Construction Health and Safety

EXCAVATION

Section 8B

8B - 1

SPECIAL ACKNOWLEDGEMENTS

Appreciation is expressed for the work of the author of this subsection:J.R. Illingworth Esq., BSc, FCIOBConsultant in construction methods and technology

Special acknowledgement is made to the Timber Research andDevelopment Association for permission to publish the table on p.8B-45and numerous illustrations, as annotated, from their publicationTimber in excavations, together with free interpretation of parts ofthe text of the same publication.

Acknowledgement is also made to the Construction Industry Researchand Information Association for its assistance, in particular forpermission to publish the table on p.8B-9, which is based on a similartable in CIRIA Report 97 Trenching practice, and to Zetica Ltd fortheir help in preparation of the guidance relating to UnexplodedOrdnance.

CONTENTSIntroduction 8B–3

Ordnance 8B–3

Ground conditions 8B–4

Method of excavation 8B–7

Basic principles of support 8B–7

Standard or designed solutions 8B–9

Systems of support 8B–9

Trenches 8B–11

Wide and single sided excavations 8B–15

Shafts 8B–15

Headings 8B–17

Permanent works as temporary support 8B–19

Related safety aspects 8B–20

Check List No.1 – Site conditions and method constraints 8B–22

Check List No.2 – Before work starts 8B–22

Check List No. 3 – Whilst work is in progress 8B–22

Glossary of terms 8B–23

Method statement checklist - groundworks 8B–24

Reference sources 8B–25

8B - 2 June 2005

EXCAVATION

8B - 3December 2007

IntroductionExcavation is an essential element of the construction

process particularly in relation to the construction offoundations, drainage work and site regrading of all kinds.

Information regarding any hazards identified prior toconstruction should be included in the Construction PhasePlan required by the CDM regulations 2007.

In carrying out an excavation, the soil conditions canvary widely, often in short distances. No soil, whatever itsnature, can be relied upon to support its own weight for anylength of time - let alone any additional loads which may beimposed by plant and materials. It should never be forgottenthat 1m3 of earth weighs approximately 1.3 tonnes. Even asmall fall of earth is capable of inflicting serious injury, evenif it does not kill. Unless, therefore, the excavation can bebattered to a safe slope, the sides will need supporting toprevent the possibility of collapse and thus:1. provide safe conditions for persons working in or

adjacent to the excavation and, in some situations, thepublic as well.

2. enable the works to be carried out without interruptionand

3. protect adjacent property and/or public services.CDM - Part 4 require all practicable steps to be taken to

prevent injury to persons due to the accidental collapse of anexcavation or by any fall of material; neither mustexcavation work be allowed to cause the weakening of anystructure. Any excavation used as a place of work must beinspected and reports completed in accordance withSchedule 3 of the CDM - Part 4. Whenever excavation hasto be undertaken, therefore, adequate prior considerationneeds to be given to the soil conditions that will be met, themethod of excavation to be used and the manner in whichany necessary support will be provided.

Traditionally, timber has been used in the support ofexcavations. However, modern methods incorporate steel,and proprietary systems are invariably of metalconstruction. It should be noted that the term ‘Timbering’ isfrequently used to describe any form of support work,whatever material is being used. A glossary of other termsused in the support of excavations is given on p.8B-22-23.

In order to assist in the proper design, planning andexecution of excavation work, a series of check lists isprovided at page 8B-23. A general checklist forgroundworks method statements is also provided at the endof this section.

OrdnanceThe most likely source of unexploded bombs (UXB) and

other ordnance (UXO) is from the thousands of devicesdropped on major cities during the Second World War. It isestimated that 10% of high explosive and as much as 50%of incendiary devices failed to explode. These continue topresent a potentially serious hazard to construction workers,property and third parties. Brown field sites that werepreviously MoD land are also high risk locations, especiallyif they were used for training purposes. Whilst thesecomprise the major areas of risk, UXO and UXB are found inthe most unexpected places, such as beaches whereordnance is washed up following post war dumping and theaction of underwater currents and wave action over manyyears.

Prior to starting Work.When excavating, piling or drilling in inner city

environments, particularly in areas that were subject tosevere bombardment such as London, Coventry, Liverpooletc, it is recommended that a risk assessment be made at an

early stage, before specialist contractors are on site, toestablish the level of probability of UXO being present.Maps are available from local authorities and commercialorganisations showing the density of impacts for specificregions. These are largely produced from informationgathered during bomb watching duties during WW2. Thisshould be available in the pre-construction Information whichthe Client has a duty to provide under CDM.

CDM Regulation 34-(3) requires suitable and sufficientsteps to be taken to reduce risk from damage or disturbanceof any underground energy distribution installations. This willapply to any ground penetrations where services may bepresent, regardless of the scale of that work.

Particular problems will arise where underground linesare in close proximity and prudent contractors are welladvised to discuss these matters with the owners of the lineduring the tendering process. If the site is in a high risk areaa qualified Explosive Ordnance Clearance (EOC) specialistshould be employed to prepare the risk assessment and amethod statement.

A surface geophysical survey using ground penetratingradar can detect ordnance and other obstructions to a levelof 3 metres which, if the ground is clear, will allowexcavation down to that depth. The radar can be then be re-employed to examine a further 3 metres of depth, a processwhich is repeated until the required depth is reached. This islaborious but it is the safest method. This process can causeparticular difficulties if the contractor is anticipating workingin an existing water course, e.g. driving piles or shafts in ariver bed which would involve the construction of coffersdams. Ground penetrating radar can operate from a boatand specialist contractors are available to carry out thisfunction if required.

The typical depth of rest of large, air-dropped UXB is15m below the surface, and could be greater in, forinstance, soft silts. These, and other ferrous objects, can bedetected using a probe-magnetometer that is driven into theground hydraulically, and can detect UXB and obstructionsdown to the no-risk depth. However, this method is invasiveand carries a potential risk of striking the UXO with theprobe.

Emergency procedure plans should also be put in placein case of unexpected discovery of ordnance because, evenwith the latest technology, detection is not 100% as outsideinfluences can mask or alter the signal reflected from theUXB.

Whilst working the siteAfter 60 or more years in the ground, bombs and other

items of ordnance generally maintain their shape, butbecome very rusty and decayed. If a suspicious object that issimilar in size and shape to a WW2 bomb is uncoveredduring the course of groundworks, the emergencyprocedures should be put into effect. These would include:• Stopping work• Evacuating to a safe distance; this may be outside of the

boundary of the site and could involve members of thepublic.

• Calling the police on 999 to notify the nearest bombdisposal unit.

• Ensuring nobody re-enters the area until told to do so bythe authorities.On discovery of an item of ordnance or a suspect object,

the location of the object should be marked, and theimmediate area evacuated in relation to the size of theobject. The object should not be touched or moved until ithas been identified. If in any doubt call the police and theywill call the bomb squad.

Ground conditionsBefore commencing any excavation, it is important

to identify the type of ground in which the excavation is to be carried out. Detailed information may be availablewith the contract documentation, or in the form of bore-hole or trial pit logs carried out as part of the site investigation. Whichever method is used, it is helpful to have a simple means of identifying the various strata that may be found. The table on p.8B-4 is a very usefulguide.

When examining boreholes or trial pit information,particular importance should be paid to the location of anywater table. If the water table is going to be exposed by theexcavation, careful consideration will need to be given tohow it may affect the stability of the excavation sides.Ground water can greatly affect the stability of any soil and,in particular, non-cohesive materials. Water can also enteran excavation as surface run-off

Whatever the source of water, effective action isnecessary, either to stop the water from surface sourcesentering altogether or, in the case of ground water,minimising its effects to the greatest possible degree.

Surface waterSurface ditches, streams etc., likely to be interrupted by

the excavation, may need diversion. Where the excavationis across a slope in the ground, cut-off ditches should beconsidered if the work is to be carried out in a rainy period.Where the location of field drains is visible, they should becut off and diverted before the main excavation starts.

It is worth noting that under the Water Resources Act1991 “pollution” of any controlled water is an offence. Theword pollution includes site run-off and silt (see section 34).

