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Steel Designers' Manual - 6th Edition (2003)
Table 27.1 Terzaghi’s constants for clayey
soils
Foundations 821
obtained for the various parameters. A factor of safety of 3 is usually applied to the
bearing strength to obtain the safe foundation bearing pressure.
27.1.3.1 Clayey soils – Terzaghi’s method
(1) Foundation long in relation to width i.e. strip footing.
q = cNc + gzNq + 0.5gBNg
where q = bearing capacity (ultimate) (kN/m2 )
c = cohesion (kN/m2 )
g = bulk density of the soil (kN/m3 )
z = depth of foundation (m)
B = breadth of foundation (m)
and Nc, Nq and Ng are constants dependent upon the angle of cohesion. These
constants are given in graph form in soil engineering references;Table 27.1 gives
approximate values as guidance only.
(2) Square and circular foundations to isolated piers or columns.
The bearing capacity of foundations rectangular, square or circular is higher
than that of strip footings. Terzaghi’s expression is adjusted as follows:
q = 1.3cNc + gzNq + 0.3g BNg
while Skempton applies a factor (1 + 0.2B/L) to the Terzaghi strip footing calculation
where B is the breadth and L the length. In the case of a square the
enhancement factor is then 1.2.
27.1.3.2 Sandy or cohesionless soils
f Nc Nq Ng
0 5.7 1.0 0
10 10 3.0 2.0
20 18 8.0 6.5
30 36 21.0 20
35 60 50.0 45.0
The appropriate site test for cohesionless soils is the standard penetration test (SPT)
in which the number of blows of a standard weight is recorded for unit penetration
of a standard cylindrical implement. According to Meyerhof the ultimate bearing
capacity is given, in kN/m 2 ,by