Simulation Of Soil-water Characteristic Curve And Hydro-Mechanical Behavior Of Unsaturated Soils Under Drying-Wetting Cycles

Most earth's surficial soils are subjected to drying–wetting cycles due to alternative rainfall and evapotranspiration in the nature.The drying–wetting cycles can significantly affect the hydro-mechanical behavior of unsaturated soils.Geotechnical engineering accidents caused by drying and wetting cycles are more and more serious,especially expansive soil engineering caused by swelling and shrinkage characteristics of expansive soils,resulting in huge economic losses.Recently,the role of drying–wetting cycles in the hydro-mechanical behavior of unsaturated soils has received particular attention.Research works on simulation of the Soil-Water Characteristic Curve(SWCC)and experimental studies on hydro-mechanical behavior of unsaturated soils under drying-wetting Cycles have important engineering and academic significance.In the thesis,on the basis of understanding the existing literatures,research on simulation of soil-water characteristic curve and experimental studies on hydraulic and mechanical behavior of unsaturated non-expansive and expansive soils under dryingwetting Cycles were conducted,and the main conclusions are drawn out as follows:1.In order to study the SWCC of Nanyang expansive soil in full suction range,three suction control or measurement methods are used: the pressure plate method,the filter paper method,and the vapor equilibrium technique.The tests results show that the SWCC has obvious hysteresis phenomenon during the drying and wetting in the low suction range.When the suction is greater than 300 MPa,the hysteresis effect of SWCC disappears;and the drying and wetting curves are almost overlapped.Test data of soil-water characteristic by the filter paper method are located between the main drying and wetting curves of the SWCC.When the suction is 367.54 MPa,the soil water content is only 0.325%,and it almost approximates to zero.Void ratio not only changes with suction,but also is influenced by the suction history and suction path.In the relation between void ratio and suction,the curve of wetting path is above that of the drying path;and they become the same at low suction.There is a clear hysteresis of the relation between void ratio and saturation degree,due to the different suction paths;and they will be consistent when close to saturation.The saturation degree increases with increasing void ratio when suction changes.The experimental data obtained by the vapor equilibrium method have an obvious linear relationship between void ratio and saturation degree;while the test data obtained by the pressure plate method are not an obvious linear relationship in the low suction range.2.On the basis of the existing experimental data about the variation of the drying and wetting curves with the numbers of drying and wetting cycles,a method is proposed by introducing a function of the number of drying and wetting cycle,which is capable of predicting the soil-water characteristic curve of soils experiencing drying and wetting cycles.Once the first drying and wetting curves and plasticity index of the soil are given,the method can predict the soil-water characteristic curve of the soils experiencing different drying and wetting cycles.The predicted results of simulation method indicate that,the method of simulation can reproduce the experimental data,and can save test's duration.Hence this can quickly provide a reliable forecast for the design and construction of geotechnical engineering.3.With the GDS triaxial testing apparatus for unsaturated soil,an experimental study was carried out to investigate the effects of suction history on the hydraulic and stress-strain behavior of unsaturated soil.With regard to maximum suction applied,the high suction was imposed by vapor equilibrium method with saturated salt solution and the low suction by axis translation technique.Test results show that even if the same path of net stress and suction during triaxial shearing,the stress-strain behavior and strength are not the same between the specimens experiencing different suction histories.When the experienced maximum suction is less than the suction of 500 kPa,the stress-strain curve of the specimen experiencing high suction is higher than that of the specimen never experiencing high suction during triaxial shearing.This is because the specimen subjected to drying-wetting cycle has experienced higher suction hence higher maximum average skeleton stress than that not having undergone such process,and becomes over-consolidated soil.On the other hand,when the experienced maximum suctions are higher than or equal to the suction of 3290 kPa,the stress-strain curve of the specimen experiencing high suction is slightly lower than that of the specimen not subjected to such history.This is because in this particular case,the high suction induces structural damage in the specimen which impacts on the stress-strain behaviour.Then the phenomenon of collapse occurred obviously during the wetting at low suction and low net stress.So the structural strength of the specimens caused by the suction history has disappeared almost.The sample's maximum average skeleton stress experienced by suction history is no longer a major factor,but the average skeleton stress at start shearing plays a dominant factor.4.With the FLSY30-1 triaxial apparatus for unsaturated soil,a series of suctioncontrolled triaxial tests were conducted on unsaturated weakly expansive soils experiencing different suction histories in a wide range of suction.With regard to maximum suction applied,the high suction was imposed by vapor equilibrium method with saturated salt solution and the low suction by axis translation technique.The test results show that at the same net stress and suction,the specimen experienced wettingdrying cycle shows a higher relationship of stress-strain and small contractive volumetric strain during shearing.This is because the swelling of the expansive soil is low during absorbing water,and the specimen volume expands slightly after the wetting-drying cycle,but its degree of saturation increases.That causes an increase in the average skeleton stress at beginning of shearing.Under other conditions are the same,the stress-strain curves of the specimens experiencing drying-wetting cycle raises higher and smaller contractive volumetric strain is observed during shearing,and its relationship of stress-strain and volumetric strain behaviour become more and more similar to the deformation characteristics of over-consolidated soil with increasing the experienced largest suction.The reason of the above phenomenon is that the specimen experienced a larger suction,that is,the specimen experienced a larger effective stress.Meanwhile the shrinkage characteristics of weakly expansive soil become obvious duo to loss of water.After experiencing drying-wetting cycle the void ratio decreases obviously,and thus the specimen exhibits the deformation characteristics of overconsolidated soil during shearing.Therefore,in order to predict the stress-strain relationship and volumetric strain behaviour of unsaturated weakly expansive soil,the specimen volume change caused by suction history should be considered.The void ratio of non-expansive pearls clay reduced slightly during the drying for high suction;but the phenomenon of collapse occurred obviously and its void ratio reduced significantly during the wetting for low suction and low net stress.So the structural strength of the specimens caused by the high suction history has disappeared.The void ratio of Nanyang weakly expansive soils reduced significantly during the drying for high suction;but its void ratio reduced slightly during the wetting for low suction.So the structural strength of the specimens caused by the high suction history still exists.The non-expansive pearls clay has characteristics of collapse,but the Nanyang weakly expansive soil has shrinkage characteristics,these result in the different effects on the two kinds of unsaturated soils by suction history.