Ground waterThe presence of ground water is more difficult to deal

with than surface water. It may affect the sides of theexcavation to the extent that, even if supported, wash out ofmaterial will occur between the sheeting. In certain soilconditions, the bottom of the excavation can becomeunstable and ‘boil’ with the inevitable total collapse of thetrench. The relationship of ground water to the soilconditions needs careful analysis before a decision is madeas to the support method to be used.

If the ground is suitable, one of several grounddewatering techniques may be used. Such methods involveeither shallow well pumping or wellpointing. In either case,

the pumping out of water has the effect of lowering theground water table to a level below that to which theexcavation is to be taken. Wells or wellpointing, for theirsuccessful use, require a proper soil analysis to make surethat the method is feasible. It must also be established, at thesame time, that no fine material will be drawn fromunderneath adjacent property with consequent risk ofsettlement.

Where water is contaminated advice must be soughtfrom environmental consultants or the Environment Agency,as re-introducing contaminated water back into the groundcould constitute a criminal offence (see Section 34).

Modern technology provides alternatives to dewateringthe ground. Stabilisation of the ground can be achieved bychemical injection or freezing (see Section 30 – GroundTreatment). Injection and freezing methods are alwaysexpensive compared to dewatering and are therefore onlyused when other methods cannot cope with the situation.They require highly skilled technologists to operate them andwill be effective only in soils suitable to their use. Before anydecision is made to use them, specialist advice from a soilmechanics expert is essential.

Where a water-bearing strata overlays an imperviousone and the depth of this impervious strata is not too great,the use of sheet piling may be more effective andeconomical. The piling, being substantially watertight, ‘cuts-off’ the water from the excavated area, thus enabling theexcavation to proceed in the dry.

Ground conditions and slopes

Notes:1 These are guide figures to slopes based on and subject

to:a Temporary conditions (ie generally 1 to 14 days)b Field safety and experiencec A safety limit of 45 degrees (but see Note 7 below)d Water seepage can cause wash out and undermining

2 ‘Dry’ site: minor or no seepage from excavated faces.Minor or no surface run-off.

3 ‘Wet’ site: submerged or widespread seepage fromexcavated faces.

4 The behaviour of soils is influenced by the grading,particle size, shape and density. In mixed soils themaximum grain size of the smallest 15 per cent by weightof the grading tends to characterise the soil.

5 Normally consolidated clays are usually stronger within a few metres of ground level due to desiccation.However, the clay crust is fissured for the

same reason (note the fissures may not be visible) andis seldom greater than 4 metres deep.Classical soil mechanics theory would suggest that claywith a cohesion greater than 30kN/m2 will standvertically to 6 metres. However, due to the fissuring, theface becomes unstable and lumps fall into the trench. Indeeper trenches there is a risk of toe failure due tounderlying softer clay. The suggested slopes are intendedto avoid danger to workmen from these risks and areoffered as a guide only.

6 Flatter slopes may be applicable or required subject tothe evidence on:a Incipient (structural) failure planesb Safety precautions against small or large falling

fragments7 Steeper slopes may be applicable or required and will be

subject to the same assessment as in Note 6.There is also the growing use of permanent construction

to provide both temporary support and deal with waterproblems. Diaphragm walls and seacant piling arefrequently used in this way. Both methods are described indetail in the paragraphs dealing with “Permanent works astemporary support”, p.8B-18/19.

8B - 4 June 2005

Use of sheet piles to ”cut-off” excavation from water-loggedground.1. permeable strata. 2. water. 3. impermeable strata.

Ground Safe Temporary slopes (1)type Field tests and descriptions (degrees from horizontal)

‘Dry’ site ‘Wet’ siteGRANULAR i Particles visible BOULDERS (>200mm) 35/45 (6) 30/40 (6)

ii Sands feel gritty COBBLES (60 to 200mm) 35/40 (6) 30/35 (6)iii Visually assess proportions GRAVEL ( 2 to 60mm) 30/40 10/30

of Boulders/Cobbles/Sand SAND (0.06 to 2mm) 30/35 10/30

COHESIVE i Particles not visible SILT (0.002 to 0.06mm) 20/40 5/20ii When damp silt and fine sand

are shaken in the hand waterappears on the surface Trench depth (m) 1.23 3-6

iii Soft clay is easily moulded by CLAY (5) Soft 30/45 20/30 10/20fingers

iv Firm is moulded by strong Firm 35/45 30/40 20/25 (6)finger pressure

v Stiff is indented by strong Stiff 40/45 35/45 25/35 (6)finger pressure see (6) and (7)

vi Fissured clay should beexamined for its structure

i Rotten or rotting vegetable PEAT Soft non-fibrous 10/20 5/10matter (black, grey Firm non-fibrous 15/25 10/15

ii Smell or brown Firm fibrous 35/40 (6) 20/25 (6)iii Fibrous or non-fibrous clayey or Stiff fibrous 35/45 (6) (7) 25/35 (6) (7)iv ‘Soft’, ‘firm’ or ‘stiff’ sandy)

FILL i All types of man-placed material As main soil type (eg clay, According to categoryii Note constituents including sand, gravel, etc) above

unnatural inclusions

ROCK i Mass stronger than the above As appropriate to the Check orientation ofsoil types predominant constituents and planes

ii Important to note cementation, characteristicsstructure and orientation,(fissures, joints. bedding,layers, etc)

iii Closely jointed rock may actas “granular” and weakweathered rock as “clay”

GROUND- i Levels water encountered Full description necessary See note 1 (d)WATER ii Rate of entry

iii Standing leveliv Flood conditions

8B - 5June 2005

This page is reproduced from the publication Timber in excavations, by coutesy of the Timber Research and DevelopmentAssociation.

Failure modesThe importance, and indeed necessity, of providing

proper support to excavations is often hard to explain to siteoperatives. The reason is not hard to find. Clays, to theinexperienced, often look very stable when first excavated.They stand up vertically, in many cases, for a surprisinglylong time and a false sense of security is built up. Rock tendsto be assumed as being highly stable, and not needing anysupport at all. Why such assumptions are dangerous isillustrated in the following paragraphs.

Saturated clayey silts present the most difficult problemas the sides will slump into the excavation. The only reallysatisfactory solution is to dewater the ground outside thelimits of the trench.

Saturated silt or sand Dewatering outside the line of theproposed trench will, in general, provide the mostsatisfactory and safe solution. If this is not possible, sheetpiling will be needed to provide a watertight support. Toachieve a safe toeing-in at the bottom of the excavation, thepiling may need driving to a considerable depth to avoid the

bottom of the excavation boiling up and causing totalcollapse of the trench support. Only if the silt or sandoverlays an impervious strata, and a cut-off is possible, will areally safe and economic solution be possible.Sand Where a degree of silt is present in the sand, thematerial may well stand up initially. The illusion of stability isfalse, as a heavy rainfall can provide the lubricationnecessary to allow the trench side to slip into the excavation.The cohesion of the material is also affected by the sides ofthe excavation drying out. Again the sides of the excavationwill collapse, usually without warning. Battered sides are thebest answer, provided that suitable surface water cut-offdrainage is provided. The alternative is to provide adequatesupport with the minimum of delay.

Soft clays Battered excavations in soft clays require carefulconsideration in relation to the slope to be used (see table p.8B–5 for suggested slopes). If too steep, failure can occur byrotation.

If battering to a safe angle is not possible, full sheeting ofthe excavation will be necessary. The method of installationshould:1. allow for the sheeting to be pre-driven before

excavation takes place, or2. be capable of sheeting to a pre-determined depth

before excavation proceeds further, or3. allow the main supporting members to be installed

horizontally as the excavation proceeds downwards(see p.8B-11 for the H pile system of support).

Firm or stiff clay These clays, with or without a degree ofsand or gravel present, will stand unsupported afterexcavation, but what is often not appreciated is that theapparent stability can deteriorate rapidly in a short period oftime; then the face will be liable to collapse without warning.

Collapse is initiated in two ways:1. When a trench is excavated, soil at the sides is relieved

of lateral restraint and tends to swell inwards. Cracksoccur causing unstable lumps. These, in turn, can breakaway and fall into the excavation without warning.

2. Many clays contain fissures, or cracks, which appeardue to the drying out of the clay. When rain, or othersurface water, penetrates the crack or fissure, the clay issoftened and lumps can readily break away. Earth fallsare therefore more frequent in wet weather.

