| Unsaturated soil may be subjected to various loads,especially dynamic loads in nature,so it is particularly important to study the dynamic characteristics of unsaturated soil.The theoretical models are adopted to improve the dynamic characteristics analysis of unsaturated soil in most of previous studies,but the theoretical model in complex load cases,especially in the validation of the true triaxial test has certain defects,mainly due to the dynamic true triaxial apparatus is relatively expensive and complicated operation,so the complex load the true triaxial condition is difficult to judge the correctness of the theoretical model.PFC3D numerical simulation can not only analyze the microscopic changes in the loading process of samples from the microscopic level,but also carry out numerical simulation analysis of unsaturated dynamic true triaxial test and large engineering problems,thus verifying the correctness of the theoretical model.Based on unsaturated clay in JiangXi province as the research object,through a lot of geotechnical test theoretical model calculation and PFC3D mesoscopic parameters calibration required necessary,using GDS unsaturated triaxial apparatus and temperature of saturated dynamic triaxial apparatus indoor triaxial test of static and dynamic load,and has carried on the tunnel vibration in an indoor model test of the influence on the dynamic characteristics of surrounding soils.In the PFC3D numerical simulation of unsaturated soil,previous studies used methods such as equal substitution or secondary development to simulate the unsaturated soil test,but the equal substitution method has a large error and is not completely consistent with the actual unsaturated soil test,and the secondary development requires high programming ability,which is difficult to implement and popularize.In this paper,the Hill contact model,which can directly give matric suction to particles,is used to simulate the triaxial test of unsaturated soil.The Hill contact model can realize the numerical simulation of unsaturated soil more conveniently and accurately.By combining the theoretical model of unsaturated soil dynamic characteristics with the hidden example package Hill contact model in PFC3D,the conventional static and dynamic three-axis numerical models and the static and dynamic true three-axis numerical models of flexible boundary were established,and the 1:1 numerical model was established based on the indoor model test chamber.The numerical simulation results show that Hill contact model can well simulate unsaturated soil materials in PFC3D,which provides a new method for the study of unsaturated soil.The research contents and conclusions are as follows:(1)In PFC3D,there are many calculation example packages of customized contact models similar to Hill contact model.Hill contact model is based on liquid bridge theory,and the program automatically determines whether the particle gap is less than or equal to 2 times the minimum contact particle radius.If it meets the conditions,matrix suction is directly applied to the particles.By combining the Hill contact model with the flexible boundary,the same numerical model as the laboratory test was established.The results show that the numerical simulation and the actual laboratory unsaturated soil test data are in reasonable error range,and generally have a high degree of agreement,which provides theoretical support for subsequent research.(2)Theoretical model calculation and necessary parameters for PFC3D mesoscopic parameter calibration were obtained through indoor basic geotechnical test,compression rebound test and shear strength index test.The numerical model is based on the laboratory test results as the calibration of mesoscopic parameters.According to the characteristics of the model,young’s modulus,Poisson’s ratio,friction coefficient and local damping of particles are used as the main mesoscopic parameters to calibrate.The obtained mesoscopic parameters are introduced into the numerical model to establish the unsaturated conventional static triaxial numerical model and compared with the laboratory test results.The results show that the errors of stress-strain curves and volumetric strain-axial strain curves are within 5%,indicating that Hill contact model can be used to simulate unsaturated soil test.(3)Based on laboratory test prototype and combining theoretical model with PFC3D established a different degree of compaction,the matrix suction and conventional static triaxial test of different confining pressure,the numerical simulation results compared with laboratory test and theoretical calculation results,found that the curve result error within 5%,the matrix suction and confining pressure,the greater the sample of deviatoric stress and strain,the greater the The greater the compactness,the greater the deviational stress and the smaller the bulk strain.The displacement diagram and force chain diagram of samples in the numerical simulation provide reliable support for the study of unsaturated soil at the microscopic level.The analysis shows that the greater the confining pressure and matric suction,the stronger the resistance of samples to deformation.According to indoor test and team members proposed three shear loading surface theory has established the different matrix suction,different confining pressure and vibration amplitude of the conventional dynamic triaxial test,compare the numerical simulation and experimental data