Creep properties of evaporite rocks with particular reference to gypsum

The bulk of the rock material in the neighbourhood of an excavation, beneath a foundation or inside mine pillars is in fact triaxially loaded over long periods. Strata exposed in mines and excavations is subjected to bending over long periods also. In many mines and underground workings the rock is nearly saturated, also information on the three dimensional creep behaviour of any rock is very limited. It was felt that studying the three dimensional creep behaviour under confinement in both dry and saturated conditions could add valuable information to the knowledge available on rock behaviour in such conditions. Gypsum was chosen as a suitable evaporite rock for carrying out this work. To assess the significance and the nature of the time-dependent behaviour of gypsum in engineering applications, triaxial compression creep tests were carried out at 10,20 and 30 N/mm2 confining pressure. Bending and uniaxial compression creep tests were also performed on the rock. Axial and lateral creep strains were measured from which volumetric creep strain was calculated. All the creep tests were carried out in dry and water saturated conditions at room temperature. Instantaneous strengths of gypsum under bending, uniaxial tensile, uniaxial and triaxial compression were found. The effects of specimen size and water saturation on these strengths were determined at room temperature. Empirical equations describing the individual creep curves in bending, uniaxial compression and triaxial compression were determined for both dry and saturated conditions, in the latter two cases equations for both axial and lateral strains were-obtained.