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5 Stability assessment, hazard ranking and reporting requirements

5.1 Overview

Methodologies for both slope stability assessment and geotechnical hazard and risk assessment are well covered in several existing publications ( e.g. Lambe and Whitman, 1979; Brunsden and Prior, 1984; Bromhead, 1986; Lee and Jones, 2004). The method of infinite slope analysis for investigating slope stability (Skempton and De Lory 1957) described below is well known and commonly employed. Only a brief summary explanation of its application is provided here.

Hazard and risk assessment for landslide investigations is thoroughly considered in Lee and Jones (2004). The overview provided here follows work being undertaken on behalf of the Scottish Executive at the time of writing this document (Winter et al, 2005). This approach is subject to change, and the overview methodology described in section 5.3 provides an approach suitable to first pass assessment of hazard and risk at a potential development site. The reader is referred elsewhere for detailed insight into approaches to Quantitative Risk Assessment (or QRA).

5.2 Slope stability analyses

5.2.1 Application of slope analyses to peat covered slopes

Fundamentally, all landslides are the result of gravitational forces causing the ground to fail. Once failure has begun, debris will travel downhill, sometimes in a highly mobile state due to mixing with water. There is potential for failure in any sloping ground but, all things being equal, the steeper the ground the more prone it is to landslides.

The likelihood of a particular slope or hillside failing is expressed as a " Factor of Safety". For any potential failure surface, there is a balance between the weight of the potential landslide ( driving force or shear force) and the inherent strength of the soil or rock within the hillside ( shear resistance) ( Figure 5.1). Provided the available shear resistance is greater than the shear force then the Factor of Safety will be greater than 1.0 and the slope will remain stable. If the Factor of Safety reduces to less than 1.0 through a change in ground conditions, the slope will fail.

The shear force is mostly a component of the weight of the rock/soil making up the potential landslide. The shear resistance is provided by the natural strength of the peat, soil or rock, which depends on the effect of water, upon the weight of the potential sliding mass and the tensile strength of fibres through the peat column, particularly at the peat surface. The field sampling methods and laboratory tests recommended in the previous chapter provide the means of quantifying these controlling parameters in a ground model of the development site.

5.2.2 Infinite slope analysis and application

Stability analysis has rarely been undertaken for peat slide failures. However, where it has been applied ( e.g. Warburton et al. 2004; Carling, 1986), the infinite slope model has provided the most informative results. The infinite slope model assumes a planar translational failure, where the shear surface is parallel to the ground surface, and the length of the slope is large in comparison to the failure depth (hence 'shallow' failure). The nature of detachment of peat landslides is most frequently by a translational mechanism, and since this is the failure type modelled by infinite slope analysis, it is the most appropriate analytical method.

The stability of a slope can be assessed by calculating the factor of safety F, which is the ratio of the sum of resisting forces (shear strength) and the sum of the destabilising forces (shear stress):

where c' is the effective cohesion, ? is the bulk unit weight of saturated peat, ? _w is the unit weight of water, m is the height of the water table as a fraction of the peat depth, z is the peat depth in the direction of normal stress, ß is the angle of the slope to the horizontal and ø' is the effective angle of internal friction.

Values of F < 1 indicate a slope would have undergone failure under the conditions modelled; values of F > 1 suggest conditions of stability.

The infinite slope model can be modified to allow use of 'slices' in the slope (Craig, 1997). These slices allow sections of the slope with differing characteristics, such as peat depth or slope angle, to be treated individually. By considering the length of the slices, a residual mobilising force from one slice, if unstable, can be brought to bear on the slice below and taken into account in the stability analysis of the lower slice. Slices are not modelled as providing restraining forces to the slices below, as this is highly unlikely to happen in practice. Sufficient slope stability analyses should be undertaken to represent the range of material, topographic and hydrological conditions at the development site. Variability in Factor of Safety can then be used as a key input into hazard zoning, as described below.