found that the stress-strain and axial strain-there was a small loop frequency curve error,error in the reasonable scope,The results show that with the increase of confining pressure and cyclic load amplitude,the hysteresis area decreases,the dynamic shear modulus increases,the matrix suction increases,and the accumulated plastic deformation and hysteresis area decrease.The change of coordination number in the triaxial numerical simulation of progressive action reflects the macroscopic mechanical behavior of the sample.With the increase of confining pressure and matrix suction coordination number,the decreasing trend and variation range of coordination number decrease in the loading process,while the amplitude of cyclic load increases,and the decreasing speed and vibration range of coordination number increase.(4)On the basis of the team members to establish three shear loading surface theory model,this paper USES the PFC3D different degree of compaction and the matrix suction was established,the different principal stress conditions of static true triaxial test,with team members to establish the theoretical model of mutual authentication determines the theoretical model calculation required principal stress influence coefficient b value,By comparing the stress-strain and bulk strain-axial strain curves,the error is found to be 7%,which verifies the correctness of the theoretical model and indicates that the true triaxial of unsaturated soil can be simulated by Hill contact model.The results show that with the increase of compactness and matric suction,the stress increases and the bulk strain decreases,while with the increase of intermediate principal stress and minimum principal stress,the bulk strain and stress both increase.Calculated by the numerical simulation of the static triaxial compaction was founded after the b value was 80%,the different principal stress condition,matrix suction and different vibration amplitude of the true triaxial numerical model,the numerical simulation results compared with the theoretical calculation results found that the stress-strain curve error within a reasonable range,increase amplitude cyclic loading,the cumulative plastic strain increases,when the minimum principal stress σ’3 and intermediate principal stressσ’2 are increased,the accumulative plastic strain decreases,matrix suction dynamic shear modulus increases,the cumulative plastic strain decreases,and the true triaxial similar to conventional dynamic triaxial coordination number change rule,the increase of confining pressure,the matrix suction and the intermediate principal stress σ’2,the coordination number increased,decreased variations in speed and reduced,increasing the minimum principal stress σ’3,the coordination number decreases,and decreasing speed change range and increase.(5)For further to explore the dynamic characteristics of unsaturated soil under complicated load,this paper established the tunnel vibration on the influence of the dynamic characteristics of soil around the indoor model box,by monitoring the internal stress of soil changes and found that the surface displacement of soil subsidence model as a symmetrical structure surrounding soils around the tunnel and tunnel section stress of soil changes before and after the basic similar,In the process of tunnel vibration,the stress of soil under the tunnel is superimposed,and the stress change is obviously greater than that of the soil above the tunnel.The further away from the tunnel,the smaller the stress change is,and the greater the buried depth,the larger the peak value and amplitude of the stress change curve are.The larger the vibration load,the larger the peak value and amplitude of the stress change curve are.The surface soil displacement above the tunnel is larger than the soil in the middle and the soil on both sides of the two tunnels.The larger the buried depth is,the smaller the surface soil displacement is.Restricted by the test conditions,the vibration load increases,and the surface soil displacement and settlement do not increase but decrease,but the overall trend is correct.The Hill contact model in PFC3D was used to establish a 1:1 numerical model,which was compared with the stress change curve of soil obtained from the laboratory model test.It was found that the results of numerical simulation were consistent with the stress change curve of soil obtained from the laboratory model test.It is verified that the Hill contact model is feasible to analyze the dynamic characteristics of unsaturated soils under complex loads.PFC3D tunnel in vibration within the soil particle displacement field and velocity field in the process of cloud cover the laboratory test is difficult to observe tunnel internal displacement of soil change,in the process of vibration displacement nephogram results showed that the soil above the tunnel to the tunnel vault,arch under the patch body to deep diffusion,the smaller the farther distance tunnel particle displacement.The results of the velocity field cloud images show that the soil around the tunnel moves at the maximum velocity,and the soil in the model moves spirally during the vibration process.The common point of the two cloud images is that the soil in the middle of the two tunnels is affected by vibration waves in opposite directions,forming an obvious separation zone,where the particle displacement and velocity are small. |
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