There are a number of natural cavities distributed unevenly throughout the English Countryside. Most have been created over vast periods of time by groundwater as it permeates through different types of geology. Permeation usually takes place at the upper levels of soluble rock types (such as limestone, chalk, and rock salt or gypsum bearing rocks) but has been known to occur at greater depths as well.
In salt and gypsum bearing rock water usually wears away the upper levels first, working downwards toward fresh layers. Where rigid surface rocks bend on the crest of slopes (as the result of historic climate conditions) to cause rock cambering, fissures can be created. Marine erosion of coastal areas creates sea-caves, often occurring along joints, bedding planes and fault lines.
The subsidence of the ground above and around a natural cavity could result from two principle causes. The first is where material moves to fill the empty space within the cavity; the second could be attributed to the collapse of surrounding host rock.
Infill is more common than collapse, and subsidence takes place when the material underneath building foundations slips away. Subsidence can be particularly rapid when large voids, filled with nothing but air, suddenly become unstable. Such instability triggers a chain reaction as roof fall progresses upwards toward surface level, and each layer of fill topples into the space vacated by the layers below.
Should mine workings intersect with a natural cavity, infill deposits can flow extremely quickly into the void and create cave-ins of tremendous proportions. This type of cave-in is prone to become even more catastrophic, should the infill continue to pour into the man-made cavities as well.
This isn’t just bad news for any unfortunate miners caught in the maelstrom, but for property on the surface as well. Loose cavity infill that becomes compacted over time could also be a problem, though it takes longer to have an effect, but can be equally devastating in the long run.
Natural cavities are often vulnerable to the very water sources that originally created them. An alteration in the water table can trigger a subsidence event, where groundwater is expended at a higher than normal rate and flowing water percolates through rock layers that were formerly saturated. Another trigger could be surface loading, where human development at surface level causes a previously unknown natural cavity to collapse without warning (though this is a less likely scenario).
From the government Review of instability due to natural underground cavities in Great Britain, we know that the most significant concentration of chalk deposit cavities can be found stretching from Wiltshire to Norfolk and Dorset to Kent.
Cavities in limestone are typically found in mountainous regions, like the Mendip Hills (Somerset), the Peak District (Derbyshire), and the north Pennines (Yorkshire, Cumbria and Durham). Localised cases of natural cavity occurrence have been noted in various other places across the country. In Essex; as an example, the largest threat of subsidence comes from the high concentrations of clay in our soil.**Nothing on this website should be confused with financial or legal advice. If you need this, or any other type of advice, please seek the help of a competent professional. In addition, because real estate laws change all the time and differ from state to state, and even city to city in the same state, everything in these pages should be considered general marketing advice and ideas. Please see link to full Disclaimer at the bottom of this page.