In India, about 98 per cent of underground coal production comes from bord and pillar workings. A large number of these workings are below built up areas, rivers and agricultural land, etc. Subsidence of ground due to these workings has, therefore, engaged the attention of mining engineers for over half a century. Presented here in this paper are the results of measurements of surface movement due to extraction of pillars with full caving, with stowing and with partial extraction in varied situations. The results indicate that the subsidence of ground, especially if the workings are at shallow depths, does not appear to follow any predictable pattern. The ground subsides in irregular depressions and in some situations the depressions may be more than the thickness of the seam. The angle of fracture generally lies close to the extraction boundary. In deep seams (> 400m), the amplitude of subsidence may be negligible in stowed workings.
En Inde, environ 98 % du charbon est exploite dans des mines souterraines, par la methode des chambres et piliers. Une grande partie de ces mines se situent sous des zones construites, des rivières et des terrains agricoles. Les affaissements provoques pas ces travaux interessent donc les ingenieurs des mines depuis plus d'un demi siècle. Dans cet article sont presentes les resultats des mesures des mouvements de surface provoques, dans differentes situations, par l'exploitation des piliers avec excavation totale et remblayage, ou par l'exploitation partielle des banes de charbon. L'etude montre que les affaissements en surface ne suivent pas de loi precise, specialement si l'excavation se fait à faible profondeur. Le terrain s'affaisse irregulièrement et parfois les depressions sont même plus importantes que l'epaisseur de la couche exploitee. Lors de l'extraction avec remblayage des couches profondes (> 400m), l'importance des tassements est neg ligeable.
Rund 98% der untertage gewonnenen Kohle in Indien werden mit der "Bord und Pfeiler" Methode abgebaut. Eine grosse Zahl dieser Baue befinden sich unter Gebauden und Zivilwerken, Fluessen, Ackerland, usw. Durch diese Baue verursachte Bodensenkungen haben seit fuenfzig Jahren die Aufmerksamkeit der Bergbauingenieure auf sich gezogen. In diesem Beitrag werden die Ergebnisse von Bodensenkungsuntersuchungen dargestellt, die durch Kohlegewinnung aus Pfeilern mit Verstuerzen des Hangenden (caving) mit oder ohne Versatz sowie durch partielle Gewinnung unter verschiedenen Bedingungen verursacht werden. Die Ergebnisse deuten an, dass die Senkungen keinem voraussehbaren Muster folgen, besonders wenn der Bau nicht zu tief liegt. Der Bruchanstellwinkel (angle of fracture) liegt in der Nahe der Baugrenze. Im Falle tief liegender Flöze (> 400 m), die mit Versatz gebaut werden, kann man das Senkungsmass vernachlassigen.
In India, about 98 per cent of underground coal production comes from bord and pillar workings. Of this 57.5 per cent production comes from development workings in which pillars are formed and 40.5 per cent from depillaring. Surface movements in all the cases have been measured to varying degrees. As the underground workings in many cases are below built up areas, or agricultural land, roads, railways, rivers etc., the problem of subsidence has engaged the attention of mining engineers for well over half a century. The first systematic attempt to study subsidence was made in 1914 by the First Subsidence Committee constituted by the Mining and Geological Institute in India, which published its report in 1923. The Committee interalia stated that "Observations were not sufficiently numerous or precise to enable definite rules to be drafted on the relation between thickness extracted and the amount of subsidence". In 1929 the same Institute constituted the Second Subsidence Committee to consider amongst other things,
"the extent and effects of subsidence resulting from continued extraction of pillars, and
geological aspects in connection with subsidence in mines".
The report of the Second Subsidence Committee was published in 1936. The main findings of the Committee were:
In the simple cases of depillaring under strata solid to the surface free from geological disturbances such as faults or dykes, the area of resulting surface subsidence lay within the area of underground goafing, provided the thickness of the subsoil was negligible.
The presence of faults and dykes disturbed the situation. Surface breaks extended towards such features. Often, in the case of a dyke or dykes bounding or in the proximity of the area that was being depillared, the draw towards them at the surface could be delayed for some time subsequently to be extended to the line of dyke. Such extension of surface subsidence occurred after 2 to 2 1/2 years of the first break.
In thick seams in virgin areas depillaring up to 182 m depth caused clean and sudden breaks and sometimes draw over the unmined pillars were noted. It was also observed that at a depth of 365 m moderately thick seams may be extracted without appreciable disturbance of the surface.