Battering must be at a shallow enough slope (see table p.8B–5). If battering is not suitable, properly assessed supportwill be necessary.

Rock At first sight, rock excavation would seem to presentthe least problems. In fact, all rock masses (including chalk,which can be classed as ‘soft’ rock) are separated intoblocks by bedding planes, cleavage planes and fissures. Allthese are planes of weakness; they can contain water or thinlayers of clay which act as lubricants which will facilitatesliding along the planes.

Excavation in steeply dipping rocks.Collapse of trench side following slide along bedding planesRemedy-heavy timbering

8B - 6 June 2005

Rock excavation, by its very nature, tends to leaveragged faces. Small but heavy fragments may be held inplace quite loosely and vibration, e.g. due to blasting, willdislodge them. All rock excavations should have such loosefragments knocked down as the digging proceeds. Furtherexaminations must be carried out daily, before work com-mences in the excavation. Chances must never be taken inrock; support must be provided if there is any doubt.

Method of excavation

Where an excavation has to be provided with a suitableform of support, the system and the method to be adoptedfor the excavation cannot be considered individually, inisolation. Each will inevitably react on the other and personsdealing with the temporary support must maintain closeliaison with whoever is making decisions on the plant to beused. Such liaison is particularly important in drainagework, when long pipes or large precast manhole rings mayhave to be lifted through any supporting struts etc.

Basic principles of support

There are many ways by which excavations can be madesafe to work in, avoid settlement to adjacent land andbuildings and allow work to proceed with minimumhindrance. There are, however, only the following fourfundamental principles involved:

Battered sidesThis is probably the safest method and accidents resulting

from the collapse of properly designed and executedbattered systems are rare. Almost all soils can be excavatedto a safe batter, provided that sufficient space is available(i.e. The batter will not affect adjacent buildings, etc.) and asafe angle of response is known and adhered to (the tableon p.8B-4 gives a guide to temporary safe batters in varioustypes of ground).

Where waterlogged ground is involved, specialist adviceshould be taken, as some system of ground dewatering willbe needed to improve stability. Only tests can show whetheror not the ground is capable of being dewatered.

Surface water can seriously affect the stability of batteredslopes. Where such flow is likely to happen in wetconditions, interceptor trenches, to cut off surface water andlead it away from the excavation, should be provided.

Double sided supportWith this method of support, the forces exerted by the

earth are transmitted from one side of the excavation to theother by walings and horizontal struts, i.e. the forcesimposed by the earth on the supporting materials areresisted by the earth on the opposite side of the excavation.Hence, if the support is adequately designed, a state ofequilibrium is maintained between the two sides.

Double sided support. All forces horizontal

Of methods using a form of structural support, this is themost satisfactory, as only horizontal forces are involved. Themethod lends itself to the use of pre-designed (“Standard”)solutions and proprietary systems, which are installed inaccordance with manufacturers’ tables provided. (Also seethe paragraphs dealing with the limitations of standardsolutions on p.8B-9).

In saturated silt and sand, where dewatering is not beingused, there is a serious risk of the bottom of the trench‘boiling’ upwards. If this happens, total collapse of thesupport system results. In such conditions, therefore,standard solutions must in no circumstances be used. Thepenetration of the sheeting is a key factor in the safety of thesupport, which requires design by a competent person.

Sheet pile solely in non-cohesive and wet soil

Penetration must be adequate to avoid “boils” or “blows”and determined by a competent temporary works designer.

8B - 7June 2006

Single sided with raking supportOnly one face of the excavation is involved. Earth

pressure from the excavated face is resisted by transferringthe load through the support material via walings to eitherraking shores or ground anchors. In the raking shoreapproach, an adequate foundation is needed to transfer theloads involved to the unexcavated ground.

When raking shores are used to resist horizontal forces,the load in the raking member is greater than the horizontalload and a vertical uplift force is introduced into the supportsystem. The design for this type of support must, therefore,allow for sufficient toeing-in of the sheeting to resist theseupward forces, as must the design of the connectionsbetween strut, waling and sheeting.

With ground anchors,’ the situation is the same, but theforces are reversed. The ground anchor system is in tension,while the vertical component is trying to pull the wholeassembly downwards. Care must therefore be taken toensure that the forces in individual anchors are not too greatfor the ground to resist their vertical component.

Whether shores or anchors are used, single-sidedsupport systems are not suitable for standard solutions. Theymust be designed by suitably qualified persons and, withground anchors, the anchors designed and installed byspecialists. It should be noted that single-sided support ismost frequently used to protect boundary conditions.

Single-sided cantilever supportWhatever variation of this principle is adopted, it should

always be designed by suitably qualified persons. Themethod can be used only:1. in ground conditions which are sufficiently stable toprovide the necessary resistance to balance the overturningforces, or2. where the effect of superimposed loads and their stabilityis not critical.

All cantilevers will deflect to a degree. Such deflectionwill create risk to adjoining installations and structures,roads, etc. and their use in these circumstances should beavoided.

The use of cantilever systems can be improved, in termsof deflection, by what is known as a propped cantilever. Thisinvolves tying back the top of the sheeting system to anappropriate anchor block. The effect of introducing ahorizontal tie-back is to reduce any deflection and at thesame time maintain a system involving horizontal forcesonly.

8B - 8 June 2005

Standard or designed solutionsIn the majority of cases, the support of excavations is

carried out by on site “Standard Solutions”, without recourseto special design. The advent of proprietary equipment hasgreatly increased the scope for such methods. The support ofsome excavations, however, will require detailed designwork by competent persons.

Standard solutionsIt is essential that clear parameters are established in the

adoption and use of standard solutions. These are:1. The use of standard solutions should be applied only in

the following circumstances:(a) Double sided, narrow trench support, not exceeding

6m deep in non-water bearing ground.(b) Shallow pits, not exceeding 6m deep.(c) Where water problems have been eliminated by

other means, e.g. wellpointing, and the excavationis within the limitations of (a) or (b) above.

(d) When an unsupported trench, not exceeding 6mdeep, is feasible.

2. When adopting the standard solution approach thefollowing points need to be followed:(a) When deciding the safe batter of an excavation,

proper account must be taken of the groundconditions.

(b) Where support is provided, the method must comply with recognised good practice in all details (see TRADA & CIRIA publications listed onp.8B-25).

(c) Where proprietary systems are used, installationmust be strictly in accordance with manufacturersinstructions.

(d) The system of work to be adopted must be explainedin detail to the persons engaged in placing orremoving supports, and others who will be workingin the excavation.

(e) If possible, the system of work should be illustratedwith drawings or sketches, or with suitableproprietary literature, together with check lists.

(f) The method of work and instructions and sketchesprovided must be strictly adhered to.

(g) Specialist advice should be sought if there are anydoubts as to the safety and sufficiency of the supportsystem.

The table on p.8B-10 provides a guide to standardapproaches in the support of trenches. Details ofconstruction are outlined on p.8B-11 and further informationis contained in CIRIA Report 97: Trenching practice.

Designed solutionsWhere designed solutions are needed, it is essential that

adequate soil data is available. This information will beprovided by bore holes or trial pits resulting from a propersite survey. The interpretation of this information and designof support methods appropriate to the situation requires theservices of a specialist.

General guidance as to where specialised design isnecessary is covered in the paragraphs on “Basic principlesof support”, p.8B-7.

Systems of supportWithin the basic principles of support, outlined on

p.8B-7, a variety of materials and methods may be used:

TimberTimber is used as a sheeting material, in the form of

poling boards and runners, and also as walings and struts.Examples of its use are given later in this section underTrenches, Shafts and Headings.

Steel trench sheetingTrench sheeting has the advantage of easy driving in

poor ground, prior to an excavation being commenced. Instiffer ground, it can be part driven initially, then furtherdriven as the excavation proceeds.

Steel sheet pilingObtainable in a variety of sectional stiffnesses, sheet

piling is interlocking and, being much stiffer than trenchsheeting, it can be pre-driven to much greater depths beforeany excavation is started.

