Model Prediction Of Soil-Water Characteristic Curves And Deformation Behaviors Of Unsaturated Soils

Unsaturated soils are characterized by multiphase and porosity.Although there is one more gas phase than saturated soils,its hydraulic-mechanical properties are more complicated.For unsaturated soils,suction or soil water potential is a key variable,and it has an important influence on the engineering properties(e.g.,strength,deformation and permeability coefficient).The relationship between suction(i.e,soil water potential)and water content can be described by the soil-water characteristic curve(SWCC).Therefore,the SWCC is one of the most important tools for studying the characteristics of unsaturated soils.However,the current applications of unsaturated soil mechanics in practical engineering are not extensive.On one hand,the measurement techniques of suction are time-consuming and laborious,and the requirements for technicians are also high.On the other hand,the assumptions derived from the model are often the simplification of the real environment around the soil,making most theoretical models semi-empirical.Therefore,it is necessary to continue to promote the further development of model theory and prediction,which is of great significance for the study of unsaturated soil mechanics and its engineering application.Taking unsaturated soils as the object,the research is carried out with the prediction of the SWCC and deformation behavior of unsaturated soils.The main research work and conclusions are as follows:(1)Three 2D equations are derived from a soil-water characteristic surface(SWCS)model and utilized to investigate the projection characteristics as well as application of the SWCS at constant void ratio e,constant suction s and constant degree of saturation S,respectively,so as to understand hydraulic-mechanical behavior of unsaturated soils.Firstly,in the logs-log S plane,as the initial e increases,the SWCC moves left-hand along the s-axis.When the difference between initial e values of the reference SWCCs and the predicted SWCCs sufficiently small,the rigid translation method can be used to predict the SWCCs at different initial void ratios.Secondly,in the loge-log S plane,as the constant s increases,the hydraulic coupling curve(HMCC)moves left-hand along the e-axis.In view of the parameters of the HMCC equation have different characteristics in different suction ranges,the conditions for distinguishing between low suctions and high suctions are given and proved,namely,1)when mn? = 1,the constant suction imposed on the HMCC is considered as high suction;and 2)when 0 < mn? < 1,the applied constant suction is low suction.The predicted HMCCs can be obtained by the translation of the reference HMCCs at high suctions,and the rigid translation method is also applicable to the range of low suctions.The last but not the least,in the logs-loge plane,as the constant S increases,the suction consolidation curve(SCC)moves left-hand along the s-axis.The proposed SCC equation can accurately describe the relationship between e and s on the plane S after appropriate correction.(2)Based on the pedotransfer functions(PTFs),the nonlinear regression equation of the parameter n of the Van Genuchten(VG)model is proposed.Combined with the regression equation of the parameter ? of the VG model,a simple prediction method of the SWCC under different initial void ratios is proposed.For a given soil,all parameters of the two regression equations can be completely calibrated by just three sets of SWCC experimental data.The proposed PTFs for predicting the parameter n is verified by the experimental data available in literature,which is applicable to not only non-deformed soils but also deformed soils.(3)When measured data are only in the suction range of 0-500 kPa,the limitation exists in the prediction of the SWCC at high suctions using the VG model and Webb iterative method.To improve the prediction accuracy,a relatively convenient and swift method is proposed in this paper to predict effectively the SWCC over the entire suction range.Firstly,the SWCC in the transitional stage shows a linear relationship on a semilogarithmic coordinate system,which is used to obtain the modified parameters of the VG model.The modified SWCC is applicable to the capillary zone.In the second place,according to the linear characteristic which is displayed by experimental data in the adsorption zone in a semi-logarithmic coordinate system,the Webb iterative method is used for predicting the SWCC in the adsorption zone.A complete SWCC is actually composed of the aforesaid two parts.The validity and applicability of the proposed method are demonstrated using an extensive set of experimental data from this study and from the available literature.(4)In the light of the three-piecewise linear model of the soil shrinkage characteristic curve(SSCC)and three-phase proportional formula of unsaturated soils,a general model describing the variation in void ratio with matric suction is derived,that is,the SCC model considering the influences of suction.Volumetric change,air inflow and volumetric strain can be predicted under the action of both setting matric suction and setting net confining pressure,through making use of both the coefficient from the model hypothesis and the water discharged from saturated specimens at a certain suction level.(5)Based on the results of the SWCC-w represented by gravimetric water content and the shrinkage test,a simple approach for modeling the soil shrinkage characteristic curve(SSCC)is proposed.Further,the predicted SSCC can be used to estimate the degree of saturation-based SWCC-S and the w-s curve.For the sake of futher short time of the shrinkage test,the shrinkage limit formula is deduced for mud specimen preparation.This formula has the advantage that only the volume change of the specimen need to be measured in the completely dry state,which means the shrinkage limit test can be eliminated reasonably.It has a clear physical meaning,which is the reciprocal of the slope of the w-S curve during the residual shrinkage stage of the SSCC.(6)The chemical reaction kinetic equation is adopted to model the swelling-time curve of expansive soils,and hence the hyperbolic model and the Logstical model are derived accordingly.For the hyperbolic model,there are two features as follows: 1)the maximum vertical expansivity has a good exponential relationship with the time that elapsed for reaching to a half of the maximum vertical expansivity,and 2)it has a good linear relationship with the initial dry density or water content.Based on these two relationships,the swelling-time curves at different initial dry densities or water contents can be well predicted.For a given dry density,the suction can be introduced into the model of swelling-time curves.The maximum vertical expansivity under different initial dry density and water content shows a good power function relationship with suction.