| Unlike previous studies where the shear strength parameter of clay is a constant value,more and more studies have shown that the shear strength of clay changes nonlinearly with increasing stress levels.The internal friction angle of clay at high stress levels is significantly smaller than that at low stress levels.Direct shear testing is a direct and effective method for exploring the relationship between the shear strength characteristics of clay and stress levels.In addition,according to the relationship between the pressure coefficient K0 of static soil and the shear strength parameters,the nonlinear increase of clay K0 value can also be regarded as the correlation reflection of the obvious decline of strength parameters.Due to the ability to explore particle scale mechanical behavior,the discrete element method is increasingly widely used to reveal the meso mechanisms of macroscopic mechanical properties of soil.This article optimizes the clay discrete element program C-DEM developed by the team in the early stage,and establishes a model at the particle scale to simulate the K0 consolidation compression and direct shear processes of clay.The phenomenon of nonlinear changes in clay shear strength is reproduced,and the underlying mechanism is studied.The main content and conclusions are as follows:1.Analyzed the irrationality of using fitting formulas to solve the mid surface potential between clay particles at present.On the basis of comparing the effectiveness of various solution methods,it is determined to use interpolation method to calculate the mid surface potential between clay particles.Quantitative analysis was conducted on the impact of occlusion on the double layer force between clay particles,and a double layer force algorithm considering occlusion was proposed and implemented.Solved the shortcomings of the original Born repulsion calculation where the normal definition was not unique,and provided a new calculation model.2.Two theories were analyzed to explain the nonlinear variation of shear strength parameters of clay with stress levels at the meso mechanism level,including the effective stress principle of clay considering electric repulsion and the electronic surface roughness theory of Santamarina et al.,pointing out the shortcomings of both theories.A new meso mechanism has been proposed.When particles are in non parallel point to surface or point to point contact with each other,due to the small electric and hydration repulsion forces between particles,actual mechanical contact will occur between mineral crystals,resulting in a higher friction coefficient.When particles are in parallel face-to-face contact,the electric repulsion and hydration repulsion between particles increase,making it difficult for particle mineral crystals to generate actual mechanical contact and reducing the friction coefficient.3.Two types of models with variable and constant friction coefficients between particles were established using the optimized clay discrete element program C-DEM,and K0 consolidation simulation was conducted on both models.The rationality of the model was verified from four aspects:comparison of simulation results with experimental data,changes in system energy,stress on the frame,and changes in imbalance ratio.A meso mechanism analysis was conducted on the simulation results from four aspects:the proportion of mechanical contact force between particles,the angle between particles,the distribution of particle orientation,and the size and distribution of indirect contact force between particles.It was found that when the mechanical contact ratio between particles is small,the simulated sample presents a"mud"state,and the K0 coefficient is close to 1.Only when the mechanical contact ratio between particles reaches 30%or more,can the value of the friction coefficient between particles have an impact on the K0 value.As compression progresses and stress levels increase,the contact configurations between particles in both models tend to be parallel.At the macro level,the K0 coefficient of the constant friction coefficient model remains basically unchanged,while the K0 coefficient of the variable friction coefficient model increases nonlinearly with the increase of stress level,consistent with the trend obtained from indoor experiments.The above results prove the rationality of the proposed nonlinear change mechanism for shear strength of clay.4.Based on the sample with K0 consolidation completed and reached a stable state,direct shear simulation was conducted using strain control method on the two types of models mentioned above.It was found that the shear strength obtained by the constant friction coefficient model is linearly related to the stress level,while the shear strength of the variable friction coefficient model shows a nonlinear change with the increase of stress level,and the internal friction angle decreases with the increase of stress level,The simulation results maintain good consistency with the experimental results,further proving the rationality of the nonlinear change mechanism of clay shear strength represented by this model. |
