Natural Cavities

England features a number of natural cavities distributed unevenly throughout the countryside. Most have been created over large expanses of time by groundwater permeating through gypsum bearing rocks, limestone, chalk and rock salt. This usually takes place at higher levels of such soluble rock, but can occur at greater depths. In salt and gypsum rocks water wears away the top layers initially, and then works downward towards fresh substances. Rock cambering, whereby slopes of rigid surface rocks bend on the crest of slopes as a result of historic climate conditions, fissures can be produced. Around coastal areas marine erosion (concentrated along fault lines, joints and bedding planes) will likely create sea-caves.

The government conducted a review of instability due to natural underground cavities in Great Britain, to address the possible threat from unidentified natural cavities. The analysis found that the most significant concentration of dissolution cavities in chalk rock deposits can be located stretching between Wiltshire and Norfolk, and from Dorset through to Kent. Cavities in limestone were common in more mountainous areas, like the Mendip Hills in Somerset, the Peak District in Derbyshire, and the north Pennines bridging Yorkshire, Cumbria as well as Durham. More localised areas with natural cavities on a smaller scale can be found dotted around here and there.

Subsidence associated with earth above and around natural cavities will be the consequence of material shifting to fill the space created by the cavity, or perhaps it could be caused by the collapse of surrounding host rock. Infill is more frequent than total collapse, and subsidence could take place as material underneath foundations slips away. More swift circumstances of subsidence take place when extensive voids, full of nothing but air, all of a sudden start to become volatile. Lack of stability results in roof falls which progress upwards right up until they break the surface, with each layer toppling to fill up the space left by the one below it.

Where mine operations intersect natural cavities, infill deposits could be triggered to flow faster to fill the void, creating catastrophic cave-ins. These circumstances can grow to be significantly worse should the infill continue to circulate into the man-made cavities, not just for the miners but for those on the surface too. Compaction of loose cavity infill causes a slower pace of movement, however both varieties can be just as destructive to property and infrastructure at surface levels allowing rise to concerns for public safety.

In the case of natural cavities, the most common subsidence trigger comes in the form of the water which initially created them. A sudden change in the water table, where by groundwater supplies are consumed at a higher rate than normal, may lead flowing water to percolate through rock layers that had formerly been saturated. This means that a change to the levels of either groundwater or surface water movement could set a subsidence event in motion. Surface loading, which occurs when we construct houses over natural cavities, could also spark a subsidence event; though this hardly ever happens.

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