Seepage characteristics of fractured limestone under osmotic pressure- stress coupling

In order to investigate the seepage characteristics of fractured rock under different confining pressures, the limestone in the Zhenfengling tunnel area of Jilin Province was selected as the research object. The multi-field coupled rock servo triaxial apparatus was used to analyze the variation of the permeability of the fractured limestone with the net confining pressure and osmotic pressure under different confining pressures. The nonlinearity fitting of the experimental data was carried out to obtain the relationship between the permeability of the fractured specimen and the change of the net confining pressure. Besides, the calculation procedure of the limestone fracture seepage-stress coupling in the FLAC3D embedded FISH language was used for the numerical calculation of specimens with different fracture angles of fractured limestone. The test results show that ① the larger the confining pressure is, the higher peak strength the limestone has, and the peak strength of the sample at high confining pressure (20 MPa) is 69% higher than the peak strength at low confining pressure (5 MPa). ② When the axial pressure and osmotic pressure are constant, the permeability of the fractured limestone decreases with the increase of the net confining pressure during the confining pressure loading phase, and recovers during the unloading phase, but lower than the confining pressure loading phase. ③ When the axial pressure and confining pressure are constant, the permeability increases when the osmotic pressure increases. ④ The inclination angle of the fracture surface of the sample decreases with the increase of confining pressure, from 90° at low confining pressure (5 MPa) to 65° at high confining pressure (20 MPa), and the fracture degree becomes more and more serious. The failure state is changed from brittle fracture under low confining pressure to ductile shear failure under high confining pressure. ⑤ The higher the confining pressure when the sample is destroyed, the smaller the angle between the crack and the horizontal, the greater the hydraulic opening, and the greater the permeability of the crack caused by the confining shear. The research results can provide some reference values for the study of the evolution mechanism of surrounding rock permeability in tunnel engineering. © 2019, Editorial Office of Journal of China Coal Society. All right reserved.