider environment.

Common applications of bioengineering occur within slopeprotection, slope stabilisation and the control of erosion onwatercourses. Other applications include the remediation ofcontaminated soils (CIRIA, 1995), and noise abatementstructures (Schiechtl and Stern, 1996; Watts, 1998). Broadreviews of many of these applications have been given byGray and Leiser (1982), Coppin and Richards (1990),Morgan and Rickson (1995) and Barker (1999a). Figure 1summarises some of these applications of bioengineering.

This literature review considers only one specific aspectof bioengineering: the use of live willow poles and theirapplication to the stabilisation of slopes. Willows arecommonly used in bioengineering applications becausethey root easily from cuttings, are quick to establish andare versatile, enabling many planting techniques to be used(Coppin and Richards, 1990). In addition to their use inslope stabilisation, live willow poles are also commonlyused in the stabilisation of river banks (see Section 2.3.6).

2.2 The role of plants in stabilising slopes

Plants improve the engineering properties of soils andinfluence the stability of slopes in many ways. Detaileddescriptions of these effects have been given by a numberof authors (for example Gray, 1978; Gray and Leiser,1982; Barker, 1986; Coppin and Richards, 1990; Styczenand Morgan, 1995; Schiechtl and Stern, 1996) and aresummarised in Figure 2. This Section presents a briefsummary of the main issues.

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Applications

Slope stabilisation

embarkments and cuttings

cliffs and rock faces

Water erosion control

rainfall and overland flow

gully erosion

Watercourse and shoreline protection

continuous flow channels

discontinuous flow channels

large water bodies (shorelines)

Wind erosion control

Vegetation barriers

shelter

noise reduction

Surface protection and trafficability

Control of runoff in small catchments

Plants as indicators

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Figure 1 The application of vegetation to engineering (from Coppin and Richards, 1990)

Figure 2 Physical effects of vegetation (from Coppin and Richards, 1990)

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2.2.1 Prevention of soil erosion and mass movementThe control of soil erosion is primarily effected byherbaceous plants although woody vegetation also plays arole. The main effects on erosion are (Coppin andRichards, 1990):

i interception of rainfall to prevent impact on the soil byraindrops;

ii restraint of soil particles by root systems and filtering ofentrained sediment from surface runoff;

iii reduction in the velocity of surface runoff by increasedsurface roughness;

iv increased infiltration since roots and plant residues helpto maintain soil porosity and permeability;

v depletion of the soil moisture through transpirationdelays the onset of saturation and runoff.

The role of woody plants is greater in preventing massmovement than is that of herbaceous plants. Gray andLeiser (1982) suggested the following possible ways inwhich woody vegetation might affect mass movement:

i mechanical reinforcement of the soil;

ii evapotranspiration and interception of rainfall to reducepore water pressure;

iii anchored and embedded stems can provide buttressingand arching reinforcements which act againstdestabilising shear stresses;

iv the surcharge loading of vegetation exerts both adestabilising downslope load, and a stress componentnormal to the slope which tends to increase theresistance to sliding;

v destabilising rotational moments may be exerted on theslope as a result of strong winds blowing against trees,especially those winds which are directed downslope.

Although the first three effects on mass movements arebeneficial, the last two effects may be detrimental to theslope stability. However, the overall effect of establishingwoody vegetation is generally beneficial. For example,Gray and Leiser (1982) cited a distinct cause and effectrelation between vegetation removal and increased slopefailures from around the world.

2.2.2 Root reinforcementRoots embedded in the soil form a composite materialconsisting of fibres, with a relatively high tensile strengthand adhesion, within a matrix of lower tensile strength.The presence of roots within the soil mass increases theoverall shear strength of the composite material. Theincrease in strength is dependent upon the root geometry,strength and frictional resistance between the soil and theroots (Wu, 1995). In addition, roots of 1-12mm diameterphysically restrain soil particles from movement inducedby gravity, raindrop impact, surface runoff and wind(Coppin and Richards, 1990).

Many theories exist in the literature regarding themechanism of soil reinforcement by vegetation (Barker,1986). One generally accepted theory is that the strainrestraint in the direction of the reinforcement increases theeffective confining stress; the soil friction angle is

unchanged (Figure 3). This effect mobilises additional shearstrength beyond that which would be generated by only theexternally applied confining strength. Another effect citedby Barker (1986) is the increased resilience of soil such thata soil containing roots has an increased ability to resistdeformation without the loss of residual strength.

Rooted soil

Root-free soil

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∆S' = increase in effective soil shear

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Figure 3 Effect of root reinforcement on the shearstrength of soil (Coppin and Richards, 1990)

The magnitude of the mechanical reinforcing effect ofvegetation is a function of the following root properties:density (i.e. the volume of root material per unit volumeof soil); tensile strength; tensile modulus; length todiameter ratio; surface roughness; alignment; andorientation with respect to the direction of principalstrain. Coppin and Richards (1990) cited the work of anumber of authors who have reported increases in soilcohesion due to roots of different plants in a variety ofsoils of between 1.0 and 17.5kN/m2. A number ofimportant points to be considered with regard to tensilestrength of roots are (Coppin and Richards):

i individual roots of shrubs and trees can have very hightensile strengths, for example up to 74kN/m2 for alder;

ii there is a very large range of root strengths for anyparticular species, depending upon size, age, conditionof root and season, for example 4 to 74 kN/m2 for alder;

iii tree root thickness and strength respond to asymmetricalloading, such as that which occurs on slopes.

Yim et al. (1988) reported three case studies from HongKong in which the reinforcing effect of tree roots used tostabilise slopes has been quantified. Analyses of thedistribution of tree roots and extensive testing of theirtensile strengths measured both in the field and in thelaboratory enabled calculation of increased factors of safetyfor the slopes. Without the ability to quantify adequately thestabilising effect of the root systems, Yim et al. concluded

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that more substantial and costly slope stabilisationworks would have been required to meet the extantsafety standards.

2.2.3 Slope buttressing and archingButtressing and arching is effected by the trunks of trees andby the volume of soil reinforced and anchored by the roots(Figure 4). Styczen and Morgan (1995) approximated theroot-reinforced soil to a vertical root cylinder for analyses ofarching effects using the method described by Wang andYen (1974). Trunks and large roots act as cantilever piles torestrain the soil from moving downslope. The extent towhich this buttressing can contribute to the stability of thesoil mass depends on the depth of the soil mantle, the

groundwater and the penetrability of the bedrock by roots(Figure 5) (Styczen and Morgan, 1995).

Where trees are sufficiently close together, the soilbetween the buttressed parts of the slope may be supportedby arching (Figure 6). Gray (1978) showed that the criticaldistance between vertical root cylinders below whicharching is able to occur was very sensitive to cohesionalong the basal sliding surface, the critical distanceincreasing as the cohesion increases. The effectiveness ofarching is also dependent upon the spacing and diameter oftree trunks, the thickness and inclination of the yieldingportion of the soil profile and the shear strength propertiesof the soil (Gray, 1978).

2.2.4 Soil moisture modificationThe presence of vegetation influences the proportion of theprecipitation that reaches the soil and affects the behaviourof water within a soil mass. The balance of water in a soildepends on the relative levels of rainfall input,evapotranspiration, surface drainage and deep soilpercolation. Only a relatively small increase in soil suction,from 10kPa to 15kPa, can prevent a slope from failing(Marsland, 1997). The ability of vegetation to modify soilmoisture is extensive and can reach beyond the physicalextent of the roots. Benefits to the stability of slopesthrough a reduction in the soil moisture can thereforeextend further laterally and to a greater depth than the soilpenetrated by the root system. The effect of vegetation onthe soil moisture depends not only upon the species ofplant and depth in the soil, but also upon the season. The

Figure 5 Classes of plant root reinforcement and anchored slopes (from Styczen and Morgan, 1995)

Figure 4 Slope buttressing by ponderosa pine tree (fromGray and Leiser, 1982)

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highest rates of moisture loss occur during the summer andthe lowest rates occur in the winter when plants aredormant and air and soil temperatures are low.

Many plants, particularly those that live in damp habitats,have a high capacity to remove soil moisture through hightranspiration rates. Such plants are referred to asphraetophytes (Schiechtl, 1980) and are potentially usefulfor relieving areas of high pore water pressure. Coppin andRichards (1990) listed a number of species which might beuseful for this application and include Tamarix spp.(tamarisk), Betula spp. (birches), Robinia pseudoacacia(black locust), Salix cinerea (sallow), S. caprea (goatwillow) and S. triandra (almond willow). However, suchplants may be intolerant of dry conditions and thereforecannot be relied upon to increase soil suction.

In some soils, particularly clays, desiccation of the soilcaused by plant growth may lead to the formation ofshrinkage cracks. Anderson et al. (1982) found that on a7m high clay embankment on the M4 motorway insouthern England, the development of cracks differed atdifferent locations on the slope. At the top of the slope,where the vegetation was very dense tussock grass, therewere few cracks. The greatest cracks were at the base ofthe slope where there was little top soil and the clay wasexposed. The most vulnerable part of the slope wasreported by Anderson et al. to be the middle of the slopesince the cracks persisted for the longest period of timeafter the onset of wet weather. These cracks allowed waterto enter the slope leading to positive pore water pressureswhich caused shallow seated slope failures. Even whencracks had closed up following a recovery of the soilmoisture, the crack sites increased the rate of response ofthe soil to precipitation. Conversely, Dexter (1991)commented that the formation of cracks can have a longterm beneficial effect on soil stability since the cracksprovide paths of low resistance along which root growth is

encouraged. This effect may be of particular value inenabling roots to penetrate compacted heavy clays.

Crabb and Hiller (1993) found that on vegetatedembankment slopes, constructed from overconsolidatedclay, a zone in which there was a seasonal cycling of porewater pressures extended to a depth of approximately1.5m. At greater depths the pore water pressure was stableat lower pressures than those near the surface. Crabb andHiller concluded that the zone of seasonally variable porewater pressures determined the depth of the shallow slopefailure surface.

In addition to the increase in strength of soils which iseffected by the reduction in moisture content, removal ofmoisture by plants reduces the bulk density of the soil.This reduction in density can be important in reducing thedisturbing force on potentially unstable slopes, but may becountered by the additional load due to the vegetationitself. This issue is discussed in the following Section.

2.2.5 Surcharge from weight of vegetationThe surcharge weight resulting from the presence ofvegetation is normally only significant in the case oftrees. Figure 7 shows an illustrative example of the effectof tree height on the equivalent uniform surcharge. In thecase of willow poles planted on highway slopes, the lowheight of the trees will keep the surcharge load generallysmall, particularly if the trees are coppiced or shrubspecies are used.

Although the additional load applied by surchargingmay be seen as detrimental, on shallow slopes, thecomponent of the surcharge acting normal to the slope willexceed that acting down slope. The overall contribution tothe stability of the slope as a result of surcharging willtherefore be beneficial. However, the destabilising effectof mature vegetation may be exacerbated by wind loading.Wind loading is only usually significant when the wind

Figure 6 Schematic of soil buttressing and arching (Wang and Yen, 1974)

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speed exceeds 11m/s (Beaufort wind scale 6) (Coppin andRichards, 1990). The forces exerted can disturb the upperlayers of the soil. Possibly the most significant effect onthe slope stability is that uprooted trees can provide a routewhich allows increased water infiltration. Again, forcoppiced trees, the effect of wind loading is unlikely to beof great significance.

3 Bioengineering using live willow poles

3.1 The potential benefits of using live willow poles

3.1.1 Slope stabilisationSection 2.2 provided a summary of the benefits which mayaccrue in stabilisation of slopes by the use of vegetation.Many of these benefits are only realised once thevegetation has become established and the roots systemshave developed sufficiently to extend to the requireddepths. This may take several years before root systemsare sufficiently well developed to improve shallowstability (Barker, 1999a).

