When water supplies are abstracted from alluvial sands, interbedded clays compact and cause ground subsidence. Entire cities are being gently lowered, with serious consequences for those on low coastal sites.

19The engineering classification of karst defines various complexities of ground conditions, in terms of the hazards that they provide to potential construction. Karst is divided into five classes (from immature to extreme). The three key parameters within the classification are caves (size and extent), sinkholes (abundance and collapse frequency) and rockhead (profile and relief). As one component of karst, caves are a hazard to foundation integrity, though natural surface collapses over caves are extremely rare. A cave roof is normally stable under engineering loading where the roof thickness is greater than 70% of the cave width. Construction can proceed over or around caves that are known. The main difficulty is finding unseen voids; ground investigation in mature karst may require extensive borehole probing, and microgravity is the most useful geophysical technique.

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With few exceptions, the ground collapses that constitute the karst geohazard in engineering activity in limestone terrains are induced by human activity. Subsidence sinkholes, formed entirely within the soil profile, constitute the most widespread karst geohazard, but are largely induced by engineered works, either directly or accidentally. Water table decline (as a result of pumped abstraction or quarry de-watering) and uncontrolled surface drainage input are the two key factors that induce subsidence sinkholes, especially where both are involved. Collapse sinkholes, formed by failure of bedrock over a cavity, are rare in natural karst landscapes, but may be induced by excessive loading imposed on limestone that lies above an open cave; the risks associated with this geohazard should be eliminated by implementation of an appropriate site investigation that includes proof drilling. Case studies to demonstrate the karst geohazard concern: (1) homes damaged by new sinkholes around a de-watered limestone quarry in Pennsylvania; (2) problems with sinkholes for a railway across dolomite karst in South Africa; (3) collapse of ground in pseudokarst in Guatemala; (4) failure of a viaduct pier into an unseen cave in Florida; (5) estimation of potential sizes of collapse sinkholes along a pipeline route over gypsum karst in Turkey. As hazardous new sinkholes in karst are almost entirely induced by either uncontrolled drainage or excessive loading, they should be largely eliminated by appropriate engineering design and works. These need to be based on a proper understanding of karst ground conditions.

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