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

Abstract

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 tall 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 gamma 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.Key words: settlement, bearing capacity, foundation behaviour.

Interference of Multiple Strip Footings on Sand Using Small Scale Model Tests

“…The reason for this difference is known and it is on account of dependency of friction angle on the stress level (De Beer 1965;Das and Omar 1994;Banimahd and Woodward 2006). It is well known that the magnitude of N c increases with decrease in the size of the footing (De Beer 1965;Das and Omar 1994;Clark 1998;Zhu et al 1998Zhu et al , 2001Banimahd and Woodward 2006;Lancelot et al 2006). The increase of N c with decrease in size becomes especially extensive when the size of the footing becomes smaller than about 1 m (Banimahd and Woodward 2006).…”

Effect of stress level on the bearing capacity factor, N γ , by the ZEL method

“…Investigations show that for foundations on relatively dense soils, the measured value of the bearing capacity is often higher than the values obtained from the theoretical equation based on a critical state friction angle; meanwhile, using a peak friction angle results in higher bearing capacities than measured values (Clark, 1998;Eslami, et al, 2004). Also, the third bearing capacity factor, N γ , increases illogically with friction angle which leads to very high bearing capacity values for large foundations.…”

“…It can be related to soil behavior under different states of imposed stress. Soil shear strength parameters have been believed to be dependent on the level of experienced stress, also known as stress level (Holtz and Kovacs, 1981;Bolton, 1986;Clark, 1998;Maeda and Miura, 1999;Budhu, 2007). This dependency may be considered as a reason of differences between experimental and theoretical approaches in determination of the bearing capacity of foundations.…”

Application of the ZEL method in the prediction of foundation bearing capacity considering the stress level effect

“…The third term suggests an increasing tendency in the bearing capacity with increase in foundation size. However, data from De Beer (1965), Bolton and Lau (1989), and Clark (1998) show that the bearing capacity of shallow foundations does not increase with size without limit [8][9][10]. Recently, other investigations showed similar results (Cerato, 2005;Cerato and Lutenegger, 2006;Kumar and Khatri, 2008;Yamamoto et al, 2009) [11][12][13][14].…”

“…Investigations showed that the soil friction angle is generally a function of the stress level, resulting in a curved Mohr-Coulomb failure envelope (Bolton, 1986;Clark, 1998; [10,16,17].…”