Study On The Mechanical Properties Of Sandstone Under Loading And Unloading At High Confining Pressure And High Pore Pressure And Its Influence On The Stability Of Deep Buried Tunnel

With the rapid development of China's transportation,large-scale hydropower projects,the deep mining of coal resources,the safety construction problems of underground engineering under high confining pressure and high pore pressure is becoming more and more serious.Under the condition of high confining pressure and high pore pressure,the mechanical properties of rock are different from those of shallow layer.In addition,the unloading effect of deep underground engineering excavation is different from that of conventional loading.However,the related research is deficient.Thus,on the basis of previous studies,based on the indoor test,theoretical derivation and numerical simulation,the mechanical properties,seepage characteristics and energy conversion of sandstone under high confining pressure and high pore pressure are studied systematically.And the derivation of the unloading constitutive model is redeveloped by FLAC3 D.The main research contents and results are as follows:?1?Uniaxial and triaxial compression tests under natural and saturated state,triaxial tests of high confining pressure and high pore pressure under loading and unloading were carried out by multi-field coupling rock triaxial test apparatus called TRIAXIAL CELL V4.0.Based on the tests of different initial confining pressure and unloading level,the test results show that in the same initial confining pressure,the higher the initial unloading level,the smaller the confining pressure is reduced,and the shorter the process of unloading confining pressure.With the improvement of initial unloading levels,the cohesive ? increases exponentially,friction angle j decreases polynomial.The lateral strain increment ??₃ in the two stages of unloading confining pressure is larger than that of axial strain increment ??₃,which shows obvious lateral expansion.There is a power function relationship between the dilatancy index dI and the confining pressure,which is the quadratic function relationship with the initial unloading level.?2?For the unloading tests high confining pressure and high pore pressure,when the initial unloading level n is kept constant,with the increase of pore pressure p,the ultimate strength under unloading decreases first quick back slow trend.And the rock is more prone to damage,more brittle,the damage variable has increased.When the pore pressure p is kept 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 ?decreases linearly.The damage variable is also decreased,especially when n is from 0.9 to 1 of the process,? reduction is more pronounced.Considering the influence of initial unloading level n and pore pressure p,the friction angle and cohesion were fitted by two element 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.Through matching the test data,the expression of Mohr-Coulomb yielding criterion for the sandstone under different initial unloading levels n and pore pressure p was modified.?3?Based on the results of triaxial compression tests of sandstone under high confining pressure and high pore pressure,a modified Duncan-Chang constitutive model of sandstone under loading condition is proposed.The parameters of the model are analyzed,and the rationality of the improved Duncan-Chang constitutive model is verified by the triaxial compression test results.?4?Based on the modified Mohr-Coulomb strength criterion,the four line elastic-plastic model is established from indoor triaxial tests under unloading.The influence of pore pressure p and initial unloading level n on parameters fa,fb,rra and b is analyzed synthetically,and the parameters of unloading state are fitted by binary function.?5?According to the theory of finite difference and good redevelopment platform of FLAC3 D software,the finite difference form of the nonlinear unloading model for sandstone is deduced.Based on which,the redevelopment of nonlinear unloading model under high confining pressure and high pore pressure is carried out in FLAC3 D using V++6.0 programs,and then the correctness and applicability of the redevelopment computer program is verified by experimental simulation and engineering example analysis.?6?The nonlinear unloading model computer program is applied to stability analysis of support engineering.According to the simulation results,the distribution of surrounding rock displacement,stress and plastic zone was discussed,and the stability of tunnel under different pore pressure and ground stress is analyzed.