Examples of open caisson sinking in Scotland

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.