石英砂岩力学特性及各向异性试验研究

Experimental study of anisotropy and mechanical property of quartz sandstone

为研究不同应力条件下层理对高孔隙率岩石材料破坏模式、弹性模量以及强度的影响,针对各向异性的石英砂岩,采用静水预压缩–增大轴向应力的加载方式,进行了多组带有不同层理角度试样的三轴试验。试验结果表明,不同应力条件下各向异性的石英砂岩试样表现出不同的变形破坏特征,以30 MPa围压为界,低围压范围内破坏模式为剪切型破坏,在该范围内随着围压增大破坏面由倾斜转向水平,并受到试样层理角度不同程度的影响,高围压范围内受到层理的影响分为剪切屈服和体积屈服两种破坏模式;随着围压的增大,新闭合的孔隙裂纹数目减少,各向异性对弹性模量的影响呈现降低趋势,弹性模量增长速率降低,弹性模量在层理分布角度?=60°时都会出现下降的现象;不同层理分布试样的强度曲线呈U型,围压在0~20 MPa范围内试样?=30°强度最低,围压在30~60 MPa范围内强度最低试样转化到?=45°附近;不同层理角度石英砂岩的强度特征系数随着围压的增大而逐渐减小,围压大于30 MPa后该系数逐渐趋于稳定。

To investigate the effect of bedding on failure mode, elastic modulus and strength of rock with high porosity under different stress conditions, several groups of triaxial tests on specimen with different bedding angles are conducted with pre-hydrostatic compression and axial stress increasing mode. The results show that the anisotropic quartz sandstone showed different characteristics of deformation damage under different stress conditions. With 30 MPa as dividing line of confining pressure, shear failure mode happened in the range of low confining pressure and as the confining pressure increased, fracture surface turned slope to horizontal and is influenced by the bedding. In range of high confining pressure, under the influence of bedding, failure mode could be divided to shear yield and volumetric yield. As the confining pressure increased, the number of newly closed crack reduce, the effect of anisotropy to elastic modulus showed reducing trend and the average increasing rate of Young's modulus decreased. Young's modulus of all specimen declined at bedding angle gamma = 60 degrees. Strength curves of different specimen with different bedding angles showed U shape. The minimum compressive strength in range of 0-20 MPa confining pressures showed at the bedding angles gamma = 30 degrees; while in 30-60 MPa transferred to gamma = 45 degrees. Feature coefficient of compressive strength for quartz sandstone with different bedding angles decreases as the confining pressure increases, and keep constant as confining pressure is over 30 MPa.