2孔間せん断試験法による岩盤強度の評価

抄録

In this paper, it is described that the estimate of the strength of rock mass can be made by means of shearing rocks between two parallel NX-boreholes.
The outline of this study is as follows;
(1) To estimate the in-situ rock strength, a new borehole-type shear instrument as shown in Fig. 1 has been designed. In the in-situ test, two NX-boreholes are drilled to the side wall of underground opening, and these boreholes are parallel to each other with the distance of 20mm or 25mm. Then the shear instrument is inserted in one side of the two boreholes and the rocks between two parallel boreholes are punched by a shear-edge (20mm×40mm) which is pressed by the oil pump. The shear strength of the rocks can be calculated from the applied shear load and the shearing area as shown in Fig. 5. The shear strength in this test is conveniently termed as “Shear strength index”. The preliminary tests in the laboratory were taken on the relatively homogeneous rock blocks, and from this tests, it will be clear that the relation between the shear strength index and the uniaxial compressive strength of the rock blocks gives essentially straight line as Fig. 7-A. The experimental equation is as follows;S_c=3.14SiwhereS_cis the uniaxial compressive strength (kg/cm²) andS_iis the shear strength index (kg/cm²).
(2) The shear tests were taken at-220mL crosscut of Shakanai mine, Akita prefecture. In this in-situ test, the shear strength indexes were obtained at 43 points in the rock mass, which was consisted of rhyolite with joints. The compressive strength of rock mass can be estimated from the cumulative distribution function of the shear strength indexes with the parameter proposed by B. Kojak. From above proposal, the compressive strength of rock mass, in which the in-situ shear tests were taken, was estimated to be about 530kg/cm².