One of the methods used to determine the long term shear strength is accomplished by conducting creep test at the laboratory. Usually the creep test is conducted by one direction loading system, which is known as uniaxial creep test. The test is very suitable when applied to the determination of long term strength of underground pillar and at roof or wall of the tunnel. But it is not representative to be applied for the determination of long term strength of the rock mass around underground opening. With three direction loading system, the laboratory creep test would be more suitable to simulate the rock mass loading condition in its natural state; hence the long term strength obtained from this test would be more appropriate. Based on this assumption, the triaxial creep test needs to be carried out. From the triaxial creep test result obtained, the axial strain condition is constant with time. Axial strain only occurred when axial load was increased (immediate strain), after that, primer, secondary and tertiary strain did not occur during constant loading within + 48 hours. The creep process did not take place because the stress and temperatures in this experiment were inadequate to induce deformation dependent to time. Hoek and Brown commented that the rock specimen used in this experiment is categorized as High Strength, so that the rock would not result in creep when subjected to axial load of 30 MPa to 132, 38 MPa and temperature of 26°-28° C. Several factors must be taken into account to acquire long term strength parameter from triaxial creep test for andesit breccia; such as the determination of preliminary axial load, constant axial load and temperature.
Yudianto (Bandung Institute of Technology, Indonesia)
