Research on Unloading Deformation Characteristics and the Law of Mechanical Parameters Based on Unloading Experiments of Marble

The stress unloading of surrounding rock in the excavation of tunnel may cause instability and failure of rock mass. There is a kind of unloading confining pressure experiments designed by using an MTS815.04 electro hydraulic servo-controlled testing machine. That is experiment of increasing axial pressure while decreasing confining pressure. Then the deformation properties of marble under unloading condition are analyzed. The results show that when axial pressure increases while confining pressure decreases, the peak of axial strain decreases compared with loading experiment, and the deviatoric stress tends to be stable in the late unloading stage of high initial confining pressure while the fracture toughness reinforces. It also shows that the failure modes of samples exhibits a relatively strong brittleness in the low confining pressure but changes toward to toughness with increase of confining pressure. The failure characteristics changes from tensile to tension-shear and the destruction of samples in unloading experiment should be more strongly than that in loading experiment. With the initial confining pressure increasing in unloading experiment, the deformation parameters of marble is no longer constants and both of them take place the effect of damage degradation in a certain degree. The elastic modulus increases gradually but has an decrease of 50%-100% compared with triaxial experiment, on the contrary, the Poisson ratio decreases gradually but has an increase of 30%-80% compared with triaxial experiment. The unloading conditions of increasing axial pressure while decreasing confining pressure have a great effect on sample deformation and there is a quadratic nonlinear relationship between elastic modulus of surrounding rock and initial confining pressure.