How to do a stability risk assessment: landfill sites for hazardous and non-hazardous waste

What to include in a stability risk assessment for a landfill site for hazardous and non-hazardous waste.

Before you do a stability risk assessment

Before you do a stability risk assessment you must:

• create a conceptual site model
• complete an environmental setting and installation design report

You only need to do a stability risk assessment if your conceptual site model confirms that you need one.

You should also read the Environment Agency reports Stability of landfill lining systems: reports 1 and 2 to help you complete your stability risk assessment.

Use this guide with the:

• application forms
• risk assessment guidance

What to include in your stability risk assessment

Your stability risk assessment must include the following sections.

1. Introduction

You must include an entry in each section, even to confirm that a specific feature is not relevant to your site. The Environment Agency accepts that you may not need to complete some of the sections.

Include information in tables in the report at the appropriate point.

Include all the information you used to complete the report in the appendices or provide references if they are within your environmental setting and installation design report.

Include all numerical and analytical models you refer to in the report, including their results, in electronic format in the appendices.

You must base your stability risk assessment on the plans or drawings in this guide.

2. Contact details and report context

Your report must include:

• the name of the proposed operator, site name and address
• the name and address of the agent who completed the risk assessment
• report context – include background information and an outline of the site
• site setting, including an outline of the proposed site and how it relates to historically operated areas of landfill or waste recovery activities
• the conceptual site model or a reference to the report it is in

3. Conceptual site model

Your stability risk assessment must include a stability conceptual site model. This must include the following.

a) Primary components

Consider the:

• basal sub-grade
• side slope sub-grade
• basal lining system
• side slope lining system
• waste mass
• capping system

Read Environment Agency reports Stability of landfill lining systems reports 1 and 2 for more detail of each component.

b) Pore fluid pressures

You must consider the pore fluid pressures relevant to each of the primary components. You must include details of:

• groundwater pressures acting from below the base and outside the side slopes of the landfill cell or development area
• leachate pressures acting on top of the base of the landfill cell or development area
• leachate pressures acting behind side slopes, such as where leachate is recirculating or where there is perched leachate adjacent to the landfill cells or development area
• landfill gas pressures acting on lining or capping components or within the waste mass itself
• excess pore water pressures generated by filling (either engineered fill or waste)
• negative pore water pressures generated by excavation

c) Settlement and strains

You must consider the impact of short-term and long-term settlement and the shear strains that may occur for each of the primary components.

You must include details of:

• groundwater pressures acting from below the base and outside the side slopes of the landfill cell or development area (for example, basal heave)
• settlement of the base and side slope sub-grade for example, compressible sub-grade material and the impact of dewatering or cavities)
• settlement of the basal and side slope lining components (for example, compressible sub-grade or waste)
• settlement of the waste on the capping system (for example, compressible waste and cavities)
• infrastructure (for example, negative skin friction, punching shear or serviceability)

d) Basal sub-grade

You must include details of the:

• geology of the basal sub-grade (such as types and structure of soils and rock)
• general form of the sub-grade (such as areas of soil and rock cut or fill)
• pore fluid pressures which could act on the sub-grade

e) Side slope sub-grade

You must include details of the:

• geology of the side slops sub-grade (for example, types and structure of soil and rock)
• range of gradients of the slopes
• general form of the sub-grade (such as areas of rock and soil cut or fill)
• pore fluid pressures which could act on the sub-grade

f) Basal lining system

You must include details of the:

• proposed mineral and geosynthetic lining elements and their interface with each other and the waste
• proposed groundwater and leachate drainage elements
• pore fluid pressures which could act on the basal lining system

g) Side slope lining system

You must include details of the:

• proposed mineral and any geosynthetic lining elements and their interface with each other and the waste
• gradient range of the slopes
• proposed groundwater and leachate drainage elements
• pore fluid pressures which could act on the basal lining system

h) Waste mass

You must include details of the:

• type of waste you intend to deposit, its heterogeneity and physical form
• type and distribution of the materials you will cover it with
• general and maximum slopes of the waste during operations, during the closure and aftercare period and at the end of the life of the landfill
• pore fluid pressures which could act within the waste

i) Capping system

You must include details of the:

• proposed mineral and/or geosynthetic capping elements and their interface with each other and the waste
• proposed restoration cover elements including drainage
• general and maximum slopes of the capping surface for pre-settlement and post-settlement conditions
• gas pressure that could act on the underside of the capping system
• pore fluid pressures which could act within the capping system

4. Stability risk assessment

After you have developed your stability conceptual site model, you must carry out your stability risk assessment.

You must classify all issues relating to stability or integrity into simple and complex categories in your stability risk assessment.

Whether the risk assessment is simple or complex will depend on the potential risks presented by the site, the degree of uncertainty in the design and the likelihood of the risk.

