Numerical Study Method Of Seepage Characteristics In Rock Damage Process

With the development of large-scale hydropower projects as well as exploration and exploitation of unconventional oil and gas resources during recent decades,investigations on permeability properties of engineering rock masses have become more and more important.Rock deformation and failure under external load is an extremely complicated mechanical process.On the one hand,the defects on mesoscale level of rock sample itself lead to the materials' non-linearity the test curves and material parameters of different lithologic samples are extremely discrete.On the other hand,rock masses usually contain a complex fracture network system owing to the presence of many discontinuous structural planes.Thus,the permeability of rock masses is controlled by the properties and behavior of fractures and joints.Because rock material is heterogeneous and the fracture network is highly discontinuous and randomly distributed,the mechanical properties of rock blocks used for laboratory tests can be very different from those of engineering-scale rock masses.It has been a problem to address how the dispersive experimental data should be correlated with the parameters of engineering-scale rock masses.At the same time,the engineering rock mass is usually stored in a geological environment formed and interacted with stress field,seepage field,temperature field,and etc.The fluid-solid coupling between surrounding rock mass and groundwater is a complex phenomenon.Particularly,when the surrounding rock mass damages and new fractures grow during the excavation unloading process,the stress field and the seepage path change continually,resulting in a dynamically changing seepage flow field.This seepage flow field variation further influences the stress field distribution.Thus,the stress and the seepage interact to determine the rock mass fracture pattern and the seepage channel development pattern.The accumulated strain energy dissipates in the form of acoustic emissions during the rock mass damage and fracture process.A coupled damage-seepage-energy dissipation model is developed and implemented into the realistic failure process analysis(RFPA)method.It is then applied to simulate the damage,energy dissipation and seepage channel formation during the failure processes of intact and jointed rock mass.A statistical distribution model may be established for joints based on field investigation,and the mesoscopic properties of the joints may be correlated with the statistical macroscopic properties.Numerical methods can be applied to study the formation of seepage channels and the variation of permeability during the rock failure process.The parameters can then be modified for different element sizes and engineering scales.The main content has been completed is as follows:(1)The RFPA method is extended to Multipath realistic failure process analysis for coupled stress-damage-flow analysis during the progressive failure process of the rock mass.Repeated damage may occur in rock masses,and joints may open and close a few times when they are under loads.Hence,the effect of loading history on the current state of the rock mass must be considered.Five states are defined for each element in Multipath RFPA,the current state of each element is determined by its damage history,the current stress-strain level and the material properties.New failure paths are developed along the elements that have undergone damage,contact and separation.Thus,conversion among various damage states of the element under complex loading paths can be considered.For instance,elements that have failed under compression may fail again under tension,or elements that have been in contact may separate again.(2)In this study,a damage process permeability model is developed,which includes the overall effect of the element damage history and the current stress and strain levels on permeability.The discrete fracture network(DFN)method is then implemented to represent random defects in rock masses using many joint elements.The complicated mechanical behaviors of rock such as the complex fracture form,the repeated opening and closing of the fracture and the interlocking,locking and re-fracture between fissure surface and fracture block are described in detail.(3)On the basis of the outstanding achievements of previous researchers,this paper introduces the modern software design method,borrowing design and development ideas from the existing mature engineering software CAE and building the REAS(Rock Engineering Analysis System)software architecture from the bottom,with the specific function realized.The user interface is developed with C#,A high-performance graphics visualization system was developed by using cross platform graphic library OpenGL.The grid subdivision,calculation and analysis programs are developed in C++/Fortran.The stress-seepage solution is based on the finite element method,the fluid-solid coupling theory and the successive over relaxation(SSOR)-preconditioned conjugate gradient(PCG)method.(4)This paper reproduces variations of macroscopic permeability of jointed rock masses with various sizes using numerical methods.The relationship between the observation scale and the seepage directionality or randomicity is presented,and the sensitivity of the scale effects of rock mass strength and permeability is compared.The paper also discusses effect of confining pressure on the scale effect of the permeability and scale effect of the permeability under triaxial loading conditions.(5)The permeability and acoustic emission activity of the complete rock mass and the joint rock mass during fracturing are compared to identify differences in the hysteresis.On the basis of the numerical simulation,it is concluded that the sharp increase of the permeability and the peak of the associated energy release immediately occur after the peak load during the failure process of the intact rock.However,in the case of the jointed rock mass,both the formation of the seepage channel through the entire jointed rock mass and the sharp increase of the permeability lag not only the peak load but also the sharp increase of the microseismic energy release.Based on the seepage hysteresis,a sharp increase in acoustic emission is identified as a precursor to the step change in permeability.The effectiveness of inrush accident prevention using microseismic monitoring is verified based on this mechanism.(6)Finally,the coupled damage-seepage-energy dissipation model is applied the Jiaozhou Bay submarine tunnel project in China to investigate the relation between the seepage channel formation and the precursor microseismic events when the tunnel passes through a fracture zone.On the basis of the numerical results,the feasibility of using microseismic monitoring to early warn water inrush disaster during tunneling is investigated.