settlement_of_shallow_foundations_on_granular_soils (Lutenegger ang DeGroot)

The shape factor is calculated as:
c, = [(1.25 1/B)/((L/B) + 0.25)] 2
[5.73]
where:
L = length of footing
B = footing width
Note that for a circular or square foundation the correction factor, C, is equal to 1.0 and for
continuous or strip footings C, = 1.56 as LIB tends to infinity.
The depth of influence factor, C 1 , is obtained from:
where:
[5.74]
H = depth from the bottom of the footing to the bottom of the compressible soil
Z 1
= depth of influence below footing
= 1.4 (B/B,)o.7s B,
[5.75]
If the SPT N 60 values generally decrease with depth, use Z, = 2B or the depth to the bottom of the
loose layer, whichever is less. The depth of influence correction factor, C" is only of concern when
a loose soil is underlain by a much denser soil and the boundary between the two layers is within Z 1
below the base of the footing.
The soil compressibility index, I" is calculated from the SPT blowcounts as:
I,= 1.71/(N 60 )r. 4
I,= 0.57/ (N 60 )r. 4
where:
(for NC soils)
(for OC soils)
[5.76]
[5.77]
N 60
=average adjusted blowcounts
The blowcount values between the base of the footing and the depth of influence are used
and should be corrected for energy only to give N 60 • No overburden correction is applied. If the soil
is a submerged dense very fine or silty sand with N 60 > 15, ij 0 should be adjusted using the
correction factor proposed by Terzaghi and Peck (1948). If the soil is gravelly sand or sandy gravel,
Burland and Burbidge (1985) recommend multiplying N 60 by an adjustment factor of 1.25.
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