Overview of underpinning and caisson shaft-sinking techniques

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.