In general, rock cavities are required to function over a period of many decades. For this reason it is at first necessary to acquire a comprehensive knowledge of the rock mass. A particular difficulty is the anisotropy of the rock. In the Rheinische Schiefergebirge, for example, the rocks frequently are characterized by a particular cleava- ge. But moreover, the geological development of the rock led to many joints affecting the mechanic behavior of the rock, as for instance bedding, cracks and faults.
In most cases the rock masses in which our structures are set up have been subject to considerable tectonic deforma- tions. Above all, these deformations are long-term proces- ses having begun with the orogeny. To the intense strain the beds at first reacted by bending and, finally, by for- ming new planes of separation and movement. Parts of the rocks could be further dislocated along faults. The fol- ding pressure can roughly be estimated on the basis of the overburden pressure existing at that time.
As regards an example taken from the Rheinische Schiefer- gebirge the extent of the deformations (1/&l) - in a certain direction, approximately diagonal to the fold axes - could be determined with ~ 20 ?. Internal stresses of the rock mass are the relics of these processes of deformation. Even nowadays they may be exi- sting.
A large amount of deformation is due to the loosening by processes of weathering. Loosening by blasting calls forth a short-term process of deformation. The duration of the processes of deformation ranges from a few seconds in case of loosening by blasting to millions of years in case of epirogenic crustal movements. It is difficult to fix exact time periods because many processes may overlap. In accor- dance with the material behavior Scheidegger (lg58) differs 191 among processes of up to 4 hours, processes ranging from 4 hours to 15,000 years with a "typical" duration of 3 years, and finally, processes exceeding 15,000 years with a "typi- cal" duration of lO0 million years. It is not intended to discuss the applicability of this subdivision but only to give an idea of the periods of time concerned.