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Proceedings of the XVI ECSMGEGeotechnical Engineering for Infrastructure and DevelopmentISBN 978-0-7277-6067-8

© The authors and ICE Publishing: All rights reserved, 2015doi:10.1680/ecsmge.60678

healing in soil-cement environments. This work is at its infancy and significant advances are expected in the foreseeable future.

ACKNOWLEDGEMENT

The authors acknowledge the financial support from EPSRC for the M4L project (EP/K026631/1).

REFERENCES Ahn, T-H. & Kishi, T. 2010. Crack self-healing behavior of ce-mentitious composites incorporating various mineral admixtures. Journal of Advanced concrete Technology, 8(2), 171-186. Al-Tabbaa, A., Liska, M., Ouellet-Plamondon, C., Jegandan, S., Shrestha, R., Barker, P., McGall, R. & Critchlow, C. 2012. Soil mix technology for integrated remediation and ground improve-ment: From laboratory work to field trials. Int. Conf. on Grouting and Deep Mixing, New Orleans, Feb., 522-532. Al-Tabbaa, A. 2013. Reactive magnesia cements. C19-Eco-Efficient Concrete (Eds. Pacheco-Torgal et al), Woodhouse Publish., 523-43. Al-Tabbaa, A., O’Connor, D. & Abunada, Z. 2014. Field trials for deep mixing in land remediation: Execution, monitoring, QC & lessons learnt. DFI-EFFC Conf. Piling & Deep Foundations, Stockholm, May. ARUP. 2010. Risk-based framework for geotechnical asset man-agement: Phase 2 Report. http://www.highways.gov.uk/know- ledge/publications/a-risk-based-framework-for-geotechnical-asset-management/ accessed April 2014. ASCE. 2009. Failing Infrastructure cannot support a healthy econ-omy, www.asce.org/infrastructure/report-card. Chu, J., Ivanov, V., Stabnikov, V. & Li, B. 2014. Microbial meth-od for construction of an aquaculture pond in sand. Geotechnique 63(10), 871-875. De Rooij, M., van Tittlebook, K., De Belie, N & Schlangen, E. 2013. Self-healing phenomena in cement-based materials: RILEM TC221-SCH State-of-the-art report. Springer Eigenbrod, K. D. 2003. Self-healing in fractured fine-grained soils. Canadian Geotechnical Journal, 40 (2): 435-449. Gilford, J. Hassan, M., Rupnow, T. & Barbato, M. 2013. Dicyclo-pentadiene and sodium silicate microencapsulation for self-healing of concrete. Journal of Materials in Civil Engineering, on-line. Harbottle, M.J., Lam, M-T., Botusharova, S.P. & Gardner, D.R. 2014. Self-healing soil: Biomimetic engineering of geotechnical structures to respond to damage. IGEG2014 – 7th Int. Congress on Environmental Geotechnics, Melbourne, Nov. (accepted). HM Treasury. 2010. Infrastructure Cost Review: Technical Re-port. www.hm-treasury.gov.uk Isaacs, B., Lark, R. J., Jefferson, A. D., Davies, R., Dunn, S. 2013. Crack healing of cementitious materials using shrinkable polymer tendons. Structural Concrete, 14 (2): 138-147. Jonkers, H.M., Thijssen, A., Muyzer, G., Copuroglu, O & Schlangen, E. 2010. Application of bacteria as self-healing agent for the develop-ment of sustainable concrete. Ecological Engineering, 36: 230-235. Joseph, C., Jefferson, A. D., Isaacs, B., Lark, R. J. 2010. Experi-mental investigation of adhesive-based self-healing of cementi-tious materials. Magazine of Concrete Research, 62 (11): 831-843. Kakuturu, S., Reddi, L. 2006. Evaluation of the Parameters Influ-encing Self-Healing in Earth Dams. Journal of Geotechnical and Geoenvironmental Engineering, 132 (7): 879–889.

