Civil Engineering Reference
In-Depth Information
which case they will be total stress. In some instances it may be necessary to consider
both cases.
18.5 Ultimate limit state: critical state and residual
strengths and factors of safety
The first fundamental requirement of engineering design is to examine the ultimate
limit state and to demonstrate that there is an adequate margin of safety against failure.
These analyses do not consider movements.
Figure 18.2 illustrates the typical behaviour of soil in a drained shear test and it is
similar to Figs. 9.1 and 9.3 in Chapter 9. The strength of the soil is the shear stress
mobilized under different conditions. There is a peak strength at relatively small strains
of the order of 1% where there are volumetric strains. There is a critical state strength
at strains of the order of 10%where the soil continues to distort at constant stress and
constant volume. For clay soils there is a residual strength at large displacement. For
soils which do not contain significant quantities of clay the residual strength is the same
as the critical state strength. Figure 18.2 illustrates typical soil behaviour in drained
tests. There will be similar relationships between shear stress, pore pressure and strain
Figure 18.2
Behaviour of soils during shearing.
