The axisymetric failure mechanism of circular shallow foundations and pile foundations in non-cohesive soils

In 1920 Prandtl published an analytical solution for the bearing capacity of a centric loaded strip footing on a weightless in-finite half-space, based on a so-called Prandtl-wedge failure mechanism. Reissner extended this solution for a surrounding surcharge and Keverling Buisman and Terzaghi for the soil weight. Meyerhof and other researchers presented correction factors for the shape of the shallow foundation, which would suggest that, the failure mechanism of circular shallow foundations, is related to the Prandtl-wedge failure mechanism. Meyerhof and Koppejan adapted this Prandtl-wedge failure mechanism also for pile foundations. The numerical calculations made in this article show that the Prandtl-wedge cannot be applied to circular shallow foundations and pile foundations in non-cohesive soils. The failure zone (plastic zone) below a loaded circular plate or pile tip, is far wider and deeper than the Prandtl-wedge. The calculations also show that there is, for these axisymmetric cases, failure both in and out of the standard x-y plane, but most of the failure is due to out-of-plane (tangential) failure. Therefore, this failure mechanism is different from the Prandtl-wedge failure mechanism. Also interesting are the circular and diagonal thin zones below the plate and around the pile tip, where there is no out-of-plane failure, although there is still in-plane failure. In these thin zones without out-of-plane failure, the tangential (out-of-plane) stresses are relatively high due to large shear strains, formed during previous shearing or sliding of the soil.