Kishi, T., Ahn, T-H., Hosoda, A., Suzuki, S. & Takaoka, H. 2007. Self-healing behaviour by cementitious recrystallisation of cracked concrete incorporating expansive agent. 1st Int. Conf. Self-healing Materials, Noordwijk, Holland. Lark, R.J., Al-Tabbaa, A. & Paine, K. 2013. Biomimetic multi-scale damage immunity for construction materials - M4L Project over-view. 4th Int. Conf. on Self-Healing Materials, Ghent, June, 400-4. Litina, C. & Al-Tabbaa, A. 2013. Self-healing capacity of hard-ened cement suspensions with high levels of cement substitution. Fourth Int. Conf. on Self-Healing Materials, Ghent, June, 201-205. Litina, C., Kanellopoulos, A. & Al-Tabbaa, A. 2014a. Develop-ment of microencapsulated healing agents for self-healing ce-mentitious composites. Young Researchers’ Forum on Construc-tion Materials, UCL, London. February. Litina, C., Kanellopoulos, A. & Al-Tabbaa, A. 2014b. Alternative repair system for concrete using microencapsulated healing agents. 5th International Conference on Concrete Repair, Belfast, Sept. Liu Q., García A., Schlangen E. & van de Ven M. 2011. Induction healing of asphalt mastic and porous asphalt concrete. Construc-tion and Building Materials, 25, 3746-3752. O’Connor, D. & Al-Tabbaa, A. 2014. Reactive magnesia cement addition in soil-cement-bentonite cut-off walls to mitigate shrink-age cracking. 7th Int. Congress on Environmental Geotechnics, Melbourne, November (accepted). Pelletier, M., Brown, R., Shukla, A. & Bose, A. 2011. Self-healing concrete with a microencapsulated healing agent, http://energetics.chm.uri.edu/?q=/system/files/Self%20healing%20concrete%20-7-11.pdf Qureshi, T.S. & Al-Tabbaa, A. 2014a. MgO-based mineral addi-tive for self-healing in concrete. Young Researchers’ Forum on Construction Materials, UCL, London. February. Qureshi, T.S. & Al-Tabbaa, A. 2014b. Effect of magnesia on the self-healing performance of PC with increasing curing time. Int. Conf. Ageing of Materials & Structures – Delft, May, 635-641. Sari, K., & Chai, J. 2013. Self healing capacity of geosynthetic clay liners and influencing factors. Geotextiles & Geomembranes, 41, 64-71. Shi, C., Booth, R. 2005. Laboratory development and field demon-stration of self-sealing/self-healing landfill liner. Waste Manage-ment, 25 (3), 231-238. Snoeck, D., Van Tittleboom, K., Steuperaert, S., Dubruel, P. & De Belie, N. 2014. Self-healing cementitious materials by the combi-nation of microfibers and superabsorbent polymers. Journal of In-telligent Material Systems and Structures, 25(1), 13-24. Van Tittelboom, K & De Belie, N. 2013. Self-healing in cementi-tious materials – a review. Materials, 6, 2182-2217. Van Tittleboom, K., Snoeck, D., Wang, J. & De Belie, N. 2013. Most recent advances in the field of self-healing cementitious mate-rials. 4th Int. Conf. on Self-Healing Materials, Ghent, June, 406-413. Wang, J., Soens, H., Verstraete, W. & De Belie, N. 2014. Self healing concrete by use of microencapsulated bacterial spores. Cement & Concrete Research. 56, 139-152. Whiffin, V., Van Paassen, L. & Harkes, M. 2007. Microbial car-bonate precipitation as a soil improvement technique. Geomicro-biology Journal 24 (5), 417-423. Wu, M., Johannesson, B. & Geiker, M. 2012. A review: self-healing in cementitious materials and engineered cementitious composites. Construction and Building Materials, 571-583. Yang, Z., Hollar, J., He, X. & Shi, X. 2011. A self-healing ce-mentitious composite using oil core/silica gel shell microcapsules. Cement and Concrete Composites, 33(4), pp.506–512.

Geotechnical engineering for wind farms on peatland sites

Géotechnique des parcs éoliens sur les sites de tourbières P. Jennings*1 and G. Kane1,

1 Applied Ground Engineering Consultants (AGEC) Ltd, The Grainstore, Bagenalstown, County Carlow, Ireland

* Corresponding Author

ABSTRACT Ireland’s commitment to renewable energy has resulted in a large number of on-shore wind farm sites being investigated and developed. The most advantageous wind conditions and locations for wind farm developments frequently coincide with upland peatland ar-eas particularly in the west of the country. This has led to the investigation and development of sometimes difficult and sensitive peatland sites. This paper uses the experience gained from a large number of wind farm developments on peatland sites to discuss the main geotech-nical engineering hazards encountered along with engineering techniques and approaches used to overcome them. The hazards, which are generally associated with low shear strength and high compressibility of peat, range from large-scale peat slides to excessive settlement.

RÉSUMÉ L'engagement de l'Irlande à l'énergie renouvelable a donné lieu à un grand nombre de sites éoliens on-shore objet d'une enquête et développé. Les conditions et les lieux de vent les plus avantageuses pour les développements de parcs éoliens coïncidents souvent avec des zones de tourbières hautes terres en particulier dans l'ouest du pays. Cela a conduit à la recherche et le développement de sites de tour-bières parfois difficiles et délicates. Cet article utilise l'expérience acquise à partir d'un grand nombre de projets de développement éolien de la ferme sur les sites de tourbières pour discuter des principaux risques d'ingénierie géotechnique rencontrés avec les techniques d'ingénie-rie et les approches utilisées pour les surmonter . Les risques, qui sont généralement associés à faible résistance au cisaillement et compres-sibilité élevée de tourbe, vont de diapositives de tourbe à grande échelle à un tassement excessif.

1 BACKGROUND

Ireland’s commitment to the European Union re-garding renewable energy has resulted in a large number of on-shore wind farm sites around the coun-try being investigated and developed. Following the 2009 Renewable Energy Directive (2009/28/EC) in relation to electricity consumption, Ireland has a re-quirement and target of 40% to be generated from re-newable sources by 2020. A large number of these wind farm sites have been developed particularly along the western seaboard to take advantage of the strong Atlantic winds. This has led to the investiga-tion and development of sometimes difficult and sen-sitive peatland sites which are covered with deep blanket peat on sloping ground.