Sheet piling is expensive in the cost of driving and in itssubsequent extraction. However, its use has the followingadvantages:1.- By pre-driving, the main structural support is in place

before any excavation is started.2. The interlocking is reasonably watertight, making its use

ideal where waterlogged or very unstable ground is to besupported.

3. Because of its structural strength, steel sheet piling can beused for any of the basic principles of support referred toon p.8B-7. It is the only really satisfactory material wherea cantilever support is used.

4. Where impermeable conditions exist below a water-logged strata, sheet piling can be used to ‘cut-off’ thewaterbearing layer from the excavation. Where no cut-off is economically possible and wet, non-cohesivematerial is to be supported, the design of the piling mustallow sufficient penetration to avoid the bottom of theexcavation heaving or ‘boiling’ (see p.8B-7).In all cases where steel sheet piling is the proposed

solution, calculations should be made by a competentperson.

Guidance on piling operations is given under Piling,p.8F-2/5.

8B - 9June 2006

8B - 10 June 2005

Waling and strut arrangements in the support of trenches(This page is based on the table on page 31 of CIRIA Report 97, Trenching practice, by permission of the Director of CIRIA).

Unsaturated ground (except soft clays)

Maximum Effective Maximum Timber Timber strut section mm*horizontal trench vertical walingspacing of depth - m spacing of section - mm Trench Trench Trenchstruts - m walings - m width width width

up to 1m 1 to 1.5m 1.5 to 2m

Up to 1.2 One set 225 x 75 150 x 75 150 x 100 150 x 150150 x 100

3 1.0 225 x 75 150 x 75 150 x 100 150 x 1501.8 1.2 150 x 100

4.5 1.0 225 x 75 150 x 100 150 x 150 150 x 1501.2 150 x 100

6 0.9 225 x 75 150 x 150 150 x 150 150 x 1501.0 150 x 1001.2 250 x 100

Up to 1.2 One set 225 x 75 150 x 75 150 x 100 150 x 150150 x 100

3 0.9 200 x 100 150 x 75 150 x100 150 x 1501.3 Twin 225 x 75 150 x 100 150 x 100 150 x 150

2.5 spiked together

4.5 0.9 Twin 225 x 75 150 x 100 150 x 150 150 x 150spiked together

1.5 225 x 150 150 x 150 150 x 150 150 x 150

6 1.1 225 x 150 150 x 150 150 x 150 150 x 1501.5 300 x 150 200 x 150 200 x 150 200 x 150

Up to 1.2 One set 225 x 75 150 x 75 150 x 100 150 x 150

3 0.9 Twin 225 x 75 150 x 75 150 x 100 150 x 150spiked together

3.0 1.5 225 x 150 150 x 150 150 x 150 150 x 150

4.5 1.0 225 x 150 150 x 150 150 x 150 150 x 1501.3 300 x 150 150 x 150 150 x 150 150 x 150

6 0.8 225 x 150 150 x 150 150 x 150 150 x 1501.0 300 x 150 150 x 150 150 x 150 150 x 1501.5 250 x 200 250 x 150 250 x 150 250 x 150

Up to 1.2 One set 200 x 100 150 x 75 150 x 100 150 x 150

3 1.1 225 x 150 150 x 100 150 x 100 150 x 1503.5 1.5 300 x 150 150 x 150 150 x 150 150 x 150

4.5 1.0 300 x 150 150 x 150 150 x 150 150 x 1501.5 250 x 200 200 x 150 200 x 150 200 x 150

6 0.8 300 x 150 150 x 150 150 x 150 150 x 1501.1 250 x 200 200 x 150 200 x 150 200 x 1501.5 250 x 250 250 x 150 250 x 150 250 x 150

Saturated ground (except soft clays and silts)

1.8 3 1.2 225 x 150 150 x 100 150 x 100 150 x 150

4.5 1.2 250 x 200 150 x 100 150 x 150 200 x 150

* Proprietary steel trench struts, of equivalent strength, may be used instead of timber struts. Steel struts should be to BS4074.

H-piling or soldier pilingIn this system, steel universal column sections are pre-

driven, at determined centres, prior to any excavationcommencing. If the ground conditions make driving difficultor, if vibration or noise is to be avoided, the steel sectionscan be positioned in pre-drilled holes with the bottom endheld in concrete.

As excavation proceeds, the exposed face is supportedby timbering or trench sheeting set horizontally between thepiles. In effect, the sheeting takes the form of horizontalpoling boards, wedged in position as shown below.

The ‘H’ piling method has a number of very importantadvantages when heavy support is needed, e.g:1. The main structural support is in place before excavation

commences.2. The sheeting can be kept tight up with the excavation,

which need not be left unsupported for any length oftime.

3. Maximum economy in sheeting material is possible; e.g.if the ground appears suitable, as exposed, hit and misssheeting may be adequate in some strata while, in otherlevels, close sheeting may be needed.

4. If insufficient sheeting has been used, i.e. if the ground isseen to be pushing through gaps in the sheeting, it canbe cut away and additional sheeting installed in thegaps.

5. Where services cross an excavation, the ‘H’ pile methodis particularly adaptable (see illustration in next column).The piles can be installed between service locations andhorizontal sheeting located above and below. If the gapresulting is excessive, vertical sheeting can be tuckedbehind the horizontal members to fill in the gaps.

‘H’ Piling can be applied to both double or single sidedmethods of support. It is not, however, applicable tocantilever support, or in ground conditions which arewaterlogged. As an engineered system, it must be designedby competent persons.

Proprietary systemsThe number of proprietary systems is constantly growing.

They are designed primarily to allow the support system tobe installed in a trench without the need for persons to enter.Some methods, however, merely provide a waling and strutassembly that can be used to support timber or steel trenchsheeting.

Examples of the use of proprietary systems are given inthe following paragraphs under Trenches.

Trenches

Standard solutions can normally be applied up to depthof 6m, provided that the excavation is in non-water bearingground. In all other situations, designed solution must beused.

Standard solutionsThe use of standard solutions in trench excavation will

mean using either “Traditional” methods, or proprietarysystems.

Traditional methodsTraditional methods use timber or steel trench sheeting as

the sheeting material, timber walings and either timber oradjustable steel struts. With these components, a number ofoptions are possible, to suit varying conditions.

The use of poling boards is dependant on the materialto be excavated standing up to a height equal to the lengthof poling board used during the boards’ installation. Twovariations in installation are possible:

The middle board method, shown opposite isinstalled by excavating to a depth equal to the length of the poling board being used. Poling boards are then set up plumb, at a distance apart equalto the length decided upon for the walings, on both sides of the trench. The walings areplaced in position and strutted against the first polingboards. The intermediate boards are next located, removing the minimum of face soil to provide asnug fit behind the waling member. When all

8B - 11June 2005

boards are in position, the final number of strutsrequired are installed. All poling boards betweenstruts must be checked and, where necessary, wedgesdriven between the poling boards and walings toprovide tight contact with the excavation.

Excavation can then proceed downwards, for the nextlevel of poling boards.

Although the diagrams show timber struts, these can, ofcourse, be replaced by metal trench struts.

The Tucking Frame method shown below, only differsfrom the middle board approach in that the walings andstruts are located at the top and bottom of the poling boards,instead of the middle.

The excavation, prior to the installation of a level ofpoling boards, needs to be slightly deeper than thepoling board length. This is to enable the board to betucked up and behind the waling.Runners are used where the ground will not stand long

enough to allow the installation of a poling board system.Runners are longer sheeting members than poling boardsand can be in timber or steel trench sheeting.

In bad ground, where continuous support is needed, therunners are pitched and held vertically in a temporaryframework (usually a lightweight version of a sheet pilingframe). They are then driven into the ground as far as theywill reasonably go, without damage. A shallow depth ofexcavation is carried out and the first level walings and struts installed. The runners are wedged tightly to theexcavation, and digging continued until the end of the

8B - 12 June 2005

(Illustration by courtesy of the Timber Research andDevelopment Association.)

runner is only just held in the unexcavated material. Thewedge on each runner is loosened separately, the runnerdriven down again, and the wedge re-tightened. Thiscontinues until the next waling is inserted.

Two sets of wedges have to be dealt with at each runnerposition. In this way, the excavated face can be continuouslysupported while excavation to the full depth of the runner iscarried out. Extra depth is possible by inserting a new set ofrunners inside the first and continuing downwards.