Hard engineering solutions, such as mini piles or soilnailing provide an immediate reinforcing effect and maybe used to stabilise slopes. The installation of live poles inslopes can provide the immediate benefits yielded by hardsolutions. The period in which other bioengineeringtechniques provide limited benefit is therefore addressedsince reinforcement of the slope occurs as soon as thepoles have been installed. Poles may be installed vertically,so that the poles mimic micropiles, or normal to the slope,such that poles can be regarded as soil nails. If appropriatetree species are used, the poles will subsequently root,providing the longer term advantages of vegetativesolutions. If the poles do not develop root systems, thenthe poles will eventually decay and the reinforcing effectwill only be short lived. On steep slopes, vegetation hasalso been used to create a living reinforced soil structureby using branches to provide the reinforcement (forexample Schuppener and Hoffmann, 1999). This type of

structure is outside the scope of the current study.To maximise the benefit of their use, willow poles

installed to improve slope stability must be inserted to adepth which exceeds the depth of the probable failuresurface. This typically requires poles to be driven to adepth of between 1.5 and 2.0m. If the poles develop rootsystems at this depth, then the maximum benefits willaccrue. Establishing a root system beyond this depth mayexceed the longer term benefits of conventional treeplanting, since the roots of such plants are commonlyconcentrated close to the ground surface (for example,Greenwood et al., 1996).

Root systems are of fundamental importance to most ofthe bioengineering functions that vegetation can perform.All plants have a mat of roots which develops mainly closeto the ground surface to absorb moisture and nutrients. Therooting behaviour of individual species varies, but there isno such thing as an intrinsically deep rooted or shallowrooted tree species (Sutton, 1969). The development ofroots is strongly influenced by the soil type, soil depth andthe groundwater regime. Roots in well drained soilsgenerally extend to greater depth than the same plantspecies growing in moister soils. A high groundwater levelor a layer of densely compacted soil, particularly clays(Dobson, 1995), will force most roots to spread laterally(Figure 8). The majority of roots are usually encounteredwithin the upper 0.5m of soil, although trees and shrubsextend a small proportion of their root systems down toaround 3m. If the use of live willow poles driven to 2mdepth will enable roots to grow and thrive along the entirelength of the pole, then this will provide an advantage overother establishment techniques. However, the literaturedoes not provide any record of this occurring: future trialsshould seek to determine the maximum depth of rootdevelopment from willow poles and whether deep rootspersist in the longer term.

Willows offer a number of advantages forbioengineering when compared with other tree species.One of the benefits of using willows in bioengineering isthat it is not highly invasive (Barker, 1996b). With theexception of goat willow (Salix caprea), willows do notpropagate easily from seed because the seed is short lived.A further potential problem with vegetation is thegeneration of leaf litter, which may block drains, causeroads and footpaths to be slippery and, in the case ofrailways, cause traction problems for trains. Barker(1996b) commented that willows are better than manyspecies in this respect because they have a low leaf to stemratio and they do not have sudden heavy leaf-falls, ashappens with species such as horse chestnut (Aesculushippocastanum).

A further advantage of willows over some other treespecies is that they are commonly pollarded or coppiced(Davison, 1994). Since willows thrive under suchwoodland management, they can be readily kept to a lowheight to avoid problems of sightline disruption andvisual intrusion.

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Figure 7 Equivalent uniform surcharge of coniferousforest (Coppin and Richards, 1990)

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3.1.2 Riverbank erosion controlOf all the options for bioengineering, this review isintended to describe mainly the stabilisation of slopesusing live willow poles. However, willow poles are used tostabilise riverbanks in ways which have a number ofsimilarities with the techniques used for slope stabilisation.Consequently a brief review of this application ispresented here.

Trees used for bank protection must be water tolerant.The principal water tolerant tree species in the UK arewillow (Salix), alder (Alnus) and black poplar (Populusnigra). Growth of trees and shrubs on the banks of watercourses provides many benefits against erosion. Inparticular, a dense root structure provides protection andreinforcement to enhance the stability of the bank bothabove and below the mean water level (Hemphill andBramley, 1989).

Willow is the most versatile and commonly used water-tolerant tree in the UK. Many varieties exist (Meikle, 1984)and it is important to select the correct variety for theparticular application. The common osier (Salix viminalis) isused extensively for bank protection because it producesabundant bushy growth. However, it requires regulartrimming to prevent it from reducing channel capacity.Crack willow (Salix fragilis) and white willow (Salix alba)are less good for bank protection because of their size and alack of growth near the roots (Hemphill and Bramley, 1989)but they are harvested and used for posts.

A variety of techniques are available for water courserepairs and protection. Reviews have been given byHemphill and Bramley (1989); Coppin and Richards(1990); and Willowbank (1998). One of the mostcommonly used techniques, and one which has somesimilarities with the use of live poles for slopestabilisation, is willow spiling. Live poles are driven alongthe river bank at approximately 500mm centres. These are

used to support live willow shoots (withies) wovenhorizontally between the posts. Usually, both the posts andthe withies grow and establish a dense root mass and topgrowth, providing protection of the bank and a silt trap(Figure 9).

3.2 Reconciling the use of willow poles with theobjectives of DETR

The objectives of woodland planting for road developmentstated in HA 56/92 (DMRB 10.1.2) are to integrate theroad with the landscape, to provide visual interest and toprovide wildlife benefits. HA 56/92 also states that treesand shrubs grow best in small groups of the same species;this is the way they generally occur naturally. Furthermore,the aesthetic and ecological value of the planted area willbe enhanced by the appropriate use of woodlandmanagement techniques. For willows the use of coppicingis appropriate: guidance is given in HA 61/92 (DMRB10.1.7). HA 62/92 (DMRB 10.2.1) addresses motorwaywidening within the Good Roads Guide. One of the designobjectives of HA 62/92 is to realise all reasonableopportunities for environmental improvement. In addition,adverse visual impact on the quality of the landscape shouldbe minimised. Section 12 of HA 44/91 (DMRB 4.1.1)provides advice on landscaping and planting onearthworks. In particular it is advised that, in general, onlyappropriate species for the area concerned should beplanted. However, other considerations may have an overriding importance, such as the spread into sight lines orneighbouring property.

The engineering benefits of vegetation are notconsidered by the Advice Notes referred to in thepreceding paragraph. HA 48/93 (DMRB 4.1.3) describesfailures of highway cutting and embankment slopes andmethods of repair. Section 6 of HA 48/93 describesmethods of improving the stability of potentially unstable

Figure 8 Modification of root distribution by site conditions (Coppin and Richards, 1990)

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slopes and includes some guidance on the use ofvegetation. It is commented that vegetation alone may beunable to provided sufficient support for steep slopeangles. In addition, potential problems arising throughenabling water ingress via planting pits and the ability ofroots to penetrate drainage systems are highlighted.These are all issues which need to be considered in theinstallation of willow poles for slope stabilisation. Thereis also a need to consider how the use of willow poles canbe reconciled with DETR’s wider objectives for plantingand landscape.

The current research aims to determine the feasibility ofusing live willow poles for stabilisation of highway slopes.The use of live willow poles rather than hard engineeringsolutions is beneficial both aesthetically and ecologically.DMRB 10.3.1 (draft) recommends that the landscapedesign should be considered in terms of aesthetic conceptssuch as enclosure, proportion and scale, unity, order,harmony, contrast, variety, sequence, colour, texture andmaterials. Furthermore, one of the design objectives of HA59/92 (DMRB 10.1.5) is the development of vegetationwithin highway land that has sustainable wildlife value andlinks with wildlife habitats in the surrounding landscape.Even where willow species do not sit entirely comfortablywithin the local environment, in many cases they willprovide a greater wildlife and visual value than thatoffered by such alternatives as granular reinstatement orconcrete retaining walls.

When assessing alternative solutions for improvingexisting roads, DMRB 10.3.1 (draft) notes that animportant consideration is ‘sustainability’. The concept ofsustainable development is inherently concerned withreconciling the benefits of development with the potentialenvironmental costs (Department of the Environment,

1996). Notwithstanding other considerations, the solutionthat achieves the best measure of sustainability should bethe favoured option (DMRB 10.3.1). There may be somedifficulty in determining the level of sustainability as thisvaries with time, perception, understanding andknowledge. On the whole it is likely that soft solutions (i.e.bioengineering) and those which draw from the locality(for example, gabions filled with locally quarried stone)are more sustainable than more traditional slopereinforcement methods. Schemes which require a low levelof maintenance can also be said to be more thansustainable than those which involve a higher maintenancecommitment (Department of the Environment, 1996).

The initial capital costs and the whole life costs ofbioengineered solutions may be significantly less than forhard options. Figure 10, taken from Barker (1999a),provides an illustration of the differences in costs of aninert system and a bioengineered system which requiresregular coppicing. If a whole life cost approach was takento the same example, then the steps in each curve in Figure10 would be reduced, by an amount determined by thediscount rate used. The bioengineered system wouldtherefore appear even more favourable than the exampleillustrated in Figure 10.

HA 13/81 (DMRB 5.2) provides advice on the plantingof trees and shrubs for highway schemes. One of thegeneral requirements of this advice note is that trees andshrubs achieve a sufficient growth rate that some visualeffect is achieved within a reasonable length of time. Thismay require the provision of 300mm thickness of topsoilto enable plants to become established. Such a thickness oftopsoil appears to be beneficial to the successful growth oflive willow poles (Section 4.1). The topsoil layer may bevulnerable to failure before root systems have developed,

Figure 9 River bank protection using willow spiling (Willowbank, 1998)

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but this may be stabilised temporarily by pinning (DMRB4.1.3). The use of willow poles may be beneficial inproviding sufficient restraining action to the topsoil untilthe plants have become established.

In summary, guidance currently provided by theHighways Agency requires that, as far as possible, planting:

i uses locally appropriate species;

ii improves the landscape;

iii improves ecological value;

iv avoids single species planting; and

v avoids monotonous road-user viewed landscapes, forexample by leaving gaps in appropriate locations so thatthe view can be appreciated.

In addition, there is a requirement to consider thesustainability and whole life costs of construction works.Some of these issues may appear to conflict with the useof willow poles, but when compared with many of theother engineering alternatives, willows may provide aviable option. Landscape and ecological value may beenhanced by interspersing willows, once established,with other species, especially at the margins of thewillow plantation. Compliance with DMRB may requirethe involvement of landscape architects during design toensure that planting is adequately integrated into itssetting (HA 44/91; DMRB 4.1.1).

3.3 Potential problems and their mitigation

Although bioengineering in general, and specifically theinstallation of live willow poles, may offer many potentialbenefits for the stabilisation of slopes, the techniques arenot without a number of difficulties and potential

problems. These may be divided into two categories:difficulties in establishing viable plants in the requiredlocation and potential adverse impacts which may arisethrough the use of vegetation.

3.3.1 Difficulties in establishing plantsThe development of plant roots is determined inter alia bythe structure of the soil. In compacted soils, experimentalwork reported by several workers (Greacen and Oh, 1972;Hemsath and Mazurak, 1974; Taylor and Ratliff, 1969) hasshown that soil strength is a key factor determining rootdevelopment. Roots tend to spread both downwards andoutwards, but when a hard stratum is encountered,downward growth is often impeded and the roots tend tospread laterally until a weakness in the stratum isencountered. This weakness may be exploited by the rootsuntil another impeding layer is reached.