The authors have been involved in over 80 wind farm developments on peatland sites in Ireland (Fig-ure 1) and on other sites in the United Kingdom, at various stages of development. The potential power generated from the 80 sites in Ireland worked on by AGEC is estimated at over 1,200MW.

This paper discusses the geotechnical engineering hazards commonly encountered on peatland sites, along with some observations on engineering tech-niques and approaches used to overcome them. Haz-ards range from large-scale peat slides to serviceabil-ity of infrastructure on weak and compressible peat.

Geotechnical Engineering for Infrastructure and Development

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Figure 1. Plan of wind farm development sites worked on by AGEC (80 nos.)

2 PEATLAND IN IRELAND

2.1 Distribution of Peatland

Three main type of peatland, or bog, occur in Ireland namely upland blanket bog, lowland blanket (ocean-ic) bog and raised bog. Peatland is generally found in areas of high rainfall under conditions of poor drain-age. About 21% of the land mass of Ireland is cov-ered by peatland (Connolly et al. 2009).

Upland blanket bog covers large expanses of most of the mountainous areas and would be the dominant peatland encountered on wind farm sites. In the west, due to the wetter climate, blanket bog occurs down to sea-level (oceanic bog). Raised bogs are commonly found in the midlands in generally flat areas. Raised bogs generally tend to be 3 to 12m thick. Blanket bogs would typically be 3m thick, but as the underly-ing surface is irregular, locally thicker deposits are commonly present. Blanket bog thickness typically thins at greater elevations.

2.2 Engineering Characteristics of Peat The engineering characteristics of peat are covered in detail in a number of papers, for example Hanrahan (1954), McFarlane (1969), Hobbs (1986), Carlston (1988), and more recently Boylan and Long (2014).

Peat is of particular engineering concern due to its low strength and high compressibility. This is signif-icant for wind farm construction where most of the engineering works are on sloping ground and shear failure of the peat can result in large-scale landslides.

Peat consists of the remains of decaying plant life and comprises an upper fibrous layer and lower hu-mified layer (Figure 2). The uppermost peat layer (acrotelm), with a thickness of typically 0.3 to 1m, contains actively growing bog plants and virtually undecayed fibrous plant matter. The acrotelm has relatively high shear strength due to its fibrous na-ture, and whilst notably compressible, provides a supportive layer above the weaker catotelm layer.

The catotelm contains dead plant matter in various states of decay. As the catotelm is essentially a wa-terlogged and oxygen deficient environment plant matter slowly putrefies insitu resulting in plant matter slowly decaying (humification). The catotelm has no-tably low shear strength (Figure 2) and high com-pressibility.

Figure 2. Undrained shear strength measurement in peat

Figure 2 shows several methods (insitu shear vane,

CPT and laboratory testing) used to determine the strength of blanket peat at a site in northwest Ireland. Distinguishing between the two peat layers using

Acrotelm Catotelm

such methods can be difficult and there is invariably scatter, due to particularly fibre content.

In practice there is variation between the results of the various testing methods and careful interpretation is required. Notwithstanding, simple methods such as shear vane testing are frequently and most commonly used to determine shear strength (Boylan & Long 2014).

3 GEOTECHNICAL HAZARDS

ENCOUNTERED ON PEATLAND SITES

3.1 Geotechnical Hazards

A number of typical geotechnical hazards are com-monly associated with construction work on peatland sites, see Table 1. Table 1 Typical geotechnical hazards and causes on peatland sites

Hazard Principal Causes Peat slide Unexpected area of weak ground conditions

Extended area of weak ground Intense rainfall event Improper construction e.g. excessive loading

Localised peat slope failure

Unexpected localised weak ground conditions Intense rainfall event Improper construction e.g. loading, undercutting

Localised bear-ing failure

Unexpected localised weak ground conditions Excessive loading Improper construction e.g. floating road, arisings

Instability of peat arisings

Excessive rainfall Overly softened arisings Concentration of surface water

Excavation slope failure in peat during con-struction

Unexpected weak ground Localised slope failure Water ingress Loading of excavation slope crest Slope too steep

Excessive or long term set-tlement of float-ing road in peat

Inadequate site investigation & insufficient un-derstanding of deformation properties of peat

Inadequate road construction Unexpected localised weak ground conditions Excessive loading

Failure of base of excavation e.g. liquefaction, piping, heaving

High groundwater Presence of sensitive soils e.g. silty sand Inadequate design and understanding of ground Loading/vibration/excavation Improper construction

Flooding of ex-cavation by groundwater

Elevated groundwater Interception of water-bearing soils Unexpected ground conditions

Most of the hazards in Table 1 are related to low shear strength and high compressibility of peat but also include high groundwater and sensitive soils be-low the peat. In terms of risk, a peat slide would be

considered the most severe and considerable ge-otechnical effort would be dedicated to ensuring that a peat slide could not occur. Other hazards would be related to serviceability issues, such as excessive set-tlement of floating roads; whilst not considered a ma-jor risk nevertheless the operational costs to a devel-opment of impaired access roads may be financially unacceptable. A number of the geotechnical hazards are discussed below.

