Study On Damage Mechanism And Creep Characteristics Research Of Quartz Sandstone Under Freeze-thawing Action

With the successive implementation of major strategic plan,such as the development of western region,the13 th five year plan and “one belt and one road” and so on,many major projects have emerged,such as mining,water conservancy,railways,highways,civil building and so on.However,these emergent major projects will stand the test of freeze-thawing effect in cold area because of freeze-thawing disasters caused by the special geological environment in western high-cold mountain area.Geotechnical engineering design and construction,and long-term use in the future will be influenced directly because of the damage and deterioration of rock strength and increasing deformation caused by the repetitive freeze-thawing.More and more freeze-thawing disasters gradually become the focus in engineering geology and geotechnical engineering field with the carry out of major projects in cold region.Therefore,a series of indoor mechanical tests have been conducted,including freeze-thawing cycle test,conventional triaxial test and acoustic emission(AE)test,triaxial creep test.The mechanical characteristics and macroscopic fracture characteristics of quartz sandstone after freeze-thawing are been studied through comparative analysis in the failure process under the instantaneous mechanical and rheological mechanical action.The fracture mechanism of rock mass is been investigated under the condition of cyclic freeze-thawing.Moreover,F-T HKBM nonlinear creep damage constitutive model is been established based on freeze-thawing environment the results of the existing indoor experiment.The main results of this study are as follows:(1)The rock X-ray powder diffraction experiment and polarizing microscope rock slice experiment have been done to study the micro structure characteristic of sandstone.The study shows that the main mineral components of sandstone contains not only hard coarse particle,such as quartz and feldspar,but also clay minerals which are sensitive to freeze-thawing,such as kaolinite,vermiculite and montmorillonite.The sandstone,which has higher strength,is mainly dominated by quartz mineral.Additional,it can be found from conducted freeze-thawing cycle test that there are no obvious macroscopic failure signs in the rock surface and macroscopic tiny rock fall is appeared.The wave velocity of sandstone decreases and the quality change is very small with increasing freeze-thawing cycles.(2)The rock failure mechanism under different freeze-thaw cycles and different confining pressure are analyzed through the conventional triaxial test.The triaxial compressive strength,elastic modulus of samples decrease and the Poisson’s ratio increases with increasing freeze-thawing cycles in the same confining pressure.Overall,the conventional mechanical parameters change little.It turned out that freeze-thawing has a role in softening and degradation on instantaneous mechanical properties of rock but not obvious because of the characteristics of high strength and low porosity of samples.In addition,it can be found by monitoring acoustic emission signals in the process of triaxial compression test that the AE peak ringing count decrease and the time corresponds to AE peak is been advanced with increasing freeze-thawing cycles in the same confining pressure.And the time corresponds to AE peak is been lagged with increasing confining pressure in the same freeze-thawing cycles.It has proved that there are freeze-thawing damage effect and confining pressure constraint action.(3)The triaxial creep test under different freeze-thaw cycles and different confining pressure has been carried out.The result shows that the creep deformation is more obvious and the load level that the sample can stand is decrease after standing different freeze-thawing cycles.The failure strength and long-term strength also decrease gradually.The effect of confining pressure on rock is mainly constraint effect but the creep of sample decreases firstly then increases because of the existence of freeze-thawing damage with increasing confining pressure in the same freeze-thawing cycles.The creep rate of sandstone has also experienced three stages under the level of fracture stress: the initial creep rate attenuation stage,the steady-state creep rate uniform stage,the accelerated creep rate stage which all are decrease with increasing freeze-thawing cycles in the same confining pressure.(4)Three failure modes appeared when failure after freeze-thawing in the process of conventional triaxial test: tensile failure,shear failure or a combination of two kinds of destruction.The crack on surface of sample increase with freeze-thawing cycles.The failure modes after freeze-thawing in the process of triaxial creep test are all shear failure.However,the broken way under long-term load transform from a single inclined shear to “X” shape conjugate shear plane gradually with freeze-thawing cycles and its degree of crushing is more severe compared to the action of conventional triaxial.In addition,the degree of wear of damaged section increase and the failure mode transform from brittle failure to ductile failure gradually.(5)Freeze-thawing damage model and creep damage model considered freeze-thawing are been established based on previous research and combined with damage mechanics theory.F-T HKBM non-linear rheological damage model that suitable for hard rock is established through introducing freeze-thawing damage and creep damage variables based on the triaxial creep test results after different confining pressure and different freeze-thawing cycles.The model is verified and the related parameter is identified by triaxial creep test curve of sandstone under different confining pressure and different freeze-thawing cycles.The result shows that the model has a high alignment with the creep test results.The model can clearly reflect damage evolution process of quartz sandstone under the effect of freeze-thawing and mechanics.