Analysis of In Situ Soil Thermal and Hydraulic Data from a Subgrade Sensor Network under a Granular Roadway

Cyclical temperature changes occurring during seasonal freezing and thawing cycles significantly impact the moisture content and matric suction of subgrade soils, contributing to severe damage to granular roadways during spring thaws. Monitoring of these data in the field can play a significant role towards development of accurate and reliable freeze-thaw prediction models. In the present study, a weather station was installed along with an extensive network of 80 subgrade temperature, matric potential, and moisture content sensors below a granular-surfaced roadway for long-term monitoring. The subgrade sensors were installed over a range of depths up to 213 cm (7 ft) in five boreholes located under the centerline, shoulders, and quarter points of the roadway. Disturbed and intact soil samples from the site were collected and tested to determine soil index, thermal, and hydraulic properties of subgrade soils. This paper presents preliminary analyses of the in situ temperature, soil moisture, and matric potential distributions under the roadway, which are found to follow similar trends in general but exhibit unsymmetrical variations across the boreholes, which may be due to differences in surface boundary conditions, compaction ratios, and soil profile changes. The matric potential and water content values were found to decrease suddenly near the freezing point, indicating the start of soil water freezing. Finally, data from the installed weather station was compared to the air temperature and wind speed measurements from the nearest road weather information system (RWIS) stations. The air temperature values generally agreed, indicating that the RWIS data can be interpolated to obtain the site temperatures with reasonable accuracy, but wind speed did not provide similar agreement and requires further analysis.