3.2 Peat Slides

Peat slides in Ireland have been dated at about 4200 BP (Murray, 1997), and with written accounts dating back to the 1400s (Feehan and O’Donovan, 1996). There is an estimated over 120 reported events of peat failures in Ireland (GSI, 2006) and undoubtedly a considerable number of unreported events.

Large-scale peat failures in 2003 at Pollatomish, County Mayo and Derrybrien, County Galway, which occurred during the construction of a wind farm, focused attention on to such events. It is now effectively mandatory to include a peat slide assess-ment as part of the planning requirements for wind farm developments. Peat failure volumes can range from a few 100m3 to in excess of million m3 (Boylan et al. 2008).

It is estimated that about 50% of Irish peat failures occur naturally (Dykes 2010) and the remainder are associated with anthropogenic causal factors such as peat cutting or construction activity. Based on the experience of the authors, triggering factors for peat failures typically relate to the type of terrain and peat cover. Where steeper slopes and peat thicknesses are generally thin (<1.0m), rainfall induced peat failures are more common (Figure 3). In contrast, on flatter sites where peat thicknesses are greater (>1.0m) an-thropogenic activity is generally the major contribu-tory factor (Figure 4).

A review of a number of peat slides associated with construction activity indicates that failure was generally initiated a short-time after construction ac-tivity. This would indicate that the operational shear strength is the undrained strength; for example this appeared to initiate the failure in Figure 4. Peat slides as a result of rainfall are likely controlled by effective strength parameters, this would account for the fail-ures in Figure 3 and the 2003 failures in Pollatomish

597

Figure 1. Plan of wind farm development sites worked on by AGEC (80 nos.)

2 PEATLAND IN IRELAND

2.1 Distribution of Peatland

Three main type of peatland, or bog, occur in Ireland namely upland blanket bog, lowland blanket (ocean-ic) bog and raised bog. Peatland is generally found in areas of high rainfall under conditions of poor drain-age. About 21% of the land mass of Ireland is cov-ered by peatland (Connolly et al. 2009).

Upland blanket bog covers large expanses of most of the mountainous areas and would be the dominant peatland encountered on wind farm sites. In the west, due to the wetter climate, blanket bog occurs down to sea-level (oceanic bog). Raised bogs are commonly found in the midlands in generally flat areas. Raised bogs generally tend to be 3 to 12m thick. Blanket bogs would typically be 3m thick, but as the underly-ing surface is irregular, locally thicker deposits are commonly present. Blanket bog thickness typically thins at greater elevations.

2.2 Engineering Characteristics of Peat The engineering characteristics of peat are covered in detail in a number of papers, for example Hanrahan (1954), McFarlane (1969), Hobbs (1986), Carlston (1988), and more recently Boylan and Long (2014).

Peat is of particular engineering concern due to its low strength and high compressibility. This is signif-icant for wind farm construction where most of the engineering works are on sloping ground and shear failure of the peat can result in large-scale landslides.

Peat consists of the remains of decaying plant life and comprises an upper fibrous layer and lower hu-mified layer (Figure 2). The uppermost peat layer (acrotelm), with a thickness of typically 0.3 to 1m, contains actively growing bog plants and virtually undecayed fibrous plant matter. The acrotelm has relatively high shear strength due to its fibrous na-ture, and whilst notably compressible, provides a supportive layer above the weaker catotelm layer.

The catotelm contains dead plant matter in various states of decay. As the catotelm is essentially a wa-terlogged and oxygen deficient environment plant matter slowly putrefies insitu resulting in plant matter slowly decaying (humification). The catotelm has no-tably low shear strength (Figure 2) and high com-pressibility.

Figure 2. Undrained shear strength measurement in peat

Figure 2 shows several methods (insitu shear vane,

CPT and laboratory testing) used to determine the strength of blanket peat at a site in northwest Ireland. Distinguishing between the two peat layers using

Acrotelm Catotelm

such methods can be difficult and there is invariably scatter, due to particularly fibre content.

In practice there is variation between the results of the various testing methods and careful interpretation is required. Notwithstanding, simple methods such as shear vane testing are frequently and most commonly used to determine shear strength (Boylan & Long 2014).

3 GEOTECHNICAL HAZARDS

ENCOUNTERED ON PEATLAND SITES

3.1 Geotechnical Hazards

A number of typical geotechnical hazards are com-monly associated with construction work on peatland sites, see Table 1. Table 1 Typical geotechnical hazards and causes on peatland sites

Hazard Principal Causes Peat slide Unexpected area of weak ground conditions

Extended area of weak ground Intense rainfall event Improper construction e.g. excessive loading

Localised peat slope failure

Unexpected localised weak ground conditions Intense rainfall event Improper construction e.g. loading, undercutting

Localised bear-ing failure

Unexpected localised weak ground conditions Excessive loading Improper construction e.g. floating road, arisings

