A series of laboratory creep tests have been performed on sandstone

shale and dolerite to investigate the creep behavior of rocks subjected to different stresses and at different initial strains. Error analysis has been performed to study how to select time increment for the calculation of creep rate. The applicability of time-dependent creep model in describing the creep behavior under complex conditions is examined. An internal state variable creep model is proposed

in which the irreversible strain is considered as the internal state variable. The fitting of test data into the model shows that the internal state variable creep model is capable of describing the creep behavior of the three rocks in laboratory tests. A primary study of the applicability of the internal state variable model in describing rate dependent behavior of rocks is performed; and test data in literatures are used to verify the applicability.