The difficulty in developing root systems into strongsoils is potentially a problem for the establishment ofwillows, or any other trees or shrubs, on embankmentsconstructed from compacted heavy clays and on cuttingslopes in stiff clays. However, some plant species arecapable of rooting in compacted soils and are used inagricultural applications where it is not practicable toameliorate compacted subsoils by mechanical means (seefor example, Elkins, 1985; Goss, 1987; Dexter, 1991). Thesite trials undertaken within the current project will need toestablish the depth to which roots on live willow poles canemerge and develop. It would be beneficial to evaluate thisboth on cutting slopes, where the ground is in situ and onembankment slopes where the fill has been placed andcompacted by mechanical means.

Net part - life

benefitInitial

capital

costs

Regular coppicing of

living components of project

over 10 - 15 year cycles

A = Subtraction of bioengineered system

excess cost during establishment3

2

1

0

Co

st

0 5 10 15 20

Major repair or replacement of

decayed inert components of

project after 20 25 years

Time (years)

Figure 10 Comparison of costs of inert and bioengineered projects (Barker, 1999a)

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The use of de-icing salt on highways can have adverseeffects on plant growth. Colwill et al, (1976), Dimitri andSiebert (1979), Thompson et al. (1979) and Colwill et al.(1982) have considered the levels of salt on the highwaynetwork and the salt tolerance of plants. In general, it isreported that plants are more tolerant of salt spray thanthey are of salt applied to the soil (Thompson et al., 1979).Colwill et al. (1982) found that, beyond 5m from the hardshoulder, concentrations of salt in the soil were belowthose at which the effects on plants are noticeable. Dimitriand Siebert (1979) found that under conditionsencountered in Germany, willows and poplars were themost resistant to salt applied either to the plant or to thesoil of those species investigated. However, there weredifferences between clones as well as species. With regardto the proposed trial of willow poles, the success isunlikely to be significantly influenced by the tolerance tosalt, since Colwill et al. (1982) found that the south ofEngland is an area where the damage or growth reductioncaused by salt is negligible to low. Salt tolerance may be ofgreater significance if the technique is subsequentlyapplied in other parts of the UK.

The fertility of the soil plays an important role in thedevelopment of roots. Roots of established trees proliferatein moist, nutrient rich soil, especially soils rich in nitrogenand phosphorus (Dobson, 1995). In general, soils with a lowfertility produce root systems characterised by long, slender,poorly branched surface roots, whereas higher fertility leadsto root systems that are well branched and descend togreater depth in the soil, provided that it is sufficientlypenetrable and oxygen is available. A chemical analysis ofthe Reading Beds and Gault Clays (Marriott 2000) showedthe nutrient contents presented in Table 1. Marriott has usedan annual dose of controlled release fertiliser containingmicronutrients to grow trees on these soils. The proposedtrials of live willow poles need to establish whether this isnecessary for poles to grow and thrive.

for willows, trees which produce fruit or other debriswhich may be harmful to livestock will preclude their usewhere they may overhang farmland.

Subsurface, tree roots can have an adverse effect onpipes. Instances of pipes being broken by the growth ofroots are rare, but blockage of damaged or leaking pipesby root growth is not uncommon (Patch et al., 1997).Patch et al. stress that roots do not break pipes through thestress exerted by increasing their diameter, nor do rootsforce their way into pipes to gain access to water andnutrients. If moisture escapes from a water-carryingunderground pipe, a moisture gradient will develop in thesoil. Tree roots in the vicinity of the leakage may flourishin the moist soil and penetrate the pipe at the seepagepoint. Roots will then proliferate inside the pipe andeventually may cause a blockage. The growth of the rootthrough the hole may not enlarge the hole: the root willexpand as it develops on either side of the hole but willremain constricted within the hole (MacLeod and Cram,1996). The force which can be exerted on buried pipes bygrowing roots, although sufficient to move the weight ofthe pipe alone, is insufficient to move a properly instatedburied pipe, due to the resistance offered by the fillmaterial (MacLeod and Cram, 1996).

A detrimental influence that trees may have, whichcould lead to damage or blockage of pipes, may occurthrough drying and shrinkage of clays as a result of treegrowth. Shrinkage of clay may cause pipes to move,opening up joints (Biddle, 1992). Modern pipes with fewand flexible joints are less susceptible to such problemsthan older pipes, particularly those with clay packed jointsor cement seals which fail when they dry out. Carefullylaid modern pipes should not be vulnerable to rootinvasion, unless they are disturbed and damaged bysubsequent adjacent excavations (Patch et al., 1997).

Although structures or buried pipes are unlikely to bedamaged by root growth, roots can exert sufficient force tolift paving slabs, cause cracking of boundary walls anddamage driveways, footpaths and cycle tracks (MacLeodand Cram, 1996). Additionally, reduction in moisturecontent through the planting of trees can cause settlementson shrinkable clay soils which may lead to more seriousdamage to property (for example, McInnes, 1984).Conversely, removal of mature trees from shrinkable claysoils can cause heave which can also be damaging tostructures (Cheney, 1988). It will therefore be necessary toconsider carefully the locations at which willows, or anyother species, are planted to improve slope stability if thereare properties in the vicinity. Planting distances on soilswhich do not contract upon drying should be determined byconsideration of the relative size of trees and adjacentbuildings; the ultimate size of the trees and any otherundesirable qualities of the trees concerned. The SubsidenceClaims Advisory Bureau (undated) provides guidance on‘safe’ planting distances for trees (see Table 2).

Where the risk of invasion of pipes such as drains orsewers by roots is considered to be a possibility, or whereit is undesirable for roots to encroach under sensitivebuildings, root barriers may be constructed. However, suchbarriers cannot be considered as a panacea applicable to

Table 1 Results of chemical analyses of clays (Marriott,2000)

Reading Beds clay Gault clay

pH (CaCl2) 7.62 7.52

%C 0.14 1.15%N 0.04 0.04P

2O

5 (mg/100g) 56.90 93.12

Exchangeable cations (meq/100g)Ca 17.17 38.28Na 0.19 1.75K 0.97 1.17Mg 8.18 4.20

3.3.2 Effects on adjacent property and servicesGrowth of trees can have impacts both above and belowground. At the ground surface, consideration may need tobe given to issues such as interruption of sight lines,shading of adjacent buildings and the shedding of leavesand brittle branches (British Standards Institution, 1991,Gellatley et al., 1995). For some species, although not so

13

every situation (Marshall et al., 1997). To be effective,root barriers should extend to the ground surface toprevent roots growing over the top and should besufficiently deep and long that roots cannot pass around orbeneath them. Root barriers should also be continuous andcontain no gaps through which roots could pass (BritishStandards Institution, 1991). Materials and methods ofconstruction should be chosen with regard to theirdurability and ease of installation. Comprehensiveguidance on tree root barriers and their use is given byMarshall et al. (1997).

3.3.3 Other difficultiesPlanting groups of plants of similar species in closeproximity to one another may encourage the rapid spreadof pests and diseases. Although it may be possible tocontrol diseases and pests through spraying or othertreatments, the costs and ecological consequences of thiswill need to be considered. It may be more satisfactory tointersperse willows with other species to reduce thepotential for the spread of disease.

Some of the pests and diseases to which willows maybe susceptible are as follows (Hessayon, 1991). Willowsprovide the food plant for the eyed hawk moth caterpillarand may also be susceptible to leaf beetle. Damage israrely severe but may be treated, if necessary, byspraying. Other problems which may be treated byspraying include willow scale, which feed on the sapcausing leaf yellowing and loss of vigour; willowanthracnose (leaf spot), a fungal infection; and powderymildew, which occurs particularly if shrubs are overcrowded. Rust causes raised yellow or brown spots on theleaves. Silver leaf causes leaves to take on a silver colour,leading to die back of shoots. Both of these problems

require removal and burning of the affected material.A potential problem with any planting regime for slope

stabilisation is that the disturbance caused to the slope mayincrease the potential for water ingress. In particular, theexcavation of planting pits to establish vegetation cancause ponding (DMRB 4.1.3). This problem is partiallyalleviated by the willow pole technique, since the livepoles must be driven into the ground in close contact withthe soil. However, if the willow poles fail to root they willeventually decay. The biopores left by decaying anddecayed poles will provide a direct route into the incipientfailure zone of the slope, which will increase the rate ofwater ingress to the detriment of the slope stability.

A design objective of HA 59/92 (DMRB 10.1.5) is thedevelopment of vegetation within highway land that hassustainable wildlife value. A problem which may arisethrough the establishment of vegetation on highway slopesis that the cover provided may attract burrowing animals,particularly rabbits, but also foxes and badgers (Barker,1997a). This has not been reported to be a problem on thehighway network, but a number of surface sections of theLondon Underground have been affected (Barker, 1997a).Several sections of track have been undermined byburrowing resulting in reduced levels of service and a needfor costly maintenance. Vegetation cover may also attractlarger animals such as deer and badgers which may presenta hazard to traffic. A review of the effects of vegetation onsurface sections of the London Underground was given byGellatley et al. (1995).

4 Case histories

4.1 M20, Longham Wood cutting

A CIRIA research project began in 1991 to undertake afield trial to develop a demonstration site forbioengineering. The trial was described in detail byGreenwood et al. (1996) and is briefly summarised here.The location for the trial was a new cutting slope on theM20 near Maidstone in Kent, excavated during motorwaywidening works. The cutting is in Gault Clay: slopes inthis geology in the area of the trial have a history ofshallow seated slope failures. Failures in the LonghamWood cutting were summarised in Greenwood et al. Theobjective of the trial was to establish a bioengineeringdemonstration facility to encourage the use of thetechniques and to provide an opportunity to compare theperformance of different types of vegetation.

The cutting was constructed to have slopes graded at 1:6and, for the benefit of the trial, a section was cut at 1:3.Counterfort drains were installed in some locations; otherparts of the slope were undrained. Top soil was placed at athickness of 50mm and 300mm; other locations were leftwithout top soil. Fertilisers were applied of various typesand quantities to different locations on the site. A total ofseven different vegetation types were used, includingwillows/alders, various shrubs, grasses and left bare. Allplanting was either as pot grown stock or, for the willows,peat plugs and was undertaken in May 1994. This trial didnot investigate the use of live willow poles, but the data

Table 2 Suggested safe distances from properties forplanting trees (Subsidence Claims AdvisoryBureau, undated)

Suggested Maximumminimum recorded

Botanical distance from rootCommon name name property (m) spread (m)

Cypress Cupressus 3.5 20.0Cypress Chamaecyparis 3.5 20.0Birch Betula 4.0 10.0Apple Malus 5.0 10.0Pear Pyrus 5.0 10.0Cherry, plum and peach Prunus 6.0 11.0Hawthorn Crataegus 7.0 11.5Rowan Sorbus 7.0 11.0Plane Platanus 7.5 15.0Lime Tilia 8.0 20.0Black locust Robinia 8.5 12.4Beech Fagus 9.0 15.0Ash Fraxinus 10.0 21.0Horse chestnut Aesculus 10.0 23.0Elm Ulmus 12.0 25.0Maple and sycamore Acer 12.0 20.0Oak Quercus 18.0 30.0Willow Salix 18.0 40.0Poplar Populus 20.0 30.0

14

provide useful guidance on root development, plantgrowth and the use of vegetation for slope stabilisation.

An instrumentation system was installed and amonitoring programme devised which aimed to investigatethree main issues:

i vegetation growth and root distribution;

ii changes in soil moisture content and pore waterpressures (including suctions);

iii modification of soil strength by the effects ofweathering and reinforcement by root growth.

