ground subsidence Any downward movement of the ground surface may be referred to as subsidence, and it can occur on a variety of scales. Regions hundreds of kilometres across may subside very slowly through tectonic movements, while a single building may be destroyed by a localized collapse over a natural ground cavity.
Large-scale crustal subsidence is essential to the continued activity of sedimentary basins and the creation of new generations of sedimentary rocks. Thick sequences of shallow water sediments, including the thousands of metres of coal measures, can only have accumulated in subsiding basins. This style of tectonic subsidence can occur either in a tension zone where the Earth's crust is ‘necking’ and thinning, or in the compressive subduction zone of a convergent plate boundary. One of the reasons why London needs its Thames flood barrier is its annual 2 mm of subsidence resulting from the extension and crustal sag of the North Sea basin.
Within sedimentary basins, ground subsidence also occurs through compaction of clay-rich sediments as they are lithified to form denser sedimentary rocks. The combined surface effect of this compaction and the crustal sag can be significant. The Po delta in Italy has a thick sediment sequence which is both sagging and compacting; the combined pro-cess is a minor but unstoppable component of the long-term subsidence of Venice.
Among the common rock materials, clay is unique in that it is compressible; it can be compacted because plastic deformation of its mineral structure allows its high initial porosity to be greatly reduced at modest pressures. Ground subsidence can be caused by clay compaction under the weight of a single building; the Leaning Tower of Pisa is the most conspicuous example. The lean to the south has been caused by differential subsidence as the clay beneath has compacted where its porosity has been reduced by pressure from the tower. It has been stabilized by drilling small-diameter holes beneath the north side of the tower. These artificially increase the clay porosity, and their slow collapse causes compaction and ground subsidence, partially correcting the inclination of the tower.
Clay also compacts when its water content is reduced, because the pore-water pressure is a significant force in keeping the clay grains apart and supporting a superimposed load. Extensive damage to buildings was caused by localized ground subsidence when the near-surface layers of the London Clay were desiccated in the series of dry summers that started in 1976. Subsidence also occurs, on a much larger scale, where groundwater abstraction from wells causes loss of pore-water pressure in clays within buried sediment sequences. Water is pumped only from sand aquifers, but the loss of water pressure affects both the sands and any interbedded clays, and the clays then compact. This is the main cause for increased subsidence and flooding of Venice in the twentieth century and also for the 9 m of ground subsidence in parts of Mexico City. At both sites the subsidence has now almost stopped because groundwater pumping has been greatly reduced by strict legislation.
Peat is the one natural material which compacts more than clay when its water content is reduced. The fenlands of eastern England were originally marshland, but have been turned into rich farmland by efficient drainage. The lowered water tables have, however, caused many metres of ground subsidence on the peat, and large areas now lie below sea level; all land drainage is now pumped up into river channels, which are maintained between high levees.
Limestone regions with karstic landscapes have a reputation for ground subsidence and collapse, related to their underground rivers and caves. Some rocky and steep-sided dolines and potholes have been enlarged by progressive collapse of the limestone and retreat of their exposed rock walls. Cavern collapse is rare and only a few limestone gorges have been formed in this way.
Subsidence sinkholes are diagnostic landforms of karst, which form in unconsolidated soils or drift deposits overlying cavernous limestone. These are created by ground subsidence, where surface soils or sediments are washed down into fissures and caves in the limestone, but they do not involve any failure of the normally strong limestone. The downwashing is normally by infiltrating rainwater, or leakage from a stream, lake, or drainage channel, and this process of sediment movement may be referred to as
suffosion. The thousands of ‘shakeholes’ in the karst of the Yorkshire dales are subsidence sinkholes, formed where the glacial till has been washed into limestone fissures. These are youthful landforms because the till is only about 12 000 years old. Rapid subsidence of new sink-holes can be a hazard in populated karst regions, especially where excessive pumping causes a water table decline and therefore increases suffosion. Florida usually loses two or three houses a year where subsidence sink-holes develop in the thick soils overlying its buried and over-abstracted limestone aquifer; most of these losses occur after rainstorms or where drains have broken.
Ground subsidence may occur on any other rock types in which cavities can form; these include chalk, gypsum, and basalt, but the subsidences are generally very localized. Rock salt is so highly soluble in groundwater that it can cause rapid ground subsidence. The ‘meres’ of the Cheshire plain are all post-glacial lakes in subsidence hollows formed by dissolution of the salt which is at rock head beneath the glacial and glacio-fluvial drift. Modern dissolution is most active where ‘brine streams’ of groundwater flow along the salt rock-head. These create linear subsidences, which generally extend for a few kilometres and can subside at rates of many metres per century; lakes suddenly formed in them are known locally as ‘flashes’.
Mining subsidence can cause serious damage to structures by its local ground movements, but is only rarely on a scale significant in terms of landforms. The inevitable consequence of modern longwall mining is a shallow bowl of ground subsidence over the panel where all the coal has been removed. Many lakes have been formed in the English coalfields where valley floors have been given a locally reversed gradient out of a subsidence bowl. Block caving is another modern method of total extraction mining, and this causes inevitable surface collapse on a grand scale. The ‘Big Holes’ left on the South African diamond pipes are hundreds of metres deep, and their floors continue to subside as the rock is dug away from below. Older methods of mining were mainly by partial extraction, leaving rock pillars for roof support; these decay over time, and frequently cause destructive ground subsidences many years after the mines have closed.
Mining is only the most obvious of the ground subsidence processes induced by man. Peat and clay subsidences are greatest where groundwater is pumped artificially, most new collapses of subsidence sinkholes are caused by drain breaks, and salt subsidences are greatly accelerated by brine pumping. Tectonic subsidence and some of the movements on clay are wholly natural, but society causes most of its own problems in ground subsidence.
Tony Waltham
Bibliography
Waltham, A. C. (1989) Ground subsidence. Blackie, Glasgow.
