ABSTRACT
This chapter is concerned with the design of potentially unstable soil masses which have been formed through either human activity (excavation or construction) or natural processes (erosion and deposition). This class of problem includes slopes, embankments and unsupported excavations. Unlike the material in Chapter 11, however, the soil masses here are not supported by an external structural element such as a retaining wall; rather, they derive their stability from the resistance of the soil within the mass in shear. Gravitational and seepage forces tend to cause instability in natural slopes, in slopes formed by excavation and in the slopes of embankments. A vertical cutting (or trench, formed of two vertical cuttings) is a special case of sloping ground where the slope angle is 90° to the horizontal. Design of selfsupporting soil systems is based on the requirement to maintain stability (ULS) rather than on the need to minimise deformation (SLS). If deformation were such that the strain in an element of soil exceeded the value corresponding to peak strength, then the strength would fall towards the ultimate value. Thus, it is appropriate to use the critical state strength in analysing stability. However, if a pre-existing slip surface were to be present within the soil, use of the residual strength would be appropriate. Section 12.2 will apply both limit analysis and limit equilibrium techniques to the stability of vertical cuts/trenches. These methods will then be extended to consider how fluid support may be used to improve the stability of such constructions (e.g. drilling of bored piles or excavation of diaphragm wall piles under slurry). In Sections 12.3 and 12.4, the analytical methods will be further extended to the consideration of slope and embankment design, respectively. Finally, in Section 12.5, an introduction to the
design of tunnelling works will be considered, where the stability of a vertical cut face deep below the ground surface governs the design. This final section will also consider how the stability of tunnel headings may be improved by pressurising the cut face (analogous to the use of drilling fluids in trench support).
