Undrained capacity of circular foundations under combined horizontal and torsional loads

Torsional loads can be significant on the shallow foundations of large onshore and offshore structures. However, the effect of torsional loads in isolation and in combination with other load directions has not been well studied. In this paper, a failure envelope for combined horizontal (H) and torsional load (T) for circular surface foundations on soils under undrained conditions has been derived using the upper-bound plasticity method. In addition, this solution is compared with finite-element model results and theoretical solutions for the uniaxial capacity for H and T. The H-T failure envelope obtained from a conventional design standard method (i.e. DNVGL-ST-0126) is also compared with the upper-bound solution. The upper-bound solution is in excellent agreement with the finite-element results, and the codified H-T failure envelope appears to be conservative compared with the other methods. This approach can help validate finite-element results and aid the assessment of the ultimate capacity of shallow circular foundations under combined horizontal and torsional loading.