Uplift behaviour of shaft foundations in loess

Based on the field uplift load test results of the shaft foundations in loess, the uplift load-displacement behaviour and mechanism of the straight-sided and belled shaft foundations in flat and sloped loess ground is presented. In general, their load-displacement curves approximately exhibit the same three-region "slow change" characteristics. The L₁-L₂ method can be successfully used to interpret the uplift resistances and may reflect the sectors of an initial linear, a curvilinear transition, and a final linear region of the load-displacement curves. The uplift resistance of the straight-sided shaft foundation is provided by the friction resistance between the shaft and the surrounding soil. However, the belled shaft foundation presents the gradual process of the soil of the enlarged bottom being compressed and compacted with the subsequent elastic deformation, the formation, development and penetration of the plastic zone of the soil around the bell, and the finally overall shear failure of the soil. The ultimate uplift resistance is composed of the self-weight of the foundation, the shear resistance of the sliding surface and the weight of the soil within the sliding surface. There are two failure modes of shallow and deep foundations for the belled shaft foundations in horizontal loess ground. The vertical displacements of the shaft head and sloping ground surface on the lower slope side is greater than those of the upper one for both straight-sided and belled shaft foundations in sloped loess ground. The failure surface is not symmetric and developed mainly on the lower slope side. The shaft head rotates slightly in the loading direction when the shaft foundation is axially uplift-loaded. The enlarged base and additional embedment can be used to increase the uplift bearing capacity of foundations in sloped loess.