The high groundwater table and extremely poor ground conditions necessitated an innovative cofferdam solution for the construction of an underground pump house forming part of a new combined heat & power (CHP) generating station at the Isle of Grain. Solutions which were considered included a thick, heavily reinforced diaphragm wall with five layers of temporary propping and a circular shaft but all were found to be either impractical or over budget. Finally, a 'peanut shaped' multicell solution was adopted using 1000mm thick lightly reinforced diaphragm walls with cells sized to suit the configuration of the permanent internal structure. Particular care in both reinforcement detailing and construction was required to ensure transmission of forces in three directions at the point where cells and cross walls meet; this resulted in four massive Y shaped panels. The foundation solution incorporated permanent pore water relief through parts of the base slab together with structural loads (pump house full and empty) and negative skin friction bearing on the diaphragm walls. With groundwater located within two metres of the top of the working platform and undrained shear strength increasing slowly from 10 to 40 kPa, great attention was paid to panel stability. Guide walls were raised above platform level and panel lengths limited to a maximum of 5 metres. In spite of this a major panel collapse occurred on cross wall panel MSI which was believed to have been triggered by ground borne vibration originating from driven piling on an adjacent site. Ground remediation using slurry bites enabled the successful completion of the diaphragm walls.
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