Figure 7.8 shows the importance of the density of the soil on the amount of settlement. For exam-ple, a footing that is 5 ft (1.5 m) wide will settle 1.0 in. (2.5 cm) if the soil density is at the bound-ary between loose and medium density (i.e., N60
=10) and the footing pressure is 1 tsf (2000 psf,
96 kPa). For the same conditions except that the sand has a soil density at the boundary between
dense and very dense (i.e., N60
=50), the footing must exert 6 times the pressure, or 6 tsf (12,000 psf,
580 kPa) to produce the same 1.0 in. (2.5 cm) settlement.
Building codes limit the allowable bearing capacity to restrict the amount of settlement. For
example, in Table 18.4 (reproduced from the International Building Code, 2009), the allowable bear-ing capacity for sands is 2000 psf (96 kPa). Using this value for sands that are in a medium, dense,
or very dense state will generally produce a settlement of 1.0 in. (2.5 cm) or less per Fig. 7.8.
However, if the sand is in a very loose to loose state, then the settlement of the footing will exceed
1.0 in. (2.5 cm) for a footing bearing pressure of 2000 psf (96 kPa). Hence building code values, such
as those listed in Table 18.4, are very conservative for dense to very dense soil, but can result in unac-ceptable settlement for loose to very loose soil.