Fracture evolution mechanism of hollow sandstone under conventional triaxial compression by X-ray micro-CT observations and three-dimensional numerical simulations

Abstract

The knowledge of the fracture evolution mechanism of hollow rock under conventional triaxial compression is very important to evaluate the stability and safety in deep tunnel engineering. In this research, a series of X-ray micro-CT observations and three-dimensional (3D) numerical simulations were conducted on hollow sandstone specimens with various borehole diameters (d = 0, 15 and 26 mm) under different confining pressures (0, 8, 16, 24 and 35 MPa). A 3D realistic failure process analysis method (RFPA3D) was established for the simulation of the deformation failure behavior of hollow sandstone under conventional triaxial compression. First, a group of micro-parameters used in RFPA3D, which reflect the macro mechanical behavior of the tested sandstone material, was calibrated using the experimental triaxial compression results obtained for intact specimens. Subsequently, a series of 3D numerical simulations was conducted on hollow sandstone specimens with various borehole diameters under different confining pressures. The quantitative comparison of the axial deviatoric stress-strain curve and peak strength of hollow sandstone obtained from the experiments and simulations indicates that the 3D numerical results agrees very well with the experimental results. The nonlinear Hoek-Brown criterion better reflects the peak strengths of intact and hollow sandstone materials than the linear Mohr-Coulomb criterion. The effect of the borehole diameter on the peak strength of hollow sandstone depends on the confining pressure, which is interpreted by radial and tangent stress distribution around the borehole. In addition, the surface and internal fracture characteristics of the hollow sandstone obtained by X-ray micro CT observations and simulations were compared. Generally, the internal cracks of the intact and hollow sandstone specimens determined using numerical simulations are very similar to those observed using X-ray micro CT. The influence of confining pressure on the fracture evolution mechanism of hollow sandstone with various borehole diameters was analyzed in detail based on the results of numerical simulations. The conclusions drawn in this study are significant for the prediction of the instability around deep well-bores in petroleum engineering and to ensure the safety of deep excavation damage zones in tunnel engineering.