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00:00In every hole that they've dug on this particular site, and let's just separate the issues here from that of Galley Hill because they are related but slightly different.
00:09On this particular site they have dug down several meters and not found any chalk.
00:15So the problems are not related to the chalk, the problems are related to the ballast material that has been laid on top of the chalk to build up the road surface.
00:26Now quite what the history of that is I don't know, whether it goes back to when the chalk pits were originally built, whether it goes back to the railway cutting,
00:34but it appears a whole load of other material dug out from elsewhere has been laid on top of the chalk and forms the foundation of the road surface here.
00:43Now unfortunately that ballast material doesn't meet modern specifications of different particle sizes, sharp stones, interlocking and everything else.
00:51So what is happening is when surface water, or water from cracked drains, or the inevitable water from Leaky Thames water pipes, gets underground,
01:02it washes out the sand and other fines from within the ballast material, just leaving the larger material and the rocks, and that's what leaves the voids which then eventually collapse.