Seepage-stress Coupling Experimental Sresearch On Deep Shaft Water-rich Sandstone Pre-freezing And Post-thawing

For the lack of research attention on freeze-thaw damage on soft rock in deep Cretaceous and Jurassic strata,after frozen wall thaws out,deterioration on rock around freezing pipe enhances the permeability,which connects deep stratum to upper aquifer and leads to water inrush into shaft.Current research on freeze-thaw can not evaluate the influence on deep rock accurrately,because stress boundary is not take into consideration.In the view of problems in shaft freezing engineering crossing Mesozoic strata and insufficiency of current research,we tested the mechanical properties of frozen intact sandstone and cracked sandstone,explored seepage-stress coupling properties and the development of pore in sandstone pre-freezing and post-thawing.Typical red sandstone,medium sandstone,fine sandstone within the water-rich stratum at depth of 500-600 m are selected as test objects,and frozen cracked sandstone with different dip angles was made.Uniaxial and triaxial compression tests on frozen intact sandstone and cracked sandstone were conducted.With temperature ranging from-5°C to-15°C,strength of frozen red sandstone,medium sandstone and fine sandstone was found to increase linearly with the drop of temperature,while temperature effect was weakened as confining pressure increasing.Frozen strength of the three kinds of sandstone increased linearly with confining pressure,which followed Mohr-Coulomb criteria.When temperature dropped from-5°C to-15°C,internal friction angle of frozen red sandstone,medium sandstone and fine sandstone increased by 9.74%,12.54% and 10.67%,and cohesion increased by 15.03%,18.69% and 15.39% respectively,which indicated that temperature affected cohesion more than friction angle.Strength of frozen cracked red sandstone increased linearly with confining pressure,while it degraded with the increase of dip angle,and remained approximately constant when the dip angle was greater than 30°.Frozen cracked red sandstone with horizontal crack presented tensile failure under uniaxial compression,while under confining pressure,it failed with sheartension compositing destruction.Specimens with dip angle ranging from 15°-45° failed with the break up of ice and shear slip along the rock-ice interface both in uniaxial and triaxial compression test.Saturated medium sandstone in depth of 450 m in Shilawusu coal mine was selected to conduct freeze-thaw experiments under different pressure and seepage-stress coupling experiments.Based on CT image,three-dimensional pore structure in medium sandstone pre-freezing and post-thawing was reconstructed and the change of pore size distribution in CT scale were analized.Pore size of medium sandstone was found to mainly fell in the range of 10-80?m,and development of pores in this range was influenced by freezethaw most.The higher the pressure in freeze-thaw process,the greater the permeability and porosity increment were,while the lower the strength under seepage condition were.Fluid pressure had critical value of 5MPa.When fluid pressure was lower than the value,permeability increased obviously with fluid pressure,while when fluid pressure was higher than the value,permeability nearly remain constant or even decreased.In hydromechanical coupling triaxial compression loading process,when confining pressure was 24 MPa,permeability of all specimens with or without freeze-thaw remained constant or decreased slightly with axial strain developing;when confining pressure was 6MPa,permeability of all specimens decreased with the increase of volumetric strain in initial stage,and increased with the decrease of volumetric strain after dilatancy occurred;when confining pressure ranging from 12 MPa to 18 MPa,permeability evolution in loading process was complex in plastic stage,it might rise,drop,or remain constant.Based on statistical damage constitutive theory,improved model which could considering the effect of stiffness degradation on Biot coefficient in damage process was established,and verified with experimental data.The improved model was capable of describe the hydro-mechanical behavior of medium sandstone with or without freezethaw process.