Characterization of Two-Layer Soil System Using a Lightweight Deflectometer with Radial Sensors

Nondestructive tests to estimate stiffness modulus, such as the lightweight deflectometer (LWD), have experienced increased popularity, but very little research has been performed to evaluate the LWD with radial sensors. Results are presented from LWD testing with radial sensors that measured the deflection bowl on one- and two-layer field test beds consisting of unbound materials. LWD testing produced a measurable deflection bowl on medium-stiffness granular materials to a radial sensor spacing of 750 mm (30 in). When limited to a stiff over soft layered system, the LWD with radial sensors demonstrated the ability to backcalculate layered moduli accurately. Backcalculated moduli closely matched laboratory-determined moduli from triaxial testing at a similar stress state as in the field. The measurement depth for the LWD with radial sensors was found to be 1.8 times plate diameter versus the measurement depth of conventional LWD testing of 1.0 to 1.5 times plate diameter. The LWD with radial sensors was able to measure deeper-than-conventional LWD testing because the radial geophones measure vertical surface deflections caused almost entirely by strains in deeper material. Compared with other configurations, the 300-mm and 600-mm (12-in. and 24-in.) radial sensor configuration is recommended for unbound materials because it produces the most accurate moduli backcalculation results and captures deflections critical to the backcalculation process.