Defining Ground Subsidence, Effects of Subsidence on Structures - Essay Example

Download file to see previous pages "Ground subsidence" is legally defined as the means or process that is characterized by the downward displacement of surface material caused by natural phenomena such as the removal of underground fluids, natural consolidation, or dissolution of underground minerals, or by man-made phenomena such as underground mining. (Colorado Geologic Survey)

Subsidence may occur abruptly-virtually instantly—or gradually over many years. It may occur uniformly in a small, confined area as shown in Figure 1a, or may occur over a wide area as local depressions as shown in Figure 1b. Subsidence is commonly associated with the dissolution of soluble rocks, such as limestone, beneath the surface while those with crystalline rocks in which most metals are mined have greater strength and are less likely to settle or collapse. The resultant landscape has closed depressions and is known as karst topography. Note that the depressions do not necessarily result in holes in the ground as shown in Figure 2.

Other major causes of subsidence include the thawing of frozen ground, compaction of recently deposited sediment, and the shrinking of expansive soils. It can also occur, to a lesser degree, due to earthquakes and the deflation of magma chambers. Human-induced subsidence occurs as a result of the withdrawal of fluids from subsurface reservoirs as shown in Figure 4, from the collapse of soil and rock over subsurface holes, such as those left by underground mining and from the draining of wetlands. (Waltham and Cushaw, 2004)

Subsidence can also occur due to expansive soils. There are clay-rich soils that shrink significantly during dry periods and expand or swell during wet periods. The swelling is caused by the chemical attraction of water molecules to the surface of very fine particles of clay. Swelling can also be caused by the chemical attraction of water molecules to layers within the crystal structure of some clay minerals. Figure 3a shows the mechanism by which expansion of soil can occur while Figure 3b shows water consumption by trees as another cause. ...Download file to see next pages Read More