The main features of the use of runners are shown below.

Where ground conditions permit, the procedure can besimplified. A shallow excavation is first cut and the pitchingframe set up in it (as shown opposite). The runners arepitched and plumbed and excavation carried out to thedepth that the earth will stand. The runners can then beallowed to drop into the excavated area, or will require onlythe minimum of driving, until they bed into the unexcavatedmaterial. They are then wedged as before and excavationcontinued. The sequence is repeated as required -

(Illustration by courtesy of the Timber Research andDevelopment Association).

Proprietary SystemsProprietary Systems fall into a number of well defined

groups:1. Hydraulic struts associated with waling systems (usually

in aluminium to save weight).2. Shields or boxes which are dragged along the trench as

work proceeds.3. Box or plate supports4. Special methods.

As there are many suppliers and varieties, the groupsabove are only outlined in this section. For further details,reference should be made to CIRIA Technical Note 95:Proprietary trench support systems.

All proprietary methods should be used strictly inaccordance with the manufacturer’s instructions.

Hydraulic struts/walingsA typical hydraulic struts/waling system is shown

below. The objective is to provide a strut/walingassembly which can be lowered into the trench andhydraulically stressed against the sheeting withoutanyone entering the trench. Such Systems are normallyonly suitable for the support of trenches up to 6m deep.

8B - 13June 2005

Use of runners.

(Illustration by courtesy of the Timber Research andDevelopment Association.)

Waling/strut assembly with steel trench sheeting.

(Photograph reproduced by courtesy of Mechplant Limited).

Shields or drag boxesAs the name suggests, these are designed primarily as

a protection for persons working in an excavation, ratherthan an excavation support. The side sheeting andstrutting, which keeps the sheeting apart, provide a rigidbox. As the work proceeds, the box is pulled forward bythe excavating machine to the new working area.

Such boxes are simple to make and, as they are rigidstructures, require little maintenance. Inevitably, though,they are heavy and the excavator has to be powerfulenough to pull them forward in the trench.

Box or plate lining SystemsBox or plate lining systems are designed for support,

not just protection and should not be confused withshields or drag boxes.

Box support systems, as shown opposite, have struttedsupport walls of a modular nature, which can bepositioned by machine, and built up vertically andlaterally. They are not designed to be dragged along thetrench.

Plate lining methods work on the principle of installinga vertical member and struts at set intervals, betweenwhich heavy plates slide into position.

8B - 14 June 2005

Typical drag box.

(Photograph shows a box manufactured by JayvilleEngineering Limited and supplied by GKN Kwikform).

Box support system.

(Photograph reproduced by courtesy of Scaffolding (GreatBritain) Limited, distributors of Krings Linings).

Plate lining method.

(Photograph reproduced by courtesy of Scaffolding (GreatBritain) Limited, distributors of Krings Linings).

With either box or plate lining systems, cross servicespresent a problem, as the run of the system has to beinterrupted. Other methods for support have to be used insuch areas.

Designed solutionsWhere the depth of trench exceeds 6m, the support

system should be designed by a competent person. To do so,an adequate soil report should be available, giving allnecessary data on water levels, the depths at which theyoccur, and whether any particular strata is liable to boil ifcut-off is not achieved.

Use of runnersWhile timber or steel trench sheeting can be used in deep

excavations, by installing one set of runners inside anothersetting, the labour intensive nature of the method tends tolimit its use.

Sheet pilingWhere water-bearing ground has to be dealt with, sheet

piling is the obvious choice. Its interlocking nature and stiffsection enable pre-driving to the full depth of the excavationand the creating of a watertight barrier. Where the strata issuitable, the water-bearing strata can be cut-off from theexcavation by the sheet piles penetrating into a non-perviouslayer below.

H-piling (Soldier piling)If water is not present in the excavation, the H-piling

method of support is normally used. In association with steelwalings and struts, considerable depths can be supported(see p.8B-11).

Wide and single-sided excavationsWhen a trench becomes very wide, problems arise with

the strutting. The struts themselves become long, andbending due to their own weight has to be taken intoaccount, as well as the compressive forces induced by theearth pressure. The effect of bending is to reduce thecompressive forces that can be resisted. In thesecircumstances, a proper structural analysis needs to bemade by a competent temporary works designer.

An alternative is to adopt a single-sided support method.Whichever approach is used, a designed solution isessential.

Single-sided supportIt is desirable that the main structure of the support is in

place before any excavation takes place. For this reason,steel sheet piling and H-piling are the only satisfactorymethods to use, unless permanent construction is beingspecified as the sheeting material. (Diaphragm walling,contiguous bored piles and secant piling, to which the contractor has to provide temporary support aredescribed under Permanent works as temporary support,p.8B-19).

The principles of single-sided support are given on p.8B-8.

ShaftsThe method of supporting a shaft, as with any

excavation, is likely to be determined by the groundconditions. In reasonably good ground, which will stand forone metre or more, support systems based on the polingboard can be used. In bad ground, systems based onrunners will be needed, unless sheet piling or soldier pilingare considered necessary.

Use of poling boardsThe commonly used technique is very similar to that used

in trenching and the same length and section of boards areused. The ground is opened up to a depth equal to the lengthof the poling boards, when the first setting of boards isinstalled and supported by walings and struts. Walings maybe installed at mid point of the poling board (middle boardmethod), or may be positioned at about the quarter points,as shown below.

8B - 15June 2005

Method of side supports in shafts.

(Illustration by courtesy of the Timber Research andDevelopment Association).

8B - 16 June 2005

As in trenching, the waling frames are supported at thecorners of the shaft by puncheons, which may be termedangle posts. Where an intermediate strut is used, thesupporting puncheon is installed as in trenching practice.

The waling frames are normally installed by positioningtwo walings which run the whole length on opposing sidesof the trench. The other walings are cut to length between theinside faces of the walings already positioned and shouldbear on timber stretchers or cleats spiked to them (seeillustration below, plan at B-B). The frame is then supportedby puncheons placed at the ends of the longer members.

When the waling frames are positioned and the polingboards tightened against the excavation by wedgesbetween the walings and poling boards, the next phase ofthe excavation can proceed and lower levels of polingboards installed.

During the excavation of shafts, and at the completion ofthe installation, waling frames should be secured andsupported by lacings, ground props and foot blocks asappropriate.

Both timber and trench sheeting may be used for thepoling boards. Timber is well suited for walings asdimensional problems can readily be overcome, while the

same applies to struts. Even so, proprietary systems areavailable and can be used when the dimensions areappropriate.

Use of runnersWhere the ground conditions are less stable, the use of

runners can ensure virtually continuous support to the shaftas the excavation proceeds.

Runners, up to 3m long, are used and are driven to formthe lower level support in much the same manner asdescribed for trenches. The ground is first excavated to asafe depth for the ground conditions. Vertical corner boardsare positioned and the first walings cut and installedbetween them. The second walings, at right angles, are thencut and positioned. Having restrained them by struts, toproduce a rigid box frame, the runners are then introducedinto the gap between the excavation and the walings anddriven down as far as the ground conditions will allow. Eachrunner is then forced against the excavation by wedgesinserted between the waling and the runner. Wedges areprogressively eased and retightened as the excavationprogresses downwards, at the same time as the runners aredriven down to cover the excavated face.

Shafts with maximum dimension up to 2. 75m showingalternative arrangement for poling boards.

(Illustration by courtesy of the Timber Research andDevelopment Association).

Notes1 Method shown is suitable for depths down to 6m.2 Timber to be strength class SC4.3 No hydrostatic head.4 Where the shaft is to be dug in cohesive ground it should

be soft or better and should have a minimum C value of30 N/mm2,

5 A uniformly distributed ground load of 10 kN/m2 hasbeen allowed.

6 Where higher loads, or point or line loads are expected.then the shaft should be designed by an experiencedperson.

Notes1 Method shown is suitable for depths down to 4.5m.2 Timber to be strength class SC4.3 No hydrostatic head.4 Where the shaft is to be dug in cohesive ground it should

be medium soft or better and should have a minimum Cvalue of 30 N/mm2.

