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Open caisson-sinking techniques permit a shaft structure to be progressively sunk, either under its own weight or with the aid of caisson jacks, in a controlled manner from the surface to a predetermined depth. The technique is suited to shaft construction through weak soils, high- to extremely high-plasticity clays, silts, and sands and gravels, particularly below the water table. In the tunnelling and public works engineering sectors, open caissons are typically circular in cross-section; those used in harbour works are commonly square or rectangular in plan. The paper briefly describes the components and method of sinking dry and wet open caissons, highlighting good practice. It then examines in detail four recent, highly successful caisson-sinking operations carried out under a framework agreement for Scottish Water Solutions in their wastewater treatment works at Crianlarich, Doune, Killearn and Drymen. Although modest in size, each contractor-designed caisson was sunk through a variety of ground and water conditions, illustrating the versatility of the technique and its advantages over more conventional methods of shallow-depth shaft construction. The importance of a comprehensive site investigation programme and the control measures required to satisfy verticality and structural integrity requirements are discussed. The paper concludes with a series of practical guidelines designed to assist those contemplating using the technique, and to remind practitioners of good caisson-sinking practice.

The paper discusses four traditional, soft ground, shaft-sinking techniques, namely: underpinning in free air; dry and wet open caisson-sinking; pneumatic caisson-sinking; and underpinning in compressed air. Each technique has been developed to suit a range of ground conditions that can be categorised by strength, stability and the influence of groundwater. Selecting the most advantageous sinking technique to mitigate design and construction risk must be based upon the results of a project-specific, comprehensive, ground investigation and infrastructure survey. Guidance is given on a suitable shaft borehole configuration and ground investigation content both in the field and the laboratory. Recently there has been a greater emphasis on reducing the risk to the workforce from falling objects by minimising man entry in shafts of 6 m diameter or less during shaft sinking. This favours open caisson-sinking, which allows the caisson lining to be erected as a surface activity and the caisson floor to be excavated using surface-mounted excavation equipment.

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