Instability of peat arisings

Excessive rainfall Overly softened arisings Concentration of surface water

Excavation slope failure in peat during con-struction

Unexpected weak ground Localised slope failure Water ingress Loading of excavation slope crest Slope too steep

Excessive or long term set-tlement of float-ing road in peat

Inadequate site investigation & insufficient un-derstanding of deformation properties of peat

Inadequate road construction Unexpected localised weak ground conditions Excessive loading

Failure of base of excavation e.g. liquefaction, piping, heaving

High groundwater Presence of sensitive soils e.g. silty sand Inadequate design and understanding of ground Loading/vibration/excavation Improper construction

Flooding of ex-cavation by groundwater

Elevated groundwater Interception of water-bearing soils Unexpected ground conditions

Most of the hazards in Table 1 are related to low shear strength and high compressibility of peat but also include high groundwater and sensitive soils be-low the peat. In terms of risk, a peat slide would be

considered the most severe and considerable ge-otechnical effort would be dedicated to ensuring that a peat slide could not occur. Other hazards would be related to serviceability issues, such as excessive set-tlement of floating roads; whilst not considered a ma-jor risk nevertheless the operational costs to a devel-opment of impaired access roads may be financially unacceptable. A number of the geotechnical hazards are discussed below.

3.2 Peat Slides

Peat slides in Ireland have been dated at about 4200 BP (Murray, 1997), and with written accounts dating back to the 1400s (Feehan and O’Donovan, 1996). There is an estimated over 120 reported events of peat failures in Ireland (GSI, 2006) and undoubtedly a considerable number of unreported events.

Large-scale peat failures in 2003 at Pollatomish, County Mayo and Derrybrien, County Galway, which occurred during the construction of a wind farm, focused attention on to such events. It is now effectively mandatory to include a peat slide assess-ment as part of the planning requirements for wind farm developments. Peat failure volumes can range from a few 100m3 to in excess of million m3 (Boylan et al. 2008).

It is estimated that about 50% of Irish peat failures occur naturally (Dykes 2010) and the remainder are associated with anthropogenic causal factors such as peat cutting or construction activity. Based on the experience of the authors, triggering factors for peat failures typically relate to the type of terrain and peat cover. Where steeper slopes and peat thicknesses are generally thin (<1.0m), rainfall induced peat failures are more common (Figure 3). In contrast, on flatter sites where peat thicknesses are greater (>1.0m) an-thropogenic activity is generally the major contribu-tory factor (Figure 4).

A review of a number of peat slides associated with construction activity indicates that failure was generally initiated a short-time after construction ac-tivity. This would indicate that the operational shear strength is the undrained strength; for example this appeared to initiate the failure in Figure 4. Peat slides as a result of rainfall are likely controlled by effective strength parameters, this would account for the fail-ures in Figure 3 and the 2003 failures in Pollatomish

Jennings and Kane

Geotechnical Engineering for Infrastructure and Development

598

and on the Shetland Islands (Dykes & Warburton, 2008).

Figure 3. View of a multiple landslides caused by rainfall (Croaghmoyle & Buckoogh Mountains, Co. Mayo, Ireland)

Figure 4. Oblique aerial overview of a peat failure caused by con-struction/ anthropogenic factors (Ballincollig Hill Wind Farm, Co. Kerry, Ireland)

Peat failures are referred to as bog bursts or more

commonly bog slides, which are described (Hutchinson, 1988) as follows: Bog flows (or bursts): Type of debris flow which

involves large quantities of water/peat debris which flow down-slope usually following exist-ing surface water channels. In many bog flows there is likely an initial shear failure on a discrete sliding surface prior to peat rapidly breaking-down into slurry.

Bog slides: Comprises a mass of intact peat that moves bodily downslope. Slides occur on a dis-crete shear plane usually located at depth in the

peat. The failed peat typically breaks into smaller pieces, and commonly evolves into a flow.

Historical accounts of bog bursts suggest that many bursts originated as slides. Indeed, inspection of many peat failure scars suggests that initial movement occurs as sliding on a discrete plane with-in the peat. To this end, many geotechnical peat slide assessments use an infinite slope analysis (e.g. Skempton & DeLory 1957). The use of the infinite slope analysis also explains a common observation of peat failures; that is the sliding surface occurs at depth and close to/at the base of the peat. Figure 5 shows the calculated factor of safety (FoS) at various depths for a peat slope for a range of insitu strength profiles (actual & simplified). This clearly shows that the lowest theoretical FoS, and most likely depth of the shear plane, always occurs at the greatest depth.

Figure 5. Range of insitu strength profiles (a) and corresponding calculated factor of safety (b) at various depths for a peat slope

For large-scale (run-away) failures to develop there is normally a large body of weak peat present that is set in motion by a triggering event, such as ex-cessive loading. A good indicator for potential areas of weak peat is the fact that there is quaking bog, bog pools or saturated mechanically cut peat on site. Quaking bog is common in level areas and is indica-tive of highly saturated peat with low strength. The buoyancy effect of the peat is as a result of a body of sub-surface water present at typically the base of the peat.

