Laboratory Testing of Open Caisson Skin Friction in Resistance to Uplift in Sands

Skin friction on open caissons is typically reduced during sinking using a bentonite-filled annulus encircling the caisson. After sinking, cementitious grout is commonly pumped into the annulus. Subsequent base construction can lead to upwards flotation (uplift) pressures due to groundwater. Uplift is typically resisted by two forces: (1) self-weight of the shaft; and (2) friction acting on the exterior of the caisson. Currently, no dedicated method exists for calculating friction in resistance to uplift due to the unique use of bentonite during the sinking phase. To address this gap, this paper presents results from laboratory testing designed to simulate (1) caisson sinking; (2) grout injection; and (3) the resulting friction during uplift. Two primary factors are explored: the age of the bentonite at the time of grouting and the time between grout injection and the onset of uplift. The spread of grout in the annulus is shown to be significantly influenced by the age of the bentonite. However, the resulting failure mechanism and friction in uplift are shown to be independent of both factors. A friction factor is derived for routine design use, and comparisons are drawn to bored-pile construction, suggesting application should be confined to coarse-grained sands. These are, to the best of our knowledge, the first documented experiments to replicate grout injection within a lubricated annulus and to directly measure uplift friction under such conditions, providing new insight into viscous-fingering behavior and filling a key gap in current design guidance.