The characterization of time-dependent brittle rock deformation is fundamental to understanding the long-term evolution and dynamics of the crust. Here we report results from a study of brittle creep in Darley Dale sandstone under triaxial stress conditions. We present new results from a series of constant strain rate experiments and conventional brittle creep experiments. We then apply a micromechanical model for brittle creep in rocks to our experimental data. This model shows that the acceleration of the strain rate observed experimentally prior to failure is due to the increasing importance of crack interactions. Finally, if we compare long-term creep failure under a constant stress with short-term failure under an imposed constant strain rate, we find a mechanical work deficit. The magnitude of the deficit in the mechanical work rate is by definition a stress deficit. Our analysis implies that rock deformation under both constant stress (creep) and constant strain rate conditions are controlled by the same underlying chemo-mechanical processes.