The Nottingham caves have been artificially excavated in the Triassic sandstone. Some of their unsupported roofs, spanning up to 5 m, exhibit progressive collapse which can develop into crown holes. Nearly all recorded roof failures have occurred where and when the rock has become saturated by water from rainfall or pipe leakages. Saturation causes the sandstone to lose rapidly about half its strength as a consequence of the breakdown of clay bridges between the sand grains.

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