Soil foundations - Geotechnical Investigation and Design Tables - page 271

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Note the above is based on Meyerhof (1956), which is approximately comparable
to the charts in Terzaghi and Peck (1967). Meyerhof (1965) later suggests values
∼

50% higher, due to the conservatism found.

Table 21.7 Allowable bearing capacity of granular soils (adapted from Meyerhof, 1956).

Allowable bearing capacity (kPa)

Foundation
width
B (m)

Very loose Loose Medium dense Dense Very dense

N 5 N 10 N 20 N 30 N 40 N 50

1 225 350 475 600

50 100

2 200 300 425 525

3 375 475

275

4 25 75 175

350 450

5 250

21.8 Settlements in granular soils
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Settlements may be estimated from the SPT N- value in granular soils.

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The settlement estimate is based on the size and type of foundation.

Table 21.8 Settlements in granular soils (Meyerhof, 1965).

Footing size Relationship for settlement

B 1.25 m 1.9 q/N <

B 1.25 m 2.84 q/N [B/(B 0.33] 2 > +

Large Rafts 2.84 q/N

•

N = average over a depth = width of footing (B).

•

q = applied foundation pressure.

21.9 Factors of safety for shallow foundations
•

Factor of Safety (FS) accounts for uncertainties in loading, ground conditions,
extent of site investigation (SI) and consequences of failure. This is the traditional
“working stress'' design.

•

=

FS Available Property/Required Property. A nominal (expected, mean or

median) value is used.

•

Allowable Bearing Capacity = q ult /FS.

•

The industry trend is to use FS = 3.0 irrespective of the above conditions.

•

For temporary structures, the FS can be reduced by 75% with a minimum value
of 2.0.