No abstract

The undrained vertical bearing capacity of a shallow foundation is a fundamental problem in geotechnics. Bearing capacity factors based on analytical and numerical methods have been published for strip and circular, surface and shallowly embedded foundations, with rough and smooth foundation-soil interfaces in soils with uniform and linearly increasing shear strength profilesalthough not comprehensively for all combinations. Furthermore, in the case of limit analysis solutions, considerable disparity can exist between upper and lower bound solutions for some foundation and soil conditions. This letter presents a coherent set of bearing capacity factors based on finite element and finite-element limit analysis, for strip and circular shallow foundations across a practical range of foundation embedment ratio, foundation-soil interface roughness and soil shear strength heterogeneity.

An experimental investigation of Mana/Gourvenec/Randolph reverse end bearing of offshore shallow foundations 2 ABSTRACT Shallow skirted foundations can mobilise uplift resistance from end bearing in the short to medium term. However, uncertainty exists over the magnitude of reverse end bearing resistance compared with resistance in compression, and how this might be affected by a gap between the external face of the foundation skirt and the adjacent soil. The study presented in this paper explored the problem through centrifuge model tests, investigating the effect of skirt embedment ratio on (i) the magnitude of reverse end bearing capacity compared to compression capacity (ii) the uplift displacement associated with spontaneous loss of suction during uplift, and (iii) the effect of a vertical gap on the external skirt-soil interface.The results show that (i) peak uplift capacity equivalent to compression capacity can be mobilised for a fully sealed foundation with an intact skirt-soil interface,(ii) suction required for reverse end bearing can be maintained through considerable foundation displacement, even for low skirt embedment ratio, and (iii) the presence of a vertical gap along the external skirt-soil interface causes abrupt loss of suction beneath the top plate after minimal foundation displacement, with subsequent uplift capacity being markedly reduced.

Kinematic soil failure mechanisms around skirted foundations, embedded in lightly overconsolidated clay and subjected to undrained compression and tension, have been investigated through digital image analysis of drum centrifuge tests and compared with predictions from finite-element analyses. Analysis of images captured in the centrifuge tests showed that rather different kinematic mechanisms govern failure in tension and compression. In tension, a reverse end bearing mechanism involving a bulb of soil beneath the foundation was mobilised even for a skirt depth to foundation diameter ratio as low as 0·1. Bearing capacity factors from centrifuge tests for a selected embedment ratio were similar in compression and uplift despite the difference in associated failure mechanism. Comparison of the failure mechanisms observed in the centrifuge tests with those predicted by finite-element analyses shows some marked differences, in spite of close agreement of bearing capacity factors.

This study reports the results of small strain finite element analyses undertaken to determine the rate of displacement of skirted foundations subjected to uplift loading, due to combined swelling and seepage. Compatibility of foundation movement with cumulative seepage of water into the skirt compartment is modelled using a layer of soft poroelastic material immediately below the foundation top plate. Performance of the model is first assessed for a range of stiffness values for the soft layer. The model is then used to investigate the rate of displacement for skirt depths ranging from 0.1 to 1 times the foundation diameter. The results are compared with available theoretical solutions and experimental results, and expressions are provided for calculating the equivalent seepage lengths and resulting uplift velocities. The effect on the seepage behaviour of the presence of a gap down the external skirt-soil interface is also discussed.