Study On Macro- And Meso-mechanical Behaviors Of Loose Sand Liquefaction Under Complex Shear Paths

As a common natural building material,sandy soils are widely used in foundations,slopes,roads and other environments.Compared with other soils,sandy soils have no cohesion,higher dispersion and anisotropy,and present extremely complex mechanical characteristics under external loading.Traditional laboratory tests and numerical simulations focus on the mechanical response of sandy soils without initial static shear and under simple shear paths.However,sand units in engineering practice often have a more complex stress history and are subjected to continuous changes in shear direction.Under such complex conditions,the macro-scale behavior of sandy soils is largely influenced by their meso-level structure evolution.Therefore,in order to better understand and predict the macroscopic mechanical response of sandy soils,the sandy soil properties need to be analyzed and studied at the meso-level.In this study,a new numerical model that can be applied to arbitrary shear paths in three-dimensional space is constructed by the discrete element method based on laboratory simple shear tests,and a series of numerical simulations of simple shear tests of sandy soils under undrained conditions are carried out using this model to reveal the sandy soil cyclic shear behavior and its liquefaction mechanism,which provides a physical basis for the establishment of sandy soil underpin constitutive theory.The main contents and conclusions of this study include:(1)Based on the laboratory simple shear test,the effects of the initial static shear stress,the direction of the initial static shear stress and the shear path on the macroscopic mechanical properties of loose sand were investigated.The results showed that the maximum value of shear stress and its variation range increased with the increase of initial static shear stress,and the samples liquefied more quickly;the peak value of shear stress decreased,and the liquefaction resistance decreased with the increase of the angle between initial static shear stress and dynamic shear stress(shear direction angle);the samples were more likely to liquefy in the bidirectional shear path than in the unidirectional shear path,and the figure-8shear path was the most dangerous.In addition,the stress paths of monotonic simple shear tests and cyclic simple shear tests have envelope effect,and the increase of initial static shear stress,the increase of shear direction angle,and the more complex shear paths all lead to the increase of the change in dynamic shear modulus,hysteresis energy dissipation,and vertical stress reduction vertical stress reduction,and the stress path envelope relationship is more significant.The sensitivity of loose sand liquefaction to the three factors in descending order is: the angle between initial static shear stress and dynamic shear stress,the shear path,and the initial static shear stress.(2)A new boundary condition was constructed based on the variable direction dynamic cyclic simple shear device,the particle shape was optimized based on the physical properties of the tested sand material,the particle size was determined through the size effect,a new discrete element method numerical model for the multi-directional dynamic simple shear test was initially established.The influence of various mesoscopic parameters on macroscopic mechanical properties was analyzed,and their relationship with stress-strain evolution was obtained.The mesoscopic parameters of the model were calibrated based on the macroscopic mechanical properties of loose sand,and a new numerical model has been constructed that can simulate simple shear tests with arbitrary shear paths.(3)Based on the contact force chain,average contact force distribution and mechanical coordination number,the evolution of particle indirect contact force at the meso-level was studied.The results show that with the increase of initial static shear stress,the increase of shear direction angle and the more complex shear path,the contact number and mechanical coordination number decrease,the contact force decreases,and the degree of anisotropy of samples increases;The coordination number of the sample decreases more rapidly at the beginning and end of shear,and the coordination number is almost the same at the initial liquefaction.In addition,the deflection angle of strong contact force during cyclic shear is different,from large to small: figure-8 shear path,circular shear path,oval shear path and linear shear path.From this,the evolution law of indirect contact force of particles is obtained,providing a theoretical basis for subsequent fabric evolution.(4)Based on the fabric tensor and anisotropic tensor,the fabric evolution of loose sand under different initial static shear and shear path conditions was investigated.It was found that the fabric tensor and anisotropic tensor of the sample oscillate and increase continuously with shear,and their amplitudes increase with the increase of initial static shear stress;As the shear direction angle increases,the anisotropic tensor of the sample may generate more peaks in each cycle,and the gap between peaks decreases,resulting in more drastic structural changes in the sample;Compared to the unidirectional shear path,the amplitude and frequency of the anisotropic tensor in the sample under the bidirectional shear path are larger,and the structural changes of the sample are more frequent and liquefaction phenomenon occurs faster.From a mesoscopic perspective,the evolution of the sample structure was further revealed,providing a physical basis for introducing the fabric evolution equation into the sand constitutive model.