Pile groups under compressive loading 269Another method of estimating the settlements of pile groups in granular soils based on static
cone penetration test values has been developed by Schmertmann(5.25)and Schmertmann
et al.(5.26)Their basic equation for the settlement of a loaded area is
(5.35)
where C 1 is a depth correction factor (see below), C 2 is a creep factor (see below), pis the
net increase of load on the soil at the base of the foundation due to the applied loading, Bis
the width of the loaded area, Izis the vertical-strain influence factor (see Figure 5.30), Ev
is the deformation modulus, and zis the thickness of the soil layer.
The value of the depth correction factor is given by
(5.36)
where is the effective overburden pressure at foundation level (i.e. at the base of the
equivalent raft).
Schmertmann(5.25)states that while the settlement of foundations on granular soils is usu-
ally regarded as immediate, i.e. the settlement is complete within a short time after the com-
pletion of the application of load, observations have frequently shown long-continuing
secondary settlement or creep. He gives the value of the creep factor as
C 2 1 0.2log (^10) (5.37)
time in years
0.1
vo
C 1 1 – 0.5
vo
p
r C 1 C 2 p^20 B
Iz
Ev
z
300
200
100
0
0 1020304050
Normally
consolidated
sand
Over-consolidated
sand (OCR > 2)
Cone resistance, qc (MN/m^2 )
Initial tangent constrained modulus,
M
o
(MN/m
2 )
60 70
Figure 5.29Initial tangent constrained modulus for normally consolidated and over-consolidated sand
related to cone resistance (after Lunne and Christoffersen(5.24)).