To provide a relative measure of the scale of weak peat on a site then insitu strength measurement using

200m

(a) (b)

a shear vane can be invaluable. Figure 6 shows the results of envelopes of insitu vane testing at a number of sites. The darker shaded envelopes are from Scot-tish bogs which have been drained; the results clearly show the effect of consolidation and increase in strength with depth. The lighter hatched envelopes are from Irish blanket bog sites which have not been drained and are prone to or have failed.

Figure 6. Insitu shear vane results used to indicate the potential for peat slides at a site

Experience suggests that sites prone to large peat

slides have a higher proportion of undrained strength at or below about 4kPa, though other factors need to be taken into account, most notably topography. For detailed assessment of peat sliding a combination of peat testing methods should be used; notwithstanding results are invariably scattered and detailed careful interpretation is required. Occasionally, weak sensi-tive soils may underlie peat; there are a few examples of shear surfaces developing in underlying weaker soils, for example Garvagh Glebe, Ireland 2008.

3.3 Localised Bearing Failure

Where in particular floating roads are constructed there is a reliance on the peat to act as a bearing stra-

tum. Peat characteristics can be spatially variable which can result in localised changes in strength and hence the ability to bear the load of a floating road. Some visual indicators of change in peat strength are for example: Quaking peat - common in level areas and is in-

dicative of highly saturated peat with low strength. The buoyancy effect of the peat is as a result of a body of sub-surface water present at typically the base of the peat.

Bog pools - encountered in level areas with peat in the pool margins notably weak and saturated.

Changes in topography - subtle changes in surface topography can result in changes in peat strength due to variations in say bog plants, degree of humi-fication, peat depth and groundwater.

Drainage - commonly drained peatland will result in an increase in peat strength (see Figure 6) but poorly maintained drainage can allow softening of peat and provide potential for tension cracks. Ten-sion cracks are potential signs of peat movement or failure.

Mechanical peat cutting (harvesting) - results in cutting of the acrotelm layer where most of the in-trinsic strength of peat lies. It is not possible to account for all changes in peat

strength by visual indicators alone and detailed site investigation is essential. Site investigation points should be as close as possible; and should be towards the closer spacing recommended in EC7 for linear structures.

3.4 Instability of Peat Arisings

A large volume of excavated peat can be generated during the construction of wind farm infrastructure. Disturbed peat strength can be 50% or less of its peak strength and in many cases behaves as a viscous ma-terial that will readily flow particularly when affected by rainfall.

Excavated peat requires appropriate engineered re-tention/storage measures to prevent potential flow failure and run-off. Disturbed peat can be spread thinly and left to dry on the surface but needs to be suitably shaped and drained to avoid saturation by rainfall. Monitoring of peat storage areas for a period after completion is considered good practice, particu-larly for peat stored on the surface.

599

and on the Shetland Islands (Dykes & Warburton, 2008).

Figure 3. View of a multiple landslides caused by rainfall (Croaghmoyle & Buckoogh Mountains, Co. Mayo, Ireland)

Figure 4. Oblique aerial overview of a peat failure caused by con-struction/ anthropogenic factors (Ballincollig Hill Wind Farm, Co. Kerry, Ireland)

Peat failures are referred to as bog bursts or more

commonly bog slides, which are described (Hutchinson, 1988) as follows: Bog flows (or bursts): Type of debris flow which

involves large quantities of water/peat debris which flow down-slope usually following exist-ing surface water channels. In many bog flows there is likely an initial shear failure on a discrete sliding surface prior to peat rapidly breaking-down into slurry.

Bog slides: Comprises a mass of intact peat that moves bodily downslope. Slides occur on a dis-crete shear plane usually located at depth in the

peat. The failed peat typically breaks into smaller pieces, and commonly evolves into a flow.

Historical accounts of bog bursts suggest that many bursts originated as slides. Indeed, inspection of many peat failure scars suggests that initial movement occurs as sliding on a discrete plane with-in the peat. To this end, many geotechnical peat slide assessments use an infinite slope analysis (e.g. Skempton & DeLory 1957). The use of the infinite slope analysis also explains a common observation of peat failures; that is the sliding surface occurs at depth and close to/at the base of the peat. Figure 5 shows the calculated factor of safety (FoS) at various depths for a peat slope for a range of insitu strength profiles (actual & simplified). This clearly shows that the lowest theoretical FoS, and most likely depth of the shear plane, always occurs at the greatest depth.

Figure 5. Range of insitu strength profiles (a) and corresponding calculated factor of safety (b) at various depths for a peat slope

For large-scale (run-away) failures to develop there is normally a large body of weak peat present that is set in motion by a triggering event, such as ex-cessive loading. A good indicator for potential areas of weak peat is the fact that there is quaking bog, bog pools or saturated mechanically cut peat on site. Quaking bog is common in level areas and is indica-tive of highly saturated peat with low strength. The buoyancy effect of the peat is as a result of a body of sub-surface water present at typically the base of the peat.

