settlement_of_shallow_foundations_on_granular_soils (Lutenegger ang DeGroot)

y = soil total unit weight
Cw = water table correction
=a\ dr/cr'v wet
where cr', is computed at D + B/2
[5.34]
The water table correction factor Cw is the ratio of the effective overbmden stress at D + B/2 of dry
soil to effective overburden stress at D + B/2 at the location of the water table. If the water table is
located below the depth D + B/2, then Cw = 1.0.
The corrected blowcount value is obtained from:
N, = (4N)/(l + 2p')
N, = (4N)/(3.25 + 0.5p')
where:
for p' ,; 1.5 ksf
for p' > 1.5 ksf
[5.35]
[5.36]
p' =effective overburden stress (in ksf) at a depth of approximately D + B/2
5.2.10 Webb (1969)
Apparently, Webb (1969) was one of the first authors to suggest that a layered approach be
used to estimate the total settlement from a footing resting on sand. Settlement is calculated from
the expression:
n
s = L (cr,/E)~zi
[5.37]
i=l
where:
s =settlement (in ft.)
cr,i =vertical stress in soil layer i produced by foundation stress q (in psf)
~zi =thickness of layer i (in ft.)
E =soil elastic modulus (in psf)
This method implies that the maximum strains occm immediately beneath the base of the foundation
where the vertical stresses are maximum values. This is contrary to results from tests on small plates
(e.g. Bjerrum and Eggestad (1963); Morgan and Gerrard (1971); and Schmertmann eta!. (1978)) as
well as elastic theory which indicate maximum strains occurring at depths of between 0.5 Band 0.75
B below the foundation base. The value of cr,i is obtained from simple Bousinesq elastic theory.
The soil elastic modulus for use in Equation 5.37 is obtained directly from the uncorrected
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