1517
Technical Committee 203 /
Comité technique 203
the peak horizontal acceleration and the relative shear wave
velocity
1
V*
s,12m
.
Idriss and Boulanger (2010) come back to the deviations
related by Seed in their actualized report “SPT-Based
Liquefaction Triggering Procedures” (Idriss and Boulanger
2010)
2
. They refer also to another recent method, developed by
Kishida et al. (2009), in which the same parameters as in the
relationship of Cetin et al. (2004) are being used. An example
for the use of those three methods with different magnitudes and
a shear wave velocity V
s
= 120 m/s is given in Figure 2.
Figure 2. Comparison of r
d
-values for M = 6.5 and for M
w
= 7.5 with a
shear wave velocity Vs = 120 m/s (Idriss & Boulanger 2010)
Relationships presented here are more accurate but also more
complicated, due to an increasing number of collected data with
the time and the introduction of new input parameters.
However, divergences as shown in Figure 2 between recent
methods, particularly those of Idriss and Boulanger (2008,
2010) and of Seed (2010), point out that further investigations
are required in this area.
4 EVALUATION OF THE CYCLIC RESISTANCE RATIO
(CRR)
In Eurocode 8, an estimation of potential of liquefaction
requires field tests, either Standard Penetration Tests (SPT) or
Cone Penetration Tests (CPT). Results of CPT and SPT are
currently used to estimate the Cyclic Resistance Ratio, as it is
mentioned in the European standard and in actual reports (Youd
et al 2001, Robertson and Cabal 2010). Annex B of Eurocode 8
includes a detailed description of the procedure based on the
SPT and some indications about the procedure based on CPT. A
method based on results from tests measuring shear wave
velocity (Spectral Analysis of Surface Waves SASW or
Multichannel Analysis of Surface Waves MASW) is also
mentioned, it is however indicated that this method is still
subject of research developments.
4.1
Methods based on Standard Penetration Test (SPT)
The standard penetration resistance N
m
measured during a
SPT is used as input parameter and firstly transformed in a
normalized value N
1
(60) for an overburden pressure of
approximately 100 kPa and a hammer efficiency of 60 %.
According Youd et al. (2001):
60 =
∙
∙
∙
∙
∙
(6)
1
The relative shear wave velocity is measured 12 m below ground
surface and divided by the needed time for a shear wave to reach the
surface.
2
See pages 65 to 68 of this report.
with correction factors for a normalization to a common
reference effective overburden stress (C
N
), for hammer energy
ratio (C
E
), for borehole diameter (C
B
), for rod lengths (C
R
) and
for samplers with or without liners (C
S
).
In Eurocode, some correction factors are not considered (C
B
,
C
R
, C
S
). Concerning the corrections C
E
and C
N
: C
N
should not
exceed 2.0 according Eurocode; Youd et al. (2001) recommend
nevertheless a maximal value of 1.7. Furthermore, Idriss and
Boulanger (2008) modify in the EERI Monograph MNO-12 the
correction factors C
N
and C
S
.
Correlations between N
1
(60) and CRR in Eurocode are
similar to those from Youd et al.(2001), as it is shown on
Figures 3 and 4. In the report of Youd et al. (2001), values can
be read from diagram or also evaluated with relationships
corresponding to the represented curves.
Figure 3. Correlation between corrected blow count N
1
(60) from SPT
and Cyclic Stress Ratio leading to liquefaction for a Surface Wave
Magnitude M
S
= 7.5 (equal to the moment magnitude M
W
for this range)
(Eurocode 8)
Figure 4. SPT Base Curves for a Moment Magnitude M
w
= 7.5 (Youd et
al. 2001)
The procedure described here is established for clean sands
and sands with fines content. For fine grained soils, see part 2.3.
An important change in the last decade is also the apparition of
probabilistic methods, developed in parallel by Idriss and
Boulanger (2010) and by Seed et al. (2003). Those methods are
nevertheless still in development.
