Comprehensive Monitoring Systems for Measuring Subgrade Moisture Conditions

Subgrade resilient modulus is highly dependent on water content, which can vary significantly with a number of environmental factors. Four sites across the state of Tennessee were instrumented with comprehensive monitoring systems that collect meteorologic, subgrade water content, infiltration, and temperature data. These data will be used to develop a rational method to account for environmental effects, such as seasonal changes in subgrade water content in flexible pavement design. Multiple-segment time domain reflectometry (TDR) probes were placed horizontally in the soil subgrade, in the stone subgrade, and in the asphalt stabilized base, and single-segment probes were placed in the asphaltic concrete of the roadway sections. Tension-free pan lysimeters were installed at three of the test sites to measure infiltration through the pavement layers. Temperature sensors were installed in the soil subgrade and in the stone base at the same elevations as the TDR probes, and at three elevations in the top 200 mm of the pavement. Each site was instrumented with a weather station that monitors relative humidity, air temperature, rainfall, wind speed, and solar radiation. The preliminary data indicate changes in water content in the stone base layer that correlate with rainfall, and a small seasonal change in subgrade water content. Data from a test site where the pavement surface layer is not in place indicate that the asphaltic layers below the surface layer are permeable during significant rainfall events and that water appears to move freely through the asphalt stabilized base layer.