Subsidence

Subsidence is the depression of the ground surface (and the structures standing on it). In the mining areas it is caused by underground mining if the roof over the void is not supported effectively. The first indication of subsidence is small cracks on the surface which gradually widen resulting in depression of patches which cover a wide area in course of time. The thinking of the DGMS and of many mining engineers in the past based on experience was that there is a safe cover beyond which subsidence is tolerable for the types of structures mentioned in Table 1 as cracks may not extend upto the surface. The safe cover was considered to be as given in the table.

The scientific studies and observations by CMRS have established that in the case of caving the theory of "Safe cover' does not hold good and that the area on the surface affected by subsidence is always more than the area from which coal (or mineral) has been extracted underground. In other words, the angle of draw is always positive in Indian mining conditions.

Angle of draw is the angle between the vertical and the final line of break i.e. the line joining the point of zero subsidence with the underground point of solid mineral from where excavation of mineral started. (Fig. 9.5).

Cracks of surface and the buildings or other structures result due to differential subsidence of the surface i.e. one part undergoing more subsidence than the other. If the structures are of such a nature that subsidence of the entire structure as a whole is permissible, subsidence can be so controlled as to take place without appearance of cracks. For this purpose the underground mining with caving or partial packing has to be planned in such a way that there is gradual but uniform subsidence of the entire surface as a whole. This is the idea behind harmonic mining, a method of controlling subsidence by proper control of underground mining, and is practised in West Germany for extraction (if coal under towns, rivers and railway lines. In our country this idea may materialise in the distant future-after prolonged studies and observations by CMRS under different conditions of mining.