Experimental Studies On Mechanical Properties And Post-Peak Non-linear Flow Of Saturated Granite Rock

As more and more high-dam hydropower and high head pumped storage projects have been constructed in China,the rock foundation of high dam and high pressure water diversion system generally sustain high water pressure and hydraulic gradient.Granite play an important role in pumped storage power,shale gas and geothermal resources exploitation and high-level radioactive waste disposal,etc.engineering construction as a series of characteristic of high density,low porosity,lithology density and high strength.Due to the construction of mining engineering,water conservancy,hydropower project and high-level radioactive waste disposal engineering,the surrounding rocks are broken.In this circumstance,not only worsening the environmental of rock mass that close to surrounding rocks which caused the changes of permeability and permeability characteristics of rock mass,but also accompany with leakage,seepage failure and water-inrush risk.A large number of studies have shown that non-linear flow occurs in the broken rock mass under the condition of high osmotic pressure and high stress.If use the Darcy’s law to research the seepage problems of broken rock mass,it will cause larger error and negative impact of engineering safety evaluation.This study experimentally investigated the mechanical properties of saturated granite rock before the peak and the transformation law of non-linear flow with the changing of hydraulic gradient and confining pressure in Yangjiang through Triaxial Cell test system.Analyzing possible causes to trigger the non-linear flow.Further,the applicability of non-linear equation which describe the seepage in broken granite and the regular of formula parameters with changing confining pressure have been discussed.The main contents of the paper are as follows:(1)Through XRD and polarizing microscope scanning,the mineral origin and microstructure of granite are authenticated.Granite main ingredients is feldspar and quartz.Granite has the characteristics of good cementation,high density,complete mineral grains.Except a small quantity of intercrystalline crack,mineral particles contact closely.In addition to a few relatively obvious crack,microcrack distribute randomly in granite and present isotropic characteristics.(2)Based on the results of triaxial compressing tests of saturated granite rock,the stress-strain curve of saturated granite rock can be divided four-stage during failure.Stress threshold can be ascertained by the relationship between volumetric strain and axial strain.The crack initiation stress of saturated granite rock enhance obviously relative to the unsaturated granite,meanwhile,the peak stress reduce.Comparing with unsaturated granite,the initiation stage of saturated granite rock go through a long process.Combined with the cracked morphology of broken granite rock and the curve of rock failure process,a series of possible causes triggering the phenomenon have been elucidated.Energy evolutionary mechanism of saturated granite rock during failure have been researched according to the energy theory.Total energy that absorbed by the granite rock and elastic strain energy increases rapidly before granite rock damaged.Saturated granite store up energy that provide by external force in form of elastic strain energy mainly.Total energy that absorbed by the granite rock increases slowly and elastic strain energy decreases rapidly after granite rock damaged.Based on the energy failure criteria,the saturated granite rock loses bearing capacity when elastic strain energy reaches 0.722~0.764 MJ?M.(3)This study experimentally investigated the non-linear flow characteristics of broken granite rock in Yangjiang subjected to a wide range of confining stresses(1-30 MPa).The characteristics of non-linear flow in broken granite rock have been elucidated,and its representation methods have been investigated.The results show that non-linear flow occurs in the broken granite rock with the condition of flow increase greater and less than linear flow.The Forchheimer equations have been adopted to characterize the non-linear flow that increase less than linear flow,respectively,which provide excellent description for the non-linear flow in broken granite rock.The power-law function between hydraulic conductivity with non-linear coefficient B has been developed.Further,based on the Forchheimer number F0,characteristics of the flow can be distinguished.The critical hydraulic gradient is less than 10 MPa/m and decreases with the increasing confining stress in broken granite rock.The Forchheimer and Izbash equations have been adopted to characterize the non-linear flow that greater than linear flow,respectively,which both provide excellent description for the non-linear flow in broken granite rock.The non-linear coefficient of Forchheimer equation experience positive and negative values during the loading process,which is possibly ascribed to the closure and extension of the cracks in the broken granite rock.The power-law function between the non-linear coefficients of Izbash equation has been developed by relating them with confining stress.The results may prove useful in proper understanding of fluid flow through fault zone and compressive zone under high water pressure and hydraulic gradient.(4)Based on the 3D model(three-dimensional model)of the YaGen secondary hydropower station,The Darcy flow numerical method and the non-Darcy flow numerical method have been adopted to analyze permeability rule of the dam under high head pressure.The calculation results show that both method can be used to assess hydropower station stability and safety of seepage.