5.3 Hazard and risk ranking

5.3.1 General

The Institution of Civil Engineers' publication by Clayton entitled "Managing Geotechnical Risk" (2001) presents the concept of risk analysis for a particular hazard as follows:

Degree of Risk = Likelihood x Effect

This approach was modified by Winter et al (2005) in their publication entitled "Scottish Road Network Landslides Study" and the study defines the understanding of the Scottish Executive's view on the management of landslide risk across Scotland. The original concept was accepted but the definitions altered such that the term "Hazard" replaced "Likelihood", the term "Exposure" replaced "Effect" and "Degree of Risk" is replaced by "Hazard Ranking". Thus:

Hazard Ranking = Hazard x Exposure

Winter et al (2005) define these terms as follows:

• Hazard: the likelihood of the (peat) landslide event occurring.
• Exposure: the impact and consequences that the event may have.

A number of possible methods exist for hazard ranking in both qualitative and quantitative terms and the specific methodology should reflect the size and cost of the scheme and the peat landslide hazards identified. However, it is advised that an initial qualitative hazard ranking matrix methodology should be considered where an expert judgement is made on hazard and exposure based on semi-quantitative rating scales, described below.

5.3.2 Determination of peat landslide Hazard and Exposure

A peat landslide hazard zonation plan and accompanying risk register should be prepared using the scale presented in Table 5.1 (below). Zoning on the plan (or map) should reflect the number of instability indicators in each zone. For example, a 'zone' of steep slope (>10°) with moderately deep peat exhibiting collapsed pipes and tension cracking and with a modelled Factor of Safety close to 1.0 would be higher on the Hazard scale than a zone of flat (<1°) terrain with shallow peat, few or no instability indicators and a high Factor of Safety. The definition of zone boundaries, and the scales applied to each zone should be determined by the competent person(s) on the basis of the site evidence and expert judgement. Such judgement is often best applied by a panel of technically competent persons with sufficient and appropriate experience of characterising peat hazards.

Table 5.1. Qualitative assessment of peat landslide Hazard over the lifetime of the development

Potential exposure should also be assessed on a similar basis, with 1 representing a very low or negligible impact and 5 an extremely high impact (Table 5.2). For the purposes of this document, Exposure relates to impacts on the environment, on the potential project or on the development site infrastructure.

If environmental rather than financial issues are the key constraint to the development, then exposure should be applied to the environment of the development site and also to the adjacent environment that may be at risk. In this case the impact refers to the potential losses in habitat (environmental damage) through construction induced peat landslides.

Table 5.2. Qualitative assessment of peat landslide Exposure over the lifetime of the development

5.3.3 Hazard ranking for peat landslides

Using the scales above, it is possible to assign a hazard ranking for each zone by multiplying the Hazard and Exposure of each geo-event (Table 5.3 and Figure 5.2). This will result in a hazard ranking of between 1 and 25 for each location on the peat landslide hazard zonation plan. Mitigation measures can then be targeted at zones with the highest hazard rankings. Suggested actions associated with each hazard ranking are presented in Table 5.3.

Table 5.3. Hazard Ranking and suggested actions

Where the hazard ranking for a zone is significant, substantial or serious, avoidance or mitigation measures are the only means by which project infrastructure can be considered acceptable within that zone at the proposed development site. Mitigation measures are considered below.

5.4 Quantitative Risk Assessment ( QRA)

Where significant, substantial or serious peat landslide hazards have been identified and the degree of hazard is uncertain for a given site, then it may be appropriate to carry out a detailed Quantitative Risk Assessment ( QRA) to reduce the level of uncertainty and aid planning and site layout design. This would comprise more rigorous quantification of hazard and exposure.

Hazard quantification requires clarification of the mechanisms, likelihood and impact of peat landslides within each identified hazard zone. Detailed coverage of assessment of landslide hazard is provided in Lee & Jones (2004).

Exposure quantification can be achieved by assessing the costs of all infrastructure, properties, human life, and other economic losses including the cost of restoring biodiversity losses. This cost is then assessed against the cost of carrying out remedial measures or management practices at the site. It is then possible to assess the benefit to cost ratio of the scheme and any proposed mitigation measures.

5.5 Mitigation

The extent of mitigation required will depend upon the scale of the project, the risk level and the nature of the risk. A combination of options may be required to reduce the risks to an acceptable level for a given scheme.

