Lateral Earth Pressure - Elements of Soil Mechanics - page 218

the silt must be regarded as a c- φ soil. The undrained strength parameter c u should be used for the evalu-
ation of active earth pressures which will be applicable to the period of during and immediately after
construction. The final active earth pressure to which the wall will be subjected can be determined from
an effective stress analysis using the parameters φ ′ and c ′ .
Rain water in tension cracks
If tension cracks develop within a retained soil and if the surface of the soil is not rendered impervious,
then rain water can penetrate into them. If the cracks become full of water we can consider that we have
a hydrostatic, triangular distribution of water pressure acting on the back of the wall over the depth of
the cracks, z 0 . The value of this pressure will vary from zero at the top of the wall to 9.81 × z 0 kPa at the
base of the cracks. This water pressure should be allowed for in design calculations, see Section 7.5.1 and
Example 7.4.
The ingress of water, if prolonged, can lead eventually to softening and swelling of the soil. Swelling
could partially close the cracks but would then cause swelling pressures that could act on the back of the
wall. The prediction of values of lateral pressure due to soil swelling is quite difficult.
Shrinkage cracks may also occur and, in the UK, can extend downwards to depths of about 1.5 m below
the surface of the soil. If water can penetrate these shrinkage cracks then the resulting water pressures
should be allowed for as for tension cracks.
7.12.2 Passive earth conditions
Granular soils
It is generally agreed that, for passive pressures in a granular soil, the operative value of φ is lower than
φ t , the peak triaxial angle obtained from drained tests, particularly for high values of φ t .
With a granular soil φ t is most often estimated from the results of some in situ test such as the standard
penetration test (see Chapter 6 ). It is suggested therefore that values for φ , to be used in the determina-
tion of passive pressure values, can be obtained from Fig. 7.31 (which is a modified form of Fig. 4.34) .
The corrected N value can be used in place of the direct blow count N.
Normally consolidated clays
As with the active state, 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 parameter is c u .
Fig. 7.31 Relationship between N and φ .