Detecting anomalies in diaphragm walls with electrical resistance measurements

Quality control of diaphragm walls prior to excavation is often difficult. One technique that can be used to detect anomalies in diaphragm walls involves electrical resistance. Electrical resistance measurements across a diaphragm wall can (within a strict framework) be used to verify the presence of leaks in diaphragm walls as a supplement to crosshole sonic logging. From measurements around a test wall conducted in this study, it is concluded that the detectability of anomalies with electrical resistance decreases exponentially with the increasing distance between the measurement electrodes and the wall. Electrical resistance setups with two and four electrodes have been compared. For usable results, a four-electrode setup must be used in which the potential electrodes need to be placed very close to the wall (less than 0.2 m away). Based upon the test experience, a field setup for verification of a building pit consisting of diaphragm walls is suggested, as well as a setup for determining the quality of the concrete covering the rebar in quay walls constructed with diaphragm walls.