Laboratory and Field Measurements of Moisture Content and Bulk Electrical Conductivity of Subgrade and Base Course Materials

Abstract

Moisture content can impact the resilient modulus of pavement base course and subgrade layers. Given its influence on the post-inundation resilient moduli of the coastal pavement and base course and subgrade layers, it is important for transportation management personnel to understand the post-inundation moisture state for short-term and long-term planning. Dielectric soil moisture sensors can be used to obtain real-time soil moisture data; however, in coastal areas these sensors will be exposed to salinity. Salinity in the soil can adversely affect the performance of various soil moisture sensor technologies. A review of numerous previous studies highlighted the difficulties associated with the use of Time Domain Reflectometry (TDR) and capacitance technology in saline environments. These studies did not follow standard means and methods for testing and calibrating soil moisture sensors in saline soils, which led to conflicting results. The performance of two commercially available dielectric soil moisture sensors, one using TDR technology and one using capacitance technology, in a laboratory environment with three different soil materials was examined. The SoilVUE10 (TDR) and TEROS12 (capacitance) both measure volumetric water content (θ), bulk electrical conductivity (σb) and temperature. Volumetric water content values from the sensors were compared to gravimetrically obtained values. The results show that the capacitance sensor was more reliable and more accurately measured volumetric water content in all three soil materials. In agreement with its product manual, the SoilVUE10 should not be used when σb > 10 dS/m. It also did not perform as well in soils that had poor contact with the waveguides. The SoilVUE10 and TEROS12 sensors, an aluminum access tube for the nuclear moisture meter, and vibrating wire piezometers were installed in pavement base course and soil subgrade at four locations along Alabama Highway 180 (AL-180), a coastal highway experiencing Sea Level Rise (SLR), in an effort to understand the fluctuations in θ, σb and groundwater levels at these locations due to SLR. The sensor installation procedure is presented, preliminary data is reported, and future applications of the data are discussed herein. The TEROS12 sensors did not perform as well in the field as they did in the laboratory; however, two sensors that were developed for agricultural use are successfully collecting θ and σb data at every desired depth at every site. International Roughness Index (IRI) data has been collected annually since 2021 on AL-180. The field moisture contents (θ), and IRI data will continue to be collected until 2025. The data will be used to evaluate the effect SLR is having on pavement performance in coastal Alabama. The findings from this study could be implemented into future pavement design to combat the effects of SLR.