Resistivity Measurement of Backfill for Mechanically Stabilized Earth Walls

Mechanically stabilized earth (MSE) walls often include galvanized steel strips as mechanical reinforcement within layers of specified backfill material. Galvanized steel reinforcement provides considerable tensile strength; however, it is potentially vulnerable to corrosion. Corrosivity of MSE backfill material is typically characterized using electrical resistivity. Many variations of the current AASHTO T288 standard exist to determine this resistivity. However, there is concern that this method may not reflect field conditions, particularly for aggregate backfills, and thus may mischaracterize the corrosivity of these backfill materials. ASTM currently has a proposed standard for coarse aggregate resistivity that addresses this mischaracterization. Five aggregate backfill materials were collected during construction from MSE wall projects located in eastern Kansas. These materials were laboratory-tested using two methods: (1) the current AASHTO standard and (2) a proposed ASTM test method for coarse aggregate resistivity. Results from the laboratory testing were compared with field resistivity measurements of two of the MSE wall backfills post-construction. The proposed ASTM test method appeared to more accurately simulate field conditions behind MSE walls. Increasing the number of soak/drain cycles substantially increased measured resistivity. Recommendations are made based on data included in this paper that should lead to more representative resistivity testing results. These include a minimum electrode spacing of 8:1 and a minimum height of the test box that is three times the maximum particle size.