The time dependent behaviour of some evaporite rocks

In practical circumstances the bulk of the rock material beneath a foundation, in the surrounding regions of an excavation, or inside mine pillars is in fact triaxially loaded over long time periods. It was felt that studying the creep phenomena of some evaporite rocks under a triaxial system of loading could add valuable information to the limited knowledge avail- able on rock behaviour in such conditions. Gypsum and anhydrite were initially chosen as suitable evaporite rocks for carrying out this work. An apparatus has been designed and constructed to enable experiments to be carried out on the chosen rocks. The axial strain of the deformed rock specimen was measured on the rock specimen inside the pressure cell. Triaxial compression creep tests were carried out at 10,20 and '30 N/mm2 confining pressure. Bending and uniaxial compression creep tests were also performe-: 1 on the chosen rocks. Instantaneous strengths of gypsum and an- hydrite under the given systems of loading were found and various percentages of the instantaneous strengths were applied in the creep tests. All short term and creep tests were carried out at room temperature. It was found that the creep behaviour of the tested rocks obeyed the following equations: Z=A+B logt and/or E= ctn, The effect of varying axial stress, confining pressure and differential stress on the creep behaviour of the tested rocks was observed and studied. A method for determining the safe creep stress, at any confining pressure, was suggested depending on the creep data available.