A study of the effects of differential loadings on cofferdams

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Virginia Polytechnic Institute and State University


Conventional design of cellular cofferdams is largely based upon semi-empirical concepts derived from classical earth pressure theories. Recent studies have suggested that most existing design methods are excessively conservative. In part, this derives from an inability to fully describe the true, relatively complex soil-structure interaction process which occurs in the flexible cellular cofferdam. In addition to the apparent conservatism in the conventional theories, they provide no means to predict movements of cofferdams. This deficiency has become more important with time, since performance and safety monitoring systems are largely oriented towards measuring deformations, and it is not possible to define reasonable levels of movements before construction. Further, finite element analysis techniques are being developed which are designed to predict cofferdam deformations, but the means for verifying the predictions are not definitive because of the lack of a broad data base on cofferdam performance. This investigation is directed towards providing a data base through the documentation and comparison of five case histories where instrumentation was used to monitor the behavior. Primary consideration was given to the response of the cofferdams under differential loading.

In the course of the investigation, the response of each cofferdam was documented and dissected in terms of the behavior at each stage of the differential loading. A strong correlation is found to exist between the lateral cell deflection and the level of differential loading, with the exact nature of the correlation depending on certain key parameters including cell foundation, cell fill material, cell width to height ratio, presence of a stabilizing berm, and nature of loading. It is found for conservatively designed cells that the response falls into a predictable pattern which can be characterized in terms of non-dimensionalized parameters for both normal and more severe levels of loading. In addition to deformations, information is provided on interlock tensions where available. The results are believed to provide a baseline for cofferdam behavior that can be useful in gaging the expected behavior of other cofferdams and for verifying the accuracy of new forms of predictive tools such as the finite element method.