A sinkhole is classed as a collapse of ground over a naturally formed void at depth. They typically occur when the ground below the surface has been dissolved and are, therefore, usually found in areas underlain by chalk, limestone, gypsum and salt. They can also occur as a result of ground collapse over man-made voids in the ground, such as mine workings or historic quarries.
Having carried out a Homecheck Professional Ground Stability report on Fontmell Close in St Albans where the latest collapse happened, it has identified Natural or Man-Made Cavities within the 250m search area, plus includes features on Ordnance Survey’s historical mapping that may indicate potential ground stability risks. Historical mapping also identifies that the area was historically used as a brick field and clay pit, with extensive local clay brick and tile manufacturing.
Geohazards expert Dr Clive Edmonds of leading development and infrastructure consultancy, Peter Brett Associates LLP, said: “At old brickworks sites dating from the 1800s it is often the case that chalk has also been mined at depth below the clay – the bricks and tiles are made of 75% clay and 25% chalk – so collapse of man-made voids at depth can cause large collapses (crown holes) to occur usually triggered by water causing the voids at depth to break down and migrate upwards to the surface. The collapse at St Albans is probably a crown hole like many other similar cases that we have dealt with.”
Rob Phillipson, Sales & Product Director at Landmark Information Group said: “In this particular instance, the results from the HomeCheck Professional Ground Stability Report identify a number of risks, based on the historical use of the land at the address in St Albans. Given the potential impact on a property, ground instability risk is clearly something for which conveyancing solicitors should be checking as part of their due diligence process. At Landmark, our residential and commercial reports contain data from multiple sources to provide risk screening and next steps.”
