Measurement of the basic friction angle of planar rock discontinuities with three rock cores

The paper presents the results of tilt tests using a large number of three-core samples, aiming to establish a method for determining the basic friction angle of planar rock discontinuities. The three-core tilt test is easy to operate, and the results are comparable with the saw-cut method after modification. Saw-cut and two-core samples were also tested for the purpose of comparison. The test results show that the cylindrical surface of the core is slightly rougher than the surface of saw-cut samples so that the friction angle measured on the cylindrical surface of the core is slightly larger than that measured on saw-cut samples. The basic friction angle of the planar rock discontinuity is obtained by decreasing 2° from the friction angle measured on three-core samples. The appropriate number for test repetitions is 10∼20 in order to obtain a reliable mean friction angle. Grinding or polishing of the cylindrical surface of cores is not recommended, since it will lead to an increase in the friction angle in most of rock types.