settlement_of_shallow_foundations_on_granular_soils (Lutenegger ang DeGroot)

4.- Use Equation 5.95 to calculate the total settlement.
5.- Apply correction factors C 1 and C 2 as appropriate.
The method of estimating settlement proposed by Schmertmann (1970) is primarily intended
for use with cone penetration test data. The CPT has the obvious advantage of providing a near
continuous record of penetration resistance, especially if an electric CPT is used and thus provides
a larger data base for delineating individual important sublayers within the compressible zone as will
as allowing statistical averaging of data within a layer.
Schmertmann (1970) suggested that based on screwplate tests, the soil modulus could be
evaluated from:
[5.100]
However, in their modification, Schmertmann et al. (1978) noted that E, = 2.5 q. would also have
been a reasonable choice to evaluate the soil modulus and recommended:
E, = 2.5 q. (for axisymmetric cases)
E, = 3.5 q. (for plain strain cases)
[5.101]
[5.102]
Normally, the CPT is more efficient to conduct than the SPT, however, it suffers from the
disadvantage of not providing a soil sample for visual classification. On most projects, an
exploration program which combines the use of the SPT and the CPT would be desirable.
Schmertmann (1970) suggested that if only SPT results were available to the engineer,
settlement predictions could still be made using his proposed method by converting SPT blowcount
values to CPT cone tip resistance values using the qjN ratio. Schmertmann (1970) recommended
that provisionally, the following qjN ratios could be used to convert N values to q.:
Soil Txve
silts, sandy silts,
slightly cohesive silt-sand mixtures
clean, fine to medium sands
and slightly silty sands
coarse sands & sand with little gravel
sandy gravels and gravel
2.0
3.5
5
8
Earlier, Sutherland (1963) had compiled a series of comparisons between q. and Nand found the
following results:
102