ENGINEERING GEOLOGY/Problematic Rocks 553 limestone Indeed, in the USA, such features have been known to form within a few years, for instance where beds of gypsum are located beneath a dam Extensive surface cracking and subsidence may be associated with the collapse of cavernous gypsum The problem is accentuated by the fact that gypsum is weaker than limestone and therefore collapses more readily Where beds of gypsum approach the surface, their presence is often indicated by collapse sinkholes Such sinkholes can take only a matter of minutes to appear at the surface However, where gypsum is effectively sealed from the ingress of water by overlying impermeable strata, such as mudstone, dissolution does not occur Massive deposits of gypsum are usually less dangerous than those of anhydrite, because gypsum tends to dissolve steadily, forming caverns or causing progressive settlement In fact, the solution of massive gypsum is not likely to give rise to an accelerating deterioration beneath a foundation if precautions, such as grouting, are taken to keep seepage velocities low On the other hand, massive anhydrite can be dissolved, leading to runaway situations in which seepage flow rates increase in a rapidly accelerating manner Even small fissures in massive anhydrite can prove to be dangerous If gypsum or anhydrite occur in particulate form in the ground, their subsequent removal by dissolution can give rise to significant settlement In such situations, the width of the solution zone and its rate of progress are obviously important as far as the location of hydraulic structures is concerned Anhydrite is less likely than gypsum to undergo catastrophic solution in a fragmented or particulate form Another point that should be borne in mind, and this particularly applies to conglomerates or breccias cemented with such soluble material, is that when this material is removed by solution the rock is greatly reduced in strength In addition, when anhydrite comes into contact with water it may become hydrated to form gypsum In so doing, there is a volume increase of between 30% and 58%, which exerts swelling pressures that are commonly between MPa and MPa and on rare occasions exceed 12 MPa No great length of time is required to bring about such hydration Rock salt is even more soluble than gypsum, and the evidence of slumping, brecciation, and collapse structures in rocks that overlie saliferous strata bear witness to the fact that rock salt has gone into solution in past geological times It is generally believed, however, that, in humid and semi-arid areas underlain by saliferous beds, measurable surface subsidence is unlikely to occur, except where salt is being extracted Perhaps this is because an equilibrium has been attained between the supply of unsaturated groundwater and the salt available for solution Exceptional cases of rapid subsidence have been recorded, such as the Meade salt sink in Kansas Karstic features, particularly sinkholes, may develop in salt formations in arid areas Salt extraction by some types of solution mining can give rise to serious or even catastrophic subsidence of the ground surface Organic Rocks: Coal Coal is an organic deposit composed of different types of macerated plant tissue, which occurs in association with other sedimentary rocks such as shales, mudstones, and sandstones Many coal seams have a composite character At the bottom the coal is frequently softer, with bright coal in the centre and dull coal predominating in the upper part of the seam, reflecting changes in the type of plant material that accumulated and in the drainage conditions Coal seams may be split, wholly or partially, by washouts Coal usually breaks into small blocks that have three pairs of faces approximately parallel to each other These surfaces are referred to as cleat The cleat direction is fairly constant and is best developed in bright coal Cleat may be coated with films of mineral matter, commonly calcite, ankerite, and iron pyrite The breakdown of iron pyrite frequently gives rise to acid mine drainage, which is associated with coal mining activity and has caused problems in mining areas around the world The heavy metals in, and the acidity and sulphate and iron contents of, acid mine drainage may pollute groundwater and surface water, and contaminate soils and sediments The unconfined compressive strength of coal varies, but generally it is less then 20 MPa Exceptionally, the unconfined compressive strengths of some coals, such as the Barnsley Hard Coal, may exceed 50 MPa and their Young’s moduli may be greater that 25 GPa Consequently, the presence of coal seams in foundations does not usually present a problem However, when coal has been mined at shallow depth, the presence of abandoned bell-pit or pillarand-stall workings frequently cause foundation problems Longwall mining of coal, which involves the total extraction of panels of coal, especially when it occurs at shallow depth, can give rise to notable subsidence at the ground surface, which, in turn, may damage buildings and structures or lead to flooding of agricultural land Another problem associated with coal is that it can spontaneously combust on exposure to air This can be regarded as an atmospheric oxidation process in which self-heating occurs (i.e an exothermic reaction emitting between kCal and 10 kCal per