Land subsidence

A land subsidence or also called a sinkhole is a movement of the earth's surface in which the descending direction predominates and which takes place in areas of different characteristics and slopes. It differs from the term subsidence due to its much smaller temporal and spatial scales. This movement can be induced by different causes and can develop at very fast or very slow speeds depending on the mechanism that gives rise to such instability.

Causes

Among the main causes of land subsidence, we can mention the dissolution of limestone, which is calcium carbonate, due to the action of groundwater. Although CaCO₃ has a relatively small solubility product constant, it is very soluble in the presence of an acid. This problem has caused many problems in many places and countries such as Prague, Mexico, Florida, Venezuela, Spain, among others.

• First, the direct action of gravity. Ground surface materials on steep slope can be moved suddenly as a response to the direct action of gravity, especially when continuous rains can contribute to the mass sliding of the unconsolidated slopes.

• Rainwater is acid by nature (by the combination of carbon dioxide (CO)₂) with atmospheric water, which is transformed into carbonic acid with a 5-6 and can become more acidic in contact with plant matter in decomposition.

• If the vertical movement is slow or very slow (e.g., centimeters/year) and affects a wide surface (km2), it is often referred to as subsidence. If the movement is very fast (m/s) it is usually talked about collapse.

In the long term, the causes of subsidence may be, inter alia:

The response of geological materials to distensional tectonic stresses, for example to the formation of tectonic trenches or which over time can give rise to fissures.

• • The response of geological materials to locally distensive tectonic efforts in a frame of epidermal tectonics with games of superficial faults with flexioned traces, for example in type basins pull-apart.
  • Liospheric readjustments by isostasia, for example at the end of a continental collision after the cessation of the cortical uprising or end of the formation of an orogen.

• Variations in the water level or in the soil humidity state, for example as a result of the exploitation of aquifers.
  • The underground mining activity, for example after the abandonment of underground galleries.

For its part, the causes of collapses involve the failure of the geological structure that supports a portion of the terrain under which there is a cavity, which may be motivated by the dissolution of the rocks (due to the effect of karstification, see Karst) to the limit of the resistance of the materials or the emptying of aquifers or in general the weakening due to physical or chemical weathering of a structure that houses a cavity. The use of natural resources (mining activity, exploitation of aquifers) can also induce collapses.

In particular, the continued exploitation of oil basins also gives rise to the sinking of large areas of irregular shapes that can cause problems such as flooding, disabling of natural drainage, alteration in land use, etc. Hurricanes Katrina in August 2005 in New Orleans and Harvey in Houston in August and September 2017 (August 23 to September 3) are notable examples of the risks of such hurricanes in oil fields exploited for decades and the resulting land subsidence. due to said exploitation.