Of most relevance to the current research was theperformance of the willows and alders. Where the plotswere top soiled, the willow growth was successful, withvery few failures. On the plots with no top soil, growth waspoor and a number of plants failed. The amount of top soilaffected the success rate of the plants, including the willowsand alders. On deep (300mm) top soiled plots, growth wassimilar on both the primary plots, which were at a gradientof 1:3, and on the secondary trial plots which were graded at1:6. Growth was consistent but less vigorous on the 150mmtop soiled plots than on the 300mm soil. Plots without topsoil performed badly, suffering from summer drought andexperiencing some plant failure.

The Transport Research Laboratory’s involvement atLongham Wood has shown that, after 5 years growth, thedifferent species and hybrids of willow had grown well onthe top soiled plots, although some individual goat willows(Salix caprea) and grey willows (S. cinera) had very poorgrowth. In particular the osiers (S. purpurea and S.viminalis) had reached a top height of 4m. The drainage ofthe plots had a significant effect on canopy coverage,being twice as high in drained plots as in undrained plotsby 1996. In the following two years, the canopy coverageon the drained and undrained plots became more similar,although the drained plots retained the greater coverage.

For reinforcement and drainage of the soil, the rootgrowth is perhaps the most important parameter. Thewillows had only developed roots down to approximately1m after 5 years and the roots at this depth were mainlyseasonal small fibrous roots. Most of the woody roots,which persist all year round, were located close to thesurface: 75 per cent of all rooting was within 150mm ofthe soil surface.

Most shallow slope failures occur at depths ofapproximately 1.5 to 2m (Perry, 1989). Therefore, at thetime the project was reported, the root systems developedin the Longham Wood experiment would be ineffective inpreventing slope failures, except than they may bind thesoil surface thereby reducing desiccation cracking.Conversely, however, the growth of plants will increasethe rate at which soil dries out and therefore mayexacerbate desiccation cracking.

A large volume of data was acquired during the study atLongham Wood. However, some of the data may be oflittle practical use in establishing how best to undertakebioengineering projects. It is important that in future trials,instrumentation is focussed on measuring those parameterswhich are most significant in establishing the effectivenessof the technique. For investigation of the performance of

willow poles, the measurements are likely to be mostbeneficial if focussed on determining the rate and extent ofroot growth and the effect of the plants on soil moisture.

4.2 A249, Iwade

During the late Spring of 1996, some 500 live willow poleswere installed in a new cutting slope on the A249 at Iwade,Kent. The 1:3 slope was cut into Woolwich Beds Clayoverlying Woolwich Beds Sands. A comprehensivedescription of this trial and the initial performance of theplanting was given by Barker (1997b).

Seven different methods of installing the willow poleswere used. These used a range of sizes of auger and insome cases two different sizes of auger were used to boreeach hole. In all methods, the pole was placed in the boredhole and then driven to refusal. A number of poles wereonly partially driven and then left exposed for some daysbefore being fully embedded. It is likely that this exposureadversely affected the survival rates, particularly since theweather during this period was hot, dry and with strongbreezes.

Following an initial growth response of 95 per cent,the willow poles suffered a die back such that by May1997 only 15 per cent of the installed poles remainedalive. Barker (1997b) attributed this to poor handling,storage and installation techniques and a reduction of theavailable moisture in the slope. The soil moisture levelwas reduced by a long period of dry weather and theconstruction of a crest drain and counterfort drains on theslope. This situation was exacerbated in a large numberof cases by the installation of the willow poles into thefree-draining granular backfill in the counterfort drains,which had been capped with 0.75m of clay, rather thaninstallation into the soil between the drains. The areaswhich had the greatest rate of successful growth werethose where the slope was wettest.

Where poles did survive, their performance, indicatedby the soil moisture content data, was promising. Theupper 0.75m of soil at the live poles was 6 to 14 per centdrier than the control panel, even though the control panelwas located directly above a drain.

Although the success of the Iwade trial was to someextent limited, it was possible to draw some usefulconclusions from the data acquired.

i The pole diameter should be between 30 and 45mmand poles should be obtained from growth that is 3 to5 years old. Closely grown plantation stock providesthe best poles because of the enforced straight growthof the poles.

ii The installation depth required is site specific, but forengineering reasons, a depth of greater than 1.5mwill generally be required. Although the long termvegetative effects of the willow poles are the primaryreason for their installation, the poles also provide animmediate reinforcing action. However, theimmediate benefit of live poles only extends to theinstallation depth.

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iii Seven different installation methods were tried. Themost successful poles were driven using a method bywhich the hole was augered, the pole was partiallydriven and then extracted and then a smaller diameterhole was driven followed by driving of the pole torefusal. The success of this technique is considered tobe coincidental. Based on present knowledge, there isno reason why this technique should be more successfulthan any other. The main influence on the rate ofgrowth is thought to be the addition of water to helpease the poles into the ground during the final drive.

iv For the two types of willow used during this trial, Salixfragilis tends to be less straight than S. viminalis,making the latter preferable, if it can be obtained inlarge enough quantity and suitable dimensions. In thistrial, the small dimensioned S. viminalis had almosttwice the survival rate of the S. fragilis.

v Sourcing of sufficient supplies of willow poles shouldcommence well ahead of their installation date to ensurethat they are installed during the optimum plantingwindow and while they are fresh. This may depend tosome extent on the location and aspect of the site, butplanting is normally required during the winter(Willowbank, 1998).

vi Synchronisation of the supply of the live poles to thesite, transferal to the installer and the installationprocess will minimise the period for which poles mustbe stored and will optimise the installation rate. Freshstock should be stored in water and transported wrappedin saturated hessian sacking covered by tarpaulins.

viiWrapping a small number of turns of light wire aroundthe tops of the poles before driving will limit crushingdamage and prevent splitting.

4.3 Great Lakes shoreline stabilisation

The Rocky Gap Bluff Stabilisation Project (Gray and Leiser,1982) aimed to stabilise clayey glacial till slopes caused byperiodic undermining of the bluffs by wave action, followedby intense surface erosion. The rate of erosion was such thatnatural vegetation was unable to colonise the slopes. Theobjective of the project was to vegetate the bluff and theupper part of the beach as quickly as possible, therebyarresting surface erosion and reducing the likelihood offurther undercutting during storms.

Of particular relevance to the current study was the useof poles set 4ft (1.2m) into the ground, spaced on 2ft(0.6m) centres to form the posts for a living dam,comprised of live willow brush and straw, at the toe of theslope. In addition, the entire slope was seeded and coveredwith a mulch. The mulch was held in place using a meshfencing secured by stapling to live willow stakes. Thesewillow stakes were 1 to 2 inches (25 to 50mm) in diameterand were driven into the slope to a depth of 12 to 15 inches(300 to 400mm), leaving 4 to 6 inches (100 to 150cm)protruding above the ground surface. All willow poles andbrush used on this scheme were cut during the winter andkept moist until used in the following March.

Approximately 30 per cent of the willow stakes used tofasten the fencing over the mulch were reported to have

grown. On the upper one-third of the slopes, described byGray and Leiser (1982) as ‘somewhat droughty’, very fewof the willows grew. The authors considered that thesuccess rate might have been increased by preformingholes for the stakes. Best growth of the willow stakes wasobserved where the slope was wettest. The growth rate ofthe living dam was less successful. Although the willowbrush and cuttings thrived, the posts did not root.

4.4 Revegetation of disturbed slopes at Lake Tahoe

The Lake Tahoe Basin is located in the Sierra Nevada,California. Revegetation of man made slopes occurs veryslowly, making erosion of highway and other cut slopes aproblem. In this example, the water quality in Lake Tahoewas being rapidly reduced by particulate and nutrientpollution (Gray and Leiser, 1982).

A restoration project was undertaken with the objectivesof identifying appropriate species to revegetateproblematic sites and to develop methods of establishinggrasses and woody plants. Trials to establish willowcuttings found that, although the native species, Salixlemmonii, could only propagate naturally on wet sites,when propagated from cuttings they would survive andthrive on relatively dry sites. In addition, althoughtreatment with hormone increased the speed of rooting andthe number of roots developed in greenhouse conditions,this benefit could not be identified in the field. The trialswere undertaken with three diameters of cuttings: ¼ inch(6mm) or less; d to ¾ inch (10 to 19mm); and f inch(22mm) and larger. In this case, the intermediate sizedcuttings gave the best results, although the authorsacknowledged that this result differed from those reportedelsewhere, which showed that in general cuttings largerthan 1 inch (25mm) in diameter rooted best.

One of the main conclusions reached by Gray and Leiser(1982) was that slopes must be reasonably stable to allowplants to become established. These authors advocated theuse of breast walls and willow wattling as inexpensiveways of stabilising slopes until the vegetation had becomeestablished. Revegetation was considered to be a practicaland attractive alternative to the more expensive mechanicalor traditional engineering solutions.

With regard to stabilisation of UK highway slopes, thestudy reported by Gray and Leiser is of interest because ofthe success of the willows on relatively dry sites. However,S lemmonii is not listed as a species existing in Britain(Meikle, 1984). Additional observations from the LakeTahoe site were the demonstration that rooting hormonedid not improve growth rates in the field and the need tohave a degree of slope stability to establish woody plants.

4.5 Rock Lake stabilisation

In 1985, a study commenced to investigate the viability ofusing bioengineering techniques as an alternative toconventional construction for the stabilisation of highwayslopes. A test site on Highway 60 in Algonquin Park wasplanted in the spring of 1986. A number of bioengineeringsolutions were investigated and reported by Harrington &Hoyle Ltd and Lumis (1991). The review presented here

16

focuses mainly on those aspects which are relevant to theuse of live willow poles.

The study site was a north facing slope cut in gravellysands. The slope was divided into three horizontalsections. Level 1 was on the lower slope and was in thearea of greatest moisture content. Level 2 was at mid slopeand Level 3 was the upper part of the slope.

In addition to the live stakes, willows were also used forbrush layers. Two types of plants were used, termed ‘oldwillow’ for branches from slower growing plants whilebranches from more vigorous plants (2 to 3 years old) weretermed ‘new willow’. Of the willows, only the old willowwas used for live stake installation, together with poplarstakes. Willow stakes were 50cm long and of two sizes:large (40 to 55mm diameter) and small (20 to 30mmdiameter). Stakes were driven into the ground to leave10cm exposed.

The most significant influence on the success of brushlayering during this trial appears to have been theavailability of moisture. The willow regenerated and thrivedmost successfully at Level 1, where the soil was wettest. It isnoteworthy that the young willow regenerated much betterthan the old willow at Level 3 where the soil was relativelydry. It was hypothesised by the authors that the youngwillow may root more quickly than the older stock andtherefore was better able to survive in the dry soil.

For the live staking, the small and large willows grewequally well at all levels, although there appeared to be aslight trend towards improved growth at Level 1 where thesoil was moist. Small willows had a higher growth rate atLevels 2 and 3 than did the large willows. The authorsconsidered that this may be because of the smaller size,younger age and greater ease of rooting of the smallerwillow stakes. Some live willow and poplar stakesproduced shoots soon after installation, but subsequentlydied at the end of the growing season. Excavation of suchstakes revealed that no roots had developed. The shootshad been able to grow because of stored carbohydrate andwater reserves within the stakes. Root regeneration in thefirst season was shown to be critical to the survival of livestakes. Harrington & Hoyle Ltd and Lumis (1991) foundthat the stakes with the most vigorous shoot growth hadalso developed roots. Survival of the stakes wassignificantly lower in Year 2 than in the first year afterplanting (Table 3).

The application of fertiliser had no effect on theregeneration of any of the plant types used either in brushlayers or live staking. However, survival in subsequentyears was considered to be enhanced by application offertiliser, lime or by mulching.

