不同初始卸荷水平和水压下砂岩卸荷力学特性试验研究

Experimental research on unloading mechanical properties of sandstone under different initial unloading levels and pore pressures

深部岩石所处的不同初始卸荷水平状态及水压环境对其力学特性影响明显,基于对砂岩进行常规三轴加载试验、不同初始卸荷水平及水压条件的卸荷试验,研究了卸荷条件下砂岩的极限强度、变形特征、变形损伤以及卸荷抗剪强度。研究结果表明：初始卸荷水平n一定时,随着水压p的增加,卸荷极限强度的降低呈现先快后慢的规律,岩样更容易发生破坏,脆性特征更明显,损伤变量ω也呈增加趋势;当水压p一定时,随着n的增大,卸荷极限强度线性增大,统一围压降参数η线性降低,损伤变量ω也减少,且n从0.9到1.0的过程中,ω降低更明显。综合初始卸荷水平n和水压p的影响,对摩擦角和黏聚力进行了二元函数拟合,利用该拟合公式可以预测不同n,p条件下的黏聚力和摩擦角,具有一定工程意义。并基于对试验数据的拟合,对不同n,p作用下的砂岩Mohr–Coulomb准则表达式进行了修正。

The mechanical properties of deep rocks are obviously affected by the initial unloading level and water pressure environment. By carrying out triaxial compression tests and unloading tests on sandstone under different initial unloading levels and pore pressures, the ultimate strength, deformation characteristics, deformation damage and shear strength under unloading are studied. The test results show that when the initial unloading level n is kept to be constant, with the increase of pore pressure p, the ultimate strength under unloading decreases quickly first then slowly. The rock is more prone to damage, more brittle, and the damage variable increases. When the pore pressure p is kept to be constant, with the increase of the initial unloading level n, the ultimate strength under unloading increases linearly and the uniform decreasing increment of confining pressure η decreases linearly too. The damage variable decreases, especially when n is from 0.9 to 1 of the process, and ω reduction is more pronounced. Considering the influences of the initial unloading level n and pore pressure p, the friction angle and cohesion are fitted by the binary function. It has a certain engineering significance that the values of cohesion and friction angle under the condition of different n and p can be predicted by the fitting formula. And based on the fitting of the experimental data, the expression of Mohr-Coulomb yielding criterion for the sandstone under different initial unloading levels n and pore pressures p is modified.