Underground mining methods can be one of the alternatives to solve high demand on commodities but they have their own common problem, that is, the opening instability. Opening instability is caused by the rock pressure in each wall and roof of tunnel. Distribution of rock pressure becomes anisotropic due to lot of kinds of joints. It makes the displacement around the tunnel different. This paper studies the effect of joints, especially the crossed ones, to the displacement distribution on the circular opening. The experiment is designed by using 45cm × 45cm × 30cm samples, which will undergo biaxial test. Biaxial test is a method of modelling the pressure in the tunnel's wall. The circular opening diameter is 8.8 cm and the crossed-joints' angles are set to be 90°, 60°, and 30°. Those joints, of which depth is as deep as the opening, have length of 30 cm. There are horizontal and vertical stress given at the test; k=1/3 and k=2 with vary horizontal stresses. Another parameter used in this test is dial gauges of 1R, 2R, 3R, 4R, and 5R (with 0.1 /-m accuracy). The biaxial test is performed to represent the physical model of the displacement. Yet, the result will also be verified by generating the numerical model from finite element method (FEM) in software Rocscience 3. For the time being, the author has hypothetically predicted that the biggest rock displacement would happen on the tunnel's wall with k=2.

1. Introduction

Underground mining can be one alternative to exploit earth resources. This method is slowly developed and used now since there are many mines left out because the companies cannot excavate deeper reserves. The other reason people start using underground mining is that it configures profitable mine planning for deeper mineralization or coal; this relates with how abundant the overburden will be if people keep using surface mining method.

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