Harrington & Hoyle Ltd and Lumis (1991) concludedfrom this study that the following factors were important inmaximising success.

i Fresh material should be used. Stakes should beinstalled such that they are in complete contact with wellcompacted soil, free of air spaces.

ii Selection of the best species has a significant effect onsuccess rates.

iii The young willow cuttings had a higher regenerationand growth rate than did the older wood.

iv Use of cultivars selected for their superior rooting abilityand grown in nurseries will maximise success ofpropagation.

v The most critical factor appeared to be the availability ofadequate soil moisture. In particular, optimumconditions are required for adequate rooting to ensuresurvival in the first year. Plant material should be asyoung and vigorous as is practicable.

In addition, the authors recorded that during planting (inApril) soil moisture froze overnight, forming a thin crustwhich had to be broken with a pick. This indicates that,providing that willow species are selected which areappropriate to the climatic conditions, the live poletechnique may be viable throughout the UK.

5 Construction methods

5.1 Selection of suitable species

For a plant species to be able to stabilise slopes,Greenwood et al. (1996) stated that vegetation ideallyrequires the following characteristics:

i rapid transpiration, winter transpiration activity and anextensive root system to reduce the soil moisturecontent;

ii rapid, deep root growth and a perennial root system toreinforce the failure plane;

iii multiple deep roots with laterals to provide buttressing;

iv a high leaf area ratio which persists through hot summerweather to provide surface shading.

Marriott (1996) presented a list of tree and shrub specieswhich may be potentially suitable for use in stabilisingslopes on over-consolidated clays. The selection criteriaincluded their suitability for growth in clay soils; waterrequirements; tolerance of wind, salt and pollution;quantity and persistence of leaf litter; perenniality; height;response to cutting; and root depth and morphology.Marriott proposed 25 species which may be useful fortrials to determine their ability to grow in overconsolidatedclays, with and without topsoil, and which would reducethe likelihood of shallow slope failures. Particularconsideration was given to the role of roots in effecting

Table 3 Survival rates (per cent) of three types of livestakes over two years at three levels on a slopereported by Harrington & Hoyle Ltd andLumis (1991)

Poplar Small willow Large willow

Level Year 1 Year 2 Year 1 Year 2 Year 1 Year 2

1 (lower) 73 22 67 37 55 222 (mid) 64 37 78 12 36 03 (upper) 55 0 56 12 20 0

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soil reinforcement and reducing the moisture content. Afurther important consideration was tolerance to theenvironment which is peculiar to the roadside, in particularresistance to pollution from exhaust fumes and salt spray

In selecting plant species for stabilisation, the nature ofthe root system is possibly the most importantconsideration. Not only do the roots anchor the plant in thesoil, they also invade the soil environment to remove waterand nutrients to sustain the plant. The form of the roots isnot only dependent upon genetic factors but also on theground and groundwater conditions in which the plant isgrowing. In well drained soils, roots generally penetratemore deeply and cover a wider area than plants grown inmore moist soils (Russell, 1973; cited by Marriott, 1996).

Moran (1949) described the benefits of using falseacacia or black locust (Robinia pseudacacia) for stabilisingrailway and highway slopes. This species has a string taproot which Moran stated would penetrate as far into thesoil as there is air present, down to 20 feet (6.1m) or more.Furthermore, it is a fast growing tree and will grow onsteep slopes and in well drained soils. It also thrives onclay soils. Moran considered that planting this tree specieson slopes produces the equivalent effect to that whichwould be obtained by driving a series of piles into theslope. One potential restriction to the use of Robiniapseudacacia for slope stabilisation is that it is a nativespecies of North America and does not occur naturally inthe UK. It may therefore not be considered suitable for usein all locations where landscape and ecologicalconsiderations are particularly important (Section 3.2).

Vegetation can be established from seed, cuttings, bare-rooted and container-grown plants. Bare rooted plantsgrown from seed are often used in forestry. Willows andpoplars are the only tree species for which the establishmentof plants from unrooted cuttings is a practical proposition(Marriott, 1996). These species are therefore likely to be theonly ones which can be used to effect an immediatereinforcement of the slope by driving live stakes.

An important consideration in selecting plant species forthe stabilisation of highway slopes is the amount ofmaintenance required. Issues to be considered are thetolerance of the plants to competition from invadingspecies, and the consequent requirement for the use ofherbicides or mechanical methods of weed control; theneed for fertiliser to sustain growth; and the frequency ofcutting required. Problems may also be caused by leaflitter blocking drains. The use of evergreen species, ordeciduous species which produce non-persistent litter, maybe beneficial but such species cannot be propagated by thelive pole method.

The aspect of slopes modifies the microclimate such thatsouth facing slopes are typically warmer and drier andhave a longer growing season than north facing slopes.Consideration should be given to the solar radiationreceived and the wind speed and direction (Coppin andRichards, 1990). Highway embankment sites arecommonly elevated above the natural ground level. Theexposed conditions may increase the risk of scorching bythe sun or damage by wind, particularly for young trees(Dunball, 1979).

The results of the trial at Iwade (Section 4.2) andinformation from the literature suggested that the greatestsuccess was with willows or poplars. In particular, ifunrooted cuttings were to be used, then these species werethe only viable options (Marriott, 1996). The number ofspecies and hybrids of willows is vast (Meikle, 1984) andmany varieties have very similar characteristics.Appropriate species for use in the trial may be Salix albacaerulea, which is an exceptionally fast growing variety.This variety is considered to be one of many hybrids ofSalix alba and S. fragilis, but in its essential characteristicsis allied to the former. This eliminates the potentialconcerns over the use of S. fragilis close to roads wherethe shedding of branches may present a potential problem.Although the rapid growth of S. alba caerulea is useful inrapid development of slope stabilisation, the disbenefit isthat more frequent maintenance (trimming and coppicing)may be required than for less vigorous varieties. Smallertree varieties or shrub varieties of willow provide thebenefit of potentially less intrusion by branches into thesurroundings. Varieties such as Salix dasyclados may beuseful in this respect.

Selection of species for any particular application maybe dictated to a large extent by the availability of sufficientsuitable poles and this may over ride the importance ofother factors. The individual properties of the availablestock should be verified by reference to Meikle (1984) toensure that the species are appropriate for use in the sitespecific conditions and for slope stabilisation.

5.2 Pole length and diameter

The use of willow poles to stabilise highway slopes isintended to provide an immediate reinforcement of theslope through the effect of soil nailing and have longerterm beneficial effects brought about by the growth of theplants (Section 3.1). In order that these benefits can accrue,the effect of the poles must extend to a depth greater thanthe depth of the potential failure surface. Perry (1989)reported that the failure surface in cutting and embankmentslopes rarely exceeded 1.5m. Other authors (for exampleBache and Coppin, 1989) have suggested that shallow slipfailures may occur at 1.5 to 2m depth. It is thereforenecessary for the effect of any plants to extend to a depthgreater than this.

The root systems of most plant species are concentratedclose to the ground surface, but their morphology isdetermined by numerous factors (McMichael andQuisenberry, 1993; Coppin and Richards, 1990; Gray,1994; Dobson, 1995). Many of these factors act to restrictthe penetration of roots from plants on slopes to a depthwhich is insufficient to extend significantly beneath the1.5m failure depth. A particular problem exists in slopeson embankments which are formed from well compactedclays and are therefore difficult for roots to penetrate.Mechanical resistance offered by the soil may impede rootgrowth, encouraging development along routes of leastresistance (Bennie, 1996). Fryrear and McCully (1972;cited by Bennie, 1996) proposed that, given sufficienttime, the roots of perennial plants would find sufficientplanes of weakness to allow them to permeate most of the

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soil volume. The detrimental effects on root growth of soilcompaction may therefore be overcome in the longer term.

Under suitable growing conditions, some tree species havebeen shown to deplete soil moisture to considerable depth.For example, Biddle (1985) reported that mature poplar treesdepleted the soil moisture in clay soils to a depth of 3.5m insummer. Coppin and Richards (1990) pointed out that theeffect of plants on reducing soil moisture was dependent uponthe season. The transpiration rate and metabolic activity isgreatly reduced during the winter, making the impact ofvegetation on the soil moisture content at its minimum whenthe rainfall is a maximum. Coppin and Richards concludedthat, during the Spring, which is the critical time for slopestability, because the soil moisture is high but the transpirationrate has not begun to rise, the ability of trees to improve thesoil strength through a reduction in soil moisture may be lessthan the reinforcing effect provided by the root system.

The use of willow poles may provide a means by whichroot growth can be extended to depths beyond those whichwould be achieved by plants established at the groundsurface. To maximise the benefits of using willow poles itis necessary to ensure that willow poles are driven to adepth greater than 1.5m. An installation method capable ofachieving this depth needs to be identified for the fieldtrials. Poles of at least 2m length will be required.

It will be necessary to determine that root growth occursand develops at the greatest depth of penetration. Thepenetration resistance of the soil may be too great for rootsto develop. If a rooting system does not develop below adepth of 1.5m, it will be necessary to determine whetherthe willow pole below the maximum root depth willendure or whether it rots in the longer term. This couldpotentially affect the long term viability of willow poleinstallations as a means of effecting slope stabilisation.

In addition to considering the length of poles to beinstalled, the diameter must also be specified. It is generallyacknowledged that the greater the biomass, the greater willbe the success rate of establishing unrooted cuttings. This isin part due to the need to withstand desiccation (Barker,1999b). There is a trade off between the size of the cuttingsand their availability (time required to be growncommercially), ease of handling and practicalities ofinstallation. Large cuttings may give the best chance ofgrowth, but will need to be obtained from fast growing stockso that they are both sufficiently large and vigorous(Harrington & Hoyle Ltd and Lumis, 1991). In addition, theweight of large poles may complicate handling on site.Conversely, thin poles may be easily handled, but they mayhave a higher rate of failure. A further limitation may be atendency for poles driven into the soil by percussivemethods to flex or break during installation if they are toothin. Poles are usually installed which have a diameter of 40to 100mm since these provide sufficient tensile strength andare suitably stiff to facilitate installation (Barker, 1999).

5.3 Harvesting, handling and transportation

Willow for live pole use should only be harvested duringthe winter period of dormancy (Schiechtl and Stern, 1996).In the UK, this means that harvesting takes place betweenNovember and March. For live planting, Willowbank

(1998) recommends that replanting should take placewithin about 3 weeks of harvesting. This means thatconstruction works should be scheduled for the winterperiod. However, Willowbank claim that chilled storage oflive poles can enable poles to be supplied and installedwell in to the summer. No data on the success rates ofpoles treated in this way have been identified.

Low survival rates at Iwade (Section 4.2) wereattributed, in part, to poor handling, storage andinstallation techniques (Barker, 1997a). Duringtransportation and storage on site, poles should be keptmoist by storing them either in a water tank or wrapped indamp hessian. In particular, live poles should be protectedfrom intense sunlight and strong winds which may causethem to dry out.

5.4 Planting method

5.4.1 Vertical or slope-normal planting

Live poles may be installed either vertically or normal to theslope surface. When installed vertically, the poles mimicmicropiles, buttressing the potentially unstable upper layersof the soil and driving failure surfaces deeper into the slope(Barker, 1999b). Live poles used in this way enhance theshearing resistance across potential failure surfaces andmove the failure surface down to a depth at which it isintended that the restraining forces offered by the soilexceed the disturbing forces of the reinforced soil mass.

When installed approximately normal to the slope, livepoles can be regarded as soil nails, acting primarily intension through the development of friction between thesoil and the surface of the poles. Poles are normallyinstalled in this fashion on steeper slopes than those wherevertical installation is used since vertical installationbecomes increasingly impracticable and ineffective as theslope angle increases (Barker, 1999b).

Vertical installation of live poles is generally morelikely to be practicable than slope normal installationbecause of the equipment used. Pre-drilling of holes priorto installation and subsequent driving of the poles into thebottom of the hole is more easily achieved if the poles arearranged vertically. No data are available to compare therelative success rates of vertical and slope-normal planting.

