Wednesday, 2 September 2020

Building houses in a worked-out sand & gravel quarry – what could go wrong?

Geologically, the sand and gravel resource underlying Straitgate Farm – a deposit apparently so attractive and profitable that Aggregate Industries has spent a good part of the last decade struggling to make a planning application for the site hold together – is categorised as being from the Chester Formation, formerly known as the Budleigh Salterton Pebble Beds and part of the Sherwood Sandstone Group.

Such deposits were laid down between 250 and 200 million years ago in the Triassic period. According to this Devon Geology Guide – Triassic Pebble Beds, Sandstones and Mudstones:
Budleigh Salterton Pebble Beds (in the Sherwood Sandstone Group) is a very prominent and distinctive gravel (or conglomerate as it should properly be called), with hard and rounded pebbles and cobbles and a substantial proportion of sand deposited by a large river flowing from a distant source. Fossils in the pebbles show that they came from mountains of much older Ordovician age in northern France. The large river continued north into the English Midlands. It ceased flowing into Devon when subsidence created an early version of the English Channel.
The Devon outcrops are closest to the presumed source, in northern France, and comprise brown, horizontally-bedded conglomerate with subordinate lenticular beds of trough cross-bedded pebbly sand and sand. The gravel is composed of well-rounded pebbles, cobbles and boulders in a coarse to fine granulestone and silty sandstone matrix.
The Chester Formation extends northwards "up to the Cumbrian coast on the west side of England, and to the Doncaster area on the east side." In Nottinghamshire, "the formation comprises pinkish red or buff-grey, medium- to coarse-grained, pebbly, cross-bedded, friable sandstone."

It is this sand and gravel deposit in Nottinghamshire – much like the one at Straitgate – that was extracted from Berry Hill Quarry, and it is this deposit that owners of the houses subsequently built in the worked out pit wish they had never heard about.

Last November, a landslide occurred after a period of intense rainfall; 32 homes had to be evacuated:
As a matter of safety, 32 homes were evacuated, and 19 remained evacuated for two weeks. During the two weeks, the council commissioned specialist consultants to deliver a programme of emergency remedial works to clear the 1,300 tonnes of material that slipped, tree removal, the creation of a stone access path at the rear of properties and the installation of blocks and bunds to give temporary protection to properties in Stone Bank and Bank End Close.


Politicians made flying visits and all sorts of promises:

Speaking on a campaign trip to the town, the Prime Minister said victims of the collapse need to receive the "maximum amount" of support from central and local government, and pledged that, if his party wins the election, he will "do everything we can" to make sure properties affected by last week's adverse weather are "restored”.


But it wasn’t surprising that steep sandy cliffs made of "friable" material might collapse when wet. As the BGS put it, in this study of the landslide:
The Sherwood Sandstone Group exhibits a wide range of engineering properties (Yates, 1992) and rockfalls and landsliding are known to occur (Bell et al., 2009). In Nottinghamshire, it is often characterised as weak rock, containing extremely weak members; its strength decreases by 30–40% when saturated (Sattler, 2018).
Last month, a review stated – what should have been obvious to planners at the time – that the Mansfield mudslide site 'should not have been developed until secure'. Mansfield District Council said it will "learn" from the findings. Residents impacted by the quarry collapse, and worried their homes are now "worthless", say they are 'none-the-wiser' following the report.



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