| Geotechnical engineering problems related to unsaturated soil are very common in arid and semi-arid climate regions,and unsaturated soil mechanics plays an increasingly important role in geotechnical engineering practice.Although unsaturated soil has been studied for decades,there are still many problems to be solved.The properties of unsaturated soil are so complex that when the soil becomes unsaturated,most of the constitutive models established for saturated soil are no longer valid.In recent years,it has become a key problem in unsaturated soil mechanics to develop constitutive relations that can explain and describe the water force characteristics of unsaturated soil.At present,most of the constitutive models are based on macro parameters.Due to the lack of micro mechanism support,it is difficult to meet the needs of scientific research and engineering.Therefore,the influence of microstructure should not be ignored in the study of mechanical properties of soil.The microstructure of soil plays a decisive role in all aspects of soil properties and the mechanical properties of soil can be understood as the macro performance of microstructure.Therefore,in order to further deepen the understanding of the engineering properties of soil,it is necessary to further study the microstructure of soil.Based on the evolution law of pore structure in soil under the mechanical and hydraulic loadings,the double pore structure model of unsaturated soil is established.Based on this model,the water contention property,effective stress and shear strength of unsaturated soil are explained and described.The main research contents are as follows:(1)Generally,the soil will shrink in the process of water decreasing.Therefore,the soil will also shrink in the process of measuring soil-water retention curve(SWRC)in the laboratory,especially for expansive soils.However,when the pore size distribution(PSD)curve obtained from mercury intrusion porosimetry(MIP)test is used to calculate the SWRC,the PSD is only represent the PSD in a certain state.Therefore,the calculated SWRC is quite different from the experimental value.In order to improve this method,it is necessary to consider the effect of shrinkage deformation.Therefore,the shrinkage curve equation which can reflect the property of shrinkage with water decreasing is substituted into the original calculation model,so that the calculation method based on PSD can reflect the shrinkage in SWRC.Finally,the test data in this study and some literatures are used to verify the improved method,and compared with the original calculation method.It can be seen that the improved prediction model of SWRC has greatly improved compared with the original model,and the calculated SWRC is in good agreement with the test value.(2)In a certain range,the SWRC is governed by the pore structure in the soil,thus some SWRC variables can be determined by the PSD curve.A large number of experimental data shows that the air-entry value(AEV),residual suction and gravimetric water content at the residual condition(RC)point can be determined accurately by the PSD curve.Due to the SC reflects the relationship between shrinkage deformation and water content,the gravimetric water content at the AEV point can be determined by SC.Therefore,almost all SWRC variables can be determined by PSD curve from MIP test and SC from shrinkage test.If the water holding curve of each part is regarded as a straight line,the parameters of the soil water characteristic curve obtained by the above method can be used to describe the soil water characteristic curve in a wide suction range.The classical SWRC can be divided into three parts through AEVand RC.If each part of classical SWRC is regarded as a straight line,the SWRC variables obtained by the above method can be used to describe SWRC in a wide suction range.This method does not need to fit the test data,can achieve the real sense of prediction,and can greatly reduce the time of measuring SWRC.However,there is a problem in this method,that is,the calculated SWRC is discontinuous at AEV point and RC point,which will be more troublesome in the mathematical calculation or embedded in the constitutive models.(3)Fredlund and Xing SWRC model is the best fitting equation and it can be used for a wide range of soil over the entire range of matric suction.However,through a series of experiments,it can be seen that the fitting parameters are quite different from the physical meaning given by Fredlund and Xing.In order to compensate for this shortcoming,the traditional FX model is improved and a two-step FX model is developed.Firstly,using FX model without c(?)to fit the test data to determine the fitting parameter ?,n and m.Then these three parameters are substituted into the FX model to fit the test data,and the last parameter Cr can be obtained.The fitting curves obtained by the two-step method are in good agreement with the experimental values,and the fitting parameters are also in line with their physical meaning.(4)The evolution laws of soil pore structure with mechanical and hydraulic loadings are studied by mercury injection test.It is found that the effect of mechanical and hydraulic loadings on soil pore is different.The shrinkage deformation with water decreasing of soil mainly comes from the shrinkage of the inter-aggregate pores(macro-pores).For the effect of mechanical loading,loading(compaction)influenced predominantly the inter-aggregate pores,which closed up with increasing load.Inter-aggregate pores(micro-pores)remained either untouched,or it slightly increased with increasing compaction effort.On this basis,the pore structure evolution models in the process of sample compression,saturation with constant volume,drying and compression are established.According to the different effect of mechanical and hydraulic loadings on pore structure,a double structure model is established.The method of distinguishing macro-and micro-pores is proposed by using the PSD curve and SC.(5)The content of pore water in soil is affected by both mechanical and hydraulic actions.If the change of soil water content is characterized by two separate processes,one is only affected by suction,the other is only affected by deformation.Based on this theory,the effective stress coefficient of unsaturated soil is derived according to the input power equation of unsaturated soil.This gives an insight into the physical meaning of effective stress parameter,and the value of effective stress parameter is the ratio of the change in water volume to the change in macro-pore void volume due to loading at constant suction.The values can be calculated by PSD curves.Shear strength data for a weakly expansive clay were used for validation.A good agreement was obtained between prediction and measurement,indicating the validity of the proposed method(6)In order to study the effect of high suction on the strength of unsaturated soil,the strengths of unsaturated soil under different suctions are tested by using the improved triaxial apparatus of unsaturated soil,and the effect of net confining pressure on the strength of unsaturated soil is also studied.It can be seen that under the same net confining pressure,when the suction is greater,the stress-strain curve is higher,that is,the strength is greater.Moreover,the soil sample will appear strain softening phenomenon under higher suction range.In the low suction range,for the same suction,the shear strength increases with the increase of confining pressure;For high suction range,the increase of confining pressure has little effect on shear strength. |
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