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.,, .~ i • III IIIIIIIIIIIIIIII 999 The Problem of Subsidence in Salt-Producing Areas Dtnc. IVa,,,,•re~; Db~ eCto~'S EXGt~ b~ f. G.J. Lewis BP Chemicals (U. K.I Limited Murgatroyd's Works E/worrh, SandbaCh Cheshire, England ABSTRACT It'hcrt salt is removed from shallow bedded de- posits of geologically recent origin, collapse of the overburden frequently occurs, leading to surface subsidence at sites remote from, mtd not clearly relatable to the point of extraction. Property dam- ngc arising front steels subsidence poses legal artd physical problems which are dealt with dtffer- erttty in the various areas concerned. A review of European practices is presented, tuitlt particularly detailed reference to those of the Ll~tited liiltgdom in w/ticlt matters of compensation altd restitution hove, for many years, been covered by Statutes. When minerals are removed from the Earth's crust, collapse of [he overlying strata and subsid- ence of the surface is alwa}•s liable to occur- particularly when the mineral is removed from shallow depths or when the over-burden is strucutr- ally weak. Both conditions are characteristic of many salt- producing areas. Bedded salt formations are fre- quently covered only by Keuper Marl and drift. This fact, combined with the readily soluble nature of the rock, makes the problem of subsidence dtte to salt extraction different Crom that caused b}• the winning of other minerals. \1'here the over-1}'ing marl has been cut through by glacial erosion and the valleys thus formed have been filled with drift, surface +vater and rainfall penetrate to the salt which is then dissolved as the water (lows done the contours, ++ith consequent collapse of weak over-lying strata. This effect is particularly marked where pre-glacial folding of the bedded salt, fol- lo++~ed by erosion, led t~ out-cropping of the salt. Such conditions are found in the I\'orth-West of England and in the Dombasle region of France, which is illustrated in Figure 1. ~= _~ -_ ~__ -- - - - Figure 1. When a mineral is extracted in solid form by conventional mining methods, the position and ex- tent of the cavity are known ++ith certainty, and any surface subsidence can be directly related to the underground workings. Cases are known in +vhich salt mining has led to clearly relatable sub- sidence of the surface (Figs. 2 and 3). These figures show the e[fects of sudden, catastrophic collapse of the roof of a salt mine in \orthwich, Cheshire, Eollow•ing the dissolution of supporting pillars by +cater which entered abandoned mine-workings. Similar phenomena in open country led to the creation of lakes +vhere none had previously ex- isted. When salt is extracted as brine, it is not kno++n with certainty from whence the salt was dissolved. The surface evidence of solution often appears at points remote from the place from which the brine 293