You must carry out detailed geotechnical analyses for the complex category. You must:

• provide a justification for the issues you classified as simple (such as sound bedrock forming a sub-grade) and not requiring detailed geotechnical analyses
• summarise the reasons for classifying other issues as complex – you must identify the governing geotechnical principles you used to decide they were complex

You must include a summary of whether you think the following components are simple or complex and whether they require analytical assessment:

• basal sub-grade
• side slope sub-grade
• basal lining system
• side slope lining system
• waste mass
• capping system

You must provide full justification if you think that any component does not require an analytical assessment.

5. Lifecycle phases

You must identify the critical phases during the development of the landfill.

Where applicable, you must consider the following operational factors. This is to make sure that your stability risk assessment addresses the main issues throughout the life of the site:

• phasing of sub-grade slopes
• phasing of engineered fill, waste placement and the rate of construction
• waste mass geometry (height, outer slope inclination and crest width) versus time
• haul roads accessing or on the waste
• leachate management
• landfill gas management
• daily cover
• temporary capping

You must provide a full justification where you consider that any component does not require an analytical assessment.

6. Data summary

You must include:

• site specific geotechnical data
• published geotechnical data with justification for its use
• assumed geotechnical data with justification for its use
• uncertainties in the geotechnical data to be used and proposals for addressing those uncertainties (such as sensitivity analyses)

Where applicable, you must include the following geotechnical data you used for the detailed analyses:

• material unit weight
• soil characterisation data (particle size distribution, plasticity index and natural moisture content)
• drained shear strength of soils and rocks
• undrained shear strength of cohesive soils
• material properties of all geosynthetic elements
• shear strength of all soil to geosynthetic and geosynthetic to geosynthetic interfaces
• groundwater pressures
• leachate pressures
• potential landfill gas pressures
• excess pore water pressure dissipation characteristics of cohesive soils
• consolidation characteristics of soils and waste
• permeability characteristics of soils and waste
• discontinuity characteristics of rock masses
• geotechnical parameters for any ground improvement methods you adopted, for example soil reinforcement
• stiffness characteristics of soil and waste
• in situ horizontal stresses in waste

7. Justification for modelling approach and software

You must include details of the:

• most suitable analytical technique to represent all the modelled phases of the lifecycle such as before (unconfined) or after (confined) waste placement or, in the case of the capping system, the placement of the restoration soils
• methodology and software that will reflect the desired goals of the analyses (for example, determination of the factor of safety or analysis of strains within liner components)

You must include details of the analytical methods you used for the stability risk assessment for:

• 2-D limit equilibrium stability analyses
• 3-D limit equilibrium stability analyses
• 3-D wedge or block analyses
• finite difference analyses to determine shear strains within liner systems and consolidation settlement
• finite difference analyses to determine pore fluid pressure distributions
• closed-form analyses (used in spreadsheets), for example interface stability for basal, sidewall lining and capping stability analyses

8. Justification of geotechnical parameters selected for analyses

You must include your assessment of the:

• quality and relevance of site-specific data
• relevance of data that is not site-specific
• methodology and reasoning you adopted to get the parameters (such as statistical analysis or conservative estimates)

For the analysis you must include a summary of the data you used to get the parameters for the:

• basal sub-grade
• side slope sub-grade
• basal lining system
• side slope lining system
• waste mass
• capping

9. Select appropriate factors of safety

You must specify the factor of safety for each element of the design. You must justify the factor of safety you have selected.

The factor of safety is the numerical expression of the degree of confidence that exists for a given set of conditions against a particular failure occurring.

It also represents the confidence in the input parameters and analysis method you used. It is commonly expressed as the ratio of the load or action which would cause failure against the actual load or actions likely to be applied.

This is more easily determined for some types of analysis (such as limit equilibrium slope stability analyses). However, you must also focus on analyses which do not report factors of safety directly. For example, a finite difference analysis of shear strains within a steep side slope structure would not usually indicate overall ‘failure’ of the model, even though the strains could be high enough to indicate a failure of the integrity of the structure. In such cases, you must define an upper limit for shear strains and express the factor of safety as the ratio of allowable strain to actual strain.

Before you determine the factors of safety for the various components of the model, you must identify key ‘receptors’ and evaluate the consequences of a failure (relating to both stability and integrity). You must consider these receptors:

• groundwater
• surface water
• other environmental receptors
• property relating to site infrastructure and third parties
• people

The factor of safety you adopt for each component of the model must relate to the consequences of a failure.

There are 2 ways you can determine the factor of safety. The traditional approach is to use material properties and loads in an unmodified state and then apply a factor of safety at the end of the analysis to allow for uncertainty.

The second is the Eurocode 7 (EC7) approach. You apply partial factors of safety early in the design to modify each parameter and load and then test for stability (for example, equality factor of safety = 1) of modified loads (Actions) and modified ground resistance.

