Electrical Impedance Needle

The needle is used to measure the impedance (electrical resistivity) of soft clay, aiming to obtain insights into the variation of density (the impedance increases with density). The design of the ETHZ electrical impedance needle is inspired by Cho et al. (2004), who suggested driving such a tool into the soil while measuring the electrical resistivity. The needle diameter is reduced to 1 mm, in order to be small enough to represent a reasonable-size prototype, and large enough so that the ratio of its diameter to the mean grain size is within acceptable limits.

The needle is a 1 mm stainless steel tube (0.8 mm inner diameter), fitted with an electrical cable at the tip of which the elec¬trical impedance is measured (Gautray 2014). A cover protects the needle from wind-induced vibrations, due to centrifuge spinning. The cover is maintained over the needle until reaching the soil surface, being pushed back while the needle is penetrated into the soil. After each sounding, the clogged (by clay particles) needle is cleaned in-flight, using a custom-developed ultrasonic bath.