Model Tests to Investigate the Influence of the Spread Angle on the Pull-Out Resistance of the Belled Pile

In the development of offshore wind farms, large tensile forces may act on submarine foundations, thereby it is important to estimate their pull-out resistance. In this study, a series of pull-out tests were conducted by varying the spread angle θ of the belled pile. The test results demonstrated that, for θ ≤ 90°, the pull-out resistance Qu,max increased with θ. For θ ≥ 90°, Qu,max decreased with θ for Toyoura sand, and was constant for Glass beads. These results suggest that the fracture mechanisms that strongly govern Qu,max are attributed to Hill’s mechanism for θ ≤ 90° and to Prandtl’s mechanism for θ ≥ 90°, respectively. In the case of θ ≥ 90°, the space between the pile tip and the ground during extraction loosens the ground around the pile. This effect was more pronounced in Toyoura sand than in Glass beads, that leads to the decreasing Qu,max with θ for Toyoura sand.