Bpv

Normally consolidated

Overconsolidated

Fig. 3.32 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 consolidated, and also tends to dilate during shear whereas a normally consolidated clay will consolidate. Hence when an over-consolidated clay is sheared under undrained conditions negative pore water pressures 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 per cent 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 7.5m. 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).

3.14 Variation of the pore pressure coefficient A

An important effect of overconsolidation is its effect on the pore pressure parameter A. With a normally consolidated clay the value of A at maximum deviator stress, Af, is virtually the same in a consolidated undrained test no matter what cell pressure is used, but with an overconsolidated clay the value of Af falls off rapidly with increasing overconsolidation ratio

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