Where applicable, you must state the approach you propose to use. Describe the relevant factors involved in the selection of the factor of safety for the:

• basal sub-grade
• side slope sub-grade
• basal lining system
• side slope lining system
• waste mass
• capping system

10. Sensitivity analyses

You must include details of how you will carry out your sensitivity analysis. This may include the use of multiple model runs to simulate different justifiable ranges of input parameter values.

Where applicable, you must include details of the approach to and results of the analyses you do for the:

• basal sub-grade
• side slope sub-grade
• basal lining system
• side slope lining system
• waste mass
• capping system

11. Assessment

You must provide a reasoned review of the results of the analyses. Include:

• a consideration of analytical limitations, the assessment of uncertainties and the potential effects on factors of safety
• an overall assessment of risk for each component

Where applicable, you must include a review of the approach and results of the analyses you do for the:

• basal sub-grade assessment
• side slope sub-grade assessment
• basal liner assessment
• side slope liner assessment
• waste mass assessment
• capping assessment

12. Monitoring

You may need to monitor the as-built structure. For example, basal settlement monitoring, pore water pressure monitoring or slope movement monitoring.

You must use your stability risk assessment to develop risk-based monitoring objectives and schedules.

This section must provide your technical reasoning for the design of a monitoring programme, to focus monitoring effort on actual risks. Where you propose geotechnical monitoring, you must specify the appropriate assessment and performance criteria within each of the appropriate sections. You must provide a full justification and a clear audit trail for each proposed criterion.

Where applicable, you must provide details of the proposed monitoring scheme for the:

• basal sub-grade
• side slope sub-grade
• basal lining system
• side slope lining system
• waste mass
• capping system

13. Interface testing

You must provide details of what site-specific shear testing you will do. For example, you might include each soil to geosynthetic and geosynthetic to geosynthetic interface.

Read the guidance on geosynthetic interface shear resistance testing in the industry code of practice (LGG 115).

Where applicable, you must provide details of the proposed interface testing for the:

• basal sub-grade
• side slope sub-grade
• basal lining system
• side slope lining
• capping system

14. Conclusion

Your stability risk assessment must confirm the structural and physical stability of the landfill or deposit for recovery activity over its entire lifecycle.

15. Tables

Tables summarise information and data.

You must:

• use tables where applicable
• put tables next to the text they relate to

The tables in your stability risk assessment must normally include:

• a summary of the geotechnical investigations you did at the site
• the results of in-situ tests relevant to the stability risk assessment
• the results of laboratory tests relevant to the stability risk assessment
• the proposed assessment scenarios (including the lifecycle phases) and how certain inputs may change with time
• the input parameters you used in the analyses for each scenario
• how you got (and justified) the range of values you used as input parameters in the analyses for each scenario
• how you got (and justified) the factors of safety and performance criteria you applied for each assessment scenario
• the results of the geotechnical analyses (including results of sensitivity analyses on values of input parameters) for each component of the model, for example basal stability and the integrity of the side slope liner
• the model validation exercise (this compares the output against any observed conditions)
• the risk-based monitoring scheme

16. Drawings

The drawings in your stability risk assessment will vary according to site-specific circumstances.

One drawing can provide information for more than one part if the drawing is clearly labelled.

Use recognised scales (such as 1:1250) for all drawings, maps or plans to show the relevant details. All levels must be in m AOD (metres above ordnance datum).

Label each drawing with the:

• drawing title
• site name
• name and address of the operator
• date the drawing was made
• drawing identification number
• scale of the drawing
• key
• grid lines and north point
• paper size of the original drawing

Your stability risk assessment should normally include the following.

a) Drawing SRA1 – stability conceptual site model

You must provide a plan (or set of plans), with appropriate cross sections, which identify all the site-specific situations you modelled scenarios for (for example, slope angles and dimensions of engineered structures).

The plan must indicate the location of relevant ‘receptors’ such as site infrastructure in relation to slopes.

b) Drawing SRA2 - proposed assessment scenarios used in modelling

You must provide schematic cross sections of each of the assessment or model scenarios you considered, including the different phases of the lifecycle of the site.

You may annotate these cross sections with the relevant input parameters and associated distributions. Each cross section should be no larger than A3 size.

c) Drawing SRA3 - monitoring

You must include a plan of all monitoring points for geotechnical monitoring installations.

d) Drawing SRA4 - details of monitoring equipment

You must include instrument and installation details where your stability risk assessment says this is necessary.

17. Appendices

You must include appendices, where applicable, in your stability risk assessment. You do not need to include information in the appendices if you have included it within the main text of the report.

You must normally include:

• information about ground investigations
• hard copies of all models and results you relied on in the assessment
• electronic copies of all models and results you relied on in the assessment
• hard copies of any data you used, including laboratory testing
• results of any sensitivity analysis you carried out for the site
• the proposed risk-based monitoring plan for the site
• the proposed monitoring equipment installation protocols