Bearing capacity of jacked piles in a layered soil profile

Peter W. Mitchell

The bearing capacity of large scale jacked piles in layered soils is developed from theoretical considerations and confirmed by project observations. When a dense sand foundation layer overlies a weaker layer, the weaker layer can affect the geotechnical base bearing capacity of the jacked pile. Whether or not a pile can be pushed through a dense sand layer with a force less than the piling rig reaction has been found to depend on the strength of the upper stronger and lower weaker layers, the depth of the weaker layer and the pile size.

The punching shear failure model first developed by Meyerhof and co-workers was used to predict the pile base bearing capacity of a jacked pile founded in a dense sand layer overlying a weaker layer. The H/B ratio, defining the distance between the pile base and the underlying weaker layer for which the weaker layer does not influence the pile base bearing capacity, was found to depend on the strength ratio of the two layers, the friction angle of the stronger layer and the geometric ratio of pile size to depth of the weaker layer. In the case of a weaker layer overlying a stronger layer the pile base bearing capacity can be taken as linearly increasing over a depth increment of 10B, beginning 2B above the stronger layer. Observations of the pile jacking behaviour at a site containing dense sand with weaker layers provided field verification of the developed concepts.