Study On Soil-Water Characteristic Curve Of Expansive Soil Under The Effect Of Stress And Wetting-drying Cycles

Expansive soil is widely distributed in more than 20 provinces(municipalities and autonomous regions)in China,with strong shrink-swell,multi-fissuring and overconsolidation,which can easily induce engineering disasters.In the natural state,the expansive soil is affected by environmental factors such as stress,rainfall,evaporation and groundwater infiltration.The soil is often in an unsaturated state.The change of water content will significantly affect the hydro-mechanical properties of the soil.The soil-water characteristic curve(SWCC)can reflect the water retention capacity of unsaturated soil under certain matrix suction.It plays an important role in analyzing and predicting the soil-water characteristics of unsaturated soil.The soil-water characteristic curve model is also an important basis for quantitatively describing and predicting soil water movement.In this paper,Nanyang weak expansive soil and Hefei weak expansive soil are taken as research objects.The SWCC of expansive soil is studied by using the geostatic pressure and the number of wetting-drying cycles as variables.The main research conclusions are as follows:Firstly,the one-dimensional stress controllable volumetric pressure plate extractor is used to study the influence of different geostatic pressures(0,40,100,200kPa)on SWCC of Nanyang expansive soil and the corresponding model is established.The conclusions are as follows:(1)Bimodal SWCCs are obtained.The water retention capacity of the expansive soil decreases with the increase of the geostatic pressure,the difference is obvious.The geostatic pressure also affects the shape of SWCC.The influence of small geostatic pressure(0~40kPa)on SWCC shape is not significant.The second boundary effect section of bimodal SWCC is significantly prolonged under the geostatic pressure of 200 kPa,and the bimodal shape is the most obvious.In addition,the influence of the geostatic pressure on the air entry value of SWCC2 is stronger than the influence on the air entry value of SWCC1.(2)The change of void ratio is more sensitive to the suction of the matrix above 50 kPa.The void ratio decrease caused by the geostatic pressure is 2.12 times of the decrease caused by the matrix suction,which can indicate that the influence of the geostatic pressure on the void ratio is more significant than the matrix suction,the intergranular pore water loss caused by the geostatic pressure is dominant.(3)A bimodal SWCC model with geostatic pressure and matrix suction as variables is established.The accuracy and reliability of the model are verified by the measured data of Nanyang expansive soil.The fitting parameters can effectively reflect the variation of the characteristic points and characteristic suction section of SWCCs under different geostatic pressures.The SWCC1 fitting parameters have a good linear relationship with the geostatic pressure.Secondly,the Hefei expansive soil is tested for wetting-drying cycles(0,1,2,3,4,5 times)to obtain soil fissures.Quantitative study of soil fissures by CT scanning and image processing is carried out.The SWCC of the fissured expansive soil under different wetting-drying cycles are measured and the corresponding model is established.The conclusions are as follows:(1)The development of fissures is evaluated by the internal surface area of the fissures and the three-dimensional fissure degree(volume fissure degree and area fissure degree)which are defined.The relationship between the three indicators and the number of wetting-drying cycles can be quantified by variation of logistic function.The volume,average area and internal surface area of fissures increase with the increase of the number of wetting-drying cycles.However,under the 5th wetting-drying cycles,the volume and average area of fissures decreased slightly due to the narrowing of the fissure width,but the narrowing of fissures does not reach the degree of fissures healing.(2)The SWCCs of the fissured expansive soil are bimodal.The water retention capacity of the soil decreases with the increase of the number of wetting-drying cycles.After the matrix suction is greater than 100 kPa,the water retention characteristics of the fissures expansive soils tend to be consistent under different wetting-drying cycles.The air entry value of SWCC2 decreases with the increase of the number of wetting-drying cycles.The influence of wetting-drying cycles on the characteristic points of SWCC2 is stronger than that of SWCC1.(3)According to the grey relational analysis(GRA),the three-dimensional volume fissure degree has the strongest correlation with the soil water retention capacity,the internal surface area is the second,and the three-dimensional area fissure degree is the weakest.(4)The bimodal SWCC model with the number of wetting-drying cycles and matrix suction as variables is established.The accuracy and reliability of the model are verified by the measured data of Hefei expansive soil.The fitting parameters have a good linear relationship with the number of wetting-drying cycles.The research has certain reference value for the engineering theory and practice of unsaturated expansive soil under high load and wetting-drying cycles fissures disturbance state,which can be used for agricultural water-saving irrigation engineering research in expansive soil area,as well as the prevention methods of soil erosion,slope instability and other disasters.