Monograph 11

Method of selection of longwall face and roadway supports for the panelling conditions

Biliński A.


Ability of counteraction of disadvantageous results of rock mass pressure in relation to the operational workings, that is to the longwall faces and neighboring roadways, consists on a knowledge of efficient measures taken against roof falls and caving in the area of longwall faces as well as against excessive convergence of near-longwall roadways.
Fall of the rocks, that make a rock mass roof over the longwall face, takes place in a result of loss of geometrical continuity of rock layers, which make the roof. Loss of geometrical continuity can occur in a result of increase of length of the given layer, that takes place during an increase of its inclination. Rock blocks, which make the layer, are loosening and are able to displace transversely till a full fall out of the layer.
Possibility of transverse displacement of rock blocks appears already at a relatively small inclination, the smaller one if the rock making the layer is less resistant. They can be treated as a boundary inclination.
In the breaking process of rock layers, the amount of roof inclination which is in a front of the face, just after exposure of roof by mining the solid rock supporting it, is of greatest importance. Good roof supporting in a geometrical continuity can be achieved when the amount of roof inclination, after its opening, that is when a unit roof inclination (zLI) is close to a boundary inclination, characteristic for the rock making a roof. A ratio of both mentioned above coefficients influences the amount of roof loading capacity indicator (g). To achieve good roof conditions in the longwall face we have to be as close as possible to g ≥ 0.8.
Preventing the disadvantageous results of the rock mass pressure in mining roadways i.e. near-longwall roadways, consists on avoiding of their excessive convergence. That means, that both roadways accompanying the longwall face should maintain their functionality ahead and close to a developed face front, and that one of them planned for servicing the next longwall face should also maintain its functionality after passing the face front to the vicinity of gob.
Result of the studies, especially those carried our in recent years, indicate that in caving longwalls there is a possibility to limit the rock mass movement in those roadways to such amounts that they do not loose their functionality. However, it can be realized only by a significant increase of total amount of load-bearing capacity of a roadway i.e. the roadway support itself, the components supporting the roadway from a gob side, which accompany it as well as by use of roof bolting technique. Here it should be mentioned that to limit the amount of real roadway convergence (zz) up to 30% in relation to the standard roadway convergence (z), that is the convergence calculated according to empiric formulae, it is required that a total load-bearing capacity of the support is two times higher than an acting load.
When using the calculation methods and formulae included in the work the following is possible:
to select the longwall face support, which would secure good roof support in given natural and technical conditions, in which the face is planed for paneling, including the rock burst hazard;
to select a protection at least one of the near-wall roadways, adequate for the natural and technical conditions in which it has to operate, such one that it should be used in the next face;
to design a proper geometry and load-bearing capacity of the face support, especially at the face front area, adequate for the natural and technical conditions, in which the face is planed for paneling, in such a way, that a good supporting is maintained along the whole face panel;
to settle the proper ranges of operation of each powered roof support, both the new one and one modified during a repair process, considering the roof and floor strength, face height, rock mass pressure and rock burst hazard; the range should be a basis for reasonable technical approval of the roof support for operation in underground mines.