5 A uniformly distributed ground load of 10 kN/m2 hasbeen allowed.

6 Where higher loads, or point or line loads are expected,then the shaft should be designed by an experiencedperson.

Use of runners in shafts up to 1.8m square.

(Illustration by courtesy of the Timber Research andDevelopment Association).

Steel or timber runners are equally suitable for thismethod and steel sheet piling may also be used where wetground conditions exist. The waling frames are usuallytimber, but proprietary systems or steel sections may also beused.

HeadingsHeadings are small tunnels cut into the sides of trenches

or shafts. They constitute the most hazardous sector ofexcavation as, in addition to the dangers associated withtrenches and shafts, they introduce the risk of trappingpersons with no alternative escape route. In such cases,rescue is difficult and dangerous.

The cutting of a heading requires a very high degree ofcareful investigation and consideration. This applies to thedesign, quality of materials, excavation techniques andworkmanship, as well as to safety and supervision.

The following recommendations apply only to headingsnot exceeding 2m either in height or width. Any headings

in excess of these dimensions must be designed by acompetent engineer, as should any heading in rock,irrespective of its dimensions.

MaterialsTimber is the most suitable material, as a great deal of

cutting will be necessary. Wherever the timbering is to be leftin place, new material should be used and treated bypressure impregnation to protect against decay, usingcopper/chrome/arsenic salts in accordance with BS 5589.

DimensionsHeight should not be less than 1.2m measured from the topof the sills to the underside of the head tree. The width shouldnot be less than 700mm measured between the inside facesof the side trees.

Excavation and supportAs the greatest reliance is placed on the experience and

expertise of the operatives engaged in the driving of aheading, no specific sequence of operations is recom-mended. Three basic support methods are used:

Box settingThis method is only suitable for small headings in firm

ground.

8B - 17June 2005

Normally, roof and side boards are not required but, inlocalised areas, they can be inserted by ‘tucking’ betweenadjacent head or side trees and the excavated face.

Poling settingThis method should be used in ground conditions where

pieces start to fall in almost immediately after excavation hastaken place. Examples are: damp or cohesive sandygravelly material, or clays which contain fissures and crackinto small pieces when drying out takes place.

A poling setting is an extension of the box setting, withroof and side boards ‘tucked’ between the head and sidetrees so that both ends are supported. The boards should bein close contact with the top and sides of the heading. Theymay be spaced apart, or be in edge to edge contact,depending on the nature of the ground.

In ground where the floor of the heading may boil orheave, then floor poling boards should also be introducedbetween sills.

Piling settingIn poor ground, of the running or flowing category,

continuous support at all stages of the excavation must bemaintained and the piling setting method used.

The method becomes more complicated when theadvancing face of the heading also needs support. Furtherdetails are given in the TRADA publication Timber inExcavations.

8B - 18 June 2005

Securing the headingRegardless of the method used, the excavation of a

heading should not cease until a setting under construction iscompleted, with head and side trees in position. The face ofthe heading should be boarded if the excavation stops formore than two hours.

SupervisionThe supervision of and routine checks on temporary

works in a heading are different from those employed inopen excavations. Routine inspection must be carried out atthe point of maximum risk, i.e. the advancing face of theexcavation.

Another important factor is that speed of the excavationhas a bearing on safety, as the rapid installation of a supportstructure is critical to the integrity of the cut face.

Inspection and supervision must therefore be carried out by experienced persons, who should be able to inspect, approve and authorise continuation, withoutimpairing the speed of advance on which the safety of thework may depend. This means that constant supervision isnecessary.

Permanent works as temporary supportThere is frequently the need temporarily to support

systems of retaining wall construction, until the permanentstructure has been sufficiently advanced to provide thepermanent support. What must be appreciated is that, whilsttemporary support is being provided, the walling system isacting as temporary works. Where there is this division ofresponsibility for temporary works, it is essential that thedesign of the supporting members is carried out by acompetent person.

The techniques are:-Diaphragm wallsContiguous bored pilesSecant pile walls

Diaphragm wallsThese are commonly used in clay and sand/gravel

areas, to achieve the perimeter support to basements. Theiradvantage lies in the ability to install the wall before anyexcavation takes place. A narrow trench, of the requiredwidth of finished retaining wall, is excavated by means ofspecial grabs. As the excavation proceeds, the trench is keptfilled with a suspension of bentonite in water. In this way,trenches can be excavated to considerable depths withoutfurther support. When the required depth is reached,reinforcement is lowered into the liquid, and concrete placedby tremie. As this is done, the bentonite solution is forced outof the trench, collected and re-used later.

Diaphragm wall construction is carried out by specialistsub-contractors. The main contractor, however, is required toconstruct the ‘guide’ wall. It should also be noted that theinside guide wall has to be removed, again by the maincontractor, as the excavation progresses downward. Itsadvantages are:1. Installation free from vibration and excessive noise.2. No support of the earth face is needed.3. Walls can be constructed with minimum disruption to

adjacent areas.4. Such walls serve a dual purpose - they avoid the need for

temporary sheeting to the excavation and become thefinal retaining wall, usually with some form of facing forcosmetic reasons. As a result, the cost of temporaryworks is reduced.

5. Where ground anchors can be used as the temporarysupport, the whole construction area within thediaphragm wall is unencumbered with supports and thepermanent construction can proceed much more

efficiently than if the support had to be by raking shoresor long struts right across the width of the excavation.

6. In water-bearing ground, diaphragm walls aresubstantially watertight.

Contiguous bored pilesBored piles can also be used to support excavations,

while providing the main element of permanent support. Inthis role, they are installed as a continuous curtain where thesupport is needed. Installation is normally carried out byboring alternate piles first and later inserting further piles inthe gaps. The method is normally used in conjunction withground anchors as the temporary support or, if practicable,with horizontal shores.

8B - 19June 2005

The unevenness of the pile means that adjacent piles arenot in complete contact. The method is not, therefore,suitable in wet conditions without special and expensiveprecautions.

Bored piles are similar in cost to diaphragm walls, andhave very similar advantages, except their inability to retainwater.

Secant bored pile wallsIn secant piling, the bored pile system has been

developed further to provide a watertight wall.The wall is formed by a series of individually formed piles

which interlock. The first stage is to construct piles at centresless than two pile diameters (female piles). Intermediate piles(male piles) are then made which, in the boring andconcreting process, cut secants out of the adjoining femalepiles.

A heavy rig hydraulically forces a tubular casing into theground by rotary oscillation, combined with vertical loading.The soil within the casing is next excavated by a hammergrab. In this way the soil is supported at all times. Asconcrete is placed, the casing is withdrawn. With the piles offixed dimensions, due to the formation within the casing,close contact is certain and a watertight junction achieved.The system is ideal in water bearing ground to provide awatertight perimeter, either on a permanent or temporarybasis.

Temporary support must be provided, as for diaphragmwalls and contiguous bored piles. Such support must alwaysbe designed by competent persons. Installation will alwaysbe by specialised firms having the necessary equipment.

Related safety aspectsApart from safety requirements directly associated with

the support of excavations, the following matters must alsobe considered:

Means of accessSafe means of access to and from excavations must be

provided, as must easy means of escape in an emergency.

Ladders must be of sound construction, of adequatelength and strength, of the correct type and be placed in thesupported part of the trench. They must be effectivelysecured, near the top, to prevent slipping sideways, bylashing, either to adjacent support material or to stakesdriven firmly into the ground. They must project sufficientlyabove the excavation to provide a safe handhold (a distanceof 1.05m is recommended).

Gangways are often needed for access purposes acrossexcavations. Where persons might fall and be injured, suchgangways must be fitted with toeboards, at least 150mmhigh, and main guardrails, at least 910mm high. Anintermediate guardrail, or other rigid barrier, must also befitted so that there is not an unprotected gap exceeding470mm in height. Gangways should be at least 430mmwide for personnel access, or 600mm for persons andmaterials.

On large sites, there may be occasions when bridgingunits are necessary to allow plant and vehicle movementacross an excavation. In such cases, proper design bycompetent persons is essential, both for the safety of theloads to be carried and to avoid excessive loads on theedge of the excavation.

