Anisotropic Characteristic Of Rock Material And Its Effect On Stability Of Tunnel Surrounding Rock

The anisotropy is an important characteristic of rock mass.The surrounding rocks of tunnel and underground engineering in stratified rock mass usually display the transversely isotropic anisotropic behaviors, which has obviously influence on the surrounding stresses and deformations. In this paper the work is mainly on the transversely isotropic behaviors, including the deformation anisotropy, strength anisotropy, anisotropic failure criterion, anisotropic elasto-plastic constitutive law and the effect of anisotropic characteristic on the deformation and stability of tunnel surroundings in the laminated rock mass in term of the regional engineering geological features that rock mass presents in horizontal and near horizontal layers. The work is done by field experiments, expreriments in lab, theoretical analyses and numerical methods with construction of twenty tunnels from Changde to Jishou expressway in Hunan province. The main conclusions are listed as follows:(1) The anisotropic elastic parameters and their variational law with lamination plane orientation of transversely isotropic sandstone and shale rock are mensurated. The validity of test results is verified by the transverly isotropic theory. The microstructure of sandstone is studied, and the micro-mechanism leading the macro anisotropy is analyzed.The anisotropic theory and elastic parameter testing methods are summarized firstly.A series of uniaxial compression tests for sandstone and shale rock has been conducted at different orientation. The deformation properties of sandstone and shale rock varying with the orientation of bedding planes are analyzed. And the test results are validated by transversely isotropic elastic theory, showing that the theoretical prediction results are in good agreement with the experimental results. The microstructure of sandstone is studied by the polarizing microscope andâ…©ray diffraction technique, and the micro-mechanism and essential reason bringing the macro anisotropic behavior of sandstone are illustrated.(2) The calculating table of stress concentration factor at the centre of the splitting specimen in the various loading directions is established, and the method used to determine the tensile strength of transversely isotropic rock is put forward. The effect of anisotropy on the tensile strength is analyzed and discussed.The influence of anisotropy on the tensile strength of rock material is studied by experimental investigation of eighty transversely isotropic Brazilian splitting numerical experimental results. The law of the stress concentration factor with the variation of passion's ratio, ratio of elastic modulus and the laminated plane direction. The parameter table of the stress concentration factor and the method calculating the tensile strength of transversely isotropic rock are proposed. The tensile strength of sandstone and shale rock at different directions is conducted by the Brazilian splitting tests. The results are contrasted with the strength obtained under the assumption of isotropy.(3) Brittle-ductile transformation character, dip effect of laminated plane, confining pressure effect and failure mode of anisotropic rock specimen are analyzed by the uniaxial and triaxial compressive tests. The distributing law of character anglecorresponding to the minimum compressive strength is presented.A series of unaxial and triaxial compressive tests for rock specimen at different dip angle have been carded out on the MTS-815 type rigid test machine system, and the stress-strain curves are obtained. The influence law of anisotropy on the uniaxial and triaxial compressive strength are analyzed and summarized. Base on the Mohr-Coulumb criterion, brittle-ductile transformation character of anisotropic rock are inquired, and the relation of brittle-ductile transformation strength and confining pressure and dip angle is obtained. The regulation of failure mode of specimen influenced by the confining pressure and dip angle of transversely isotropic plane is presented by investigating the failure progress during the test and last failure pattern.(4) A modified transversely isotropic failure criterion is brought forward based on the discussion of discontinuous criterion, the method of determining the parameters in failure criterion is discussed, and the modified strength theory are validated by experimental data.Based on summary of three kinds of transversely isotropic failure criteria, the discontinuous failure criteria are discussed, and the new modified failure criterion describing the anisotropy of rock is proposed coupling the Mohr-Coulomb criterion, weakness plane strength theory and maximum axial strain theory. The reliability of failure criterion is verified by the comparative analysis of theoretical prediction strength and experimental results presented by author and other scholars.(5) The computational procedure is developed by VC++6.0, and the modified transversely isotropic failure criterion is embedded into the FLAC numerical simulating software o Engineering instance is analyzed by applying the program.Applying the VC++6.0 langague, the subprogram describing the modified transversely isotropic failure criterion is developed, which might be called straightly by FLAC software, and the program flow is introduced. Stability of tunnel bulited in stratified rockmass is studied by the instance, and then the application of subprogram is realized in rock engineering.(6) Monitoring and field experiment'about the parameter optimization of bolting and shotcreting of tunnels in layered rock is conducted with construction of highway from Changde to Jishou. The influence of layered anisotropy on stresses and deformations of surroundings is analyzed, and the optimizing method about the bolting and shotcreting in stratified rock is proposed.The time-dependment laws of vault subsidence, horizontal convergence, bolt axial stress, and etc, are obtainded by monitoring of typical tunnel section, and the construction is guided by the feedback of informations. Field experiment about the parameter optimization of bolting and shotcreting is carded out in Tongmuwan tunnel, the optimizing method about the bolting and shotcreting for tunnel in stratified rock mass is proPosed combining with the numerical simulating results.