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GPR monitoring of volumetric water content in soils applied to highway construction and maintenance

Dept. of Civil and Environmental Engineering, UC Berkeley, Berkeley, California, U.S.

S. Hubbard

Lawrence Berkeley National Laboratory, Berkeley, California, U.S.

Y. Rubin

Dept. of Civil and Environmental Engineering, UC Berkeley, Berkeley, California, U.S.

Corresponding author: krgrote@lbl.gov

The first 20% of the full text of this article appears below.

Detailed knowledge of the subsurface water content is important for highway design, maintenance, and repair. Transportation engineers can monitor the water content of subasphalt soils to estimate the soil stiffness as an index of the likely performance of a pavement and to evaluate the need for subsurface drainage retrofits. Conventional approaches for measuring water content include gravimetric sampling, time-domain reflectometry (TDR), and neutron probes, all of which are time-consuming and invasive. Additionally, each of these methods provides only point measurements; because soil moisture content can vary greatly over space and time, point measurements are of limited value when surveying over a large area and over a period of time. An alternative to these conventional methods is ground-penetrating radar (GPR), which can be used to quickly collect continuous, high-resolution water content estimates. GPR techniques can be used to estimate water content due to the sensitivity of electromagnetic velocity to water content.

The two experiments described here, a controlled pit study and a transportation application in subasphalt soils, are based on measuring the velocity of common-offset GPR reflections, which allows estimation of water content over deeper intervals than is possible with groundwave data and more quickly than can be accomplished with common-midpoint surveys. To estimate the velocity from GPR reflections, both the travel path and the traveltime of the electromagnetic energy must be known. With common-offset GPR techniques, the two-way traveltime of the energy from the surface to the reflective interface and back is measured. To estimate electromagnetic velocity, the depth of the reflective interface must be determined. For engineered materials, the depth of a reflective layer might be known from construction records; for natural soils, calibration boreholes could be used to estimate the depth of soil layer interfaces.

After the electromagnetic velocity has been calculated, it can be converted to the . . . [Full Text of this Article]

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