Quality assurance of diaphragm walls by sonar measurements – model study

Diaphragm walls are used to provide a stable and water-tight confinement for the construction of tunnels, underground railway stations, deep basements and other structures requiring deep excavations. These walls consist of individual panels, cast piece-wise in the ground, separated by water-tight joints. There are several methods to improve the water tightness of these joints, including rubber bands, metal sheets or precast concrete elements. In most cases, diaphragm walls perform very well. Quality problems, which occur in a few occasions, may be due to uncontrolled concrete flow between the joints, unexpected geological conditions or poor workmanship. Different methods of quality assurance are applied on construction sites. Sonar instruments can be used in a bentonite-filled trench to evaluate the shape of the excavation. An improved prototype instrument has been developed, including digital data-processing features, which allow the detection of even small anomalies at the stop-end surface before concreting the adjacent panel. The sonar sensor can be used in depths up to 200 m. Model experiments show that a centimetre resolution can be achieved. Set-up, model experiments and their results are shown. The prototype might be the basis of an easy-to-use commercial device.