Geoscience Reference
In-Depth Information
3. SPT-based correlation for residual strength
3.1. CORRELATION OFS
r
WITH
(
N
1
)
60cs
-
Sr
Seed (1987) was the first to suggest back-calculating a residual strength, S
r
, of lique-
fied soil from case histories of liquefaction failures and to correlate the S
r
to equiva-
lent clean sand SPT corrected blow counts,
(
N
1
)
60cs
-
Sr
. A direct correlation between S
r
and
N
1
)
60cs
-
Sr
was considered logical based on critical state concepts (e.g., steady state
strength at large strains is a function of void ratio alone) and established correlations
between the overburden corrected penetration resistance and relative density.
The values of S
r
obtained by Seed (1987), Seed and Harder (1990), and Olson and Stark
(2002), are listed in Table 1.1 and presented in Figure 1.4. A design relationship that has
been widely used in practice over the past 10 years for estimating S
r
based on median
penetration resistance isalso shown in thisfigure.
(
For stability analyses, it was recommended that the residual strength from this relation-
shipberestrictedtolessthanorequaltothesoil'spre-earthquakedrainedshearstrength.
Thisrestrictionavoidedtheuseofshearstrengthsthat,usuallyatshallowdepths,relyon
negative pore water pressure.
50
Group 1— Case histories with an adequate amount
of in-situ measurements (e.g., SPT, CPT) and
reasonably complete geometric details
Group 2 & Group 3 (see text for more details)
40
Seed (1987)
Seed & Harder (1990)
30
Olson & Stark (2002)
20
10
Relationship for
Estimating Residual Strength
Using Median Measured SPT
0
0
4
8
12
16
20
Equivalent Clean Sand SPT Corrected Blowcount, (N
1
)
60cs-Sr
Fig. 1.4. Residual shear strength, S
r
, of liquefied sand versus equivalent clean-sand,
corrected SPT blow count based on case historieslistedinTable 1.1
