Study On Shear Strength Test Of Unsaturated Soil-structure Interface And Its Application To Earth Pressure

The analysis of interaction between unsaturated soil and foundation is one of the biggest issues in geotechnical engineering.Due to the great difference between the deformation of soil and structure with external loading,it is essential to clarify the mechanical characteristics and deformation mechanism between soil and structure and to estimate the shear strength reasonably.At the same time,there is mostly unsaturated soil in the actual engineering structure,and the structural failure often occurs when the matrix suctions drop a lot.Therefore,the concept and method of unsaturated soil mechanics should be applied to the calculation of shear strength and earth pressure between soil and structure precisely and reasonably,which will achieve more effective analysis of the design,construction,monitoring and other stages of retaining structure.Therefore,this paper discusses the strength characteristics of unsaturated soil and structural plane and its influence on soil pressure,introducing SWCC test,large-scale shear test of soil and structure interface,modeling research and theoretical analysis of unsaturated soil pressure.The main research work is as follows:(1)Direct shear test study of unsaturated soil and structure interface.By measuring the soil-water characteristic curve of unsaturated sand,and carrying out 48 clusters of large-scale direct shear tests of unsaturated soil and unsaturated soil with diverse concrete interface with different roughness,the effects of matrix suction and interface roughness on shear strength and dilatancy characteristics are discussed.The test results show that,with the increase of suction,the friction angle between unsaturated sand and concrete interface increases apparently,while the total interfacial cohesion shows improvement before its decline.The interface matrix angle decreases during the increase of suction,and its strength envelope exhibits nonlinear characteristics.The dilatancy angle increases with the increase of suction and interface roughness.The peak shear strength trend of interface is related to the interface roughness.During the increase of the roughness,the shear strength of the interface shows the upward trend.(2)Study on the theory of unsaturated soil-structure interface shear strength and the calculation method of unsaturated earth pressure considering interface shear strength.On the basis of the shear test phenomenon,fully consider the influence of the dilatancy angle on the apparent friction angle,a modified formula for interfacial shear strength is proposed.By comparing the research with the experiment,the rationality of the model is verified.Based on the interface shear strength formula,the traditional soil wedge analysis model is improved under the premise of considering matrix suction and limit equilibrium method.And the unified solution of extended active and passive Coulomb earth pressure is proposed,which includes the characteristics of unsaturated soil and interface of unsaturated soil.The classical Coulomb and Rankine earth pressures can be degraded to its exceptional cases.(3)Model test study on interaction between unsaturated soil and rigid retaining wall.Taking the water content of unsaturated soil and the roughness of soil structure interface as variables,the modeling test of active failure state of rigid retaining wall under translational displacement mode was carried out.The distribution tendency of matric suction and earth pressure during the test was measured by piezometer and earth pressure cell respectively,and the development law of soil fracture surface was analyzed by DIC image correlation technology.The test results show that a wedge is formed through the soil surface from the heel of the wall when the filling behind the wall is destroyed actively;And the earth pressure tends to be stable when the displacement of the wall reaches about 10 mm.Comparing the distribution of earth pressure and the position of rupture surface obtained from the test with the unsaturated earth pressure calculation method and the classical Coulomb earth pressure method,the results show that the calculated results are closer to the experimental results,which can be used as a reference for engineering design.