7.6 Embankments and filled areas

7.6.1 Design of embankments and filled areas

7.6.1.1 General

The overall stability of embankments should be designed in accordance with the requirements of BS EN 1997-1:2004, 12.5 and the deformation of the embankment or filled area should satisfy the requirements of BS EN 1997-1:2004, 12.6.

Embankments and filled areas should be designed to have adequate stability against shear failure and to ensure that any deformation is within acceptable limits.

NOTE The information required before the cross section of the embankment can be designed includes:

• a) ultimate width of top of embankment;
• b) loading on top of embankment;
• c) geotechnical properties of the foundation and fill materials;
• d) restrictions on width of land available;
• e) special conditions to which the embankment would be subject, for example, tidal waters, active mining operation and natural cavities, and environmental and other economic factors which could influence the final choice of cross section, e.g. earth banks for sound screening or flattening of slopes to allow them to be returned to agriculture.

7.6.1.2 Stability

Calculation of the stability of the embankment should be undertaken using the methods of analysis described in 7.3.1. In some instances, it may be desirable to analyse embankment deformations using, for example, finite element methods to determine whether deformation is acceptable.

COMMENTARY ON 7.6.1.2

Parameters of the shear strength of the fill appropriate for use in the stability calculations are usually obtained from laboratory tests on recompacted samples. Where an embankment is built of rockfill or other granular material with side slopes not exceeding the angle of repose of the fill, it is inherently stable for all heights as long as the foundations are capable of sustaining the loads. However, the angle of shearing resistance of a well compacted granular fill can be considerably greater than the angle of repose and consequently the laboratory determination of this parameter and its use in the stability calculations can lead to a more economic embankment cross section. For rockfill embankments, where laboratory determination of the angle of shearing resistance of the fill material might be difficult, reference can be made to the approach set out at BS 8002:1994, Table 3 and Table 4. The shear strength and pore pressure parameters of clays and silts can be measured in laboratory triaxial compression tests. If the natural moisture content of the material in the field is high but the permeability characteristics are such that it can be readily reduced, the design could take advantage of the resulting improvement in shear strength.

Where embankments are constructed on sidelong ground and a layer of impermeable material underlies a significant thickness of permeable material, a perched water table can form, causing saturation of the coarser material with possible erosion or slumping where the water table emerges onto the side slope. The stability of an embankment depends not only upon the strength of the fill material from which it has been formed but also upon the strength of the material on which it is founded. An assessment is necessary to check the ability of the foundation material to carry the required superimposed load without shear failure or unacceptable deformations. The factors governing the behaviour of soils and rocks in cuttings generally apply also to their behaviour as foundation materials for embankments. If the site contains geological features such as faults or slip surfaces resulting from previous movements, due regard has to be taken of these during the evaluation of the stability of the embankment. Techniques are available for improving the strength properties of fill and foundation materials. The effects of embankment loading on materials of low shear strength can be mitigated by various methods including, excavation and replacement, staged construction, the use of berms, or flattening the side slopes, use of geosynthetics, or undertaking ground improvement prior to construction.

7.6.2 Materials

The characteristics of the materials given in Table 7 should be taken into consideration in their use for foundations and embankments.

The strength, deformation and moisture susceptibility of foundation and fill material should be established by means of:

• a) in situ testing as part of site investigation;
• b) laboratory tests;
• c) instrumented field trials;
• d) information from previous performance.

In the case of soft ground engineering or rock embankments, field trials should be considered in order to determine the best procedures both for excavation and for forming a satisfactory embankment.

Some materials, such as silty sands, silty clays and chalk, have a critical level of moisture content above which they rapidly become unsuitable for normal methods of earthworks construction. Laboratory examination should be made of the relationship between moisture content, density and undrained shear strength or CBR values for all types of soil exhibiting predominantly cohesive properties.

The selection of parameters for design should take account of both the soil grading and, in fine grained soils, the nature of the fines content, which is commonly defined by the soil plasticity.

COMMENTARY ON 7.6.2

In the field of earthworks there are differences in the fines content required for a soil to be classified as fine grained (cohesive) which reflects the different situation under consideration:

• for geotechnical design (e.g. slope stability or settlement analysis) a soil with more than 35% fines is normally considered to behave as a "fine grained" soil (although it is important to realise that some soils with lower fines contents can still behave as a fine soil;
• for classification of fill materials the change point occurs at 15% fines (above which the fill is defined as cohesive under the SHW [1]), this reflects the tendency for the materials to trap excess porewater pressures during compaction; pavement foundation layer designs follow a similar approach and designers normally aim for the granular soil to be "non-plastic" as defined by plasticity testing.

See Table 1a), Table 1b) and Table 1c) for information on soil descriptors in different earthworks circumstances. In practice the point at which the materials behaviour changes from granular to cohesive can vary resulting in what may be best defined as "intermediate soils" as illustrated in Table 1c).