7.2 Factors governing the stability of slopes

7.2.1 Introduction

As part of the design of earthwork slopes, the designer should assess aspects that can be considered by calculation (e.g. stability of slopes, potential for adverse settlement, scour), and also other potential modes of failure that might require engineering judgement and precedence to be used rather than relying entirely on calculation (e.g. erosion, influence of animals or vegetation). The site observation, modelling of ground conditions and the risk assessment process is an important part of the design of earthworks to help identify potential modes of failure.

The overall stability of slopes should be assessed based on the requirements given in BS EN 1997-1:2004, 11.5, and designers should consider whether it is appropriate to assess deformation of the ground as detailed in BS EN 1997-1:2004, 12.6. The applicable actions for different design situations should be considered (BS EN 1997-1:2004, 11.3) which for earthworks can include external influences such as earthquakes and pile driving.

7.2.2 Materials and ground conditions

For the purpose of making a preliminary assessment of stability conditions and for guidance in formulating a field or laboratory testing programme, consideration should be given to previous relevant experience and published information to obtain an indication of the behaviour of a particular type of soil when excavated to form slopes and platforms. The parameters used to define shearing resistance should be obtained from back analysis or from appropriate field or laboratory tests which take account of the permeability of the mass of material and also of the stress changes which take place in the material, both in the short and long term, as a result of excavating for slopes and platforms.

The designer should review the nature of the materials that form or influence the stability of the slope and consider which attributes of the material could have a significant influence on the potential modes of failure:

  • grading/permeability of the soil (coarse soil or fine soil, or intermediate soil that may show attributes of both) and groundwater conditions, these factors tend to dominate the likely form of instability within soil slopes in most general cases; further discussion on this topic is provided at 7.2.4;
  • previous stress conditions are of particular importance to fine grained soils which can be in a normally consolidated or overconsolidated state;
  • presence of geological structure within the soil or rock, such as bedding planes, laminations, fissures or other discontinuities;
  • presence of zones of contrasting permeability;
  • presence of historical slip surfaces where previous movement was sufficient to generate smooth (slickensided) surface with a "residual" shear strength in clay soils;
  • weathering of fine grained soils or rocks leading to a reduction in strength of the material often resulting in zones of weakness (e.g. along fissures in soil or rock, and can lead to karst conditions in limestone), and the leaching of minerals under prolonged seepage or other weathering phenomena can lead to the development of sensitive soils prone to collapse on disturbance in some normally consolidated fine soils.
  • influence of human activities in the form of mining.

Once the potential modes of failure have been identified, each should be assessed by a method suitable for the material type and reflecting the geotechnical category of the structure. Details of methods of analysis are provided within soils mechanics references such as Bromhead [22].

7.2.3 Actions

NOTE 1 Load cases for earthworks design usually comprise externally applied actions and the self-weight of the earth structure itself.

For externally applied actions, details should be obtained of static, transient and dynamic loads that might be applied to the earthworks. A minimum surcharge of 10 kN/m2 should be applied to the surface at the top of embankments and cuttings where the external action might adversely affect the stability of the slope.

NOTE 2 This requirement is not in addition to any specific live loading of equal or greater magnitude that is included within the slope design model.

The minimum surcharge should be considered as a permanent load and appropriate partial factors should be applied to the action.

Additional surcharge loading should be applied to take account of actions resulting from loads imposed on the earthworks during construction and during the design life. The surcharges applied to the earthworks may be classified as:

  • uniformly distributed load (UDL) consisting of a continuous load on the surface, this may be a defined load case (e.g. railway industry RL or RU loading), or a general surcharge to represent construction plant, stored materials (10 kN/m2 minimum);
  • concentrated loads (e.g. pad foundations);
  • line loads (e.g. strip footings);
  • dynamic loads (e.g. impact loads), these are generally modelled as a UDL. In some specific cases impact loads may be modelled as point loads.

The combination of these load cases can result in a combination of applicable surcharge being applied to the slope; the designer should identify the applicable surcharges to be modelled. This situation is illustrated in Figure 3 as a possible design scenario.

Figure 3 Example of possible surcharge combination on a slope
Example of possible surcharge combination on a slope

In the absence of more exact calculations, the nominal loads due to live load surcharge may be taken from Table 3.

Table 3 Nominal load due to live surcharge
Standard load Uniformly distributed load UDL
Typical applicable design cases
No specified load case 10 Earthworks slopes where maintenance equipment might present an adverse load case.
Typical highway loading 10 Common practice is to assume this value. Extreme cases agreed on a site-specific basis.
RL loading 30 on area occupied by tracks London Underground and other light rail systems
RU loading 50 on area occupied by tracks (see Note 3) "Rail universal" used for all standard UK railways

NOTE 3 RU loading is included in Table 3 because this has formed the basis of design for many years. However, in future UK railways will change their standards to apply surcharge loads in accordance with Eurocodes. It remains important that the designer considers the likely distribution of load below the track and the dispersal of load with depth through the soil.

A clear understanding of the loads that will be applied to the earthworks should be obtained from the asset owner; otherwise, the designer should identify to the asset owner the restrictions on surface loading of the earthworks.

The designer should give consideration to the duration of applied loads and the selection of appropriate soil parameters for the assessment of slope stability, i.e. whether drained or undrained conditions are appropriate for the duration of the load. The designer should consider that some surcharges might exist for short periods of time only, in which case, the ground can respond in an undrained manner during the entire period of the application of the surcharge.

The dead load may be assumed to be the load of the earthwork.

NOTE 4 BS EN 1997-1:2004 distinguishes between permanent loads and variable loads that are applied to the slope in so far as different partial factors are applied to each type of load. Permanent loads will, for example, comprise structures and buildings whilst variable loads will normally consist of traffic or rail loading or other transient loads. Loads imposed by structures and buildings will consist of both the dead load from the structure and live load applied to the structure; unless the live load forms a significant proportion of the total load from the structure the load applied to the earthworks by the structure may be assumed to be permanent. An example of where variable load from a structure might need to be applied separately to the dead load is in the case of a service reservoir located at the crest of a slope. In this case the loading from the water might be greater than the dead load of the structure and the load might fluctuate throughout the design life of the structure.

Care should be exercised when applying surcharge loads to the slope face since these loads can act either favourably or unfavourably (i.e. they might contribute to either destabilizing forces or to restoring forces) depending on their position on the slope.