Measurement of Matric Suction In Thin Membrane Surface Highways Using Thermal Conductivity Sensors

The performance of Thin Membrane Surface (TMS) highways is directly related to the strength of the subgrade. The subgrade of these highways consists unsaturated soil whose strength is a function of not only normal stress but also matric suction. Therefore matric suction should be a major consideration in the analysis and design of these highways. The consideration of matric suction in the design of TMS highways requires a reliable and practical methodology quantitatively measure soil suction in the subgrades of these highways. A study was conducted starting in September 2000 to develop a practical installation procedure for the new thermal conductivity sensor developed at the University of Saskatchewan, Canada. The new sensor, called the University of Saskatchewan Thermal Conductivity Matric Suction Sensor, is able to indirectly measure soil suction in TMS highways. Two highway subgrades in southern
\nSaskatchewan, Canada were instrumented with the new sensors. Sixteen of the new
\nsensors were installed at each site at various depths and distances from the highway
\ncentreline. The sites were monitored for a period of nine months. The results of the study showed the sensors reaching equilibrium at varying times depending on the sensor characteristics and soil conditions. The results also indicated that highest matric suction values were achieved during the winter months and the lowest values were exhibited during the spring. The time of maximum soil suction values recorded ranged from early November 2000 for the shallowest sensors to late March 2001 for the deepest sensors. Minimum suctions were witnessed in early April 2001 in the shallowest sensors while the deepest sensors exhibit a steady decline in suction magnitude from the maximum value in late March 2001 until the
\nend of the study period. In so far as matric suction largely controls bearing capacity in highway subgrades, maximum and minimum bearing capacities will correspond with maximum and
\nminimum matric suctions.
\nFreezing conditions in the soil around the sensors was witnessed in early November 2000 in the shallow sensors while the soil around the deepest sensors did not exhibit freezing conditions during the study period. The results from Site 2 South of Torquay, Canada exhibited suction values nearly twice that of the values obtained from Site 1 North of Bethune, Canada.
\nDuring the monitoring period the Data Acquisition System (DAS) at one of the
\nsites was contaminated by meltwater; therefore, some suction readings from early
\nspring at the site were unavailable. The damaged system was replaced and the data
\nacquisition resumed on May 4, 2001. The DAS used in conjunction with the new sensors functioned well during the study period and was not adversely affected by the wide range of climatic conditions experienced at both sites during the study period. The wireless communication system utilized for the transmission of field data from the DAS functioned very well during the study period for the collection of the suction readings from the field.
\nThe study describes a practical installation procedure developed for the new sensors in order to obtain appropriate data. The ability of the new sensor to measure soil suctions in highway subgrades at remote locations was verified as was the DAS and communication system utilized in conjunction with the sensors. The effect of matric suction on the bearing capacity of highway subgrades was reviewed. This study illustrated that soil suction readings obtained by the new sensors can be analyzed within the context of unsaturated soils for use in highway analysis and design.

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