| In recent years,with the rapid development of China’s infrastructure as well as the economy,urbanization construction and above-ground and underground transportation construction have been going on,and buildings and rail transit foundations built on sandy soil foundations have been mushrooming,so the study of dynamic properties of sandy soil has been one of the hot spots.In the field of geotechnical engineering,sandy soil,as the foundation of a large number of engineering structures,is frequently subjected to dynamic loads,which may change from solid to fluid state under the action of dynamic loads,causing catastrophic consequences such as foundation settlement and collapse of buildings and bridges,soil landslides and sinking of underground pipes.A lot of research has been done by domestic and foreign scholars on fluidized deformation models for saturated sandy soils,but most of them are combinations of basic rheological elements or empirical models,which leads to redundant element parameters and limited applicability.Thus,in this paper,considering the continuity of sandy soil deformation,based on Maxwell’s rheological model,the state function is introduced,a unified coupled state evolution theoretical model is proposed,and the following studies are carried out for the rheological properties of saturated sandy soil combined with indoor tests and numerical simulation analysis.(1)The physical and mechanical properties of Fujian standard sand were studied using particle analysis test,specific gravity test and relative compactness test,and the physical property parameters of standard sand were obtained;the hardware composition,software loading system and operation procedures of the triaxial system of GDS hollow cylindrical torsion shear were introduced;the indoor conventional consolidation undrained triaxial test of Fujian standard sand was carried out,and the stress-strain curves under two sets of different surrounding pressures were obtained,which provided the test data for the calibration of parameters in the following PFC numerical simulation.(2)Based on Maxwell’s rheological model,considering the continuity of sandy soil deformation,a continuous state function θ is introduced to describe the evolutionary characteristics of its properties during sandy soil deformation,and a unified coupled state evolutionary theoretical model is given:σ’=-θσ’+Gγ,θ=1/γc(Vs/Vcγ2-γθ).According to the state evolution theory,the model introduces a key parameter:characteristic strain γc,which is used to consider the influence of historical strain on the current state.The evolution of strain with time calculated by the theoretical model is compared with the results obtained from static triaxial and cyclic dynamic triaxial tests,and it is found that the calculated results of the theoretical model are consistent with the test results,thus verifying the applicability of the model proposed in this paper;The test results show that under the test conditions of effective confining pressure of 0.1 MPa,deviator stress of 100 kPa,and load frequency of 0.1 Hz,the critical amplitude of the sample is close to 40 kPa,and the critical velocity Vs=3.5 m/s;the influence of historical strain on the current state gradually decreases with the increase of deviator stress,while the influence becomes greater with the increase of dynamic stress.(3)The fluidization deformation characteristics of saturated sandy soil under axial cyclic loading were investigated through the GDS dynamic triaxial test system,and the theoretical model was verified from different test conditions,and the effects of different test conditions on the parameters γc(characteristic strain),θ0(initial state),Vc(critical velocity),θ(state function),and η0(initial viscous coefficient)were discussed in depth.The main conclusions are as follows:the characteristic strain is positively correlated with the dynamic load amplitude and load frequency,and negatively linearly correlated with the effective confining pressure and relative compactness;higher and faster dynamic stresses lead to higher initial strain rate,while higher effective confining pressure and relative compactness decrease the initial strain rate and reduce the initial fluidity of the specimen;the value of the state function gradually decreases with the increase of the dynamic load amplitude action time,and the overall fluidity of the specimen,the value of the state function will gradually decrease with the increase of the action time of the dynamic load amplitude,and the overall fluidity of the specimen system decreases,and the state value of the granular material will become smaller when increasing the effective confining pressure and relative compactness;the increase of dynamic stress and loading frequency will make the critical velocity decrease,which means that the critical damage value of the whole specimen system decreases,making the specimen reach the yield state faster,while the specimen system is more stable when the effective confining pressure or relative compactness is increased,which makes the critical velocity increase,and the critical damage value of the system increases accordingly.The initial viscous coefficient of saturated sandy soil gradually decreases with the increase of load amplitude and load frequency,and there is a significant increase of initial viscous coefficient when the effective envelope confining and relative compactness are increased.(4)The PFC3D numerical simulation of cyclic dynamic triaxial test was carried out to calibrate the fine-view parameters of Fujian standard sand,and a set of feasible PFC3D numerical simulation of the fine-view parameters of Fujian standard sand was derived;the fluidization deformation characteristics of saturated sand under cyclic dynamic load were qualitatively analyzed,and the validity of the model was verified by comparing the simulated strain evolution law with time with the theoretical calculation results by changing the dynamic load amplitude and load frequency and other factors. |
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