Experimental Study On Creep Characteristics Of Saturated Sandstone Under Pore Water Pressure

The rock at the " Hydro-Fluctuation Belt" in the Three Gorges Reservoir area has suffered a long-term cyclical effect of the water level fluctuation,which has caused major safety hazards to the stability of the rock slope in the reservoir area.According to the water level changes in the reservoir area,it is of great significance to carry out the creep test of the slope rock mass.This article is funded by the key items of the National Natural Science Foundation," Deformation and failure mechanism and protection of reservoir bank slope under complex conditions "(51439003),the sandstone grading loading creep test under the conditions of saturation,pore water pressure and cyclic pore water pressure are conducted with an aim to study the creep mechanical properties of slope sandstone under different conditions by taking typical sandstone as test object.The conclusions are as follows:Based on the instantaneous mechanical test,the deformation curve,deformation characteristics,creep rate,long-term strength under different confining pressures are obtained by performing the creep test of saturated sandstone.The results show that:Following with the increase of stress level,the instantaneous strain,creep strain and creep rate shows an increasing trend,while the instantaneous strain increment shows a negative tendency;with the increase of confining pressure,the restraining effect of confining pressure on sandstone strain decreases and yet the long-term strength increases;The hoop creep enters the accelerated creep stage earlier than the axial creep.In practical engineering application,more attention should be paid to the expansion of hoop creep deformation.The deformation characteristics,creep rate and other mechanical parameters of saturated sandstone at different confining pressures is analyzed by carrying out the creep test of sandstone under pore water pressure cycles.The results show that:The existence of pore water pressure will have no effect on the deformation law of creep,but will reduce the initial creep strain,long-term strength,cohesive force and internal friction angle,of which the decrease of cohesive force and internal friction angle are 9.1%and 5.6%respectively.From the perspective of effective stress,the phenomenon of initial creep strain reduction is explained with the End Effect Theory.The deformation characteristics,creep rate,long-term strength,cohesion,internal friction angle and other mechanical parameters of sandstone under 5MPa confining pressure are obtained with the creep test of saturated sandstone under pore water pressure cycles.The results show that after the first pore water pressure cycle,the creep strain reaches the maximum value due to the instantaneous stress loading.With the increase of pore water pressure cycles,the creep strain,elastic modulus and single strain degradation decrease;with the increase of stress level,the single degradation of elastic modulus and creep rate increase.Comparing the deformation characteristics and creep rate under the conditions of saturated state,pore water pressure and pore water pressure cycles,it is found that the saturated sandstone under pore water pressure cycles enters the steady creep stage at the latest and yet the nonlinear accelerated creep failure occurs at the earliest.Based on the analysis of the creep deformation law of saturated sandstone and K-B model,a new nonlinear constitutive model is built by introducing the Weibull distribution function to optimize the viscosity coefficient.According to the model as well as taking the damage of elastic modulus and viscosity coefficient caused by pore water pressure cycles into consideration,a creep constitutive model of saturated sandstone considering pore water pressure cycles and mechanical cumulative damage is established by introducing the damage variable d_En?d_?n.The L-M iterative algorithm in Origin software is used to identify the model parameters and fit the curve.The fitting degree is high,which indicates the rationality and accuracy of the model.