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 acros s the state of Tennessee were instrumented with comprehensive monitoring sy stems that collect meteorologic, subgrade water content, infiltration, and temperature data. These data will be used to develop a rational method to a ccount for environmental effects (e.g., seasonal changes in subgrade water content, in flexible pavement design). Multiple-segment TDR probes were pla ced horizontally in the soil subgrade, in the stone subgrade, and in the as phalt stabilized base, and single-segment probes were placed in the asphalt ic concrete of the roadway sections. Tension-free pan lysimeters were insta lled at three of the test sites to measure infiltration through the pavemen t layers. Temperature sensors were installed in the soil subgrade and in th e stone base at the same elevations as the TDR probes, and at three elevati ons in the top 200 mm of the pavement. Each site was instrumented with a we ather station that monitors relative humidity, air temperature, rainfall, w ind speed, and solar radiation. The preliminary data indicate changes in wa ter content in the stone base layer that correlate with rainfall, and a sma ll seasonal change in subgrade water content. Data from a test site where t he pavement surface layer is not in place indicate that the asphaltic layer s below the surface layer are permeable during significant rainfall events and that water appears to move freely through the asphalt stabilized base l ayer.