Volume 34, Issue 6 (November 2011)
Microstructural Interpretation of Water Content and Dry Density Influence on the DC-Electrical Resistivity of a Fine-Grained Soil
In geotechnical engineering, there is a need for an in situ technique for measuring density and water content of soils quickly, accurately and, preferably, in a non-destructive way. DC-electrical resistivity provides a method capable of addressing this need. A new resistivity-cell and a new experimental procedure were developed to assess the resistivity of compacted samples of a silty soil. The study was carried out over moisture content and compaction level ranges that are consistent with those acceptable in earthwork construction. In this laboratory work, the effects of dry density and gravimetric water content on the soil bulk resistivity are considered separately. Furthermore, the effect of air index on electrical resistivity is demonstrated. Our data exhibit two distinct trends separated by a “critical” water content which appears to fall very close to the standard Proctor optimum moisture content for this soil. This promising result cannot be generalized as it needs to be investigated on other soils. The experimental results are further analyzed within the context of a previously published soil microfabric model which distinguishes water phase, air phase, particles assumed to be “inert” and clayey aggregates. Two types of electrical conduction enable explaining the results: first, conduction occurring in the intra-aggregate voids, controlled by the charge density on the surface of clayey particles; and then, inter-aggregate conduction controlled by interstitial water resistivity. The relative effect of each type of conduction is shown to strongly depend on water content. Finally, an extrapolation at zero air void allows for presenting an interpretation of the intra-aggregate volume contribution to electrical conduction in dry states. This interpretation suggests a dilution effect on the adsorbed cation concentration within bound water, that is shown to be consistent with recently published research.