To provide better description for the volume deformation of rock, a lateral-axial strain fractional dashpot is presented on the basis of the Abel dashpot with a constant viscosity coefficient and a variable viscosity coefficient. A new model of volume-axial strain relationship is proposed based on fractional order calculus. The analytical solution for the model of volume-axial strain relationship is given theoretically. The new models have the advantages of solving volumetric strain of rock. Moreover, a mul...