3.3.3. Subsidence due to mining
Subsidence is defined as any movement at any place in the earth's crust or on the surface due to any natural and/or manmade activity (Saxena et al. 1989). Such situation may develop due to many causes, natural or manmade, among which those related to mining are as listed below :
* Underground excavations for mining
* Open excavation for mining.
* Pumping and withdrawal of liquid from UG.

Subsidence due to UG mining is an often-heard phenomena. It takes place due to disturbance caused in the superincumbent strata due to extraction of materials from UG resulting loss of support in some cases. It may happen also due to the disturbances in the aquifers due to UG excavation, aquifer compression or aquifer loss detailed next.

Pumping of water from aquifers is an activity that causes subsidence. Such pumping may be due to pumping of ground water that flows into the site of excavation for any mineral, if the excavation behades any aquifer. Extensive ground water withdrawal for water use for any human purpose also may cause alike problems. The aquifers hold water at a certain hydrostatic head and the stresses are in the state of equilibrium in natural condition. As soon as pumping of water is started from the aquifer, a drawdown zone develops around the pumping site causing reduction of hydrostatic pressure around. As a result of this the stress equilibrium is disturbed. If such situation retains years after years the equilibrium is regenerated by some grain to grain rearrangement to remove the fine pores which were providing spaces in the strata produced by removal of water due to lowering of WT, thus to compensate the loss of hydrostatic pressure. Under such circumstances the aquifers get compacted under the stress due to the load of superincumbent strata. The compaction manifests on the surface in the form of land subsidence. Once the aquifer attains such situation it cannot be recharged by natural process because the porosity to hold the water had already been lost through compaction.

Similar situations may occur also due to withdrawal of petroleum from the pore space of the reservoir rock. Examples of subsidence due to such fact are common world over. Mention may be made of San Joaquin Valley, California, the world's largest subsidence area effecting a land area of 13500 sq km, through an average lowering of 1m, the maximum amount of lowering being 10m (Coates, 1981).

Subsidence due to GW withdrawal has been predicted at Kolkata by a group of Scientists (Sikdar, et al., 1996).

In OC mining subsidence, in technical sense, is caused due to slope failures. Such situations if occur in land below the general contour in the region it amounts to slides of quarry faces (of area strip mines) and hence degradation of that much land only, and loss of support of the land beside it. If such situation occurs in case of contour strip mining, the subsided matter falls on the regional contour, damages its physical and chemical characters and disturbs the landuse and landcover of the region.

Whatever may be the cause and amount of subsidence, it disfigures topography of the region and alters the drainage conditions, runoff pattern, hence transportation pattern of soil particles and hence the water regime. The quality (physical and chemical) of the land obviously gets disturbed. In case of subsidence below ground, i.e. due to UG mining and ground water withdrawal the lowering of land by subsidence may be a phenomena to manifest long after the withdrawal and in some cases beyond the scope of calculation. This may produce limited time and hence limited scope to reclaim the land damages.


3.3.4 The cumulative effect
Effects of mining etc.
Whatever impacts of mining and its related activities on land and land related environmental attributes have been explained so far in the previous sections, all ultimately produces any of the three effects i.e., damage to land, damage to greenery or high erosion. These three occupy important positions in the cycle of land degradation (Fig. 3.1 & 3.2) already explained. These are continuous cycles, which continue to work in repetitive manner so long not stopped by some strongly effective action, and thus push the land towards desertification.

It is hence very clear that all impacts of mining on land and land related environmental attributes join together and multiply the effects to push the land towards desertification.

The matter could be explained in other way, that mining damages greenery as well as water resource is a well accepted fact now a days, while these two natural resources are capable to protect all aspects of environment. Plants improve air quality, water resources, aesthetics, provide food and source of earning, hence in turn improve the level of biodiversity, quality of life and quality of ecosystem. It prevents noise and vibration, flood and draught and hence protects land.

Utilities of greenery in preventing land degradation can be listed as follows :
* When properly utilized, forests act as climatic stabilisers, inhibit flooding and sedimentation.
* Cooling capacity of one full-grown tree equals that of five average air-conditioners operating 20 hours a day (Coates 1981, pp 512).
* By taking part in evaporanspiration plants keep the hydrologic cycle running.

* Tree roots provide extra shear strength to the soil particles to resist gravity movement and surface erosion on slope as well as on flattish land.

* The green canopy dampens raindrop impacts on land.
* Trees' litter makes a spongy cover on land and hence a barrier between eroding agents and land, this reduces erosion potential
* The decaying vegetal matter enriches the nutrients in top soil, increases permeability and allows more infiltration, hence lesser run-off and lesser erosion.
* Tree roots provide avenues for additional infiltration. Hence these together with decaying vegetal matter minimise direct runoff and prevent sheet flooding and erosion.

Deforestation negates all the above helps that forests provide for land protection. Hence the deleterious influences on land that are created by damage to greenery are
* increased erosion & soil loss
* increased sedimentation with all its accompanying damages
* disturbing the hydrologic regime by lowering water table and
* loss of soil nutrients.



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