Previous work has suggested that the external skin friction for the portion of a suction caisson installed by pumping out the water inside the caisson (referred to as suction) is reduced compared to the external skin friction for a driven or pushed pile. The primary reason given for this reduction is that the suction penetration tends to displace most of the soil at the caisson tip towards the inside rather than outside of the caisson. Presently, however, a consensus does not exist regarding the proposed movement of soil inside a suction caisson and the resulting loss in skin friction. To better understand the soil displacement, four caissons installed in the Gulf of Mexico in mostly normally consolidated soils were examined. By comparing the observed penetration outside the caisson with the point where an inner plate stopped penetration, the amount of displaced soil that moved inside the caisson at the caisson tip was determined. The four caissons were 6.50 m in diameter and were installed to about 24.0 m below the mud line. The external wall of the caisson was flush. The wall thickness at the caisson tip was 51 mm. However, only the inner 22 mm was flat. The remainder (29 mm) had an external 4:1 taper. The results from the field measurements suggest that during suction penetration, as a minimum, all the soil displaced by the 22 mm flat portion of the tip moved inside the caisson. This observation, therefore, tends to support the basis for reduced external skin friction.
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