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Overview

Land subsidence is an environmental hazard which is caused by overdraft of groundwater, coal mining or oil extraction and results in gradual settling or sudden sinking of the earth’s surface owing to subsurface movement of the materials of the earth. Subsidence is an inevitable phenomenon of ground movement caused by various manmade and natural activities. The economic prosperity by the exploitation of the hidden resources in the earth is always accompanied by the adverse impacts of subsidence.

Groundwater related subsidence:

Land subsidence occurs when large amounts of groundwater have been withdrawn from certain types of rocks, such as fine-grained sediments. The sediment compacts because the water is partly responsible for holding up the ground. Decline of water table or piezometric surface results in vertical compression of the subsurface materials. Along with vertical compression, lateral compression may also take place due to initiation or acceleration of lateral flow of groundwater. This lateral movement also results in subsidence of the land surface. Any flow or overdraft of groundwater in unconsolidated material should produce some movement of the land surface. This movement is generally small, but may become very significant where subsurface materials are thick and/or compressible and the groundwater level declines appreciably.
Land subsidence is either individual or a combined effect of inelastic compression of the confining bed and/or elastic compression of the solid matrix of the aquifers. Land subsidence due to groundwater overdraft is essentially irreversible in case of inelastic compression of the overlying confining clay bed. It can be stopped only by halting the decline of groundwater level. However, rebound of the land surface is generally insignificant even if the groundwater levels are restored to the height, prior to subsidence.

Coal Mining Related Subsidence:
Every year loss of life occurs due to subsidence of land in coal mining areas. Subsidence is very prevalent in the Raniganj coal field which is a major coal producing area. The occurrence of thick coal seam at shallow depth is the main reason for the subsidence and as a result collapses have occurred in many coal mines in recent years.
Subsidence due to underground coal mining has been reported from almost all parts of the world and India being a major coal producer has been facing very severe problems of subsidence in some of its coal fields.

SUBSIDENCE PROBLEM

Raniganj (RCF) coal fields is facing significant subsidence problem due to underground coal mining. Extraction of thick seams at shallow depths has damaged the ground surface in the form of subsidence and formation of cracks reaching up to the surface, enhancing the chances of spontaneous heating of coal seams leading to mine fires. The board and pillar method of underground mining is very common, this method is used in more than 90% of Indian coal fields. Mining activities in RCF have degraded a sizable land area and have endangered about 50 localities (townships, villages and other residential areas). The subsided area in the Raniganj coalfield covers about 43.43 km2.
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