Environmental Engineering Reference
In-Depth Information
Figure 11.7.
Selection of foundation strength - Normal case. Notes: (1) Discard, probably disturbed.
(2) Discard, probably affected by shells, roots etc. (Ferkh and Fell, 1994).
Figure 11.8.
Selection of foundation strength - influenced by shells, roots etc. and large scatter of data
(Ferkh and Fell, 1994).
and 11.8 show typical soft ground strength profiles with an overconsolidated crust over the
near normally consolidated soil.
In these circumstances it is recommended that where the data plots as in Figure 11.7 the
median strength be used after discarding outlying results but, where there is considerable
scatter as in Figure 11.8, a more conservative e.g. lower quartile strength be adopted.
In the overconsolidated crust Ferkh and Fell (1994) adopted an empirical approach, using
the median strength line - which uses a strength equal to the median strength up to where the
liquidity index - (water content - plasticity index) / (liquid limit - plastic limit) - is less than
1.0. These guidelines were developed for soft clay deposits, which are not usually found
where dams are being built, but the principles are applicable. Rather than use the empirical
approach detailed above it would be better to assess the fissured strength for the overconsol-
idated crust, as detailed in Section 6.1.8.
For embankments:
(a) The undrained strength varies within and between the layers of the compacted fill, reflect-
ing varying degrees of softening as the soil wets up with seepage from the reservoir.
 
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