Study On Soil-Water Characteristic Curve Of Hunan Clay And Its Hysteretic Effect In Full Suction Range

The soil-water characteristic curve(SWCC)is the relationship curve between unsaturated soils suction and water content/degree of saturation,which is the important basis for the study of strength,permeability,constitutive relationship and consolidation theory of unsaturated soils.A large number of studies have shown that SWCC is greatly affected by soil type,initial water content,dry-wet cycle,pore structure,stress history,particle size distribution,initial dry density,temperature and mineral composition.Because soils are subjected to rainfall and climate change in natural state,they have been always in the dry-wet cycle state.Therefore,the study of soil-water characteristic curve during dry-wet cycle has become one of the hotspots in geotechnical engineering.In addition,under the natural condition,the variation range of soil suction is large,which makes it necessary to obtain SWCC over full suction range to analyze the hydraulic characteristics of soil in practical geotechnical engineering.At present,the research is mostly based on single test data because SWCC test is time consuming and sensitive to human factors.However,the measurement ranges of suction in different test are also different due to the different test principles,and it is difficult to obtain the SWCC over full suction range from single test.To explore the variation law of SWCCs in the whole suction range during drying-wetting cycle,a set of tests were carried out on Hunan clay under the drying and wetting cycle conditions.Firstly,based on pressure plate test,SWCCs with full suction range during drying and wetting cycle is obtained.Secondly,these SWCCS were fitting to empirical model and fractal model,and then the fitting results were analyzed.Finally,according to the contact angle test data of samples with different water contents,the hysteretic effect of contact angle and the relationship between water content and contact angle were analyzed.Then,a method of predicting soil-water characteristic hysteresis curve was proposed from contact angle hysteresis effect.The related research work and conclusions are as follows:(1)Taking Hunan clay as the researched object,seven groups of samples with different initial dry densities were prepared for filter paper method and saturated salt solution vapor equilibrium method under drying-wetting cycle conditions.Combined with the experimental data of pressure plate instrument,it is found that the variation law of water content and matric suction was same in three kinds of test,that is,the water content decreases gradually with the increase of suction.Under the same matrix suction,the larger the initial dry density,the higher the corresponding moisture content,and it is more difficult to loss water.In addition,the SWCC over full suction range was obtained by integrating test data of three experiment,and it is found that the three groups of test data could be well connected.The experimental SWCCs showed obvious hysteretic phenomenon,which gradually weakened with the increase of suction.(2)Different classical empirical fractal models were fitting to experimental data of three tests.It is found that for the dewetting test of pressure plate instrument,the fitting results of six models within the measured suction range showed little difference,and the fitting correlation coefficient of VG model was highest.For filter paper dewetting/wicking test,the fitting results of six models in the measured suction range are quite different,and the fitting result of empirical model was better than fractal model,among which Gardner model had the highest fitting correlation coefficient.For the dewetting/wicking test of saturated salt solution vapor equilibrium method,the fitting results of six models changed greatly,but gradually decreases with the increase of suction,where the VG model fitting correlation coefficient was highest.(3)Based on the integrated seven groups of hunan clay test data with full suction range,six typical empirical models and fractal models are fitting to them.Compared the fitting results of different models,it is found that the fitting accuracy of empirical model was higher than fractal model as whole.Among them,the correlation coefficient of the VG model was highest and in good agreement with the measured value.In addition,SWCC over full suction range cannot be accurately predicted from the limited low-suction range test data,while the prediction accuracy can be effectively improved by appropriately adding the high-suction range test data.(4)Based on the contact angle test,the contact angle of Hunan clay with different water contents were obtained.On this basis,a new method for predicting absorption SWCC was proposed from contact angle hysteretic effect and dewetting SWCCs.The comparison between the measured results and the predicted results shows that the coincidence degree of them is relatively high,indicating the effectiveness of the proposed method.