settlement_of_shallow_foundations_on_granular_soils (Lutenegger ang DeGroot)

the compression of the layer is calculated. The general expression for settlement is:
z
s = L (1/C) /);z log [(cr'vo + L'lcr'~)/cr'vol
0
[5.16]
where:
s = settlement (in ft.)
C =bearing capacity index= (1 +e,)/C,
Llz = thickness of the layer (in ft.)
cr'vo =initial vertical effective stress at the mid-height of the layer
L'lcr'v =change in vertical effective stress at the mid-height of the layer
z = thickness of the compressible zone
The change in vertical effective stress resulting from the foundation loading is obtained from elastic
theory simple stress distribution charts such as Bousinesq. The thickness over which compression
takes place is assumed to be equal to the depth where significant stress increases, i.e., Llcr'jq is equal
to I 0%. Alternatively, it was suggested that one may use an approximate stress distribution method
to obtain L'lcr'v as:
L'lcr'~ = p/(B+h) 2
L'lcr'v = p/[(B+h)(L+h)]
where:
p =applied footing load
L = length of footing
h= depth
(for square footing)
(for rectangular footing)
[5.17]
[5.18]
No mention was made by Hough (1959) to apply any correction factors to the SPT
blowcounts and therefore the bearing capacity index, C, was originally presented by Hough (1959)
as a function of the uncorrected field SPT blowcounts as shown in Figure 5 .2. Even in his later
textbook on soil mechanics, Hough (1969) makes no mention of correcting SPT blowcounts but
presented a new chart showing the relationship between SPT blowcounts and bearing capacity index.
The new chart is shown in Figure 5.3 for various soil types.
There is a significant difference in the Charts presented in Figures 5.2 and 5.3, therefore, it
is important that users of this method identify which chart is being used. For example, for aN= 20
in a "well graded silty sand and gravel", Figure 5.2 gives a bearing capacity index of approximately
78 while Figure 5.3 gives a value of about 50 which is a difference of almost 50%.
60