Study On Engineering Geological Properties Of Red-bed Sandstone In Gaolan Mountain, Lanzhou

The red-bed is widely distributed in southwest, south, southeast and northwest of China. The red-bed sandstone softens and disintegrates quickly in water. Weak planes exist in rock mass. In order to ensure the engineering construction carried out successfully in red-bed, it is of great significance to ascertain the engineering geological properties of the red-bed sandstone, especially the influence of bedding angle on the strength.In this paper, uniaxial compression tests, triaxial compression tests, acoustic testing experiments and numerical simulation were carried out on the red-bed sandstone samples of different bedding angles. The conclusions are as follows.(1) The failure modes of samples under uniaxial compressions are anisotropic. With the increase of the coring angle, the failure mode of samples is firstly vertical fractures, and then shows slip shear fracture along the bedding plane and gradually translates into tensile fracture intersecting the bedding because of pressure.(2) The mechanical properties of samples under uniaxial compressions are anisotropic. The uniaxial compressive strength of vertical bedding direction are biggest. The uniaxial compressive strength of parallel bedding direction is smaller than that of vertical bedding direction. The uniaxial compressive strength is smallest when the coring angle is 30°. The secant young’s modulus and Poisson ratio decrease with the increase of coring angle.(3) The peak strength and residual strength of samples under triaxial compressions increase with the increase of confining pressure. The secant young’s modulus also increases with the increase of confining pressure. The difference value between peak strengths decreases with the increase of confining pressure. It shows that the anisotropic property of samples becomes weaker with the increase of confining pressure. For the sandstone sample obtained in Gaolan Moutain, Lanzhou, it turn into isotropic material when the confining pressure reaches to 8MPa. The failure mode of samples follows the Mohr-Coulomb criterion. The calculated angle by Mohr-Coulomb failure criterion is close to the fracture angle after destruction and the actual fracture angle can be predicted on the basis of the failure criterion.(4) P-wave velocity and S-wave velocity obtained at free condition are anisotropic. P-wave velocity and S-wave velocity decrease with the increase of coring angle. The value of the VP/VS ratio also decreases with the increase of coring angle. There exists relation between P-wave velocity and S-wave velocity. The P-wave velocity increases with the increase of S-wave velocity. There is regular change of dynamic elastic modulus, dynamic shear modulus, dynamic bulk modulus and dynamic lame constant which are calculated from the velocities of wave. These dynamic elasticity parameters decrease with the increase of coring angle.(5) P-wave velocity and S-wave velocity increase with the increase of confining pressure. The difference value between velocity of wave decreases with the increase of confining pressure. It shows that the anisotropic property of samples becomes weaker with the increase of confining pressure. There is regular change of dynamic elastic modulus, dynamic shear modulus, dynamic bulk modulus and dynamic lame constant which are calculated from the velocities of wave. These dynamic elasticity parameters increase with the increase of confining pressure. P-wave velocity and S-wave velocity are strong correlative to differential stress of samples under triaxial compressions. The change of velocity can reflect the strength of samples.(6) Uniaxial and triaxial compressive strength obtained by numerical simulation are high when the coring angle is 0° or 90°. The uniaxial compressive strength is low when the coring angle is 30°, which is consistent to the result of the mechanical tests.