BarriersThe edges of all excavations, where anyone might fall

and be injured, must be protected by rigid barriers or,alternatively, excavations must be securely covered. It is awise precaution to protect even shallow excavations.

Non-rigid barriers, which may be used in areas whereaccess for persons and transport is not required, should givean obvious warning by being immediately apparent andbrightly coloured.

Spoil heaps can constitute effective barriers, providedthat they are set back a safe distance from the edge. Ropebarriers can also be effective, again provided they are setback far enough.

Barriers may be omitted to allow access of persons, or forthe movement of plant, equipment or materials, or where ithas not yet been practicable to erect such a barrier since theformation of that part of the excavation.

Wherever barriers are set back, all access to theexcavation should be confined to the proper paths and nomaterial should be stacked in the space between barriersand the edge.

Barriers, or stop blocks, are also necessary to stopcranes, dumpers, lorries, tipping vehicles, etc. frommanoeuvering too close to the edge of an excavation andendangering its stability. Timber baulks are effective againstsmall wheeled machines, but taller barriers are needed forlarger wheeled equipment and tracked machines (also seeSection 19 - Site Transport).

8B - 20 June 2005

8B - 21June 2005

LightingDuring darkness, edges of excavation should be

illuminated, especially where they are adjacent to publicthoroughfares.

During the winter, or in deep excavations, shafts andtunnels, lighting will be necessary to enable operations toproceed safely. Such lighting must be installed withequipment suitable for use in the excavation.

VentilationExcavations must be kept free from toxic or explosive

gases and it must be remembered that any gases which areheavier than air will tend to settle in excavations. The gasesinvolved may be natural, like methane and sulphur dioxide,or they may arise from nearby internal combustion engines(carbon monoxide), leakage from liquefied petroleum gasequipment, or underground storage, or from sewers.

One of the most effective methods of keeping theatmosphere healthy, is to use ventilating equipment to blowclean air into the excavation, shaft or tunnel, in sufficientquantities to dissipate the foul atmosphere.

Tests must always be carried out in advance of workstarting, and throughout the period of the work. For furtherinformation, see Section 23 Confined Spaces.

Means of escapeAll persons working in an excavation should be instructed

in escape and rescue procedures should an emergencyarise. This is particularly important where unhealthyatmospheres might arise. Instruction should be given on theuse of rescue equipment (see Section 23 - Confined Spaces).

Damage to underground servicesIt is essential, at the planning stage, to ensure that proper

precautions are taken to avoid damage to undergroundservices. Detailed guidance is given in Section 20 -Overhead and Underground Services and in HSE GuidanceBookletHS(G)47.

NoiseOccupational and environmental noise is a hazard in

excavation, particularly in the case of pile driving. Stepsshould be taken to ensure that noise from plant is reduced atsource as far as possible, that noise levels are assessed andear protection worn where necessary. Further informationand guidance on compliance with the Noise at WorkRegulations 1989 is given in Section 32 - Noise.

Check List No.1 - Site conditions and methodconstraints. (Design stage)

Knowledge of the ground and local constraints (adjacentbuildings etc.) must be available in order to determine themethod of excavation and the need for support. In addition,in the case of trench work in particular, what has to go intothe trench or excavation will be important. The handling ofsuch items may well affect the form of support that can beused. The following questions need to be answered beforeany decisions are taken:1. What are the soil types and groundwater conditions?2. Is there any knowledge of work carried out previously

in the area?3. Is the excavation adjacent to existing structures or

roads?4. Are there any obstructions to the line of the excavation?

e.g. public utility mains or cables, sewers, industrialmains, overhead cables, or limited clearance on theline of the excavation which may restrict choice ofexcavating equipment?

5. Has the condition of the adjacent buildings, roads etc.been recorded by the client or his representative? (e.g.cracks etc.)

6. Is there any risk of surface flooding while the work is being carried out? Is flooding from services possible?

7. Is the excavation within the site boundary (inside thehoarding area), or are special precautions necessary toprotect the public?

8. If the excavation is a trench, what are the maximumdepths to be supported and what size and length ofpipes have to be handled? Can pipes be tested in shortlengths or does the specification require testingmanhole to manhole?

9. In the case of wide excavations, what will be theimplication of the support system to the permanent work- and vice versa?

10. What surcharge loads are likely to arise?11. Will the method involve vibration? Will it affect

the stability of the excavation and/or adjacentbuildings?

When answers to the above questions have beenobtained, the following need to be asked:

(a) If the ground is saturated, are the conditions suitable for a dewatering system to be used? If so, can battering be adopted without endangering adjoiningproperty?

(b) Is the ground of the type that may ‘boil’ whenexcavation is commenced?

(c) How long will the excavation be open? (If capable ofbattering this will be the economical solution, especiallywhere the excavation has to be open for a long periodof time).

(d) Where surface water may be a problem, can anadequate run-off system be achieved from any cut-offdrains or ditches that may be needed?

(e) What action is needed to support and safeguardservices etc. which cross or are adjacent to theexcavation? How will this relate to the support systemenvisaged?

(f) Is the position of the works likely to lead to surfacewater pollution?

(g) Is the dewatering method likely to lead to a possiblepollution threat to ground water or an aquifier? If soadvice must be sought from an environmentalconsultant or the Environment Agency

When all these questions have been answered, adecision can be made as to the use of battering, proceeding

with standard solutions, or whether the support will need tobe designed.

Check List No.2- Before work starts1. Is the person directly supervising the work fully

experienced and competent in the support ofexcavations?

2. Have all services been located and proved as toposition? (see Section 20 - Overhead and UndergroundServices).

3. Are the necessary drawings or sketches (wherestandard solutions are to be used) available? Have theoperatives been properly briefed and instructed as towhat is required of them?

4. Are all materials necessary available on site and inaccordance with the drawings/sketches?

5. Has the excavator to be used a certificate of exemptionfor lifting material in connection with the excavationand is a copy available on site? (see p. 9-33)

6. What arrangements need to be made in relation to sitesecurity, particularly in relation to stopping childrengetting on to the site?

7. Is there adequate working space for plant to be used, inaddition to the requirements for spoil heaps? (Spoilheaps should not be less than 1m from the edge of theexcavation).

8. Is material for barriers available and, where workingon the highway, approved traffic signs? (see Section 26Roadworks).

9. Will bridges or gangways be needed? If so, is theequipment available?

10. Are sufficient ladders on hand for access to and fromthe excavation?

11. Will lighting be required?12. Is appropriate protective clothing and equipment

available?13. Are operatives experienced in the type of excavation,

or will extra supervision be needed?14. Are water pollution prevention measures in place?15. Is the site in a high risk area for unexploded ordnance?16. Are emergency procedure plans in place for use in the

event of the discovery of unexploded ordnance?

Check List No.3- Whilst work is in progressThe following list covers the main items which need to be

checked in carrying out the inspections required bySchedule 3 of the Construction (Design and Management)Regulations 2007:1. Is access to and from the workface sufficient and

secure?2. Are all working faces secure, wedges tight and support

material free from damage?3. Is there any sign of movement or deflection in the

support system?4. Is the soil condition as predicted? If not, what action

should be taken?5. Are spoil heaps the correct distance back from the

trench edge?6. Are pipes, bricks and other materials, plant etc., well

clear of the edge so that there is no risk of falling intothe trench or of vibration causing damage to thesupport?

7. Are the drawings/sketches being properly followed ininstalling the support? (This is particularly important inrelation to the spacing of walings and struts).

8. Are walings and struts, or proprietary equipment,supported against falling downward - by hangers,puncheons, lip blocks, etc?

9. Is there any risk of gases, or noxious fumes, getting intothe workings?

8B - 22 June 2007

8B - 23June 2007

10. Are regular tests for gases or fumes being carried out?(This is particularly important in shafts and tunnels). Isventilation required?

11. Has any risk of flooding been properly assessed?12. Is resuscitation equipment available and a nominated

person trained to use it?13. Have all persons been instructed in evacuation

procedure and the correct rescue procedure to follow ifsomeone is overcome by gases or fumes in the trench?

14. Have all support materials been checked beforeinstallation in the excavation? In particular, are thecorrect pins provided in the trench struts?

15. Is the work adequately protected and marked duringthe day? Is it fenced, or covered, and lit at night? Arewatchmen needed?

