1. Introduction
Subsidence is displacement of ground surface vertically over a broad region or at localized areas. It may be either a gradual lowering or a collapse. This can have costly effect on facilities and structures over a subsiding area.
Subsidence results from a number of different mechanisms. It can occur as a consequence of natural processes. The dissolving of limestone, salt, or other soluble materials creates underground openings that may collapse. Collapse may also occur in the roofs of lava tubes in areas underlain by volcanic rock. Withdrawal of fluids from subsurface reservoirs can create human-induced subsidence. This type if subsidence has resulted from extracting oil, gas and ground-water. Underground mining is another mechanism for creating subsidence by creating subsurface openings.
Natural solution of rock leading to collapse of the overlaying surface can be a rather spectacular form of subsidence. A single sinkhole 324-ft wide and 100-ft deep was formed by collapse on May 8-9, 1981, in Winter Park, Florida. It destroyed a house, several cars, streets, parts of neighboring buildings, and the city swimming pool, causing losses estimated to exceed $2 million.
Groundwater withdrawal for surface uses or dewatering of quarries and mines causes a general lowering of the water table. Construction activities are a less-common cause of collapse. Subsidence because of construction can result from loading the ground surface over a cavity or from the diversion of surface water, thus changing the groundwater system and increasing sinkhole development.
Subsidence caused by underground mining results in severe economic losses in some areas. It is estimated that damage amounting to $30 million annually results from subsidence over abandoned coal mines. Underground mining notably coal mining, creates subsurface openings. Rock layers bridging these voids may fracture and collapse into the opening, with resultant lowering of the ground surface. Subsidence causes differential settlement, with the greatest amount near the center of the opening. Associated with this differential settlement are ground cracks. The extent and size of cracks will change until subsidence is complete in an area. Withdrawal of oil, gas and water has produced subsidence that has resulted in extensive losses in Arizona, California, and along the gulf coast of Texas.
Subsidence causes damage in several ways. The most obvious causes are in tilting, cracking, and shearing of structures where subsidence produces differential settlement. Large-scale collapse can completely destroyed some structures. Destruction results when water-containment structures such as reservoirs and canals are breached. Subsidence causes damage impairing the function of some surface facilities. It can create low points in pipelines and alter the alignment of microwave transmission stations. A more-subtle consequence of subsidence is ground lowering that makes more land subject to flooding.
2. Evaluating Subsidence Processes
It should be clear from our discussion that evaluation of subsidence processes depends on a variety of methods. Evaluating possible underground openings will require a very different approach than estimating subsidence from fluid withdrawal.
The potential of solution-caused subsidence depends on the presence of limestone or other soluble rock type. Examining the natural subsidence occurring where these rocks are present serves as an initial indicator. For example, a sinkhole-density map compiled by means of aerial photography is useful measure of relative collapse potential. Similarly, the potential for subsidence from underground mining exists only where mining is active or was conducted in the past. Historical records or the details surface surveys normally conducted by many present day mines provide a basis for evaluating subsidence potential. The extent of subsidence depends on factors such as: 1) thickness of mined coal, 2) mine geometry and mining methods and 3) thickness, lithology, structure, and hydrology of bedrock and surficial material in the mining area.
Detecting solution cavities or abandoned mine openings is mostly reliably done with the drilling. However, expense limits such drilling over large areas. Earth-resistivity surveys can detect openings at depths up to 25m. Gravity surveys are marginally successful; only large openings near the surface are detectable. Subsurface radar techniques proved too unreliable for practical use.
There are two field-based techniques for subsidence prediction where fluid is being withdrawn. One is the depth porosity method, and other is aquitard-drainage method. Estimating subsidence in an area where fluid withdrawal is being initiated is best done with the depth-porosity method. For the more complicated situation in which subsidence is already active, the aquitard-drainage method is recommended. Ground failures in areas subsiding owing to fluid withdrawal can be predicted. These ground failure range from tension cracks to surface faults. Prediction of ground failure in areas where deformation is underway requires monitoring of surface conditions for sign of failure. In areas not yet subject to subsidence, prediction requires determining the particular surface conditions conducive to failure.
3. Mitigating the Effects of Subsidence Processes
Controlling land use to avoid large-scale changes in the regional water table is one way to avoid subsidence in areas underlain by soluble rock. Avoiding withdrawal overdraft from compressible groundwater aquifers is equally effective in avoiding subsidence. Reservoirs from which oil, gas, or geothermal fluids is being withdrawn can be re-injected with water to compensate for the lost fluids. In some instances, subsidence over mines need not result in structural damage. This requires knowing the specific factors that influence subsidence at that locality and conducting mining in a manner that permits a general lowering of the entire area in which a structure is situated. This minimizes the differential settlement responsible for most of the distress to structures.
Structures damaged or impaired by subsidence can be restored in many cases, this involves sealing the cleaned-out sinkhole, restoring the ground surface, ensuring that surface water to site is minimized, and promoting groundwater flow down gradient from the repair location. Some problems with solution-related subsidence are human related. This is especially true where natural sinkhole is used to drainage. Diverted water can increase the groundwater gradient in areas, leading to greater subsidence.
Subsidence controls over mined areas generally takes the form of either providing selective support for the structure or filling the underground space to halt further subsidence. In an effort to control subsidence affecting an electric substation in Pennsylvania, both approaches to dealing with subsidence were used. Selective support involved placing drilled piers and piling seated into rock below the base of the mined coal seam. Fly ash was injected through drill-holes to fill some underground openings. In 15 selected locations, grout columns were constructed.
