Operative values for f and c for passive pressure

Granular soils

It is generally agreed that, for passive pressures in a granular soil, the operative value of (f> is lower than 4>u the peak triaxial angle obtained from drained tests, particularly for high values of <j>t.

With a granular soil fa is most often estimated from the results of some in situ test such as the standard penetration test. It is suggested therefore that values for fa to be used in the determination of passive pressure values, can be obtained from Fig. 6.31 (which is a modified form of Fig. 3.34). The corrected N' value can be used in place of the direct blow count N.

Normally consolidated clays

As with the active state, the strength of this type of clay is at its weakest when in its undrained state, i.e. during and immediately after construction. For a normally consolidated clay the operative strength parameters are c = cu and 0 = 0°.

Over consolidated clays

With this soil its weakest strength occurs once the soil has reached its drained state. The operative parameters are therefore c = 0 and (¡> = <p', although this is an over-simplification for the case when the level of soil in front of the wall has been reduced by excavation. In this instance there will be a relief of overburden pressure which could result in softening occurring within the soil. When this happens some estimation of the strength reduction of the soil must be made, possibly by shear tests on samples of softened soil.

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