The effects of capillary hysteresis on the measurement of matric suction using thermal conductivity sensors
Matric suction has proven to be a key parameter in the study and application of soil mechanics for unsaturated soils. Field measurements of suction are necessary in many engineering analyses, such as the prediction of total heave, the analysis of the slope stability due to changes in soil suction, and the monitoring of moisture flux through a soil cover or barrier structure used to impede contaminant transport. There are a number of methods to measure soil suction in the field. The thermal conductivity sensor proves to be one of the most promising means of in situ suction measurement. The thermal conductivity sensor measures matric suction by measuring the rate of dissipation of thermal energy in the ceramic sensor tip. The thermal diffusivity. of the ceramic is dependent upon the water content of the ceramic. The water content is a function of the matric suction in the surrounding soils. This function is referred to as capillary function or soil-water characteristic curve and exhibits hysteresis. In other words, the same value of matric suction may correspond to different ceramic water contents, thus different sensor outputs, depending upon the drying and wetting history. The objective of this study is to investigate the properties of capillary hysteresis of the sensor ceramic and its effects on the measurement of matric suction. Two groups of laboratory tests involving drying and wetting processes were carried out; one group measured the relationship between water content and matric suction of the sensor ceramics, the other group measured the relationship between sensor output and matric suction for a newly developed sensor. The result shows that, although the hysteresis loop is relatively narrow compared with those of coarse-grained materials found in the literature, the effects of capillary hysteresis on suction measurement using the thermal conductivity sensor are not negligible. If the capillary hysteresis is not taken into account, the maximum possible relative error of suction measurement caused by the capillary hysteresis is from 24% to 50% for the sensors used in the tests. The problems associated with the conventional method of calibration are also discussed in the thesis. To make the suction measurement more accurate, the sensor output versus suction relationship of each of the possible wetting and drying processes should be measured in calibrating the sensor. However this calibration is impractical. Therefore, it is desirable to predict the hysteresis curves from limited measured data using a mathematical procedure. There are a number of models found in the literature to simulate the capillary hysteresis of a porous material. Some of these models were examined using the experimental data of the sensor ceramic. It was found that the models in the literature either require a large amount of measured data to make the prediction, or fail to reproduce the measured curves of hysteresis. Therefore, an analytical approximation was developed which used a curve fitting method to fit the measured main drying curve and to predict the main wetting curve and the primary scanning curves. Based on the above experimental and modeling studies, suggestions were made on the calibration of the sensor.