Critical skirt spacing for shallow foundations under general loading

Finite-element limit analysis is used to identify the critical internal skirt spacing for the undrained failure of shallow skirted foundations under conditions of plane strain based on the criterion that the confined soil plug should ideally displace as a rigid block, such that optimal bearing capacity is realized. General loading (vertical, horizontal, and moment) is considered for foundations with skirt embedments ranging from 5 to 50% of the foundation breadth in soil having either uniform strength or strength proportional to depth. The results explicitly identify the number of internal skirts required to ensure soil plug rigidity under arbitrary combinations of horizontal and moment loading expressed as a function of the normalized skirt embedment and the maximum expected level of vertical loading as a fraction of the ultimate vertical bearing capacity. It is shown that fewer internal skirts are required with increasing normalized foundation embedment, butmore internal skirts are required with increasing soil strength heterogeneity. The results also indicate the potential for a significant reduction in capacity if insufficient skirts are provided, such that plastic deformation is permitted to occur within the soil plug.</p

Failure mechanisms of skirted foundations in uplift and compression

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.</p