Problems of stress-strain states of a rock mass during coal exploitation by using the method of longwalling are still assessed prevailingly on the basis of the classification of rocks occurring in the effective hanging wall of the seam. It has been proved that it is also the main roof mainly reaching as far as the previously worked out seam that affects these problems. Determining the stress state of the rock mass exploited is very difficult and the majority of methods are considerably expensive. One of the methods that we may use in the assessment of development of deformation, and thus the state of stress in the rock mass, is observing the formation of a subsidence trough on the surface in time. Paper deals, on the basis of measurements of the formation of a subsidence trough in time and in dependence on mining geometry.
Various problems that we meet in the course of underground mining are often caused by great acting stresses. The determination of values of stress in particular places of the exploited rock mass it thus highly desirable. However, up to now any method that would Satisfy requirements of the wide range of practical uses has not been developed. This is the reason why we make efforts to utilise other observations and measurements that are not so demanding as for the assessment of acting stress.
In this Contribution we are concerned with the study of the state of disturbance of the undermined roof, on the basis of which we are able to draw certain conclusions concerning stress distribution, If the disturbance of the roof is stopped then the undisturbed rocks owing to their weight cause a substantially greater load on the surroundings of the worked out area when compared with the state after their failure, which will manifest itself in the larger subsidence of the surface.
When mining under the firm roof, the maximum height of broken rocks depends on the width of worked-out area. At a small width of the worked out area, the breaking of the undermined roof will be stopped at a certain height, which will show itself in the minimum subsidence of the surface. In the undermined roof an arch will be created that can be self-bearing for a very long time, and the next disturbance of the roof will mostly occur as late as after widening the worked out area by other faces.
The fact whether or not the arch will be created and the next disturbance of undermined roof will be stopped depends, in addition to the worked out width, on the strength of the roof, the thickness of firm undisturbed layers, the existence of higher lateral stress, and others. Under certain specific mining-geological conditions, a certain width of the worked out space always exists that represents a limit for arch formation. The next disturbance of the roof is stopped until this width is reached.