Engineers Australia

Bearing capacity of footings in layered clay by the punching shear method

Peter W. Mitchell


The undrained bearing capacity of shallow and deep square and circular footings and piles by the punching shear model in layered clay consisting of a stronger layer cs overlying a softer layer cw is examined. It is shown that for a footing founded in the stronger clay layer an important concept is the critical depth, defined as the footing depth when the softer clay first affects the bearing capacity. Equations for both the bearing capacity and the critical depth based on the punching shear model are derived. These are compared to solutions by the computer program FLAC for surface and deep footings, published computer solutions for circular footings and spudcans and experimental test results on model footings, to confirm the validity of the punching shear model. The critical depth ratio H/B is a function of cs/cw and the equation relating H/B and cs/cw derived for surface footings is also valid for deep footings. The maximum H/B is 1.55. Pile design methods that use H/B = 10 are therefore significantly conservative for a layered clay soil profile. The application of these concepts to a variety of geotechnical problems is illustrated by eight worked examples. Good agreement was found with computer derived solutions, and the punching shear model is free of the computational inconsistencies that affect some existing finite element methods. The equations derived from the punching shear model provide a very rapid and convenient means of obtaining the bearing capacity of footings in layered clay in a sufficiently accurate manner, suitable for routine geotechnical practice.