Stability of bucket foundations under non-coplanar combined loading

Using combined load-displacement searching method, the stability of skirted bucket foundations in saturated soft clay subjected to horizontal load (H), moment (M) and torque (T) non-coplanar combined loading is studied through three-dimensional finite element numerical analysis. The soft soil is assumed to be undrained and obeys elasto-perfectly plastic constitutive relationship following the Tresca failure criterion. The failure locus of bucket foundations in various loading spaces is investigated. The results show that the shape of failure envelopes of bucket foundations subjected to non-coplanar combined loading varies considerably from that under planar combined loading. Under non-coplanar combined loading, the shape of failure envelope does not depend on the aspect ratio of bucket foundations, which may be approximated as a circle or ellipse in normalized load space. Based on finite element analysis, the existing equation of failure envelope in three-degree-of-freedom loading condition is extended to the six-degree-of-freedom case, then an equation of failure envelope involving all six-degrees-of-freedom is suggested, which can be used to evaluate the bearing capacity of bucket foundations in soft soil under combined loading condition. Values of the eccentricity parameter in the proposed equation are also presented for a range of practical embedment ratios.