5.5.1 Avoidance

Areas exhibiting serious or substantial hazard ranking associated with peat landslides should be avoided, for example by relocating infrastructure within the development area or by relocating to an alternate site. Where complete avoidance is not possible, the proposed design should be modified to incorporate engineering measures to reduce or eliminate the risk.

5.5.2 Engineering mitigation measures to minimise landslide occurrence

Many of the site specific ( e.g. peat depth, slope angle) and site independent variables ( e.g. weather) that contribute to the incidence of natural peat landslides are beyond engineering control without significant damage to the peat itself. However, a number of engineering measures exist to minimise the risks associated with potential triggers (such as short term peaks in hydrogeological activity):

• Installation of drainage measures: installation of targeted drainage measures would aim to isolate areas of susceptible peat from upslope water supply, re-routing surface (soakaways/gullies) and subsurface (pipes) drainage around critical areas; drainage measures need to be carefully planned to minimise any increase in instability caused by creating discontinuities in the peat mass;
• Slope re-profiling: this is not normally considered an acceptable mitigation measure but has been used effectively in the past where environmental costs have been outweighed by safety benefits to the public;
• Soil nailing: this is not normally acceptable for peat slopes, since peat materials may readily deform around the soil nails;
• Construction management: site specific procedures aimed at minimising construction induced peat landslide hazards should be identified and implemented and followed rigorously by site construction personnel. These may include work method statements subject to an environmental check to monitor compliance. These checklists should incorporate a weather forecast to minimise peat working during heavy rain and to allow environmental mitigation measures to be put in place where construction work is on-going. Weather forecasts can be obtained using data available from numerous web-sites or provided at a cost by commercial organisations or the Met Office.

5.5.3 Engineering mitigation measures to control landslide impacts

A number of engineering measures are available for reducing the impacts (or Exposure) associated with residual peat landslide hazards. These include:

• Catch wall fences: where the potential for peat landslides has been identified, catch-fences positioned down-slope of the suspected or known landslide prone area can slow or halt runout (Tobin, 2003). Catch fences should be engineered into the peat substrate. Fencing may require periodic inspection for removal of debris;
• Catch ditches: similarly, ditches may also slow or halt runout, although it is preferable that they are cut in non-peat material. Simple earthwork ditches can form a useful low-cost defence. Paired ditches and fences have been observed (Tobin, 2003) to slow peat landslide runout at failure sites.

5.5.4 Monitoring and review

A peat hazard management plan incorporating a risk matrix should be prepared and updated regularly, with the frequency of the review contingent on the hazards identified and the status of the proposed development. Monitoring of all stages of analysis and assessment and prioritisation as the plan evolves and is implemented will provide feedback for the reassessment of risk.

It should be noted that factors that affect the likelihood of peat landslides and their consequences may change with time. Thus, ongoing review of the peat hazard management plan is essential. Design of stabilisation measures may be reviewed and risks may be reassessed during construction as the process of construction yields further data.

5.6 Reporting

The multidisciplinary team responsible for reporting the peat landslide hazard assessment should prepare both the factual and interpretative reports for the study. Recommendations as to the contents of these reports follow.

5.6.1 Factual report structure

The following is a suggested structure for the factual report, detailing the minimum content:

(i) Introduction: a brief statement indicating who the work has been carried out for, a summary of the site location and extent, and a brief summary of the scope and purpose of the investigation;

(ii) Design of Ground Investigation: an explanation of the size and scale of the investigation, a rationale of why the locations and methods have been selected and a statement on the measures undertaken to mitigate and/or reduce potential site risks;

(iii) Field investigation and testing: a summary of the field work undertaken including, detailed mapping, detailed site investigation plans, and length of time taken to carry out the field investigation. A summary of the logging procedure and a summary of the different peat types identified onsite. Details of the samples taken and the procedures used should be included and the location and reasons for any installations should be included;

(iv) Laboratory testing: a summary of the laboratory testing that has been undertaken including summary tables of physical, chemical and geotechnical properties of each peat layer identified onsite. The information displayed should be sufficient to populate any slope stability model used in stability analysis;

(v) Appendices: A detailed series of appendices should be included to present all of the supporting information for the above sections. At a minimum this should include:

• A desk study summary plan for the site summarising all supporting mapping and observations;
• Details on the sites investigation locations including logging sheets, elevations, photos of excavation;
• A list of all samples taken including sample reference numbers, dates taken and the tests undertaken on each sample, and
• All supporting laboratory results that have been used to characterise the specific site materials and their geotechnical properties.

