Laboratory Testing of Open-Dug Caisson Skin Friction During Sinking in Sands

The construction procedure for open-dug in-situ caisson shafts involves concurrent casting of reinforced concrete walls and excavation of soil from within the structure. This process causes the shaft to ‘sink’ into the ground, driven by the self-weight of the walls. A key consideration during construction is frictional stresses (skin friction) exerted on the exterior surface of the caisson walls by the surrounding soil. Frictional stresses are reduced by using a steel cutting shoe to form an annulus, which encircles the caisson and is filled with a lubricating fluid, such as bentonite slurry. This chapter presents the results from experimental testing carried out using a novel apparatus that simulates the sinking process and measures the skin friction that develops on the caisson during sinking. Results from four tests are described, focussed on exploring the influence of annulus formation and lubrication on the development of skin friction.