Shear Strength of Rocks along Weak Planes

case of schist, mudstone and granite

Abstract

The strength of a rock in situ essentially differs from that of a rock of laboratory size. Experiments were undertaken to show the difference.
It was presumed that the rock in situ is weaker than the rock specimen, since the former contains many weak planes, and the experiments were prepared mainly to abtain the shear strength of the rock in situ along a weak plane.
The crystalline sandstone-schist of Shikoku, the alternation of mudstone and sandy tuff or tuffaceous sandstone of Miocene (in Yamagata Prefecture) and the granite along Tenryu River were taken up as the objects.
Assuming that the shear strengths of these might be expressed as a sum of the cohesion and the friction, results were as follows.
sandstone-schist;τ=n
alternation of mudstone and sandstone;τ=0.5-1.0+0.3n
granite;τ=0.5-1.0+0.6n, where τ is the shear strength and n the normal stress on the sheared plane expressed in t/m². The areas of sheared planes were 0.2-2m².
To check the reliability of results the strength obtained by experiments for the mudstone of Miocene was compared with that which was estimated by the rock failure at Kobato Apl. 27. 1957. It was found that the former was as large as about twice of the latter.
The reasons of the difference might be 1), the deviation of the strengths of the rock at various points around the mean value of them, 2) the deterioration of test pieces during shaping, 3) the error in calculation of the failure or 4) the essential difference of the mechanism of rock failure in various cases.
Accordingly, the results must be carefully applied in the practice and the mechanism of rock failure should be studied further. Especially, the process of the failure at various parts of a test piece should be observed to clarify the mechanism.