Publication Abstract

Proceedings of the 25th Annual Members' Conference and Eighth International Conference and Exposition, 2000, New York NY, (DFI)

An innovative foundation technology for the construction of retaining walls in urban areas
F. Pagliacci, M. Bertero

Retaining walls for the construction of underground urban infrastructures have to be executed considering the impact of a deep foundation job-site within an urban network. Traditional excavation systems, based on the use of grabs and bentonite mud, involve serious problems such as: the opening of trenches adjacent to existing buildings; the cleaning of the working area; the site installation for the production and storage of the bentonite mud; the transport and disposal of soil contaminated with bentonite mud. Alternative solutions have therefore been developed in order to overcome these problems. For temporary works soil consolidation methods, combined with the use of sheet piles, have been implemented, by a combination of traditional mixing methods and jet grouting systems to speed up the operations, consequently reducing the construction time. For permanent workds two tehcniques named respectively CSP (cased secant piles) and CDW (continuous diaphragm walls) have been studied, applied and tested. The basic concept of the two systems involves the execution of concrete retaining walls, without the need of fluids to support the excavation, also in presence of soft or loose soils by using equipment suitable to operate in urban areas with minimum environmental impact. CSP is the execution of secant piles, by using a continuous flight auger housed inside a steel casing. Auger and casing are driven by two separate rotary heads; the concrete casting is performed throughout the hollow auger during the extraction phase. CDW, an innovative and tested technology in diaphragm wall construction, makes use of equpment fitted with a customised trencher which, after its first introduction into the soil, is shifted along the longitudinal axis of the retaining walls to be executed, by cutting the soil to the required thickness and depth. The same equipment pours the concrete simultaneously during the trench excavation, avoiding joints. The execution of the wall takes place in a continuous working cycle, resulting in minimum downtime. The characteristics of excavation systems, equipment and the sequence of construction phases are discussed. Case histories of projects in urban areas focused on retaining walls for underground car parks and projects describing an innovative technology for improving the capacity of existing tunnels while assuring traffic continuity are reported.

 article #905; publication #54 (AM-2000)