Study On The Effect Of Temperature On Soil Water Characteristic Curve For Unsaturated Soils

The unsaturated soil water characteristic curve is usually defined as the relationshipbetween the soil matrix suction and gravity water content(w)、volumetric water content(θ)or saturation(S). It is closely related to the pore structure and pore size distribution and canreflects the soil water storage and release properties. That curve can be used to predictphysical and mechanical parameters of the unsaturated soil, such as the moisture diffusioncoefficient and shear strength. So it has a important significance in the unsaturated soilmechanics. There are many factors affecting soil water characteristic curve, such ashysteresis、the liquid viscosity、stress state、stress history、grain composition and so on.The factors mentioned above have a great influence on soil water holding capacity, thus itcause the attention of more scholars. In comparison, the influence of temperature on soilwater holding capacity in the natural environment is not significant, thus it is also beenignored. But with the development of science and technology, the geotechnical engineeringproblem become more and more complex, such as the disposal of underground nuclearwaste、carbon sequestration、geothermal resources development、oil drilling、high voltagecable buried and so on. More works will be under high temperature change. In this field,the influence of temperature on the soil water characteristic curve no longer can be ignored.But scholars have not come to an agreement on how to obtain the soil moisture propertiesunder the impact of temperature and what is the influencing mechanism.This study developed a high precision automatic measurement system on soil watercharacteristic curve, which is used to carry out soil water characteristic curve test underdifferent temperature. At the same time, the changing rules of the unsaturated soil watercharacteristic curve under the impact of temperature and the temperature influencemechanism were discussed and analyzed. On the basis of the previous conclusion, aprediction model of soil water characteristic curve which is considering the effects of thetemperature was established and verified. The conclusion is used to carry out the researchon soil water movement under the condition of evaporation. Main contents and results ofthis research are as following:1）A set of high precision automatic measurement system on soil water characteristiccurve is developed. The system realizes the automatic control of the whole test process. Atthe same time, increasing the control precision on the gas pressure、correcting drainagevolume of each pressure level and controlling the test temperature. So the system not only can meet the test requirements but also improve the test efficiency and ensure the accuracyof the test. It is especially suitable for coarse granular soil water characteristic curve test.Compiling the procedure of the instrument and introducing operating considerations inorder to achieve the standardization of the instrument operation, which will service forfollow-up studies.2）The silty sand soil water characteristic curve tests under the four kinds oftemperature are carried out. Connected with different test data, the changing rules of theunsaturated soil water characteristic curve under the impact of temperature and thetemperature influence mechanism are discussed and analyzed. The results show that theinfluence of temperature on unsaturated soil water characteristic curve cannot be ignored.The air entry value and residual water content are reduced with the increase of thetemperature. At the same matrix suction, the soil volume water content linearly reducedwith the increase of the temperature. The influence of temperature on the soil watercharacteristic curve of silty sand、silt、clay and expansive soil appear local differenceswhich is caused by soil particle size and pore size distribution.3）Considering the temperature effect on surface tension、wetting angle and suction，and with VG prediction model，the temperature effect prediction model of soil watercharacteristic curve is established and verified which is based on the theory ofthermodynamics and unsaturated soil. With the help of this model, we just need to knowany two groups of soil water characteristic curve in different temperature, the soil watercharacteristic curve of this soil in any other temperature can be predicted. The predictionmodel is proven to have a universal applicability by comparing the predicted results withmeasured data. But for the cohesive soil, because of ignoring the influence of temperatureon the particle surface hydration, a small deviation appears in the transition zone andresidual zone.4）One-dimensional soil column unsaturated evaporation test was conducted with theautomatic measurement system of soil evaporation in variable environmental temperatureconditions. At the same time, the numerical simulation about unsaturated soil watermovement under the conditions of evaporation was done with heat and water migrationmodel and related predictive parameters. The model is proven to be reliable by comparingthe predicted results with measured data. The results show that unsaturated soil evaporationprocess can be divided into three stages: stabilization stage、decelerating stage、residualstage. As the evaporation continues, wet-dry interface begin to form from the soil surface and continuously move towards the bottom of the soil column.The innovation points of this paper: Experimental study on the temperature effect ofunsaturated soil water characteristic curve is carried out and the mechanism of temperatureeffect is analyzed；The temperature effect prediction model of soil water characteristiccurve is established, which is applied to the numerical simulation research on soil watermovement under the conditions of evaporation.