Near surface soil properties using electromagnetic and seismic waves

Earthwork compaction evaluation is fundamental to geotechnical engineering. The conventional approach to compaction control makes use of measured water content and dry density. Field measurements of these can be made by nuclear or other standardized test methods. The measured water content and dry density for a given soil are indicators for parameters used in design such as shear modulus, shear strength, and hydraulic conductivity, but do not measure any of these directly. For the design of pavement systems and many other facilities, the shear modulus of the soil is of great value. Both seismic waves and electromagnetic waves are convenient tools for characterizing particulate materials. This paper introduces a technology which provides near surface water content, dry density, and shear modulus by the combined use of electromagnetic and seismic waves. In the test setup, four spikes are driven into the ground in a configuration that simulates a coaxial cable where the soil between the spikes acts as the dielectric medium. An electrical pulse is then applied to the spikes and from the observed reflected electrical signal, soil dielectric properties are determined. These are used to estimate water content and dry density of the soil (Drnevich et al., 2002). For the measurement of shear modulus, one of the spikes is selected as the excitation spike and is tapped with an instrumented hammer. The other spikes, with accelerometers amounted on their heads, act as the receivers. The steel spikes are much stiffer than the surrounding soil. Hence, the excitation spike acts as a source of vertically polarized shear waves along the length of the spike and the receiver spikes act like waveguides for measuring the horizontally traveling waves. Travel time analysis is used to determine the shear wave propagation velocities which are used with the soil density measured by electromagnetic waves to determine shear modulus. Another critical parameter, the water content also is measured in the process. In addition, increasing the magnitude of tapping, increases shear strain levels of the propagating waves and reduces the shear wave propagation velocities. Shear modulus reduction curves can be obtained from corresponding particle velocity (integrated from recorded accelerations) and shear wave velocity. The test setup was evaluated both in the field and in the laboratory. The results show the measured shear moduli are reasonable and the influence of soil disturbance by spike insertion is minimal. The modulus reduction curves measured by this method are consistent with those in the published literature. With improved system design, the testing and data analysis can be done quickly and can provide parameters needed for engineering design.