Slope Stability Analysis / Retaining Wall

Slope Stability Analysis

Slope stability analysis is the assessment of any dipping surface to determine if it will remain intact or move under its own weight or under the application of a load. Slopes, natural or manmade, have an inherent tendency to degrade to a more stable form, in essence to flatten out and become horizontal.  There are four principal ways of degradation or failure – termed mass movement: falls, rotational slipes, transitional slide, and flow. The causes of these failures can be:

• Unsuitable geometry
• Change in ground-water regime
• Presence of weak planes
• Progressive deformation
• Increase in effect slope height
• Additional imposed load

A slope can have multiple potential areas of failure or slip surfaces, it is considered stable if the shear strength along each potential slip surface is greater than the shear stress. Therefore, by analysing each of slip surface within the slope an overall assessment of its stability can be determined.

Stability Assessment

Slope stability analysis is conducted by creating a model of the slope and performing analytical methods to determine the likelihood of failure. To create the initial model of the slope the different strata layers and material properties of each layer needs to be determined.  This is generally achieved by drilling boreholes in selected areas, typically at the top for natural slopes whereas manmade slopes require further drilling at the bottom and often in the middle depending on the size.  The stratum information gathered is combined with measurements of slope length and angle to create a model.

Using the model and geotechnical software, the different strata layers are assigned properties and the software is run using Bishop’s method of slices, the grid method or others to assess the stability.  If the slope stability is below the factor of safety (1.3) then the slope is likely to fail at some point in the future.  Therefore, methods to secure the slope would be required such as rock bolts, retaining walls, etc.

Retaining Walls

Retaining walls are vertical to near vertical structures designed hold material behind it and prevent its lateral movement.  They are typically used on sites that have undergone level changes producing naturally unstable slopes.  The retaining walls are used to stabilise these slopes by countering the active pressure or thrust of the soil and porewater pressure behind the wall with the passive pressure (resistance) of the wall.  Retaining walls are often used in road cuttings, housing estates built on hills and landscaping.

Retaining wall design

There are several types of retaining wall, that are suited to different jobs and sites depending on end us and ground conditions, but the most common types are

• Gravity
• Sheet pilling
• Reinforced
• Cantilever
• Anchored earth walls
• Gabion

To gather information for the retaining wall a ground investigation is undertaken to access the properties of the stratum on site behind and beneath the retaining wall.  This typically involves the drilling of several boreholes to provide this data.  This is combined with topographical surveys of site levels and construction materials as discussed with the client.

Based on the information gathered the retaining walls can be designed using software to assess the construction of the retaining wall and determine its height and width per meter of length.  Analysis of safety factors ensure that possible failure does not occur.

Overturning moment – the forward collapse of the wall blocks or at the base due to the soil behind the wall, this is resisted by the weight of the wall.

Bearing capacity – the ground supports the weight of the wall and earth behind it and ensures settlement is within tolerable limits.

Global stability – the earth around the wall has adequate strength to avoid an overall slip failure.

Shear – the wall does not move at the base; this deepens on the friction between the wall and ground at the base.

Detailed outputs from the software and AutoCAD drawings created by GES staff are provided to aid in construction details.