Bearing capacity of shallow footings in sand over clay by the punching shear method

Peter W. Mitchell


The bearing capacity of shallow strip, square and circular footings by the punching shear model in stronger sand overlying weaker clay is examined by using published computer solutions and experimental test results on model footings. The Meyerhof punching shear coefficient is shown to be dependent on the strength ratio f’bw/fbs and the footing type, and is used to derive the punching shear model equations for the bearing capacity for sand friction angles ranging from 30˚ to 50˚, and for f’bw/fbs ranging from 0 to 1. The equations are used to explain the effect that several variables have on the bearing capacity, including the sand thickness, the clay strength, the sand strength and the surcharge. The application of these concepts to a variety of geotechnical problems is illustrated by six worked examples. Provided f’bw/fbs>0.4 to 0.5, the simple equations derived from the punching shear model provide a very rapid and convenient means of obtaining the bearing capacity of footings in sand over clay in a sufficiently accurate manner, suitable for routine geotechnical practice.