岩石のせん断強さについて

抄録

In order to find out the methods for measuring the shear strength of rocks, the several tests, namely the direct shear, the uni-axial and the tri-axial test, have been undertaken on five kinds of rocks.
A summary of the results is shown below.
1) Based on the concept postulated by Griffith, the diameter of a maximum stress circle through S_t-point, which represents the uni-axial tensile strength on Mohr's diagram, is equal to 4S_t. Consequently, the Mohr's envelope is expressed in a common tangent drawing from 4S_t-stress circle to S_c-stress circle, which represents uni-axial compressive strength. In this case, the shear strength is approximately obtained from an intersection of τ-axis and the common tangent, and can be calculated by using the following equation: if Se>3S_t
S_s-5=S_c·S_t/2√S_t (S_c-3S_t)
2) The shear strength of rocks under the various normal pressure acting on the shearing plane is given by a single shear test-device, which can be arbitrarily adjusted the inclination angle of the shearing plane by setting a pair of rotary dies. Satisfactory results in this test are usually to be obtained with the inclination angle ranging from 15°to 45°. Moreover a zone which we shall call “failure zone”, represents the relationship between the shear strength and the normal pressure, is illustrated on τ-σ plane. It will be seen that the shear strength of rocks may be approximately estimated by the failure zone, because the failure zone approaches to a Mohr's envelope in the vicinity of τ-axis.
3) The shear strength by the double shear test is calculated larger than that by Mohr's envelope, because the normal stress acting on the shearing planes in the double shear test is compressive.