The settlement and bearing capacity of very large foundations on strong soils: 1996 R.M. Hardy keynote address

Strong soils are not typically problem soils, and hence their behaviour has not been extensively studied. Strong soils are best defined on the basis of their geologic history, but for this paper they can be roughly defined as cohesive soils with an N value of about 15 or over and cohesionless soils with N values over 30. Settlement of Call buildings on strong soils has always been of interest. The means of estimating settlement of the large foundations or pile foundations associated with these structures varies but is generally understood to be predominantly elastic. Although predictions of settlement based on laboratory tests or in situ tests may vary as much as an order of magnitude, there now exists a reasonable data base which suggests that large buildings will settle similar amounts regardless of the size or bearing pressure of the foundations or, for that matter, the type of foundations. No data base exists for quantifying the maximum bearing pressure that will be tolerated by large foundations without failure. The angle of internal friction is known to be critical and to decrease with increasing pressure. It is difficult to measure the undisturbed strength of strong soils, since undisturbed samples are very difficult to secure. Centrifuge model tests of large foundations of different shapes confirm that the bearing capacity factor N, decreases with increased size of footing, but the decrease of N gamma may not be accounted for entirely by the friction angle change with pressure. Selection of a friction angle to determine the peak capacity of very large foundations must be done very carefully and with a great deal of judgement, since it cannot be accurately measured.