Subsidence
Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement,[1][2] which distinguishes it from slope movement.[3]
Not to be confused with Atmospheric subsidence.
Processes that lead to subsidence include dissolution of underlying carbonate rock by groundwater; gradual compaction of sediments; withdrawal of fluid lava from beneath a solidified crust of rock; mining; pumping of subsurface fluids, such as groundwater or petroleum; or warping of the Earth's crust by tectonic forces. Subsidence resulting from tectonic deformation of the crust is known as tectonic subsidence[1] and can create accommodation for sediments to accumulate and eventually lithify into sedimentary rock.[2]
Ground subsidence is of global concern to geologists, geotechnical engineers, surveyors, engineers, urban planners, landowners, and the public in general.[4] Pumping of groundwater or petroleum has led to subsidence of as much as 9 meters (30 ft) in many locations around the world and incurring costs measured in hundreds of millions of US dollars.[5] Land subsidence caused by groundwater withdrawal will likely increase in occurrence and related damages, primarily due to global population and economic growth, which will continue to drive higher groundwater demand[6].
Causes[edit]
Dissolution of limestone[edit]
Subsidence frequently causes major problems in karst terrains, where dissolution of limestone by fluid flow in the subsurface creates voids (i.e., caves). If the roof of a void becomes too weak, it can collapse and the overlying rock and earth will fall into the space, causing subsidence at the surface. This type of subsidence can cause sinkholes which can be many hundreds of meters deep.[7]
Mining[edit]
Several types of sub-surface mining, and specifically methods which intentionally cause the extracted void to collapse (such as pillar extraction, longwall mining and any metalliferous mining method which uses "caving" such as "block caving" or "sub-level caving") will result in surface subsidence. Mining-induced subsidence is relatively predictable in its magnitude, manifestation and extent, except where a sudden pillar or near-surface tunnel collapse occurs (usually very old workings[8]). Mining-induced subsidence is nearly always very localized to the surface above the mined area, plus a margin around the outside.[9] The vertical magnitude of the subsidence itself typically does not cause problems, except in the case of drainage (including natural drainage)–rather, it is the associated surface compressive and tensile strains, curvature, tilts and horizontal displacement that are the cause of the worst damage to the natural environment, buildings and infrastructure.[10]
Where mining activity is planned, mining-induced subsidence can be successfully managed if there is co-operation from all of the stakeholders. This is accomplished through a combination of careful mine planning, the taking of preventive measures, and the carrying out of repairs post-mining.[11]
Land subsidence causes vertical displacements (subsidence or uplift). Although horizontal displacements also occur, they are generally less significant. The following are field methods used to measure vertical and horizontal displacements in subsiding areas:
Tomás et al.[45] conducted a comparative analysis of various land subsidence monitoring techniques. The results indicated that InSAR offered the highest coverage, lowest annual cost per point of information and the highest point density. Additionally, they found that, aside from continuous acquisition systems typically installed in areas with rapid subsidence, InSAR had the highest measurement frequencies. In contrast, leveling, non-permanent GNSS, and non-permanent extensometers generally provided only one or two measurements per year[45].