Research On In-situ Matrix Suction Measurement Of Unsaturated Soil Based On Thermal Conductivity Sensor

The accurate measurement of matrix suction is the prerequisite to understand the mechanism of unsaturated soil.The correlation between matrix suction and moisture content is the key parameter to explain the mechanical phenomena and characteristics of unsaturated soil,which is called soil water characteristic curve(SWCC).Soil matrix suction is generally determined by experimental methods.At present,there are a variety of laboratory techniques for measuring soil matrix suction.However,laboratory measurements are limited by sample availability and sample disturbance.Moreover,the laboratory analysis operation is complicated and the experiment period is long.Tensiometer can relatively accurately measure the in-situ matrix suction of soil,but the measurement range is limited.Therefore,there is still room for improvement in soil matrix suction measurement technology.This study innovatively used the thermal conductivity sensor to measure the insitu matrix suction of soil.The existing in-situ thermophysical parameters tester of the research group was improved.The in-situ thermophysical parameters and matrix suction of the soil were obtained by adding a porous casing to the probe.The physical properties such as the material and pores of the casing were determined through experiments.The thermal conductivity and bond strength of different coupling agents were compared to determine the bonding method between the casing and the probe.Silty clay and sand soil-water characteristic curves(SWCC)were measured by the GEO-Experts soil-water characteristic curve measurement system,and the correlation of moisture content and matrix suction were obtained.The experimental values were simulated by Gardner model,Van Genuchten model and Fredlund and Xing model.Considering the limitations of single algorithm in terms of operation speed and generalization performance,the heuristic algorithm was used to optimize the algorithm.The SWCC prediction model was established based on particle swarm optimizationextreme learning machine(PSO-ELM)algorithm,extreme learning machine(ELM)algorithm and support vector machine(SVM)algorithm.Comparing the computational efficiency and simulation accuracy of the classical model and the machine learning model,the model with the best simulation performance was selected as the SWCC prediction model.Sand and silty clay samples with gradient moisture content were configured and the matrix suction balance test was carried out.Combined with the SWCC prediction model,the discrete relationship between matrix suction and casing thermal conductivity was obtained.Based on the Linear-Source theory and the experimental results of sand suction balance test,the functional model of soil matrix suction and casing thermal conductivity was established.The obtained function model was verified by the experimental values of silty clay,which proves whether the function model can accurately describe the functional relationship between soil and casing,and whether the functional relationship was limited by soil type.The field experiment in Changchun area was carried out with the instrument based on the embedded optimal function,and the designed instrument and tensiometer were embedded in the same position.The results of two instruments on soil matrix suction were compared,and the reliability of the measurement was analyzed.Finally,the error propagation and synthesis theory based on random error was used to analyze the systematic error of the designed instrument.The experimental results show that the designed instrument can effectively describe the change of soil matrix suction caused by saturation or dry.After error analysis,the systematic error of the instrument is 9.85%,which can accurately and effectively measure the soil in-situ matrix suction.Meanwhile,the designed instrument based on the thermal conductivity sensor can obtain the soil temperature,the thermal conductivity and the thermal diffusivity while obtaining the soil matrix suction.The designed instrument can also effectively avoid the influence of atmospheric temperature,and solve the limitation of insufficient range of tensiometer,easy to operate and carry,and the measurement results are stable and accurate.Therefore,the designed instrument can provide researchers with an accurate measurement method of soil in-situ matrix suction.