5.4.2 Length of pole remaining above groundThere is a need to leave a length of live poles standingproud of the ground to enable shoot development andgrowth. Consideration should be given to the potential fordesiccation to occur on exposed sites, particularly if toogreat a proportion of the total length of a live pole remainsabove ground. Typically, an exposed length of 300 to500mm is used (Gray and Leiser, 1982; Barker, 1996;Barker, 1999b). The top of this length should be trimmedand any damage caused by the installation process shouldbe removed (Barker, 1997b).

5.4.3 Spacing and planting patternPlant spacing will depend on the type of vegetation used.For rapid tree or shrub cover, Coppin and Richards (1990)

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suggested that staggered rows with 1.5m between plantsmay be used, but such close spacing will require thinningat an early stage. Planting for wooded areas is typically at2 to 2.5m spacing, but is species dependent.

There is little guidance available in the literature on therequirements for planting patterns and the spacing requiredbetween live willow poles. Willow and alder rooted stockplanted at the Longham Wood trial (Greenwood et al.,1996) were planted at staggered 1m centres. Barker(1996a) illustrated willow poles installed at typically 1 to2.5m spacing with the rows staggered. Barker (1999b)stated that live poles are generally planted as orthogonalarrays at 0.75 to 1m centres. Spacing can be varied suchthat the down slope interval varies, increasing to up to 5mso that the installation is effectively contour planted.

Spacing may need to be dictated by the requirements forimmediate improvement of the slope stability at a site andtherefore may be dictated more by slope stability calculationthan by the requirements of the established plants. Soil nailsmay be used over a fairly wide range of spacings. At theupper limit the nails (usually drilled and grouted) may belong, say 10m, at 2m or 3m vertical and horizontal spacings.In this case the design philosophy tends towards that ofground anchorages with a small number of widely spacednails providing the restoring force by anchoring the activezone of soil to the passive one (Johnson and Card, 1998). Atthe lower limit, a larger number of smaller nails (typicallysteel bar or steel angle, driven directly into the ground), whichare more akin to willow poles, are used. These are normally1m to 2m long and installed at 1m (or less) horizontal andvertical spacing. This division of nails into two types is alsoused in France under the headings of ‘widely spaced nails’and ‘nails at close centres’ (also termed ‘Hurpin’s method’,Clouterre, 1991) in which the nail density is given as 1 or 2nails per square metre of face. The advice given in HA 68(DMRB 4.1) for soil nail spacing only covers the maximumspacing between nails. The maximum recommended verticalspacing is 2m and, for any given design, the horizontalspacing should not exceed the vertical spacing.

In designing the layout of planting, it is necessary toconsider guidance given in DMRB, in particular the GoodRoads Guide (DMRB 10.1). Section 1 Parts 2, 3 and 4provide guidance on the integration of roads within thelandscape. Advice is also provided which is relevant toimproving existing roads in HA 63/92 (DMRB 10.3.1).With particular relevance to the use of willow poles tostabilise highway slopes, The Good Roads Guide advisesagainst the use of extensive groups of single species andthe abrupt ending of wooded slopes. Planting of otherspecies, particularly shrubs, around the area of willowsused to stabilise slopes will assist in complying with theserequirements. One of the key issues in establishingwoodland given by HA 56/92 (DMRB 10.1.2) is that treesand shrubs grow best in small blocks of the same species,which is the way they are found in nature.

5.4.4 Use of tree guardsTree guards should be installed around willow poles onsites which may be subject to rabbit or vole attack. Fencingcan be used to deter rabbits, but vole collars must be

placed around plant stems to prevent voles from removingthe bark of young plants (Sangwine, 1996), which areparticularly susceptible because they have thin bark(Barker, 1997b). Guards also provide protection againstdamage caused by mechanical flailing to clear invasivegrowth from around the willows. Selection of appropriateguards must consider the potential for closed types ofguard to cause overheating due to high solar gain and poorcirculation; the open mesh type might prove moresatisfactory, particularly on south and west facing slopesexposed to the sun.

5.4.5 Equipment and working methodMany slopes on which stabilisation by willow poles maybe a suitable remediation technique will be at angles whichmay make working hazardous. Safety in working shouldbe the main determinant of the kind of installation andmaintenance operations which are carried out on a slope.In turn, the kind of operations which can be carried out ona slope will determine the suitability of the use of willowpoles, or any other stabilisation technique. There istherefore a strong interdependence between slope angle,management programme and vegetation type whichdetermines the potential roles for vegetation in anysituation (Coppin and Richards, 1990). In general, thedangers of working on slopes increase with increasingslope angle. Guidance on safe working on slopes withagricultural vehicles has been given by the Scottish Centreof Agricultural Engineering. However, familiarity withequipment, assessment of site conditions and operatorexperience must determine whether a particular operationis safe or not (Coppin and Richards, 1990).

Installation of cuttings will require a hole to be madeinitially into which the pole can be driven. Schiechtl andStern (1996) commented that the pole can then be drivenin further than the base of the preformed hole withoutcausing harm to the pole, provided that the basal cut on thepole is at a slant. These authors also pointed out thatmechanical hammers can be used to install large poles.When holes are bored to accept the poles, the boreddiameter should be as close as possible to the polediameter, or possibly slightly smaller. It is important forthe growth of the pole that as much of the pole as possibleis in close contact with the soil (Harrinton & Hoyle Ltdand Lumis, 1991). The trial at Iwade (Section 4.2) showedthat a few turns of light wire wrapped around the head ofeach pole will prevent crushing damage and splitting of thepole (Barker, 1997b). Barker (1997b) also suggested thatthe introduction of a small quantity of water to each holeimmediately before the pole is installed may provide somelubrication to assist installation.

6 Instrumentation for performanceassessment

Whatever the application, geotechnical instrumentationsystems need to be designed, specified and installed with aclear knowledge and understanding of the purpose of themeasurements. All instruments should be targeted at both

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the parameters of interest and, since there will inevitablybe budgetary restrictions, the locations at which theinstruments are installed should be chosen carefully tooptimise the value of the data.

For the investigation of the use of willow poles in slopestabilisation, quantification of two aspects are of primaryimportance. The moisture content of the soil is the mainvariable related to the soil conditions which affects thestability of highway slopes. With regard to the vegetation,it is the development, growth and survival of the rootsystem which has the greatest effect on survival of theplants and the effect of the plants on the slope stability.

6.1 Instrumentation for soil moisture monitoring

Critical to the stability of highway slopes is the pore waterpressure. Porewater pressures in slopes are likely to besub-atmospheric and the growth of vegetation will act toincrease the magnitude of these suctions. Therefore,measurement of the changing porewater pressure willenable the effect of the growing willows on the soilmoisture content to be established. Measurement ofnegative pore pressures, or tensions, is undertaken using atensiometer (Hanna, 1985).

The tensiometer is essentially a hollow porous ceramicprobe which is filled with water to exclude air. When theinstrument is installed in the soil, the water in the tubeequilibrates with the soil water pressure. Piezometers areconventionally installed in a cell of single sized sand. Forreliable measurement of negative porewater pressures, thesand cell installation technique is inadequate (Crabb andHiller, 1993): the tip of tensiometers should be pusheddirectly into the clay at the bottom of the borehole inwhich it is installed. Good contact between the instrumentand the soil is essential for an accurate reading(Greenwood et al., 1996).

Two fundamental types of tensiometers are available.Automatic electronic tensiometers include a pressuretransducer in the head, the output from which can belogged continuously by automatic data loggers. Manual (orseptum) tensiometers are similar to the electronic type, butthe head of the instrument is replaced with a speciallyformulated rubber bung which seals the top of the shaftwhich holds the water. This bung, or septum, allows aneedle type pressure sensor to be inserted into thetensiometer shaft to make measurements. The septumreseals itself once the needle is removed and can be usedfor many readings. The manual method clearly requires thesite to be visited for each set of measurements, but a singlesensor can be used to take readings from an unlimitednumber of septum tensiometers at different sites. Whereonly long term changes in porewater pressures are ofinterest, the manual tensiometer is likely to be a more costeffective solution than an electronic system. In order thatthe depth to which the effect of plant growth on theporewater pressures can be established, an array oftensiometers will be required, installed at a series ofdifferent depths. For example, Greenwood et al. (1996)installed tensiometers in each instrumentation cluster, atdepths of 0.25m, 0.5m, 1.0m and 1.5m.

In addition to increasing the negative porewaterpressures in the vadose zone, the growth of trees on slopesmay also lower the groundwater level. Measurement ofnegative porewater pressures alone is insufficient todetermine the depth to the water table. Therefore it isnecessary to install piezometers. For the purposes of thisstudy, standpipe piezometers read using a manual dipperwill provide appropriate information. When daily, or morefrequent records are required automatic logging systemsare available, such as that described by Crabb and Hiller(1993).

6.2 Monitoring of root growth

There are two considerations to assess with regard to rootgrowth:

i whether roots develop and thrive along the entire lengthof the willow pole;

ii the density of roots in a volume of soil.

Methods of investigating root systems were reviewed byBohm (1979). The discussion here considers mainly theinstrumentation systems which may be used for acquisitionof the required data. Root growth may be investigated bydestructive or non destructive methods. Carefulexhumation of plants and measurement of the root systemprovides one method of root system inspection, but at theexpense of destroying the plant. Excavation usingcompressed air offers potentially the least damagingmethod of intrusive inspection.

Less damaging than complete excavation of the plant,but still highly invasive, is the use of trenching, blocksampling or acquisition of samples by coring (Adlard,1990). A detailed description of one such method wasgiven by Matthews (1990), in which an advancing trenchmethod was used. Samples were taken at several depths ina progressively deepened trench. The method isdestructive, but less so than complete excavation of aplant. It is also labour intensive, both in terms of acquiringthe samples and in treating the samples to obtain therequired data.

The preferred methods of investigating rootdevelopment are non destructive. Two methods describedby Taylor et al. (1990) are the use of rhizotrons andminirhizotrons. Rhizotrons are covered undergroundwalkways or rooms that have clear walls on at least oneside and are one of the earliest non destructive methodsused to investigate root growth in soil. Some of theadvantages of rhizotrons described by Taylor et al. are thatsuccessive measurements are easily made on the sameindividual root; it is easy therefore to make a series ofobservations of the same root; sensors can easily beinstalled to monitor the chemical and physical conditionsin the soil; there is also commonly sufficient space tomount a camera to facilitate time lapse photography. Theprincipal disadvantages of rhizotrons are their high costand the soil structure and environment are modified by theexistence of the rhizotron.

A less expensive and less disturbing method of in situinvestigation of root development is through the use ofminirhizotrons. This technique involves inserting a tube

21

fabricated from a transparent material into the ground andlowering a device into the tube to allow the roots at the soil-tube interface to be observed. Minirhizotrons wereoriginally described by Bates (1937), who used a mirror andbattery operated lamp mounted on the end of a rod as anobservation device. Modern technology has enabled closedcircuit colour television to be used. Mackie-Dawson et al.(1989) used a miniature borescope to view rootdevelopment in pot-grown plants via 22mm internaldiameter tubes. These authors commented on the advantagesof having video images which were in colour rather thanmonochrome for differentiating root types. Mackie-Dawsonet al. were interested in counting new white roots, since theuptake of nutrients by white roots is generally greater thanthat by brown roots. Mackie-Dawson et al. (1995) used thesame miniature borescope in a subsequent study of rootgrowth. In this later study, the authors coated the tops of theglass inspection tubes with black paint to prevent lighttravelling down the tube to the roots.

