Tunnels in London Clay
The London Clay is an ideal medium for driving tunnels, which is why the London Tube railway network expanded quickly North of the Thames, but South of the Thames the London Clay is much deeper, and the stratum is water-bearing sand and gravel (not good for tunnelling) at tube level, which is why there are few tube tunnels there. London Clay has a stand-up time long enough to enable support to be installed without urgency. It is also almost waterproof, resulting in virtually no seepage of ground-water into the tunnel. It is over-consolidated, which means that it is under pressure, and expands upon excavation, thus gradually loading the support, i.e. it is not necessary to stress the support against the ground. Sometimes the London Clay is prettily coloured, and is called shepherd's plaid, but generally the colours disappear when the clay is fired.
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