Influencing Mechanism And Regulating Method Of Swelling Anisotropy On Expansive Soil Slope And Its Retaining Structure Stability

The stability and reinforcement of expansive soil slope is a key technical problem in the construction and operation of transportation infrastructure in expansive soil area.Revealing the lateral swelling characteristics of expansive soil under different constraints and humidification conditions,establishing corresponding constitutive models,and putting forward the regulating principle and method for the stability of expansive soil slope and its retaining structure are the key scientific issues to be solved in this paper.In this paper,research was carried out through research and development of equipment,laboratory test,constitutive model,numerical simulation,model test and mechanism analysis,innovations have been made in the swelling test device and method of expansive soil under saturated and unsaturated conditions,as well as the constitutive model and numerical calculation method of unsaturated expansive soil reflecting the swelling anisotropy.The main achievements are as follows:(1)Based on the conventional consolidation oedometer and unsaturated consolidation oedometer,a retractable cutting ring integrated with a mico-shaped earth pressure sensor was developed.The vertical swelling strain and lateral pressure of the expansive soil generated from specimen during inundation could be measured synchronously by using it in combination with the conventional consolidation oedometer and unsaturated consolidation oedometer.The equipment is stable in performance,reliable in operation,easy to operate,with sufficient control accuracy and measurement accuracy,and can be used for the experimental study of expansive anisotropic behavior of expansive soil under saturated and unsaturated conditions respectively.(2)Based on a series of indoor saturated and unsaturated swelling tests,the difference of two-way swelling law and its influencing factors are revealed,and the influence mechanism of expansive soil microstructure on expansive anisotropy is analyzed by SEM observation.The results show that the swelling anisotropy is constantly changing due to the continuous stress redistribution during humidification.The constant volume lateral swelling pressure measured under saturated water condition is about 0.27～0.69 times of the vertical swelling pressure.Compared with the initial water content,the initial dry density has a greater influence on the swelling anisotropy.The horizontal orientation of medium-sized clay minerals is the reason for the macroscopic swelling anisotropy.The denser the soil sample,the higher the horizontal orientation degree of microstructure,and the more obvious the swelling anisotropy.(3)The nonlinear elastic model of unsaturated soil and its parameter determination method proposed by Fredlund and Morgenstern are improved,so that it can reflect the anisotropy of swelling and become more reasonable when calculating the lateral pressure of expansive soil.The experimental results in this paper preliminarily verify the validity and rationality of the model.Based on this,the calculation method of modulus non-uniform field of unsaturated expansive soil is put forward,and the stability of expansive soil slope and its retaining structure is analyzed.The results show that the swelling pressure will make the principal stress axis of soil rotate,and local passive failure will occur at the foot of the slope,which will lead to the instability of retaining structure.Compared with the case of swelling anisotropy,the maximum lateral stress calculated with isotropic swelling and without swelling will be reduced by 20.8%～38.3%and 73.9%～88.3%,respectively.(4)The swelling pressure ellipse model is extended to represent the no-load swelling rate in all directions.Based on the relationship between swelling pressure and swelling deformation under saturated conditions,the swelling pressure regulation method of expansive soil retaining structure is put forward.Laboratory scale model test reveals the interaction mechanism between expansive soil and retaining structure,and verifies and analyzes the control method of expansive pressure of expansive soil retaining structure and the calculation method of modulus non-uniform field of unsaturated expansive soil,which preliminarily shows its effectiveness and rationality.This study provides theoretical support for the stability and reinforcement of expansive soil slope,and can provide reference for the research and development of new technology.