Subsidence map development in an area of abandoned salt mines

Abstract

Purpose of the study Morphological mapping is not generally associated with mining and subsidence, having traditionally been used to define the nature and extent of surface features such as landslides and landforms. However, detailed morphological mapping, linked to subsurface investigation, can provide a preliminary indication of potential subsidence hazards in some areas of abandoned mine workings. This example describes how mapping was carried out in an undeveloped part of the Cheshire saltfleld, prior to any subsurface investigation, with the aim of establishing the likely extent to which a number of mines had collapsed. The site The site is in the Northwich area of Cheshire, England, and has had a long history of rock salt mining and brine pumping. The site is largely undeveloped, although locally important infrastructure routes cross the area, and it has potential for use as a community woodland and open space area. At the site, rock salt (halite) occurs in two discrete horizons within the Triassic Mercia Mudstone sequence, separated by around 10 m of marls: the 20–30 m thick Top Bed (at c . 40 m b.g.l.) and the 23–28 m thick Bottom Bed (at c . 75 m b.g.l.). This sequence is overlain by glacial till. Salt was extensively mined at the site from around 1777 to 1933, with the one Top Bed and seven Bottom Bed mines opened over this period, varying in size from 0.8 to 13 ha.