Civil Engineering Reference
In-Depth Information
Figure 4.4
Stages of erosion and deposition during changing sea levels.
As the ground is eroded the soils become overconsolidated (see Chapters 16 and 18)
due to unloading, but they do not recover their original state. Overconsolidated soils
have stiffnesses and strengths which are more or less uniform with depth and which
are larger than those of normally consolidated deposits at the same depth.
Figure 4.4 illustrates a sequence of falling and rising sea levels; this is a highly
simplified model of the recent geology of the Thames estuary north east of London.
In Fig. 4.4(a) London Clay is deposited in a shallow sea; notice an element at A just
above the Chalk. Figure 4.4(b) shows a glaciation with a very low sea level and a
nearby glacier. Much of the London Clay has by now been eroded so the element A
is nearer the surface. Meltwater from the glacier has eroded a river channel which has
been partly filled with outwash gravels. Figure 4.4(c) shows the present day; rising
sea levels have led to deposition of soft soils in the valley. The soil at B is about
the same depth as that at A, but it is normally consolidated and so is relatively soft
and weak.