Taylor et al. (1990) suggested that minirhizotrons shouldbe installed in the soil at an angle of typically 30 to 45degrees from the vertical to reduce the risk of rootsfollowing the tubes for long distances. However, acomparison reported by Mackie-Dawson et al. (1989) foundthat tracking of roots along the tubes occurred for similarlengths on both vertical and inclined tubes. Taylor et al.reported that roots will not normally follow the uppersurface of an inclined tube, but that if intimate contact is notestablished between the tube and the soil, then roots mayfollow the underside of the tube. Tracking of the root alongthe tube may be problematic for data acquisition if roots arecounted by counting the number of intersections of rootswith grids since a root tracking along the tube may becounted several times. Mackie-Dawson et al. (1989)suggested that a counting method based on the first points ofcontacts of roots with the tube would be more successful.

For the purposes of the current research, instrumentationis required which will enable the maximum rooting depthof the willows to be established. In particular it isnecessary to determine whether the roots develop to adepth greater than the depth at which failure surfacesmight occur. Taylor et al. (1990) commented that theaccuracy of data obtained from minirhizotrons may be lowwhere rooting density is low. The maximum depth of rootdevelopment may therefore not be reliably established.However, if roots are shown to exist at a depth sufficient tointercept potential failure surfaces then the maximumrooting depth will not be required.

An alternative method of mapping tree root systemswithout the need for any intrusion of the ground has beenproposed by Hruska et al. (1999). These authors usedground penetrating radar (GPR) to study the threedimensional distribution of root systems of large oak trees.While this approach appears to have been reasonablysuccessful, the resolution possible is insufficient to make itappropriate for the current research. GPR has an inherenttrade off between the maximum resolution and thepenetration depth. Hruska et al. found that roots ofdiameters less than 10mm could not be detected.

7 Maintenance requirements

The key maintenance task in establishing plants is theeradication of competing vegetation (Sangwine, 1996).The Highways Agency’s specification requires a 900mmcircle, free of vegetation, around each plant throughout theestablishment maintenance phase of the planting contract.Sangwine reported that the most cost effective method ofachieving this is by the use of herbicides. Considerablecare should be exercised to ensure that the willow polesare not adversely affected by the herbicides. An alternativemethod is the use of tree mats, which can be installed atthe time of planting willow poles. The use of tree matsmay prove to be the most economic option for weedcontrol in small trial areas. A further option is careful andfrequent maintenance to remove growth around the plantsas required (Barker, 1997b).

Secondary maintenance of vegetation follows the periodof establishment maintenance and usually commences inyear seven in the life of a typical scheme with early thinningand coppicing work (Sangwine, 1996). On conventionallyplanted sites, thinning is aimed at permitting thedevelopment of vigorous safe climax trees. Where willowpoles have become established, thinning or coppicing maybe required to ensure healthy growth of plants.

In later years, planting schemes may suffer from aninvasion of natural regeneration. In some circumstances,this may be undesirable, since the original objective of theplanting scheme may be compromised. However, once amonoculture of willows has become established, theintegration of additional species may be beneficial inecological and aesthetic terms. Sangwine (1996) pointedout that gorse (Ulex europeus) and broom (Cytisusscoparius) can reach such an intensity that they suppressother species. Furthermore, they can become a fire risk indry periods and therefore they should be controlled.

A further maintenance consideration is whether fertiliseris required. Sangwine (1996) noted that the widespread useof poplars and willows on early motorways was a responseto the poor growing conditions. Top soil was often verythin or absent. Greenwood et al. (1996) reported thatwillows developed more quickly on top soiled slopes thanwhere no top soil was used. It may therefore be concludedthat fertiliser need not be applied for the growth of willowpoles to be successful, but a layer of top soil of the order of300mm thick may be beneficial.

8 Conclusions

The evidence from the literature has shown thatbioengineering in general, and in particular the use of livewillow poles to stabilise slopes, is potentially a versatileand cost effective alternative to more traditionalengineering methods. Bioengineered approaches also havethe advantage of offering improved ecological andaesthetic benefits.

To yield the maximum success of the live willow poletechnique, care is required in the selection of speciesappropriate to the local conditions; in the harvesting,

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handling and transportation of the poles; and in theinstallation procedure. The experience and opinionspresented in the literature have been used as a basis for thepreparation of a draft installation procedure which ispresented in Appendix A of this report.

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Appendix A: Draft specification: Installation of live willow poles for stabilisinghighway slopes

CONTENTS

PROJECT DESCRIPTION 26

1. SITE SELECTION 27

1.1 General characteristics 27

1.2 Soil characterisation 27

1.3 Survey of existing vegetation 27

2. SOURCING OF LIVE WILLOW POLES 28

2.1 Suppliers 28

2.2 Cuttings 28

2.3 Harvesting 28

2.4 Storage and transportation 28

3. PREPARATION AT TRIAL SITES 28

3.1 Site layout 28

3.2 Vegetation 28

3.3 Installation equipment 29

3.4 Preparation of poles 29

4. INSTALLATION OF LIVE WILLOW POLES 29

4.1 Supervision of installations 29

4.2 Direct installation in weak soils 29

4.3 Installation in pre-formed holes 29

4.4 Installations records 30

5. MONITORING EQUIPMENT 30

5.1 Installation of monitoring equipment 30

6. PROTECTION OF INSTALLED POLES 31

6.1 Protection from invasive vegetation 31

6.2 Protection during maintenance 31

6.3 Interference from animals 31

7. LONG-TERM MONITORING 31

7.1 Monitoring requirements 31

8 MAINTENANCE 32

8.1 Maintenance requirements 32

by D H Barker (Geostructures Consulting) and D J MacNeil (TRL Limited)

27

PROJECT DESCRIPTION

Installation of live willow poles appears to offer an easy, relatively rapid and cost effective method of ensuring thatvegetation is successfully established both at surface level and at depth within a slope. Whilst it is assumed that root growthoccurs rapidly from the live pole at depth, it is not known what conditions control root development during establishmentand in the long term; earthworks slopes generally have a 60 year design life requirement, although it may be longer.Therefore, there is a requirement for specific trials to study the mechanism of root development of live willow poles undera range of differing soil types and moisture regimes. Four trial sites will be established on the Highways Agency’s network.The information gained from the trials will not only enhance knowledge of the effects of vegetation, but is also essential iflive poles are to be considered as a form of vegetated soil nailing in the prevention, and repair, of shallow slips on highwayembankment and cutting slopes.

1. SITE SELECTION

1.1 General characteristics

1.1.1 Prior to selecting trial sites, the following characteristics of potential sites shall be recorded:

1. slope details: natural or formed, cutting or embankment;

2. crest elevation;

3. slope inclination;

4. orientation of slope fall line;

5. geometry: vertical height, width, and slope length.

6. location and extent of significant repairs.

1.1.2 The sites selected for the trial installations of live willow poles shall be safe for slope working and have wellmanaged access procedures. Care shall be taken to ensure that working on the sites shall pose no significantincrease in risk to the safety of users of the network.

1.1.3 The slopes selected for the trial installations shall have a slope inclination of between 1 (horizontal) on 2 to 3(vertical), equivalent to a slope angle of between 20 and 30 degrees. The trial sites shall comprise single, open,unshaded slopes, and shall have slope lengths, measured from the crest to the toe, in excess of 10m.

1.1.4 The selected slopes shall be of sufficient area to permit the setting out of a trial area of approximately 11m (width)by 8m (slope length). A control area of comparable dimensions shall also be prepared at each site.

1.1.5 The selected slopes should not have significant on-slope drain systems, such as counterforts or herringbone drains.Both shallow crest and toe drains are acceptable. If significant on-site drainage systems are installed on theselected slopes, the drainage systems shall be accurately located and mapped prior to setting out each trial.

1.2 Soil characterisation

1.2.1 The soils at the trial sites shall be soft plastic clays, preferably over-consolidated, with an effective shear strengthtypically less than 40kN/m2 and a minimum soil moisture content of 60% field capacity. Field capacity moisturecontent occurs when excess water has drained away under gravity after the soil has been nearly saturated.

1.2.2 The soils at the trial sites shall be characterised as follows:

1. soil type, colour, structure, and weathering;

2. particle size distribution;

3. plastic and liquid limits;

4. in situ moisture profiles;

5. in situ shear strength;

6. fertility and organic contents.

1.3 Survey of existing vegetation

1.3.1 Prior to commencing installations at the trial sites, the existing vegetation cover shall be surveyed. Details of thespecies of grasses, shrubs and trees, and the percentage cover of each class of plant shall be recorded.

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2. SOURCING OF LIVE WILLOW POLES

2.1 Suppliers

2.1.1 All cuttings shall be hardwood willow species (Salix alba, S. fragilis and other S. species of specified size),sourced from an approved supplier or location.

2.2 Cuttings2.2.1 All live pole willow cuttings incorporated into the project shall be:

1. healthy, free from defects and freshly harvested or brought from cold store;

2. dormant, i.e. no buds shall have burst before installation;

3. labelled and kept moist, and protected after delivery on site from frost, heat, wind and damage from impacts.

2.2.2 The dimensions of live willow pole cuttings shall be:

1. length: 2.25-3.0m,

2. diameter: butt end: 65-90mm, upper end: 40-60mm,

3. shape: straight, or only slightly bent, smoothly tapering or nominally cylindrical, with no bends or branchpoints forming large bifurcations. As a check for acceptable shape, poles shall fit into a 3m long, 125mminternal diameter plastic tube. The edges of the tube shall be rounded to limit damage to the poles duringinsertion.

4. regularity: as the willow poles are to be installed for most of their length into the slopes, they shall be free ofbumps or angularity, other than small protuberances at branching points, which prevent or hinder entry into theprepared holes. Subject to approval from the Project Site Manager, it may be permissible to cut branchprotuberances flush to the surface of poles.

2.3 Harvesting

2.3.1 All live willow pole supplies shall be freshly harvested after autumn leaf-fall in a sustainable manner and preparedfor direct transportation to site, or taken to an approved cold store. In general, it is preferable in terms of viablepropagules if live poles are harvested from the middle third of stems. If stems of sufficient length are not available,advice should be sought from the Project Site Manager.

2.4 Storage and transportation

2.4.1 At all times and locations prior to installation, it is important that the live willow poles are wrapped or covered inwetted hessian or other fabrics and are kept shaded, cool and moist. In particular, after harvesting, the live polesshall be transported direct to site, or stored in cold store maintained at 1oC until required at a project site. The livepoles shall be transported under tarpaulin cover, wrapped in moist hessian sacking or geotextile and sheltered fromthe wind and airflow. At the project site or depot, the live poles shall be stored under water and in shade, at anambient temperature not exceeding 12oC, until the next section of the site and its array of holes has been preparedfor installation.

2.4.2 Immediately prior to installation into the slope, small bundles of live poles shall be taken from site storage tanks,wrapped in wet hessian cloth or similar strong absorbent fabric, and taken directly to the installation location on the site.

3. PREPARATION AT TRIAL SITES

3.1 Site layout

3.1.1 The site shall be set out in full at the start of the work. A typical layout of a trial site is presented in Figure 1. The finallayout of the trial and control areas shall be adjusted locally by the Project Site Manager to suit site conditions.

3.2 Vegetation

3.2.1 Prior to commencing installations, any existing vegetation which, in the opinion of the Project Site Manager, hasthe potential to provide significant competition for the installed willow poles during the period of earlyestablishment shall be cut back to ground level.

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3.3 Installation equipment

3.3.1 Prior to the start of the work, the contractor shall bring to the site all of the following equipment:

1. hole pre-forming equipment:

a. mandrels:

b. hand and power augers, with auger diameters and lengths sufficient to pre-form/drill to a depth of 1.75min diameters from 75mm up to 150mm, in 4 equal increments;

c. window sampler equipment, in diameters ranging from 80mm up to 150mm.

