Civil Engineering Reference
In-Depth Information
design values of shear modulus, by factoring down the small strain modulus according
to strain levels expected in the soil. Typical shear strain levels around a single pile under
working conditions will be less than 0.5% adjacent to the pile, with the shear strain
level decaying inversely with distance from the pile. Within a pile group, however,
interaction of displacement fields will lead to a reduction in the average shear strain
level, and a working range of 0.05 to 0.1%may be assumed typically. The appropriate
shear modulus for consideration of settlements will therefore be close to the small strain
modulus,
G
0
.
For clays, an early correlation of soil modulus with the undrained shear strength was
presented byWeltman and Healy (1978), who showed that the soil modulus applicable
for pile loads corresponding to an average axial stress in the pile of 5 MPa was very
high. A conservative fit to their data (expressed in terms of the shear modulus,
G
) gave
25
(
c
u
/
p
a
)
MPa
G
≈
0
.
6
×
(5.22)
where
p
a
is atmospheric pressure (100 kPa). For
c
u
in the range 100 to 150 kPa,
G
/
c
u
would increase from 150 to 500.
Jardine
et al
. (1986) report ratios of small strain modulus to shear strength which
range from
G
0
/
c
u
=
500 to 1000. This range is consistent with data presented by
Kagawa (1992), who found that the initial shear modulus in normally and lightly
overconsolidated clays was directly proportional to the consolidation (or yield) stress,
with a typical ratio of
G
0
/σ
y
=
160. Assuming a strength ratio of
c
u
/σ
y
=
0
.
25, the
implied rigidity index is
G
0
/
640. Other work has suggested a variation that varies
with effective stress level to a power less than unity, for example (Viggiani, 1991)
c
u
=
A
(
OCR
)
m
p
p
a
n
G
0
p
a
≈
(5.23)
with typical values of
m
close to 0.25,
n
ranging from 0.65 for low plasticity clay to
0.85 for high plasticity clay, and corresponding modulus numbers,
A
, ranging from
500 down to 200.
For sands, Carriglio
et al
. (1990) presented data that are consistent with a
relationship of the form
2
I
D
p
0
.
5
400
1
G
0
p
a
≈
+
(5.24)
p
a
where
I
D
is the relative density,
p
is the mean effective stress and
p
a
is atmospheric
pressure (100 kPa). The value of the modulus number (400) and the exponent (0.5)
will vary for different soils, with the exponent generally in the range 0.4 to 0.55, and
the modulus number decreasing with increasing silt content of the soil.
Correlations of small strain shear modulus with cone resistance,
q
c
, and effective
overburden stress,
σ
v
0
, have been given by Baldi
et
al
. (1
989) and Robertson (1991),
p
a
σ
v
0
as shown in Figure 5.20.
Correlations of shear modulus with SPT value,
N
, have also been proposed.
Wroth
et al
. (1979) have discussed a range of correlations, based on the work of
where the ratio
G
/
q
c
is taken as a function of
q
c
/
