Defects in soil-bentonite (SB) slurry trench cutoff walls may reduce the effectiveness of the engineered structure to minimize groundwater flow and contain pollutants. One potential construction defect identified for SB cutoff walls is the presence of granular material in the wall due to sidewall collapse or sedimentation on the backfill slope and/or in the trench key. However, post construction methods that could be used to detect these defects are relatively untested or the analysis of the results is very complicated. In this study, electrical resistivity (ER) was used in unique configurations in an experimental SB cutoff wall installed in a well characterized alluvial formation to detect and characterize designed defects placed during construction. Defects were placed in the middle of wall at depths of 1 m or 4 m below the top of the wall, well above the trench key at ~7 m, and were also placed on the trench key. The granular defects ranged in size from 0.02 m3 to 0.3 m3 and were predominantly permeable sandbags with a clean, well sorted (poorly graded) medium sand. The largest defect installed in the wall was an impermeable limestone boulder. Custom electrodes were developed that could be pushed into the SB cutoff wall and thus collect ER data both at the ground surface and at depth within the wall. The ER data indicate that while it is possible to detect the defects, the sidewalls of the trench impart a significant effect on the ER data and thus affect the maximum distance between the observation electrodes and the potential defect.