ENGINEERING GEOLOGY/Problematic Rocks 551 Figure (A) A sinkhole developed as a result of groundwater lowering at a miner’s recreation centre at Venterspoort, South Africa (B) Mechanisms of ravelling may be capable of bridging a void developed in an underlying limestone As the void is enlarged, the clay cover ravels until it eventually fails to form a dropout sinkhole (Figure 9B) Ravelling (breaking away) failures are the most widespread and probably the most dangerous of all the subsidence phenomena associated with karstic carbonate rocks Rapid changes in moisture content lead to aggravated slabbing in clays In particular, lowering of the water table increases the downward seepage gradient and accelerates downward erosion It also reduces capillary attraction and increases the instability of flow through narrow openings, and gives rise to shrinkage cracks in highly plastic clays, which weaken the mass in dry weather and produce concentrated seepage during rains Increased infiltration often initiates failure, especially when it follows a period when the water table has been lowered Suffusion sinkholes develop in noncohesive soils where percolating water washes the soil into a cavity in the underlying limestone Buried sinkholes occur in limestone rockhead and are filled or buried with sediment They may be developed by subsurface solution or as normal subaerial sinkholes that are later filled with sediment Unfortunately, the appearance of sinkholes at the surface is commonly influenced by human activity, especially groundwater withdrawal or an increased flow of water into the ground from a point source As an example, more than 4000 sinkholes have been catalogued in Alabama as being caused by human activities, with the great majority developing since 1950 The largest is called the ‘December Giant’ because it developed suddenly in December 1972 It measures 102 m in diameter and is 26 m deep Sinkholes are particularly dangerous when they form instantaneously by collapse, and they often occur in significant numbers within a short time-span They have resulted in costly damage to a variety of structures and are a major local source of groundwater pollution Areas underlain by highly cavernous carbonate rocks possess the most sinkholes; hence, sinkhole density has proven to be a useful indicator of potential subsidence Solution voids preferentially develop along zones of high secondary permeability because these concentrate groundwater flow Data on fracture orientation and density, fracture intersection density, and the total length of fractures have been used to model the presence of solution cavities in limestone Accordingly, the locations of areas at high risk of cavity collapse have been estimated from the intersections of lineaments formed by fracture traces and lineated depressions Aerial photographs have proved particularly useful in this context Subsidence-susceptibility maps can be developed using a geographical information system that incorporates the relevant data in a spatial context The chalk in southern England exhibits a wide range of dry density values, ranging from as low as 1.25 Mg m up to 2.5 Mg m Generally, chalk in northern England (i.e Yorkshire) is denser than that in the south-east (Kent) The average porosity of chalk ranges between about 25% and 40% The dry unconfined compressive strength of chalk varies from moderately weak to moderately strong; the chalk in Yorkshire tends to be appreciably stronger than that in Kent When saturated, chalk undergoes a notable reduction in compressive strength, frequently by over 50% The deformation properties of chalk in situ depend on its strength and on the spacing, tightness, and orientation of its discontinuities In addition, the values of Young’s modulus are influenced by the amount of weathering that the chalk has undergone Six grades of weathering have been recognized, varying from completely unweathered material to a structureless me´ lange consisting of unweathered and partially weathered fragments of chalk in a matrix of deeply weathered chalk Fresh chalk has a low compressibility and compresses elastically up to a critical pressure, which has been termed the apparent preconsolidation pressure (the yield stress) Marked breakdown and substantial consolidation occur at