Detection of Anomalies in Diaphragm Walls

Abstract

If a calamity with a retaining wall occurs, the impact on surrounding buildings and infrastructure is at least an order of magnitude more severe than without the calamity. In 2005 and 2006 major leaks in the retaining walls of underground stations in Amsterdam and Rotterdam occurred. After these cases had been thoroughly studied it was concluded that the diaphragm walls had anomalies in the joints between the D-wall panels. These anomalies are hard to locate using regular leakage detection systems and may cause major damage to the surroundings if an erosive leak progresses. Such a risk is hard to handle for a project and may stand in the way of the application of D-walls close to adjacent buildings. Between 2009 and 2014, within the GeoImpuls program, research was done to develop measurements to detect these anomalies. Three techniques have been found to be effective in evaluating the quality of the concrete in the joint area between D-wall panels. During D-wall production the slurry refreshing operation and concrete casting can be verified using Distributed Temperature Sensing (DTS). After curing of the concrete the Cross-hole Sonic Logging (CSL) method, applied to the joint area, can be used to assess the concrete quality in the joint. Anomalies can be localized both vertically and horizontally. The size and contents of the anomaly can be estimated based upon laboratory reference tests and site experience of several projects. If verification of anomalies detected with CSL or DTS measurements is required, Electrical Resistivity Tomography (ERT) can be considered to evaluate the hydrological parameters of the anomaly, although the reliability of these results is much lower than with the DTS and CSL measurements. Based upon the information of the tests, it can be decided if mitigating repair works prior to excavation of the building pit are necessary or that reparation of the anomaly can safely take place while excavating the building pit. The paper will give a short description of all three methods and the executed validation measurements. The paper concludes with practical recommendations for implementation of the measurements.