Time-dependent Deformation Of Sandstone Under Cyclic Unloading And Loading Stress

Buckling failure of mining structures,for instance surrounding rock mass of tunnels,mining rooms and mining pillars,due to time-dependent deformation is a serious problem faced in the research of stability analysis for underground engineering.There are a lot of reasons why surrounding rock of tunnels time-dependently deform to fail,a common one of which is the influence of neighboring mining operations,such as excavating and supporting of adjacent tunnels and cutting coal at the mining face.These operations result in the variation of induced stress burdened by surrounding rock,which is easy to accelerate the failure of rock with time-dependent deformation.For example,reciprocating mining activities,like cutting coal at the coalface,are easy to bring about the cyclic changes of mining induced stress on the surrounding rock and the surrounding pillars.Therefore,it is worth to do some in-depth researches about the effect mechanism and the features of these cyclic stress variations on the stability of neighboring rock mass with time-dependent deformation.Laboratory test is an efficient and reliable way to investigate the time-dependent deformation of rock.But,as for the state of the art relevant to the current technology,it is not enough efficient to investigate the evolution of rock failure from microscale to macroscale,only relying on the method of laboratory tests.However,laboratory experiments coupled with numerical simulations is a very popular method to study the time-dependent failure of specimens from micro-crack evolution to macro fractures.In order to obtain the viscoelastoplastic behavior of rock,and simulate the damage evolution of rock failure from microscale to macroscale during the simulation of the effect of cyclic loading and unloading stress on the time-dependent deformation,a suitable numerical model is needed.Even though,few researchers have done some effort in the numerical model about the time-dependent visco-elastoplastic mechanical behavior of rock,it still needs more improvement in regard tothe time-dependent model considering the path of loading and unloading stresses.Furthermore,reflecting the micro heterogeneity of rock and regenerating the macro failure based on the damage evolution of micro elements in a numerical model also should be paid more attention.To solve the above question,plastic yielding function,plastic potential function and strength criterion of elements for Neijiang sandstone,were firstly determined by the experimental data obtained from conventional uniaxial and tri-axial constant strain rate tests.And the instant(time-independent)elastoplastic damage constitutive model considering the anisotropic elements was built based on the non-associated flow rule constructed by the plastic yielding function and the plastic potential function.The elastoplastic numerical model was validated by the comparison with analytical solution of stress distribution around the circular hole.Meso heterogeneous properties of numerical model were determined based on the mechanism of micro mineral particles in sandstone.Then some conventional uniaxial and tri-axial constant strain rate tests were simulated using the time-independent elastoplastic damage model to repeat the macro failure process of Neijiang sandstone on the basis of damage evolution of micro-elements.Secondly,the maximum applied stress used in the cyclic time-dependent deformation tests of rectangular stresses was determined based on the analysis of volumetric strain in the multistep time-dependent deformation tests of sandstone.And then some time-dependent deformation tests with different unloading stress amplitudes and different loading paths of rectangular stress were carried out to investigate the time-dependent deformation characteristics.Then,determined-previous plastic yielding function and strength criterion were adopted and coupled with the non-associated flow rule to construct a modified Nishihara creep damaged model.And the loading path of cyclic loading and unloading stress was also considered into the modified Nishihara model to develop a time-dependent deformation damaged model which could perform time-independent elastoplastic behavior,timedependent viscoelastic behavior and time-dependent visco-plastic behavior of rock.And the built time-dependent deformation numerical model was validated by the comparison of the stress distribution and the time-dependent deformation around the circular hole with the analytical solution.Based on the visco-elastoplastic creep damaged model,some multi-step creep tests and previous(rectangular)cyclic loading and unloading creep tests were simulated to study the evolution characteristics of micro-cracks till the macro failure of sandstone.Finally,some conventional uniaxial single-stress creep tests were performed on sandstone to determine the creep stresses used in the linearly cyclic loading and unloading creep tests.And then loading stress level,unloading stress amplitudes and loading rates were changed during cyclic creep tests to explore the influence of linearly cyclic loading and unloading on the time-dependent deformation characteristics of sandstone.Besides,the viscoelastoplastic time-dependent deformation damaged model used for sandstone was determined by the experimental data from some conventional uniaxial and tri-axial constant strain rate tests.In the end,applied stress levels,time of constant applied stress within cycles and confining pressures were changed within linearly stress cyclic loading and unloading creep tests to study the characteristics of time-dependent deformation of sandstone and the macro failure process of sandstone from the damage evolution of micro-element.