Research On Estimating Soil Matric Suction Based On Electrical Resistivity Tomography

Matric suction is a key controlling factor in water movement and soil stability in unsaturated soils.Matric suction test methods are mainly tension meter,axis shift technique,electric / heat transfer sensor,contact filter paper technology and so on.These methods can get more accurate matrix suction value in a soil local point,but they are most widely believed as high cost and invasive techniques for the evaluation of spatial distribution of matrix suction.In recent years,electrical resistivity tomographyis a non-invasive and dynamic measurementrepresentative geophysical method,which has been widely used in environmental geology,hydrogeology,andengineering geology.Moisture content is the main factor controlling the matric suction and soil resistivity.Therefore spatial distribution of the matrix suction can be evaluated by the resistivity obtained by the electrical resistivity tomography through the relationship between the matrix suction and the resistivity by the moisture content,which can be used as an important supplement to the conventional matrix suction test method.In this paper,according to the National Natural Science Foundation of China project of landslide matrix suction time-space distribution of electrical resistivity tomographyNO.41402268,Chengdu silty loam and Gansu loess are studied.We obtained closed spatial variations of soil matric suction mapping by electrical using resistivity tomography,which combining the Archie's law and the van Genuchten expression.Firstly,we derived a form model between soil matric suction and electrical resistivity with water content increasing.And then change dry density and pore waterto obtain the impact of these factors on the matrix suction-resistivity mathematical model.Then the loess is studied at the dry density of 1.25 g/cm3 with intermittent artificial rainfall and at the same time sensor are installed to measure the moisture content and the matrix suction in the vertical direction.Electrical resistivity tomography method is used to obtain the spatiotemporal change processof resistivity.At the same time,in order to improve the accuracy of electrical resistivity tomography method for predicting matrix suction,we will perform the correlation between the matric suction and the electrical resistivity at the same position of the different sections of the soil column.The electrical resistivity tomography was used to predict the matrix suction distribution of different sections of soil column during water infiltration by using this mathematical model.The main results in the research on estimating soil matric suction based on electrical resistivity tomographyare as follows:(1)The matrix suction of silty loam and loess are negatively correlated with volumetric water content,which is consistent with Van Genuchten model.The resistivity of silty loam and loess are also negatively correlated with volume water content,which is in accordance with Archie model.The resistivity of silty loam and loess are positively correlated with the matric suction.The Van Genuchten model and the Archie formula are combined to derive and establish the relationship model between resistivity and matric suction.In general,the relationship between resistivity and matrix suction of loess is better than that of silty loam.(2)Under the condition of intermittent artificial rainfall in the loess column,the water content from shallow to deep soil is increased graduallyand the maximum water content was about 0.4.Eventually,the soil moisture content in the shallow and middle part is about 0.2and the bottom soil moisture content is about 0.4.Soil matrix suction from the shallow to the deep is gradually reducedwith the infiltration of water and the minimum minimum of about 10 KPa.To the end of the test,the shallow soil matrix suction is greater than the lower soil matrix suction.(3)The relationship between the volume water content of the soil column and matric suction is consistent with the Van Genuchten model.In general,the relationship between the volume water content and matric suction of the lower soil is in a good agreement with the Van Genuchten model than others.This is because the intermittent artificial precipitation causes the upper soil to be in the process of repeated moisture absorption and dehumidification,so that the lagging effect of the soil water characteristic curve is significant,but the lower soil is always in the process of moisture absorptionand the lag effect of the characteristic curve of soil water is not obvious.(4)The results show that with the continuous infiltration of water,the resistivity value decreases gradually from shallow to deep,which show the movement process of water wet front.So we can get that there is obvious heterogeneous characteristics on the same plane.After the wet forward,the soil resistivity tends to rise.It is found that the correlation between soil resistivity and matrix suction is significant by analyzingand the model is used to evaluate the spatial and temporal distribution of matrix suction.(5)Based on the matric suction-resistivity model,electrical resistivity tomographyis used to evaluate the spatial and temporal distribution of matric suction at the bottom of soil column.The matrix suction value gradually expands from the middle area to the entire profile,which is similar to the change in resistivity,and the matrix suction has heterogeneous characteristics in the same profile.Until the end of the test,the substrate suction to a smaller value,about 10 Kpa,which is the same as the value measured by the matrix suction sensor.The relationship between soil resistivity and matrix suction was studied by mechanism analysis,laboratory test and mathematical statistics.The results showthat Thereis a significant positive correlation between matrix suction and resistivityand the established model could be used for the evaluation of matrix suction during moisture absorption.This result is a useful complement to the existing method of measuring the matric suction.