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Innovative quality assurance of diaphragm walls

Diwaq will develop innovative instrumentation and procedures for the quality assurance of diaphragm walls. Their use will prevent hazardous events, construction delays and cost increase at deep excavations (tunnels, underground stations. Basements).

Diaphragm walls are used all around the world 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. As diaphragm walls are reaching depths of up to 60 m, they have to withstand large earth and water pressures. In most cases, diaphragm walls perform very well. Nevertheless, quality problems may cause a mixture of water and soil particles to flow into the excavation. This may lead to a lack of performance (especially water inflow) or the failure of a single panel, the entire wall or even adjacent buildings. A recent accident in GERMANY has cost two lives and unaccountable damage to cultural heritage. These quality problems may be due to uncontrolled concrete flow between the joints, unexpected geologic conditions or poor workmanship. As diaphragm walls are dug with the use of bentonite slurry to inhibit water flow into the hole or soil blocks falling into it, direct visual control of the joints between the wall panels is not possible from the surface. Thus quality control is so far limited to measuring the depth and inclination of the slots and recording the mass of the removed soil and poured concrete. Ultrasonic methods adopted from pile testing are limited to the interior or a single panel, but are not capable (or at least not validated) to check the integrity of joints between panels. An indirect non-destructive testing procedure for this purpose would provide the possibility of quality assurance and remediation before excavation. This would lead to an increase of safety and savings of remediation costs. We will develop an innovative two stage quality assurance system including the necessary instrumentation, which will provide objective data on the shape and state of the joints between the panels. The first stage will allow corrections in the construction process, while the second one will be done as a final check after completion of the wall before excavation: * STEP 1: Profiling the shape of the joint (adjacent concrete panel wall) of the secondary panel by newly developed ultrasonic/sonar equipment or other means after soil removal before concreting, especially the shape of the adjacent concrete panel wall. * STEP 2: Investigation of the joint after concreting by cross-hole sonic logging with sensors deployed in steel access tube mounted to the reinforcement cage. This testing procedure (ASTM D6760-08) is currently used for checking the internal part of a single pile or panel, but is not used for joints. Some preliminary work has been published by one of the partners on measurements made on a model wall. It has shown the potential of the proposed procedures, but has not been applied in practice so far. Other parties (partially with involvement of partners of this consortium) performed field trial, but not with a systematic and validated approach. Additional testing methods will be tried, namely thermal logging, electrical conductivity, etc. These methods may prove superior to the ultrasonic one or serve in combination to define joint anomalies. With this two-step approach, i.e. profiling the primary face first and then cross-hole sonic logging across the completed joint, a quality assessment of the overall joint is possible prior to excavation of the diaphragm wall. As such, the risk of defective joint is significantly reduced. If the hardware and software is made available to contractors and consulting firms, the method will be incorporated into national piling specification. Consequently, future contracts for construction of diaphragm walls will require these tests on the joints of as state-of-the-art quality assessment.
Acronym: 
DiWaQ
Project ID: 
6 286
Start date: 
01-04-2011
Project Duration: 
24months
Project costs: 
810 000.00€
Technological Area: 
Other Non Destructive Testing
Market Area: 
Construction Services

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