Needle probe application for high-resolution assessment of soil spatial variability in the centrifuge

This paper addresses the development of an electrical resistivity needle probe to be deployed during centrifuge model testing to assess with high-resolution the spatial variability of soil electrical resistivity. The probe is able to detect thin layers and accurately resolve interfaces between soil layers. The prototype probes are made from thin, stainless steel needles, with an insulated wire inserted into the needle, and bonded to it with epoxy resin, to form a coaxial probe (Cho et al. 2004). Different tip shapes including single-wedge, double-wedge and cone have been developed to optimize the spatial resolution of porosity, soil interfaces and layering. The calibration and testing of the needle probe has been conducted both at Georgia Tech and UC Davis. This paper presents results of resistance versus depth and porosity versus depth, and compares the porosity based on measurement of mass and volume with the calculated porosity based on the needle probe measurements. Consequently, insertion effects are discussed and theoretical explanations are given. For its deployment in the centrifuge, a special needle probe tool has been developed to be operated by the new NEES robot. Instrumentation, data acquisition and data processing issues associated with the needle probe robot tool are discussed. The basis for selection of materials, probe tip geometry, and the optimum frequency of AC electrical measurements is explained.