Finite element analyses of cellular cofferdams

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Virginia Polytechnic Institute and State University


Cellular cofferdams have primarily been used as temporary systems which serve to allow construction of facilities in open bodies of water. Applications for these structures have been increasing and today they may serve as permanent retaining walls or as navigation or waterfront structures. Conventional design methods for cellular cofferdams are based on semi-empirical approaches largely developed in the 1940s and 1950s. None of the available traditional procedures are capable of predicting cofferdam deformations, a parameter of key importance to the cofferdam performance, and which is often observed during construction for purposes of safety monitoring. Also, there is evidence that much of the conventional design technology is conservative, in some cases predicting loading by more than twice that which actually occurs. Recently, the finite element method has shown promise as a tool which can be used to help resolve some of the outstanding problems with cofferdam design.

There are three primary objectives of this work: (1) enhance existing finite element program to allow for more accurate and refined analysis of cellular cofferdams, (2) use the enhanced finite element programs to assess the degree of conservatism in conventional design methods for cofferdams founded on sandy soils, and (3) use the results of parametric studies of cofferdams founded on sandy soils to develop a simplified procedure to predict cofferdam movements and determine potential for internal failure. The first of the objectives involves adding better bending elements to the program SOILSTRUCT to represent the sheet pile system In axisymmetric and plane strain analyses. Also, in the case of the plane strain program, a new method is developed to allow shear transfer through the sheet pile system. Through case history and theoretical analyses, the enhanced programs are demonstrated to yield accurate and realistic results.

Parametric studies using the axisymmetric program show that conventional design methods overpredict, in some areas strongly, the interlock forces which develop during filling of the cofferdam. Parametric studies using the plane strain program suggest that there is also considerable conservatism in design methods to predict internal stability of the cofferdam. A new, simplified method is proposed for this type of analysis. In addition, it is shown that the deformations of cofferdams on sand follow consistent trends and can be set into a nondimensionalized context which can be used to predict future cofferdam movements.