5.6.2 Interpretative reports

The following is a suggested structure for the interpretative report, detailing the minimum content:

(i) Introduction: a statement indicating for whom the work was done, the nature and scope of the investigation, and a summary of the site location and extent ;

(ii) Description and History: a detailed description of the site based on the observations made by the Competent Person during their site review and reconnaissance. It should be referenced to a plan of the site showing national grid co-ordinates and to a scale no smaller than 1:2,500;

(iii) Ground Conditions: descriptions of the ground conditions found during the investigation and an interpretation of their relevance to the stability of the site and surrounding area should be provided, informed by the factual report. Anomalies in any of the data collected should be noted and their impact on confidence in interpretation clearly stated. The following items should be discussed where appropriate: geological conditions; local climate and hydrology/hydrogeology; history of past landslide events and ground movement rates; soil and peat properties, land-use history and natural environmental change. They should be supported by interpretative geological cross sections of the peat environment;

(iv) Evaluation of Stability: the stability of the site and relevant adjacent areas should be evaluated with respect to the proposed development components (structures, communications) and any associated stabilisation measures. It is expected that particular attention be paid to the gradients of cut slopes and fills, drainage measures, retaining structures, failure mechanisms and the design criteria applied;

(v) Discussion regarding mitigation measures: A detailed discussion should be presented of the main conclusions of the investigation and of the resulting mitigation measures that are recommended for each infrastructure component. This should include details of the recommendations to ensure both the long term peat stability of the site (taking account of the anticipated life of the development) and the short term peat stability of the site during construction. Peat landslide hazard zonation plans should be included and peat landslide hazard management plans referred to with appropriate mitigation measures, this should be achieved by the use of a risk register. It is expected that particular reference be made to matters such as: provision for free drainage of groundwater; minimising drainage diversions and specification of drain linings where site conditions require them; avoidance of natural drainage pathways ( e.g. gullies, soakaways) and provision of flexible jointed pipes capable of sustaining small movements without leakage; The discussion of mitigation measures should also include a summary of any environmental damage that may be caused by the development and proposals for remediation.

(vi) Conclusion and Recommendations: Single line conclusions and recommendations should be presented stating the outcome of the above discussion.

5.6.3 Early exit from the peat landslide hazard assessment process

Figure 3.1 provided a means of exiting the peat landslide hazard assessment process in the event that site conditions indicate very low potential for peat landslides. If this exit option becomes available and is taken by the developer, then the desk-study report must justify in its closing chapter the reasons for exit from the hazard assessment process. Justification should be referenced to data collected for the desk study that verifies the conditions outlined in Figure 3.1.

5.7 Further reading

This document has provided a brief overview of peat landslide hazards, the means by which they may be managed, and the risks associated with their occurrence mitigated. However, a considerable range of published material is available that may be of use to developers in undertaking the works described in this document.

The form, mechanisms and causes of peat landslide are considered in detail in a paper by Dykes and Warburton (2006), and in two books that consider wider issues of peat degradation (Evans and Warburton, in press; Martini et al. 2006).

The geotechnical properties of peat are well described in a chapter on organic soils by Bell (1999), with detailed discussion of peat physical and chemical properties provided by Hobbs (1986).

Methodologies for landslide investigation and management are provided in DoE (1996), while landslide hazard and risk assessment is considered in detail in Lee and Jones (2004).

5.8 Acknowledgements

We gratefully acknowledge the input of Halcrow Geotechnical staff in the compilation of this document and the guidance given by Dr Jeff Warburton of Durham University

Thanks are given to NPower Renewables Ltd. for their kind permission to use the diagrams and data from the geotechnical input to Farr Wind farm.

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