FIELD MEASUREMENT OF SOIL SUCTION USING THERMAL CONDUCTIVITY MATRIC POTENTIAL SENSORS
van der Raadt, Paul
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The performance of railway embankments are directly related to the mechanical behaviour of unsaturated soils. The mechanical behaviour of unsaturated soils can be significantly influenced by changes in matric suction. Matric suction, therefore, should be considered in the analysis and design of railway subgrades or any earth structures consisting of unsaturated soil. The equations for relating matric suction to the mechanical behaviour of unsaturated soils are known. In order to incorporate these equations into design, the range of values of suction expected under field conditions must be known. A study was undertaken in the fall of 1984 to evaluate a commercially available thermal conductivity sensor which is designed to measure matric suction indirectly. Five railway subgrades were instrumented and monitored in western Canada. Six sensors were installed at each site at various depths below subgrade period of up to one year. The sites were monitored for a The seasonal variations in matric suctions indicated that sensors near surface indicated the largest matric suction during the winter months with the minimum values recorded in spring. Both the minimum and maximum recorded matric suctions occurred later in the year for the sensors at depth. During the course of the study, problems arose with the thermal conductivity sensors. Many of the sensors failed during the field monitoring period, several in a non recoverable fashion. It was subsequently discovered that the sensors will fail from prolonged submergence in water. After the field monitoring program, a recalibration study was undertaken in order to explain the erratic results displayed by many of the sensors. The results of the recalibration study indicated a significant difference between the shapes of the actual calibration curve and the calibration curve supplied by the manufacturer. It was discovered that below approximately 200 kPa the overall shape of the calibration curve was in close agreement with the manufacturer, however above 200 kPa the slope of the calibration curve increases sharply slope approaches a vertical line. In several cases the slope approaches in a vertical line. Because of this the reliability of the sensors for measurements above 200 kPa is in doubt. Overall the study showed that matric suction can be measured in the field as long as the matric suction falls within the operational range of the sensors.