Thursday 24 December 2020

Homes evacuated following mudslide in Mansfield, England.

Several families have been forced to evacuate their homes following a mudslide at a housing development in Mansfield in Nottinghamshire on at about 11.40 pm local time on Wednesday 23 December 2020. The incident happened at Bank End Close in the Berry Hill Quarry in the south of the town following several days of heavy rainfall in the area. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall. A similar incident in November 2019 resulted in 35 households being forced to evacuate, with 19 unable to return home for more than two weeks.

Mudslide behind houses in Berry Hill Quarry, Mansfield, this week. Nottingham Post.

Residents of the housing development claim that Mansfield District Council had failed to respond to concerns raised about the stability of the cliff face from 2017 onwards. Photographs of the area do appear to show a cliff-face comprised largely of unconsolidated sediments close to the rear of the houses, something which was apparently not picked up by a planning process that began with an application for planning permission in 1999 and ended with houses being built in 2011.

An independent enquiry carried out by an experienced former public sector solicitor, Jayne Francis-Ward, found that that stabilisation works or detailed reports on the stability of the cliff face should have been sought earlier in the development process, and that the council should improve its due diligence procedures when considering future planning applications. Mansfield District Council has since carried out remedial work intended to stabilise the cliff, which was completed in December 2019, although this now appears to have been ineffective.

A separate investigation into the November 2019 landslide by the British Geological Survey found that this was probably caused by a block fall in a block fall in the upper part of the slope followed by an earth/debris flow that, in turn, caused further failure in the underlying, weathered Chester Formation. It is also possible that the landslide was initiated by failure in the superficial deposits, which may have been overhanging the underlying bedrock. It ran-out approximately 17 m from the cliff, bringing trees and other vegetation down with it. The debris covered nearly all of the rear gardens of three properties.

Geological cross-section through the landslide after the debris had been cleared away. The surface from the stone wall through the overburden shows some noise caused by vegetation in this area. The depth of the overburden and glaciofluvial deposits are inferred from a LiDAR scan and what was observed visually from the ground. Borehole evidence is required to confirm this. BGS/UKRI.

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