Experimental And Theoretical Characterizations On Damage And Permeability Evolution Of Brittle Rocks

Anisotropic damage and permeability evolution of brittle rocks under complex geological environment and engineering disturbance are key issues for optimization design and performance assessment in underground powerhouses of large-scale hydropower stations and water-sealed underground oil storage caverns,it is also a hot topic of international rock mechanics.Taking the underground powerhouse projects as the engineering background,this study aims to investigate the anisotropic damage and permeability evolution of brittle rocks under complex stress conditions.The main research work and achievements are as follows:(1)Triaxial compression tests and hydrostatic compression tests were performed on granite,fine sandstone and coarse sandstone.Corresponding to the distinct regions in the stress-strain curves,the permeability of granite evolves with a clear permeability decrease in the initial microcrack closure region,a constant permeability value in the elastic region,a stable and a dramatic permeability increase in the stable and the following unstable crack growth regions.The closure of pre-existing microcracks in the initial loading stage leads to a reduction of permeability within one order of magnitude.The permeability at the failure state increases drastically up to 2 orders of magnitude.The permeability of fine sandstone is almost constant or slightly decreased during the initial loading region,then increase rapidly after the stable crack growth region.The permeability at the failure state increases about 2-3 times compared with the initial state.While for coarse sandstone,the permeability decrease during initial loading process,and the permeability at failure state increase slightly at low confining pressure,or decrease sustainedly at high confining pressure.For hydrostatic compression tests,the permeability of all rocks decrease with increasing hydrostatic pressure,and the decrease of granite permeability is the most obvious,which can be close to one order of magnitude,followed by fine sandstone and the permeability decrease of coarse sandstone is relatively small.Based on the experimental results,the relationship between threshold stresses,acoustic emission events and permeability evolution of brittle rocks during triaxial compression tests were analysed.And a threshold stresses-based permeability variation model for brittle rocks was proposed.The proposed model has advantages of clear physical meaning,easy laboratory parameterization and well reproduced the main features of permeability variation observed in the laboratory for brittle rocks under hydrostatic/triaxial loading.(2)The anisotropic mechanical behaviors and failure mechanisms of slate were systematically examined by performing a series of uniaxial and triaxial compression tests on the slate specimens with varying inclination angles(θ=0-90°)under different confining pressures(σ3=0-20MPa).The variation of the compressive strength as a function of the foliation angle θ at different confining pressures follows a typical U-shaped trend,with the maximum strength attained at θ=90° and the minimum strength at θ=45°.The slate samples were observed to fail in three typical modes,i.e.shear failure across foliation planes,sliding failure along foliation planes and axial-splitting from foliation planes,depending on the loading direction(θ)with respect to the foliation.Based on the experimental observations,a micromechanical damage-friction model was proposed for the foliated slate by characterizing the slaty rock as a homogeneous elastic matrix containing arbitrarily-distributed penny-shaped microcracks and a family of parallel foliation planes.The good agreement between the model predictions and the laboratory measurements shows that the proposed model satisfactorily describes the stress-strain relation of granite and sandstone,and the anisotropic deformational and strength behaviors of the foliated slate.(3)Based on the proposed micromechanical damage-friction model and the threshold stresses-based permeability variation model for brittle rocks,the influences of initial geostress and tectonic development characteristics on the damage and permeability variation in the surrounding rock of underground caverns were studied.Then the excavation-induced damage and permeability changes of the underground powerhouse on the right bank of Baihetan Hydropower Station were analysed.The research results can provide a reference for the deformation and seepage control of large underground caverns.