The performance of diaphragm type cellular cofferdams

Abstract

The construction of water management and navigation structures often requires temporary works to exclude water to facilitate construction in the dry or at least under a lower water level within the construction area. The use of cellular cofferdams for both temporary and permanent earth/water retaining works is very common. A number of theories are presented that describe the failure mechanisms involved. These failure mechanisms were identified from model studies on circular type cofferdams are applied independent of cofferdam geometry. It is common to use diaphragm type cofferdams. To asses the validity of common failure mechanisms associated with cellular cofferdams when applied to diaphragm type cofferdams, a series of numerical modelling analyses were conducted. These were validated using field monitoring results of a large scale diaphragm type cofferdam constructed at St. Germans, Norfolk, UK.

A series of plane strain analyses of a diaphragm type cofferdam were conducted using the geometry from a critical section of cofferdam. These analyses identified the lowest factor of safety based on the drained strength of the clay on which the cofferdam was constructed. The water level within the river was increased to accommodate flooding and soil strength was reduced to identify the general failure mechanism. The structural forces were calculated using both 2D and 3D models for a larger width section (13m wide) to allow comparison with wall bending moments and displacements measured in field. The 3D analyses used actual tie spacing and membrane effects whilst reducing the lateral stiffness of the wall to accommodate the stiffness reduction due to variation in the interlock forces.

To measure the bending moment in sheet piles, resistance type strain gauges were installed on a 13m wide section of cofferdam. The cell deflection, river and cell water levels were also monitored to identify tidal river effect and the influence of cell water level on performance. Comparison of field and numerical results highlighted a number of important design and construction detail related to diaphragm type cellular cofferdams.

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