Diaphragm Wall Repair, Lessons Learned Although diaphragm or slurry wall tech- nology and equipment has vastly improved since its development in the late 1950s, significant problems still occur. Key issues are repairs related to panel joints between wall panels, concrete defects associated with tremie concrete problems, leaks and other defects at tieback reservations or inserts, wall leaks within the plane of the wall, and surface repairs. Potential problems need to be addressed in the design stage to allow for flexibility in repair work. The worldwide use of diaphragm walls (structural slurry walls) has accelerated even though the construction process is challenging. The work is largely done in the blind, and often in difficult underground geologic and hydraulic site conditions. Casting the wall against fills and/or native ground using tremie concrete methods does not enhance chances for success. Today most projects have little time for design, detailing and construction that can lead to imperfections and defects. The project owner, engineer and contractor often have different points of view and expectations of the end product. We describe some problems and their potential resolution based on our “lessons learned” during our 80 years of collective experience working on diaphragm walls. Communications. All parties in a project must communicate effectively and fully understand the design process and construction steps, including the intended structural and cut-off functions of the diaphragm wall. They also need to consider the tools available and the site constraints, and the contractors’ and inspectors’ roles in quality control and quality assurance. If value engineering changes are offered during the bid process, there must be a rapid exchange of information about necessary design changes. All parties should agree that the proposal is an equivalent substitution. Sometimes, the disadvantages are not apparent during review. When such submissions are only reductions in scope and/or quality, rather than innovations in design and construc- tion, they should be implemented with caution. 38 • DEEP FOUNDATIONS • JAN/FEB 2012 Figure 1. Diaphragm perimeter wall for Four Seasons and Legg Mason headquarters Design Decisions. Engineers select the wall thickness, concrete strength and methods to reinforce the wall system, including reinforced cages, structural beams and pre-stressing. Practical considerations such as equipment limitations, depth to rock or a suitable cut- off layer usually dictate wall panel depth. Precast concrete panels can be used where panels can be excavated without interrup- ting utilities or interference by adjacent structures. Sufficient lay down space for precast panel casting on site is essential. The precast panel depth is generally less than 50 ft (17 m) because of delivery and erection restrictions. A complete descrip- tion of the various types and sizes of panels is summarized in the DFI Slurry Wall/ Trench Committee’s paper (2005). AUTHORS: Raymond J. Poletto, P.E. Senior Associate Mueser Rutledge Consulting Engineers, New York, NY George J. Tamaro, P.E.,Consultant Mueser Rutledge Consulting Engineers, New York, NY Contractor Decisions. Contractors have the choice of clamshell buckets, grabs, hydromill excavators and drills to excavate the soil and drill rock to construct panels to the required design thickness, length and depths. Environmental restrictions may limit the use of specific tools. One critical feature is the contractor’s method of joining successive panels to achieve a “watertight” wall system. The joint detail is usually not well defined in the construction docu- ments. This approach may be inappropriate for conditions at the site, skills of the contractor and details of the design. The improper placement of tremie concrete within the open panel excavation can lead to serious problems. If the panel’s rein- forcing system, embedded items (tieback trumpets or structural beams), and geotechnical monitoring devices are not properly designed, detailed and connected, and do not permit the concrete to flow throughout the panel and to the end stop joint, the situation can be problematic. Removing slurry fluids within the panel should be done so as to not trap debris and solids that may weaken the structural elements and joints. A complete description