16. Are operatives wearing safety helmets? Is any otherprotective equipment needed?

17. In shafts and tunnels, in particular, is adequate lighting provided? Is the temporary lighting systemsafe?

18. Do gangways or bridges comply with the requirementsof the CDM - Part 4 and the Work at Height Regulations2005 in relation to the provision of safe movement forpedestrians and vehicles and the prevention of falls.Have access bridges for plant and vehicles crossing theexcavation been designed by competent persons?

19. Where backfilling is required, are stop blocks inposition?

20. Is there an agreed system of support withdrawal andhave those carrying it out been properly instructed?

21. Where pumping is necessary, is a proper watch beingkept to make sure that fine material is not being drawnout from behind the support system?

22. Is the water from a pumping process being dumped outonto grassland or, if it is being pumped into a watercourse, aquifier or drain, is there an EnvironmentAgency consent in place and are the conditions beingcomplied with?

Glossary of terms

Batter (or rake) An artificial, uniform steep slope. Intrenching, the angle of batter must beless than the angle of repose of the soil.

Bitch A fastening of iron or steel used forsecuring heavy timbers which cross eachother. Similar to a dog but with one of itsends at right angles to the other.

Chogs (or chocks, Timber blocks used as distance piecesor blocking) or packing, e.g. between a waling and

the temporary or permanent lining of an excavation.

Cleat A block of timber fixed to a member toprevent the movement of other abuttingtimbers.

Dog A fastening of iron used for spiking largetimbers together and having both endsbent down and pointed.

Folding wedges Wedges used in pairs, overlapping eachother and driven in opposite directions inorder to hold or force apart two parallelsurfaces.

Foot block A timber pad used to spread a load froma ground prop or side tree.

Hanger A steel rod or bolt sometimes used(or tie rod) instead of lacings between successive

frames to take their weight and prevent movement of the timber.

Head trees Horizontal timbers, at least 225m x75mm, in the roof of a heading, whichrest on the side trees and support thehead boards.

Lacings Vertical timbers spiked to the sides ofstruts and walings, tying them together tocarry the weight of the lower frames asexcavation proceeds.

Lip (lipping block, A short length of timber fixed and spikedor lipping piece) to the top of a strut, and projecting

sufficiently beyond its end so as to rest ona waling. It supports the weight of the strutwhile wedges are being driven.

Pinchers Pair of poling boards, strutted apartacross a trench, to support the excavatedfaces where the ground is good.

Poling boards Timbers, 1m to l.5m long, 32mm to 50mmthick and usually 225mm wide, orequivalent steel sheeting, placed verticallyin an excavation to support the sides.

Puncheons Vertical props used to support higher(or props) walings or struts from the ones

below.Runners Vertical timbering, usually approximately

2.5m to 3m long, 50mm thick and225mm wide, with the lower ends chisel-shaped. Used in unstable ground insteadof poling boards and driven in advanceof digging.

Side boards Boards, at least 150mm x 38mm,forming the sides of a heading.

Side trees Timbers, at least 225mm x 75mm, whichsupport the head trees and side boardsin a heading.

Sills Timber, at least 225mm x 75mm, laidacross the bottom of a heading or trenchand carrying at its ends the feet of theside trees.

Soldier piles Vertical supports holding horizontal(or H piles) sheeting or walings in place. They are

secured by struts across the excavation or by ground anchor tie backs.

Steel sheet piling Steel sheets, capable of being inter-locked, driven to the full required depthbefore digging begins. Normally usedin loose, soft or waterlogged soil.Sheeting is either tied back, struttedacross the width of the excavation, orused with raking shores or in cantilever.This type of sheeting can be re-usedmany times.

Steel trench sheets A lightweight form of steel sheet pile (seeabove), which has largely replacedtimber poling boards and runners insupport work.

Stretchers (or liners) Timbers, at least 150mm x 38mm, drivenbetween the ends of opposing membersof a frame to lock them in position andspiked to members against which theyrest.

Struts Horizontal members in compressionwhich resist the thrust from the sides of anexcavation.

Walings Horizontal members supporting polingboards, runners, vertical sheeting orsoldiers.

8B - 24 June 2005

Groundworks method statements shouldnormally include the following so far as isrelevant in the circumstances :-– Name and address of the groundworks contractor.– Name and address of the site to which the method

statement relates.– Names of the supervisor/foreman and appointed safety

adviser and arrangements for monitoring the work.– Description of the works to be carried out including

confirmation that the site has been visited and dueaccount taken of surrounding conditions, access, etc.

– Details of the personnel to be used and their training/experience.

– Details of PPE to be provided to personnel.– Sequence of operations detailing method of dealing with

specific hazards.– Details of methods of locating and avoiding any existing

underground services, whether or not their presence isknown.

– Details of Environment Agency consent conditions inrelation to any likely impact of the works on surfacewater, ground water or an underlying aquifier.

For all excavation works :-– Nature of the soil and moisture conditions.– Measures taken to check for toxic contaminants in the

ground.– Dimensions of the excavation.– Method of excavation.– Proximity of services or structures.

– Duration of work.– Details of the precautions to be taken to prevent collapse

of the sides of the excavation e.g. battering or support. Ifsupport is proposed this should include details of thesystem to be used.

– Details of access/egress to and from the bottom of theexcavation plus any crossings over the excavation.

– Details of any fencing around the open excavation.– Details of plant and equipment to be used together with

confirmation that all necessary records will be providedon site.

– Method of storing and dispensing fuel oil includingprecautions to prevent pollution of groundwater.

– Quantity of petrol to be stored on site and method ofstorage.

– If any roadworks are involved, the name of the supervisortrained in accordance with the New Roads and StreetWorks Act 1991.

Note :-An additional method statement will be required for any

work involving entry into confined spaces e.g. sewerconnections.

This checklist is intended to aid the production andapproval of method statements. It is not an exhaustive list ofevery possible issue that may need to be addressed for anygiven task.

Refer to Section 1 for general information on methodstatements.

CHECKLIST FOR GROUNDWORKS METHOD STATEMENTS

8B - 25June 2007

REFERENCE SOURCES

LegislationThe Water Resources Act 1991Management of Health and Safety at Work Regulations 1992Construction (Design and Management) Regulations 2007Work at Height Regulations 2005Lifting Operations and Lifting Equipment Regulations 1998Provision and Use of Work Equipment Regulations 1998The Groundwater Regulations 1998

British StandardsCP 2004 FoundationsBS 4074 Specification for metal props and strutsBS 5268 Code of Practice for the structural use of timberBS 5930 Code of Practice for site investigationsBS 6031 Code of Practice for earthworks

GuidanceConstruction Industry Research and Information

Association:Technical Note 95: “Proprietary trench support systems”(2nd Edition 1982). An illustrated summary which includestabulations of each system’s characteristics andcapabilities.Report 97: “Trenching practice” (1983). A guide to safepractice in the design and use of temporary support fortrenches not deeper than 6m.

Timber Research and Development Association:“Timber in excavations” (1981). A guide to assist sitestaff and planners to assess site conditions and to chooseappropriate support for trenches, shafts and headings.“Simplified rules for the inspection of second-hand timberfor load bearing use” (1981). A valuable pocket guideshowing how to assess the value of second-hand timber.

Health and Safety Executive:HSE Leaflet CIS 8 Safety in excavationsGuidance Booklet HS(G)47: Avoiding danger fromunderground services

National Joint Council for the Building Industry:“Site Safe and You”: A pocket booklet on all aspects ofsite safety, which includes a checklist for work inexcavations and with excavation plant.

Training CoursesConstruction Industry Training CentreBircham NewtonNr. Kings LynnNorfolk

Construction Health & Safety GroupJohn Ryder Training CentreSt. Anne’s RoadChertseySurrey

Training aidsConstruction Health and Safety Group slide/tape

programmes

CIRIAvideo:Trenching - Good Practice

Environment AgencyBuilding a cleaner future

All of the above reference material is available from:Construction Industry Publications Ltd.,17 Gatelodge Close, Round Spinney,Northampton NN3 8RJ.Tel: 0870 078 4400Fax: 0870 078 4401e-mail sales@cip-books.com

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