2 down-hole scraper for removing clay smearing in pre-formed holes;

3. open tubes (variety of appropriate lengths and diameters) to line any pre-formed holes which tend to close;

4. pole driving equipment:

a. sledge and post hammers;

b. electric or hydraulic powered impact hammer and suitable driving tool attachment.

5. small diameter rammer for manual compaction of soil around live poles.

3.4 Preparation of poles

3.4.1 Prior to installation, the live willow poles shall be prepared in the following manner:

1. the extreme 200-250mm length of butt ends of the poles shall be shaped into a point;

2. the upper end of each pole shall be protected from splitting by wrapping 25mm from the end with 2 turns of1mm diameter galvanised wire, which shall be secured in position by a minimum of 3 twists.

4. INSTALLATION OF LIVE WILLOW POLES

4.1 Supervision of installations

4.1.1 Live willow poles shall be installed at locations on the slopes as set out and approved by the Project Site Manager,to spacing and depths as indicated on the approved plan drawings. The live pole spacing will nominally be 0.75malong contour and 1.0m along slope fall line (see Figure 2). Following instruction from the Project Site Manager,poles shall be installed following the method detailed in either Clause 4.2 or 4.3.

4.1.2 Live poles shall not be removed from the temporary storage area until at least 6 holes have been prepared. At notime shall more than 12 holes be prepared in advance of installation. Each live pole shall be inserted and driveninto the slope during a single shift. Live poles shall not be left undriven and exposed at the end of a shift. Anyundriven poles remaining at the end of a shift shall be removed and stored in accordance with the conditionsdetailed in Clause 2.4.1.

S S

S S

S S

S S

20 - 30

degressControl section

Toe of slope

Crest of slope

Trial section

Plan view

T

TT T

TT

10.5m

T

Willow pole

Tensiometer clister

Standpipe piezometer

T

S

Figure 1 Typical layout of a trial site

30

4.2 Direct installation in weak soils

4.2.1 If the slope soils are sufficiently weak, the poles may be driven to refusal directly into the slopes, either verticallyor inclined, without the use of pre-formed holes.

4.2.2 Each pole shall be inserted butt end first and driven into the slope, using the pole driving equipment described inClause 3.3. Each pole shall be driven to refusal or until an undamaged length of 300-500mm remains exposedabove ground level.

4.2.3 On completion of driving, the exposed end of each live pole shall be trimmed cleanly at an angle of 60° to thelongitudinal axis of the pole. Any splintered portions at the end of the pole shall be cut off cleanly prior totrimming, and the end re-wrapped with wire in accordance with Clause 3.4.1 (2).

4.3 Installation in pre-formed holes

4.3.1 For all slope soil conditions which do not permit the direct installation detailed in Clause 4.2, the poles shall beinstalled in the base of pre-formed holes in accordance with the guidance in Clauses 4.3.2 to 4.3.8.

4.3.2 Vertical holes shall be prepared to depths varying from half to 85% of the length of live poles, at locations detailedin the site layout plan, or as specified by the Project Site Manager.

4.3.3 Inclined holes shall be prepared to lengths varying from half to 85% of the length of live poles, at locationsdetailed in the site layout plan, or as specified by the Project Site Manager. The Contractor shall submit a methodstatement for this work to the Project Site Manager two weeks prior to commencement of the work on site.

1.5

m0

.5m

0.5

m

1.5

m0

.5m

0.5

m

Unreinforced critical

failure surface

Live poles

1.0m

1.0m

1.0m

1.0m

Figure 2 Cross section of willow pole array

31

4.3.3 Pre-formed holes shall be created to specified depths using either:

1. mandrels (i.e. spikes or large-diameter closed or open tubes) driven by manual or mechanical hammer,

2. hand or powered augers,

3. window sampling equipment of appropriate diameter.

4.3.4 After completion of the hole, any clay smeared down its sides shall be recovered by light scraping with a groovedscraper.

4.3.5 In the event that pre-formed holes close up before poles can be inserted and driven, open tubes shall be used totemporarily line the holes.

4.3.6 Each pole shall be placed butt end first into the base of a pre-formed hole and driven into the slope, using the poledriving equipment described in Clause 3.3. Each pole shall be driven to refusal or until an undamaged length of300-500mm remains exposed above ground level.

4.3.7 In dry or stiff soil, following instruction from the Project Site Manager, a maximum of 2 litres of fresh water maybe added to the each hole before or after the installation of the pole.

4.3.8 On completion of driving, the exposed end of each live pole shall be treated in accordance with Clause 4.2.3.

4.3.9 Any voids around the pole after installation shall be backfilled with fine dry sand or loam soil to within 250mm ofthe surface of the slope. The final 250mm void shall then be backfilled with the clay soil removed during theforming of the void. Where there is insufficient clay soil available, a suitable alternative source shall be identifiedby the Project Site Manager.

4.4 Installations records

4.4.1 Records of each installed live willow pole shall be maintained in a project register. The following details shall berecorded:

1. identifying code number;

2. harvesting date;

3. location on site;

4. installation date;

5. species;

6. live pole dimensions: length and diameter at both ends;

7. installation details: depth of hole, driven length, whether water added, whether sides of hole required to bescraped to remove smearing.

4.4.2 Each live pole shall be labelled with a durable label, detailing the identifying code number and date of installation.

5. MONITORING EQUIPMENT

5.1 Installation of monitoring equipment

5.1.1 At each trial site, access holes shall be prepared for the following monitoring equipment:

a. tensiometers, for monitoring soil suction;

b. standpipe piezometers, for monitoring groundwater level;

c. transparent access tubes, for video monitoring of root development.

5.1.2 The locations and depths of the access holes shall as detailed in the site layout plan, or as specified by the ProjectSite Manager.

32

5.1.3 Tensiometers of 30mm diameter shall be installed in clusters of four, to depths of 0.5m, 0.75m, 1.0m and 1.5m.Two tensiometer clusters shall be installed in each trial area, and one cluster in each control area.

5.1.4 Holes for a minimum of two standpipe piezometers of 30mm diameter shall be formed to depths of 2.5m belowground level in each trial area, typically at the toe and crest ends of the trial area. An identical array shall beformed in each control area.

5.1.5 A maximum of six vertical and inclined holes for 75mm diameter transparent access tubes shall be formed oncompletion of installing the array of willow poles. The locations of the holes will depend on the geometry of thecompleted installation.

5.1.6 All monitoring equipment shall be installed and operated by experienced personnel.

6. PROTECTION OF INSTALLED POLES

6.1 Protection from invasive vegetation

6.1.1 Biodegradable natural fibre (excluding tarred roof felt) tree spats or mats of 0.45m minimum diameter shall beplaced around each live pole and pinned in place, in accordance with the instructions of the Project Site Manager.

6.2 Protection during maintenance

6.2.1 Tree guards shall be installed to protect live poles during grass cutting operations. The guards shall be an openpolymer mesh type of approximately 300mm in height and 12mm square mesh. The guards shall be secured tostakes driven into the ground or by other methods approved by the Project Site Manager.

6.3 Interference from animals

6.3.1 At sites subject to rabbit or other pest infestation, open polymer mesh tree guards of 600mm minimum height shallbe used. At sites subject to deer attack, taller guards shall be supplied and installed in accordance with localpractice, as instructed by the Project Site Manager.

6.3.2 As an alternative to using tree guards, site fencing may be installed. At sites subject to rabbit or other pestinfestation, the fencing shall incorporate rabbit fencing, buried to an appropriate depth. At sites subject to deerattack, the installed fencing shall be of sufficient height and robust construction.

7. LONG-TERM MONITORING

7.1 Monitoring requirements

7.1.1 The survival rate and stem growth of the above ground sections of the willow poles shall be monitored eachquarter at all trial sites.

7.1.2 Soil suction data from the tensiometer clusters and groundwater levels for the standpipe piezometers shall berecorded at each site, at appropriate intervals during the duration of the trials. The minimum frequency forrecording these data shall be on a monthly basis.

7.1.3 The development of root architecture of the installed willow poles shall be monitored over the duration of thetrials. In particular, the depth profile, spread and density of the root systems shall be evaluated. A combination ofthe following techniques shall be employed:

1. visual inspection, using a video camera placed down the transparent access tubes;

2. excavation around selected poles, either by hand for small scale excavations, or by slit trenching using amechanical excavator for larger excavations;

3. use of window sampling equipment to recover soil and root matter from depth.

33

7.1.4 Towards then end of the project, consideration will be given to undertaking the following procedures:

1. exhumation and inspection of entire poles;

2. investigation of decay rates in any dead willow poles.

8 MAINTENANCE

8.1 Maintenance requirements

8.1.1 The slope ground cover shall be clipped annually to minimise competition with live poles.

8.1.2 Live pole growth coppicing shall be carried out where instructed after 5 years in three coups in successive years,each comprising only one in three of the live poles.

34

Prices current at September 2001

For further details of these and all other TRL publications, telephone Publication Sales on 01344 770783 or 770784,or visit TRL on the Internet at www.trl.co.uk.

Abstract

This literature review was undertaken with the objective of using previous experience to guide the preparation of adraft specification for the use of live willow poles to stabilise highway slopes. This draft specification, which ispresented in Appendix A of the current report, will subsequently be applied to the establishment of site trials on themotorway network. The proposed trials are intended to verify the potential engineering benefits which may accruethrough the use of live willow poles to stabilise slopes.

The project is intended to focus mainly on the engineering aspects of the live willow pole technique. However,consideration is also given to the aesthetic and ecological consequences of slope stabilisation by this method. Inparticular, the consequences of how the method impinges on the wider requirements of the Highways Agency, toensure integration of planting schemes within the local landscape, are discussed. A number of case histories aredescribed in which live willow poles have been used in a variety of applications both in the UK and abroad.Potential problems which may be experienced in establishing viable plants from live poles and which may arisethrough the planting of a monoculture of willows are discussed.

In general, the live willow pole technique appears to offer many advantages over traditional repair and palliativemeasures for slope stabilisation. However, before it is used widely on the highway network, it is necessary toundertake validation trials to optimise the installation process and to confirm that the method is viable.

Related publications

TRL515 Vegetation for slope stability by D J MacNeil, D P Steele, W McMahon and D R Carder.2001 (price £35, code J)

TRL506 Establishment of vegetation for slope stability by C A Marriott, K Hood and J R Crabtree (MacaulayLand Use Research Institute. 2001 (price £35, code J)

TRL493 Analysis of performance of spaced piles to stabilise embankment and cutting slopes by D R Carder andM R Easton. 2001 (price £35, code J)

TRL373 The use of soil nails for the construction and repair of retaining walls by P E Johnson and G B Card.1998 (price £35, code H)

RR257 Determining the age of failure of motorway earthworks from aerial survey photographs byR D Andrews. 1990 (price £20, code B)

RR199 A survey of slope condition on motorway earthworks in England and Wales by J Perry.1989 (price £20, code C)

RR30 Maintenance and repair of highway embankments: studies of seven methods of treatment byP E Johnson. 1985 (price £20, code AA)

LR1061 An assessment of the conditions for shrubs alongside motorways by D M Colwill, J R Thomson andA J Rutter. 1982 (price £20)

SR513 The impact of road traffic on plants. The proceedings of a Symposium organised by the BritishEcological Society and TRRL, 11-13 September 1978 edited by D M Colwill, J R Thompson andA J Rutter. 1979 (price £20)

SR217UC Studies of conditions for vegetation in the central reserves of motorways: a preliminary report byD M Colwill, J R Thompson and P S Ridout. 1976 (price £20)