To provide a relative measure of the scale of weak peat on a site then insitu strength measurement using

200m

(a) (b)

a shear vane can be invaluable. Figure 6 shows the results of envelopes of insitu vane testing at a number of sites. The darker shaded envelopes are from Scot-tish bogs which have been drained; the results clearly show the effect of consolidation and increase in strength with depth. The lighter hatched envelopes are from Irish blanket bog sites which have not been drained and are prone to or have failed.

Figure 6. Insitu shear vane results used to indicate the potential for peat slides at a site

Experience suggests that sites prone to large peat

slides have a higher proportion of undrained strength at or below about 4kPa, though other factors need to be taken into account, most notably topography. For detailed assessment of peat sliding a combination of peat testing methods should be used; notwithstanding results are invariably scattered and detailed careful interpretation is required. Occasionally, weak sensi-tive soils may underlie peat; there are a few examples of shear surfaces developing in underlying weaker soils, for example Garvagh Glebe, Ireland 2008.

3.3 Localised Bearing Failure

Where in particular floating roads are constructed there is a reliance on the peat to act as a bearing stra-

tum. Peat characteristics can be spatially variable which can result in localised changes in strength and hence the ability to bear the load of a floating road. Some visual indicators of change in peat strength are for example: Quaking peat - common in level areas and is in-

dicative of highly saturated peat with low strength. The buoyancy effect of the peat is as a result of a body of sub-surface water present at typically the base of the peat.

Bog pools - encountered in level areas with peat in the pool margins notably weak and saturated.

Changes in topography - subtle changes in surface topography can result in changes in peat strength due to variations in say bog plants, degree of humi-fication, peat depth and groundwater.

Drainage - commonly drained peatland will result in an increase in peat strength (see Figure 6) but poorly maintained drainage can allow softening of peat and provide potential for tension cracks. Ten-sion cracks are potential signs of peat movement or failure.

Mechanical peat cutting (harvesting) - results in cutting of the acrotelm layer where most of the in-trinsic strength of peat lies. It is not possible to account for all changes in peat

strength by visual indicators alone and detailed site investigation is essential. Site investigation points should be as close as possible; and should be towards the closer spacing recommended in EC7 for linear structures.

3.4 Instability of Peat Arisings

A large volume of excavated peat can be generated during the construction of wind farm infrastructure. Disturbed peat strength can be 50% or less of its peak strength and in many cases behaves as a viscous ma-terial that will readily flow particularly when affected by rainfall.

Excavated peat requires appropriate engineered re-tention/storage measures to prevent potential flow failure and run-off. Disturbed peat can be spread thinly and left to dry on the surface but needs to be suitably shaped and drained to avoid saturation by rainfall. Monitoring of peat storage areas for a period after completion is considered good practice, particu-larly for peat stored on the surface.

Jennings and Kane

Geotechnical Engineering for Infrastructure and Development

600

3.5 Excessive Settlement of Floating Infrastructure

Peat is highly compressible with notable creep set-tlement. A review of peat settlement (Long & Boylan 2013) showed that consolidation theory provides a reasonable prediction of settlement, which the au-thors generally found to be the case in practice. In order to predict settlement detailed investigation with high quality peat sampling and testing is required. As a rule of thumb, under typical floating road loading conditions the underlying peat can consolidate by up to one third of its thickness. However, there are many localised instances where continued excessive creep and unpredicted settlement occurs. As such, it is al-ways prudent to include for maintenance of floating infrastructure on peatland.

4 CONCLUSIONS

(i) In pursuit of Ireland’s commitment to renewable energy targets a large number of on-shore wind farm sites have been investigated and developed in loca-tions which invariably coincide with upland peatland. (ii) Upland blanket bog covers large expanses of most of the mountainous areas and is the dominant peatland type encountered on wind farm sites. Peat depths are typically 3m but can vary significantly. (iii) There is a combination of specific geotechnical hazards commonly associated with construction work on peatland sites (Table 1), which are as a result of the low shear strength and high compressibility of peat. Hazards range from peat slides to excessive set-tlement. (iv) The general types of peat slides, failure mecha-nisms and analysis are discussed. Slide shear surfaces generally occur at the base of the peat layer and as such site investigation should notably target this depth. Occasionally shear surfaces can develop in underlying weak soils. (v) Peat strength and compressibility characteristics need to be carefully selected from a thorough ground investigation. Test results are invariably scattered and detailed interpretation based on experience is re-quired. Some guidance is provided in Boylan & Long (2014). (vi) Due to the natural variability of peat, site inves-tigation points should be as close as possible, espe-cially where floating roads are envisaged. There are

many localised instances where there are serviceabil-ity problems due to continued excessive and unpre-dicted settlement. Allowance should be included for inspection and maintenance of infrastructure during the operational phase of any wind farm on peatland. (vii) The selection of a particular technique or ap-proach for a given wind farm development requires careful planning and consideration and the engage-ment of experienced personnel. Given the variability of ground conditions on peatland sites a particular technique or approach cannot be universally applied to all sites.

