Slope Stability Assessments

What is a Slope Stability Assessment?

A slope stability assessment is exactly what it says – it is the assessment of the stability of a slope. Or how likely is the slope to fail if conditions change – conditions such as greater loading on the slope (foundations or pavements etc.) or a change in the pore water pressure (if it becomes saturated etc.) or if a natural or man-made slope is made steeper.  

A range of testing and calculations are undertaken to determine, essentially, if the slope is too steep for the proposed end use or if the existing slope could be at risk of failing. 

What is Slope Failure and what Causes Slope Failure?

The term fail can be slightly ambiguous or subjective when looking at slope failure. It can mean a slight slippage of the slope where a section slides slowly down. It can mean a significant slippage where a large portion of a slope collapses – in extreme circumstances think mudslide. 

There can be a number of causes of slope failure and understanding this is an important factor in the assessment of potential failure or of a slope that has already failed.  

Increase in pore water pressure – think saturation of the soils or material the slope is made up of. There are voids or cracks and fissures in the soils that make up pretty much all natural and engineered slopes. These may be tiny ‘pores’, but as water fills these pores, combined with gravitational effects, the pressure increases to a point where the matrix holding the particles together break apart and create a slip plane and the soil to collapse. A simple example of this might be building a sand castle then watching it collapse when you pour water. 

Greater loading on the slope – essentially, the heavier the load placed on the ground at the top of a slope, the more likely it will destabilise. To achieve a stable slope, the shear strength of the soil must be higher than the shear stress, which includes the weight added to top of the slope. The load can include buildings, roads, vehicles etc. The further from the slope’s crest the load is, the higher the possibility of preventing slope failure

Types of Slope Failure

The most common type of slope failure is  rotational failure (or rotational slide) which is identified when failed soils move outwards and downwards. There are three types of rotational failure – face, base and toe failure. 

Translational failure occurs when adjacent areas of soils of differing strength slide pass each other. The sliding mass often travels long distances before coming to rest.  

Compound failure is a combination of rotational and translational. This generally occurs when the surface curves at both ends but maintains a flat central point. This distinctive topography is a primary indicator of a compound failure. 

A flow slide occurs when soils behave like a fluid which spreads across and down even very shallow slopes.  

Wedge Failure (or block failure) occurs when weak layers, joints and fissures are broken apart and material moves down the slope in wedges or blocks.

Phases of a Slope Stability Assessment

The first step in any geotechnical project, including slope stability assessments, is a Phase I Desk Top Study. This will include a review of available geological information and history of the land use. Omnia will also recommend completing a site walkover to visually inspect the slope and commission a topographical survey to obtain accurate measurements of the land. If our engineers can’t access the slope, a drone can be employed to survey the area and assist with the walkover. 

Following the completion of the desk study, a geotechnical site investigation will be conducted. The aim of this investigation is to determine the materials that make-up the slope and surrounding ground, and to establish groundwater conditions. The quality of data from the ground investigation can be critical and it is essential that a well scoped and robust set of investigation and testing is completed, as the parameters used in the assessment are derived here. Getting them wrong can cause a bit of an issue down the line.  

Access on to the slope is also key here. Assessing an existing slope for integrity or to determine a re-grade/design parameters will need information from the slope itself, which often comes with challenges – walking up some slopes can be difficult, let alone putting specialised plant on to it. This can become even more of a challenge if dealing with remediation of a failed slope. 

Omnia have experience in planning and executing these type of complex investigations and are fortunate enough to have a strong and diverse supply chain able to support us in completing even the most difficult access projects. 

Using the findings from the site investigation, and taking the time to understand the client’s requirements, our consultants will conduct a slope stability analysis to the requirements of EC7. The subsequent report will include all findings from the investigation and present a suitable set of design documents appropriate for the project in hand. This might include re-profiling as part of access creation or level adjustment, design drawings as part of an earthwork construction, broader assessment to support a cut and fill exercise. It might include a completely revised design to assist in the remediation of a failed slope including recommendations to rectify and prevent further failure. 

Our consultants have the knowledge and experience to assist with slope stability assessments across the UK. Please get in touch with our team if you have any questions or you would like us to take a look at your site.