Ultimate Uplift Capacity of Multi-Underreamed Anchors in Soft Soil

The ultimate uplift capacity of multi-underreamed anchors (MUAs), a key design parameter in anchor engineering, is influenced by the parameters of the surrounding soil, anchor, and anchor–soil interface. To explore the ultimate uplift capacity of MUAs in soft soil, this study establishes mechanical models of the arch-shaped and cylindrical failure modes of MUAs. These two mechanical models follow a series of basic assumptions. The formulas for calculating the bearing capacities of MUAs in the two failure modes are then derived based on the equilibrium balance theory. The methodology to derive the ultimate uplift capacity of MUAs proposed here is in close agreement with model test data. The ultimate capacity of MUAs comprises the end and side bearing capacities. Because of the superposition of the side and end resistance effects, the ultimate capacities of MUAs are higher than those of non-underreamed anchors. The arch-shaped and cylindrical failure modes of MUAs can influence each other via a variety of underream parameters. With increasing relative underream parameters, the ultimate uplift capacities of MUAs with arch-shaped and cylindrical failure modes both increase, linearly or nonlinearly.