Environmental Engineering Reference
In-Depth Information
FIGURE 3.58
Quick-release drop hammer used in
Israel and Europe for SPT sampling.
Natural Factors Affecting N Values
Soil type and gradation:
Gravel particles in particular have a very significant effect on
N
as
shown in
Figure 3.59.
Compactness and consistency:
Compactness of granular soils (
D
R
) and consistency of clay
soils affect
N
values.
Saturation:
In gravels and coarse sands, saturation has no significant effect. In very fine
or silty sands, pore-pressure effects during driving are corrected approximately for test
values
N
15 by the expression (Terzaghi and Peck, 1948; Sanglerat, 1972)
N
15
1/2(
N
15)
(3.45)
Depth correction factor
(
C
N
)
:
Effective overburden pressures cause a fictitious increase in
N
values with increasing depth as shown by the studies of various investigators (Gibbs and
Holtz, 1957; Bazaraa, 1967; Marcuson and Bieganousky, 1977a, 1977b). Various investiga-
tors have proposed relationships to correct
N
for depth (
C
N
) (Peck et al., 1973; Seed, 1976,
1979; Liao and Whitman, 1986; Kayen et al., 1992):
N
1
C
N
N
,
C
N
(1/
σ
v
o
)
0.5
(Liao and Whitman, 1986)
(3.46)
C
N
2.2/(1.2
σ
v
o
/
P
a
) (Kayen et al., 1992)
(3.47)