REFERENCES

2009/28/EC. Directive 2009/28/EC of the European Parliament and of the Council of 23 April 2009 on the promotion of the use of energy from renewable sources and amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC. Boylan, N. & Long, M. 2014. Evaluation of peat strength for sta-bility assessments. Proceedings of the ICE, Geotechnical Engi-neering, 167 October 2014 Issue GE5 Pages 421–430 Boylan, N., Jennings, P. & Long, M. 2008. Peat slope failure in Ireland. Quarterly Journal of Engineering Geology and Hydroge-ology , 41, 93-108. Connolly, J. & Holden, N.M. 2009. Mapping peat soils in Ireland; updating the Derived Irish Peat Map. Irish Geography. 3, 343-352. Dykes, A. and Warburton, J. 2008. Characteristics of the Shetland Islands (UK) peat slides of 19 September 2003. Landslides. 5(2), 213-226 Dykes, A.P. 2010. Peatlands and Peat Instability Presentation. Pre-sented at: Peat Stability Seminar at the Geological Survey of Ire-land, 7th October 2010. Feehan, J. and O’Donovan, G. 1996. The Bogs of Ireland - An In-troduction to the Natural, Cultural and Industrial Heritage of Irish Peatlands. UCD, School of Biology and Environ. Science. Geological Survey of Ireland (GSI) 2006. Landslides in Ireland. Irish landslides working group. Hanrahan, E.T. 1954. An investigation into some physical proper-ties of peat. Geotechnique, 4, 108-123. Hobbs N.B. 1986. Mire Morphology and the properties and behav-iour of some British and foreign peats. Quaternary Journal of En-gineering Geology and Hydrogeology, v.19, 7-80. Hutchinson, J.N. 1988. General report: Morphological and ge-otechnical parameters of landslides in relation to geology and hy-drogeology. Proc. 5th Int. Symp. Landslides, Switz., vol.1, 3-35. Long, M. & Boylan, N. 2013. Predictions of settlement in peat soils. Quarterly Journal of Engineering Geology and Hydrogeolo-gy, 46 (3) 2013-08-08, 303-322 MacFarlane, I.C. 1969. Muskeg engineering handbook. National Research Council of Canada. University of Toronto Press. Murray, C. (1997). Tipperary 1997:539 DER218, Derryfadda. Da-tabase of Irish Excavation Reports. http://www.excavations.ie. Skempton, A.W. & DeLory, F.A. 1957. Stability of natural slopes in London Clay. In, Proc. 4th ICSMFE, Rotterdam, 72-78.

Value Engineering as a basis for safe, optimized and sustainable design of geotechnical structures

Principe d´analyse de la valeur est la base pour un dimensionnement sécurisé, optimisé et durable de structures géotechniques

R. Katzenbach*1, S. Leppla1 , M. Seip2 and S. Kurze2 1 Technische Universität Darmstadt, Institute and Laboratory of Geotechnics, Germany

2 Ingenieursozietät Professor Dr.-Ing. Katzenbach GmbH, Germany * Corresponding Author

ABSTRACT Safety, optimisation and sustainability are the most important aspects for the design of geotechnical structures. The principle of Value Engineering is the basis for a safe, optimised and sustainable design reducing the expenditures and saving the resources material, money and time. For example the special foundation system Combined Pile-Raft Foundation (CPRF), load tests on the construction site and advanced monitoring systems for in-time detection can be used to reduce the financial and environmental impact of new structures during construction and service time. Value Engineering can be applied for any type of geotechnical structures like foundation systems, deep ex-cavations, slopes, dams and tunnels. This paper explains the principle of Value Engineering and presents its application in engineering practice.

RÉSUMÉ Sécurité, optimisation et durabilité font partie des aspects les plus importants dans le dimensionnement de structures géotech-niques. Le principe d’analyse de la valeur est la base pour un dimensionnement sécurisé, optimisé et durable réduisant les dépenses et éco-nomisant les ressources matérielles, financières et temporelles. Il est par exemple possible dans le cas spécial de fondations mixtes semelle-pieux de réaliser des tests de charges sur le site de construction et d’utiliser des systèmes de contrôle avancés pour une détection en temps réel afin de réduire l’impact financier et environnemental durant le temps de construction et après la mise en service de nouvelles struc-tures. L’analyse de la valeur peut être appliquée pour tout type de structure géotechnique telle que les systèmes de fondations, excavations profondes, pentes, barrages et tunnels. Cet article explique le principe de l’analyse de la valeur et en présente des applications pratiques.

1 VALUE ENGINEERING

For sustainable construction an economic and envi-ronment-friendly design is necessary (Vaniček 2007, Katzenbach et al. 2011). The World Commission on Environment and Development defined sustainability as follows: “Sustainable development meets the needs of the present without compromising the abil-ity of future generations to meet their own needs” (WCED 1987). A scheme of the main constituent parts of sustainable development is shown in Figure 1. By means of Value Engineering the design of geotechnical structures leads to reduction of con-struction material used, construction time and money spent and energy consumed during construction and service time (Katzenbach et al. 2010b, 2010c).

Figure 1. Scheme of sustainable development.

Value Engineering is a planning and design meth-

od for the optimization between technical and finan-cial efforts and the biggest benefit (DIN EN 12973,

Social

Environment Economic

Bearable

Viable

Equitable