Undrained bearing capacity factors for conical footings on clay

INTRODUCTION The bearing capacity of circular foundations on undrained clay is of fundamental importance in many geotechnical problems. In particular there are a number of designs of offshore foundations where the foundation can be treated approximately as a large circular footing, for instance some gravity bases, the spudcan foundations of jack-up units, and the more recently developed caisson foundations. In most cases the footing is not placed at the ground surface, and it is important to take into account the depth of embedment. Furthermore, the base of a spudcan is generally not flat, but approximates a shallow cone. For foundations on soft clays, the effect of the increase of strength of the soil with depth needs to be taken into account, and this is particularly important for large foundations. The purpose of this note is to present calculations of bearing capacity factors for shallow circular foundations, accounting for embedment, cone angle, rate of increase of strength with depth, and surface roughness of the foundation. The results have widespread application, particularly in the offshore industry. The soil is assumed to be rigid-plastic, with yield determined by the Tresca condition with an undrained strength su. The method of characteristics is used for the bearing capacity calculation, as described by Shield (1955), Eason & Shield (1960), Houlsby (1982) and Houlsby & Wroth (1982a) for application to undrained axisymmetric problems. Some previous results have been published for this problem using similar numerical techniques (e.g. Houlsby & Wroth, 1982b; Salencon & Matar, 1982; Houlsby & Wroth, 1983; Tani & Craig, 1995; Martin, 2001), but the study presented here involves a much more comprehensive coverage of the parameters. Where comparisons can be made with the previous solutions, the factors differ by up to about 0·5%, which gives some indication of the level of accuracy attainable with this numerical technique. Exceptionally, the rough footing results given by Tani & Craig (1995) are higher by up to about 5%, but this may be due to a problem with their numerical integration procedures (see Martin & Randolph, 2001).