Seepage And Stress Coupling Algorithm Of Fractured Rock Mass By Composite Element Method

There is an important and complicated coupling between seepage and stress fields in the fractured rock mass. On the one hand, the stress field is influenced by the seepage load; on the other hand, the stress field affects the permeability characteristics and in turn the seepage field as well. Due to the West-East electricity transmission project and South-North water transmission project (the west line), the construction of the hydraulic and electricity engineering has been shifted to the high canyon area in the west in China, where the active tectonic movement, atrocious weather, well developed fractures, and high geostress are found playing important roles. Therefore, the control of rock seepage、deformation and stability mainly depends on the analysis level of seepage and stress coupling for fractured rock mass. European and American scholars also work hard on the seepage and stress coupling for fractured rock mass in order for the problems of nuclear waste deposit, deep oil exploitation, deep long tunnel and so on.So far, there have been some accomplished researches on the seepage and stress coupling for fractured rock mass, mainly including experimental technology, hydraulic parameters, numerical models, numerical algorithm, and so on. However, there are still some difficulties to be overcome, such as how to actually and conveniently simulate the complicated configuration (fractures, drainage holes, bolts, etc.) with simplified preprocess work, how to consider the coupling mechanism for fractured rock mass as actual as possible, how to take the flow exchange between fracture and adjacent rock block into account, how to deal with the problem of fractured rock mass containing drainage holes, and so on.As for the problems mentioned above, a composite element algorithm on the seepage and normal stress coupling for fractured rock mass firstly has been proposed in this dissertation. Then the coupling mechanism for fractured rock mass during the shear process has been established, as well as the coupling algorithm on the seepage and stress for fractured rock mass containing drainage holes. Finally the proposed algorithms have been taken into application to the Xiaowan arch dam foundation system with complicated configuration. The main work in this dissertation can be briefly summarized as follows:At first, based on the composite element method (CEM), the seepage and normal stress coupling algorithm of fractured rock mass by the CEM has been presented, which is an important extension of CEM. The rock fractures are assumed as a filled medium, and it enables to treat the rock fractures with or without fillings in a unified way. The coupling mechanism has been carried out through iteration algorithm between the seepage and stress field, and the flow exchange between the fracture and adjacent rock block has been taken into consideration. The FORTRAN programs have been compiled for the proposed algorithm. And finally the proposed algorithm has been verified by one simple numerical example. The computation results have been compared to the conventional finite element method (FEM), where the advantages and reliability of the proposed algorithm have been well shown, and the obtained seepage velocities along the fracture are much closer to the actual ones than those obtained by the FEM.Next, considering the different levels of normal stresses, the relationship between shear stress and shear deformation of rock fracture has been established. This relationship can be divided into three phases:shear shrink, shear dilation up to peak value, and residual shear strength. Then, based on the CEM, the seepage and stress coupling algorithm of fractured rock mass during shearing has been carried out, by which the variation and relationship among the shear deformation, fracture aperture, conductivity of rock fracture, seepage field as well as stress field are illustrated. Finally the numerical example indicates that, with the constant mechanical parameters of filled medium in the fracture, the seepage velocity of fracture remain stable with variable shear deformation and normal stress, but if the fracture aperture changes, the flow rate per unit width through the fracture will change accordingly.And then, the CEM is formulated for the seepage and stress coupling algorithm of fractured rock mass containing drainage holes. Each fracture or drainage hole segment is considered as a special sub-element with definite seepage and deformation characteristics, which is located explicitly within the composite element. According to the variational principle the governing equation for the composite element containing both fractures and drainage holes is established and implemented in the software. Based on the CEM developed in this dissertation, the fractures and drainage holes can be simulated explicitly but do not participate in the computation domain discrete, in this way the generation of the computation mesh is not restricted strongly by the position and the orientation of the fractures and drainage holes, which is of significance in the optimal design of seepage control system. If there are no fractures and drainage holes, the CEM will automatically be degenerated to the FEM. The coupling mechanism on the seepage and stress for fractured rock mass containing drainage holes has been carried out through the iteration algorithm between the seepage and stress field. The FORTRAN programs also have been compiled for the proposed algorithm. And finally the validity and reliability of the proposed algorithm have been verified by the numerical examples. The application and comparative study for the Baozhusi dam foundation have been presented also.Finally, the proposed coupling algorithms based on the CEM have been taken into application to Xiaowan arch dam foundation system with complicated configuration, and their advantages and reliability of the proposed algorithms have been further proved. The proposed algorithms can be applied conveniently in the complicated engineering with simplified mesh generation preprocess, and the obtained seepage velocities through the fracture and drainage holes are much closer to the actual ones than those by the FEM. The uneven hydraulic behavior of the fractured rock mass resulting from the stress is remarkable when considering the seepage and stress coupling, and the importance of the coupling analysis has been further emphasized for fracture rock mass or fractured rock mass containing drainage holes.The innovation of this dissertation can be briefly summarized as follows:Based on the CEM, the seepage and normal stress coupling algorithm of fractured rock mass has been proposed, which takes the flow exchange between the fracture and adjacent rock block into account, and can be used for both filled and unfilled fracture; The composite element algorithm on the seepage and stress coupling for fractured rock mass during the shear process has been established, and the shear process has been divided into three phases, starting from the shear shrink phase, then into the shear dilation phase until the peak value phase, and ending with the residual shear strength phase; The seepage and stress coupling algorithm of fractured rock mass containing drainage holes has also been proposed by the CEM, where each fracture or drainage holes is considered as a special sub-element with definite seepage and deformation characteristics, and embedded explicitly into the composite element; The FORTRAN programs have been compiled for all the proposed algorithms; And finally all the proposed coupling algorithms have been taken into application to Xiaowan arch dam foundation system with complicated configuration, where the advantages and reliability of the coupling algorithms have been shown obviously.