Geoscience Reference
In-Depth Information
the post-earthquake dissipation of earthquake-induced excess pore water pressures, as
discussed previously. This relationship can be represented bythe following equation:
exp
0
(
3
S
r
σ
vo
=
(
N
1
)
60cs
-
Sr
16
N
1
)
60cs
-
Sr
−
16
φ
+
−
3
.
≤
tan
(1.2)
21
.
2
The upper design relationship in Figure 1.5 corresponds to a condition where the effects
ofvoidredistributioncanbeconfidentlyjudgedtobenegligible. Thiscouldincludesites
wherethestratigraphywouldnotimpedepost-earthquakedissipationofexcessporewater
pressures, such that the dissipation of excess pore pressures would be accompanied by
densification of the soils at all depths. In this case, the available experimental data and
correlations between relative density and
(
N
1
)
60cs
-
Sr
indicate that the undrained shear
resistance would increase rapidly as the
N
1
)
60cs
-
Sr
approaches 16 or 17 (corresponding
toarelativedensityofabout60%).Thisrelationshipcanberepresentedbythefollowing
equation:
(
exp
0
(
3
S
r
σ
vo
=
(
N
1
)
60cs
-
Sr
16
N
1
)
60cs
-
Sr
−
16
+
−
3
.
21
.
2
1
exp
(
6
N
1
)
60cs
-
Sr
2
φ
×
+
−
6
.
≤
tan
(1.3)
.
4
The potential role of void redistribution or other strength loss mechanisms in the case
histories is not fully clear at this time. Physical and analytical models indicate that void
redistribution is potentially most severe for loose sands, and is likely to have played a
role in many of the currently available case histories. This would suggest the two design
relationships should be somewhat different at the lower penetration resistances, but the
current state of knowledge does not provide a basis for incorporating any difference at
this time.
The appropriate S
r
/
σ
vo
ratios for
σ
vo
>
400kPa are expected to be smaller than those
recommendedinFigure1.5.Forexample,triaxialcompressiontestresultsbyYoshimine
et al. (1999) showed that S
r
/
σ
vo
ratios at a given D
R
were relatively independent of
σ
vo
up to
σ
vo
. This effect
maybeapproximatelyaccountedforbyusingthestatenormalizedpenetrationresistance,
(
σ
vo
of about 500kPa, but decreased at higher values of
N
1
ξ
)
60cs
-
Sr
(Boulanger, 2003), inplace of
(
N
1
)
60cs
-
Sr
inequations (1.2) and (1.3).
4. CPT-based correlation for residual strength
The development of a CPT-based correlation for S
r
/
σ
vo
requires estimating CPT pene-
trationresistancesfor14ofthe18casehistoriesinTable1.1,andthereforetheprocedures
for estimating CPT penetration resistances are also discussed in this paper. Correlations
betweenCPTandSPTpenetrationresistancesthataccountfortheeffectsoffinescontent
and relative density are used to: (1) convert the previously derived S
r
/
σ
vo
-
(
N
1
)
60cs
-
Sr
