Subsidence and Sinkholes

Subsidence is movement in the ground which causes a property to move or sink downwards causing a shift in the foundations of a property.

This usually happens when a home is built on clay soil, when the water table drops due to a long, dry spell or when water is sucked out of the soil by trees and bushes. The clay starts to shrink and it pulls at the foundations of buildings causing structural damage. It also occurs if mining has previously taken place in the surrounding areas.

A sinkhole is usually formed by erosion, caused by the frequent exposure to water. There are two basic types of sinkhole; a cover-subsidence sinkhole which is created slowly, over time and a cover-collapse sinkhole which will appear suddenly.

Both are usually the result of what is known as the Karst process where acidic water, which is created by rainfall seeping through the soil, absorbing carbon dioxide and reacting with decaying vegetation, causes the erosion and dissolving of the soluble rock layers such as limestone or gypsum, beneath the surface – eventually creating a cavity below the surface.

The soil or sand over the limestone collapses into a sinkhole when it can no longer support the weight above because of the void below. This final collapse of the surface might take anything from a few minutes to several hours.

In March this year residents and business were evacuated from an estate in Northfleet, Kent following a hole which appeared in the ground. There were worries of the surrounding buildings subsiding so Lawrence Square was fenced off, people were evacuated and services were shut off. Businesses were shut down as the site was deemed too dangerous for people to enter

In April Coal Authority experts were called to a sink hole that appeared in a road in Gateshead. After investigations the Coal Authority found that the 2ft wide, 5ft deep hole was caused by historical mine works. Police and council officers had to be called to make the area safe.

In Britain, the BGS (British Geological Survey) says the carboniferous limestone of the Mendip Hills, the north of the South Wales coalfield, the Peak District, the Yorkshire Dales, the northern Pennines and the edges of the Lake District all host well-developed karst landscapes. Karstic features are also common in the UK on the chalk of south-east England, on salt in the centre and north-east of the country, and particularly on the gypsum that underlies parts of eastern and north-eastern England, especially around Ripon and Darlington, and in the Vale of Eden.

Tin Mining and China Clay Mining was predominant in Cornwall, Dorset and West Devon in the past.

Our mining report will provide your client with the confidence of knowing whether a property has been subject to any mining related issues and whether the property is likely to be affected by subsidence.

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