| The response of the subgrade soil owing to the high-frequency and lowamplitude vibration induced by the high-speed rail is complex.At present,on the study of soil properties under vibration load,scholars have discussed the liquefaction characteristics of soil under the actions of different factors through a large number of traditional vibrational triaxial tests,and the vibrational frequency does not exceed 20 Hz,the mechanical response of soil under vibration larger than 20 Hz or even higher remains to be evaluated.Therefore,based on the characteristics of vehicle-induced high-frequency and low-amplitude vibration loads,this paper developed a modified triaxial test apparatus and systematically explored the stress-strain response of the soil through different testing conditions.The main research contents and conclusions are as follows:(1)The stress-strain response of standard sand under coupled static and highfrequency cyclic loading were systematically studied through the self-developed triaxial apparatus.The results show that when the sample is monotonically sheared to a near-critical state,if the vibration intensity a/g ≥ 0.05,the high-frequency cyclic loading causes the shear strength reduction,which can be regarded as a dynamic relaxation process(DRP).In addition,apparent volumetric and axial compression are also observed,while the vibration-induced excess pore water pressure is not significant.(2)When a soil specimen reached a near-critical state,by changing different test conditions,the responses of shear resistance reduction,volumetric compaction,axial compaction and excess pore pressure influenced by initial relative density,effective cell pressure,loading duration and vibrational frequency were obtained.The results show that the existence of water is not the main reason triggering the flow characteristics of sand under the action of vibration,but the high-frequency and lowamplitude vibration causes the local flow of sand particles,which eventually leads to the shear strength reduction and volumetric compression.It is also found that,the vibration acceleration is the main controlling parameter controls the DRP response of saturated sand under the vibrational frequency of 60 Hz to 120 Hz.There is a vibration acceleration threshold,below which the dynamic response of soil stress and strain states are unchanged.If the vibrational frequency beyond the acceleration threshold,the response of the shear strength reduction,volumetric compression,axial compression and excess pore water pressure of soil increase linearly with the increase of vibration acceleration.In addition,the dynamic responses of sand under DRP are independent of the initial relative density,loading duration and vibrational frequency,but are influenced by the effective cell pressure.(3)When a soil specimen stay in a non-critical state,the effects of stress state,initial relative density,effective cell pressure and vibrational frequency on the DRP response were obtained.The results show that the vibrational acceleration threshold of sand at non-critical state is consistent with the threshold at critical state.When the vibrational acceleration is higher than the threshold,the shear strength reduction of sand increases linearly with the increase of vibration acceleration.While the volumetric compression,axial compression and excess pore water pressure of sand increase exponentially with the increase of vibrational acceleration.It is also found that the DRP responses of the sand mainly depended on the initial relative density,and independent of the stress state,effective cell pressure and vibrational frequency.Based on the stress-strain responses of sand at critical state and non-critical state,the vibro-critical state of sand is proposed.Similar to the critical state,the vibro-critical state is a special state of sand in the DPR state,under which the vibro-critical state stress ratio is only related to the shear strength reduction induced by vibration,and the shear strength reduction ratio is controlled by the testing conditions.The response mechanism of sand under high-frequency and low-amplitude vibration is further analyzed.(4)Based on the DRP characteristic of sand,a mesoscale model considering the local characteristics of bulk materials,such as the Shear Transformation Zone Model,is introduced.Based on the first law and the second law of thermodynamics describing the energy flow of the bulk material system,the kinetic equilibrium equation of the internal configuration potential energy of the material is established.Considering the changes in the number of STZs caused by the formation and annihilation of STZs under external forces,the internal variables of the volumetric strain are introduced,and the motion equation describing the basic deformation element of the internal system of the material is established,which is the vibrationinduced strength reduction model.Through the analysis of the stress-strain response of the system under vibration,it is verified that the model can effectively describe evolution law of the stress-strain response of bulk materials at mesoscale,and the evolution of the configuration potential energy of the internal configuration subsystem under vibration is clarified.Finally,based on the critical state of the bulk material system,the influence of vibration intensity on the vibration-induced shear resistance reduction characteristics is discussed,which is comprehensively compared with the test results. |
