Fluid-Solid Coupling Analysis And Engineering Application Of Fractured Rock Mass Under Unloading Confining Pressure

With the increasing depth of excavation of underground mines,coal mines and oil exploration,and the construction of tunnels in water-rich karst areas,more and more engineering construction involves water environment.The instability of rock mass engineering induced by water-force coupling is also becoming more and more prominent,which not only affects the smooth construction of the project,but even endangers human life and property safety.Therefore,it is of great significance to study the fluid-solid coupling of fractured rock mass.Based on the previous studies,this paper studies the unloading mechanical properties,seepage characteristics and failure characteristics of sandstone under different confining pressures and different osmotic pressures by using the combination of laboratory test and numerical simulation,and uses FLAC^3D software.The fluid-solid coupling characteristics of fractured rock mass under different confining pressures and osmotic pressures are simulated.Finally,the tunnel engineering excavation is simulated to analyze the variation of the stress field,displacement field and pore water pressure of the surrounding rock under the effect of seepage and without considering the seepage.The main research contents and results are as follows:(1)The sandstone was subjected to conventional triaxial tests,unloading tests and fluid-solid coupling tests under different confining pressures and different osmotic pressures using the"rock multi-field coupling triaxial apparatus"to analyze the strength and deformation characteristics of sandstone.For the strength characteristics,it is found that the peak strength has a significant confining pressure effect,that is,increases with the increase of confining pressure;at the same time,it has obvious pore pressure effect,and its peak strength decreases with the increase of osmotic pressure.For the deformation characteristics,the influence of confining pressure and osmotic pressure on the elastic modulus of sandstone is also opposite,that is,the confining pressure increases and the elastic modulus also increases;while the osmotic pressure increases,the elastic modulus decreases.(2)Analysis of the seepage characteristics of the sandstone shows that the permeability of the rock is lagging behind the rock failure stage,which is due to the influence of fracture initiation and development on the rock permeability.When the initial confining pressure increases,the greater the degree of compaction of the internal crack channel of the rock sample,the smaller the permeability;when the osmotic pressure becomes larger,the higher the seepage velocity in the crack channel,the greater the permeability.(3)Using FLAC^3D software to numerically simulate the fluid-solid coupling of sandstone specimens,and compare the stress-strain curves of numerical simulation with the indoor test results.The curves of the two are similar.At the same time,the numerical simulation results are analyzed,and it is found that the peak intensity increases with the increase of confining pressure and decreases with the increase of osmotic pressure.This characteristic is also in good agreement with the test results.(4)In the numerical simulation of the tunnel,the seepage field of the surrounding rock is redistributed as the tunnel is excavated.Before the tunnel is excavated,the head distribution of the rock mass obeys the law of hydrostatic pressure distribution.During the tunnel excavation,groundwater continuously flows into the tunnel to form a seepage water funnel.Among them,the pore water pressure at the bottom of the tunnel and the left and right arches is the largest,and it is most prone to water inrush and water leakage.(5)Numerical simulation of tunnel excavation under two conditions considering seepage and no consideration of seepage,and comparative analysis of the distribution of stress field and displacement field under tunneling conditions.After the tunnel excavation stress is redistributed,stress concentration occurs at the arching position.In addition,the stress of each node is considered to be greater than the calculation result of fluid-solid coupling without considering fluid-solid coupling.By analyzing the displacement fields in the vertical and horizontal directions,it is known that the displacement at the dome is almost formed by the excavation of the upper step,and there is basically no influence when the lower step is excavated,and the horizontal displacement at the arch is excavated by the upper step and the lower step.Excavation is accumulated;at the same time,the displacement of each node under the consideration of fluid-solid coupling is larger than the calculation result without considering fluid-solid coupling.Therefore,in the analysis of rock mass stress field and displacement field,it is necessary to consider the influence of fluid-solid coupling.