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Technical paper: Design and performance of a large sheet piled cofferdam in soft clay

A Bracegirdle, Geotechnical Consulting Group, H Sloan, Roger Bullivant and KM Dight, Henry Boot. This paper was first published in GE’s December 2001 edition.

Excavation for the construction of a pumping station at Zetland Park began in October 1999. The 30m diameter, 6.5m deep excavation was completed in January 2000 and was supported by a circular sheet-piled cofferdam. The site is on mudflats at Grangemouth, within 10km of the Bothkennar soft clay test site (Figure 1). A very simple ground investigation identified ground conditions similar to those at Bothkennar (Hight et al, 1992).

The project involved construction of a conventional circular reinforced concrete pumping station, supported on piles. The sheet-piled cofferdam was to provide temporary support to the excavation, with the sheet-piles being extracted on completion of the pumping station structure. Very little time was available to investigate alternative forms of construction, nor was there a budget for a numerical analysis or further ground investigation. It was decided to proceed with the design of a cofferdam with a deep cut-off to enhance the base stability of the excavation. Uncertainties were engineered out rather carrying out further investigation with a view to achieving a more elegant or efficient solution.

A very simple monitoring programme was undertaken to ensure that displacements of the sheet-piles remained within sensible bounds. The results of the monitoring give useful insight into the behaviour of the cofferdam and the design methods used.

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