ABSTRACT
When an optically sensitive material is used for investigations, it should be remembered that its mechanical properties do not reflect the real properties of stratified rocks. Development of the friction dynamometer followed by semi-conductor devices has made it possible in recent years to carry out stress measurements on a mass scale and to establish laws governing their distribution around mine workings. The nature of stress distribution in the rock mass in the vicinity of the extraction monitored by dynamometers confirmed the existing general concepts regarding isolated seams. Collapse of the rock mass during the first cave-in often occurs over large areas and at considerable speed, which leads to failure of hydraulic supports.
