Shear Strength of Soils - Elements of Soil Mechanics - page 122

Fig. 4.32 Strength envelope for an overconsolidated soil subjected to a drained shear test.
Fig. 4.33 Typical results from consolidated undrained shear tests on saturated clays.
An overconsolidated clay is considerably stronger at a given pressure than it would be if normally con-
solidated, and also tends to dilate during shear whereas a normally consolidated clay will consolidate.
Hence, when an overconsolidated clay is sheared under undrained conditions negative pore water pres-
sures are induced, the effective stress is increased, and the undrained strength is much higher than the
drained strength - the exact opposite to a normally consolidated clay.
If an excavation is made through overconsolidated clay the negative pressures set up give an extremely
high undrained strength, but these pore pressures gradually dissipate and the strength falls by as much
as 60 or 80% to the drained strength. A well-known example of overconsolidated clay is London clay,
which when first cut, will stand virtually unsupported to a height of 7.5 m. It does not remain stable for
long, and so great is the loss in strength that there have been cases of retaining walls built to support it
being pushed over.
Several case histories of retaining wall failures of this type are given in Clayton ( 1993) .
4.13 Operative strengths of soils
For the solution of most soil mechanics problems, the peak strength parameters can be used, i.e. the
values corresponding to maximum deviator stress. The actual soil strength that applies in situ is depend-