Research On Dynamic Modulus Characteristic Test And Prediction Model Of Unsaturated Frozen Soil

With the strategies of Transportation Power and The Belt and Road Initiative developed,the focus of infrastructure construction has gradually shifted to high-cold areas and frozen soil has become a key component that affects the safety and stability of the entire engineering system.Compared with unfrozen soil,the mechanical properties of frozen soil are extremely sensitive to changes in environmental factors such as temperature and water content.And the test cost for obtaining engineering mechanical parameters is relatively high.It is of great significance for reducing test cost and the failure risk faced by the project that how to obtain the engineering mechanical parameters of the unsaturated permafrost quickly in the complex hydrothermal state.Therefore,with the combination of laboratory experiments and theoretical analysis,the mesophysical mechanism of unsaturated frozen soil under freezing is studied,and a small-strain shear modulus prediction model of unsaturated frozen soil under complex hydrothermal conditions is established in this paper.Subsequently,in order to simulate the transformation of the frozen soil element from the undisturbed non-destructive state to the failure state driven by external forces,a correlation model suitable for the initial dynamic modulus and the accumulated strain is constructed under a relatively simple mathematical framework.(1)Considering the initial water content and temperature as the test factors,the small strain dynamic characteristics tests of unsaturated compacted soil in a state of freezing and melting are carried out with the bending element test system.The freezing temperature and the soil-water characteristic curve are measured at the same time,and then the shear wave velocity changes with temperature are studied.By using the small strain shear modulus as an index to describe the dynamic performance of unsaturated frozen soil,the influence of initial water content and temperature on the shear modulus are analyzed,and the prediction model of shear modulustemperature is built.(2)Based on the test results of the bending element test of unsaturated frozen soil under small strain conditions and combined with the unfrozen water prediction model,soil-water characteristic curve and small strain shear modulus-temperature prediction model,and used the small strain shear modulus under normal temperature condition as the reference value,matrix suction and void ratio are characterized the physical state of cryostructure.By applying the two physical state of cryostructure,the freezing characteristics of unsaturated frozen soil are studied.Based on the Hardin-Richart classic model,a prediction model between the void ratio after pore ice filling,the matrix suction and the small strain shear modulus is constructed from the level of frozen soil strengthening mechanism in order to reveal the freezing mechanism of unsaturated frozen soil.Then the mechanism can realize the unified prediction of the shear modulus under different complex hydrothermal conditions.(3)The cyclic triaxial tests of frozen soil under cyclic stress with varying amplitudes are carried out systematically.The effect of varying amplitude loading combination characteristics and cyclic stress amplitude on the dynamic modulus of frozen soil was studied by using three stages of cyclic loads with different amplitude.Based on the test results,the variation law of the axial accumulated strain with the number of cycles under each step loading,the stress-strain curve and the dynamic modulus under the typical number of cycles are studied in detail,and the coupling effect of the accumulated strain history and the amplitude of the cyclic stress on the cyclic dynamics are analyzed.Subsequently,based on the Hardin-Drnevich hyperbolic model,the initial dynamic modulus of unsaturated frozen soil under different strain accumulation history conditions is obtained,and the correlation model between the initial dynamic modulus and the accumulated strain generated in the process of cyclic loading is established.Finally,the prediction model is verified by experimental results.