Vibro compaction of granular soils 65
Therefore, the next step is to evaluate the liquefaction resistance of the
soil or CRR taking into account the actual soil characteristics that are
responsible for this phenomenon.
The difficulties and associated costs for retrieving undisturbed samples
from water-bearing granular soil are prohibitive for most projects. Only
in exceptional cases and with specialized sampling techniques can sufficiently undisturbed specimens be collected; these are subsequently tested
in the laboratory, where the seismic loading conditions can be modeled
adequately. Instead, field tests have replaced this procedure and are widely
used for routine liquefaction investigations. NCEER recommends four
methods: (1) the SPT, (2) the CPT, (3) shear wave velocity measurements,
and (4) the Becker penetration test. Since, for the latter two, only limited or
sparse test results from liquefaction sites are available, the following deals
only with the CRR value measured by SPT and CPT methods.
Figure 3.22 depicts the CRR values as a function of the corrected blow
count of the SPT (N 1 ) 60 developed by Seed et al. (1985) from empirical
data as recommended by the NCEER for 5%, 15%, and 35% fines in the
sand. However, these are only valid for magnitude 7.5 earthquakes. The
blow count (N 1 ) 60 is the measured blow count N m , corrected to account
for the influencing factors, overburden pressure (C N ), energy ratio (C E ),
borehole diameter (C B ), rod length (C R ), and sampling method (C S ) by
Equation 3.8:
( )
1 60
m
N
E
B
R
S
N
N C C C C C
=
⋅
⋅ ⋅ ⋅
⋅
(3.8)
Suggested ranges of correction factors can be taken from Table 3.6. To
account for the overburden pressure, the C N factor is calculated by using
the effective overburden pressure ′
σ v0 in Equation 3.9 that acted at the time
that the SPT test was carried out,
C
p
N
a
v0
5
= ′
σ
0.
(3.9)
with p a representing the atmospheric pressure of 100 kPa.
Table 3.5 Earthquake magnitudes and number of significant stress cycles
Earthquake magnitude M
Number of significant stress cycles N c
7
10
7.5
20
8
30
Source: After Seed, H.B. and Idriss, I.M., J. SMFD, ASCE, 97(SM9), 1249, 1971.
Therefore, the next step is to evaluate the liquefaction resistance of the
soil or CRR taking into account the actual soil characteristics that are
responsible for this phenomenon.
The difficulties and associated costs for retrieving undisturbed samples
from water-bearing granular soil are prohibitive for most projects. Only
in exceptional cases and with specialized sampling techniques can sufficiently undisturbed specimens be collected; these are subsequently tested
in the laboratory, where the seismic loading conditions can be modeled
adequately. Instead, field tests have replaced this procedure and are widely
used for routine liquefaction investigations. NCEER recommends four
methods: (1) the SPT, (2) the CPT, (3) shear wave velocity measurements,
and (4) the Becker penetration test. Since, for the latter two, only limited or
sparse test results from liquefaction sites are available, the following deals
only with the CRR value measured by SPT and CPT methods.
Figure 3.22 depicts the CRR values as a function of the corrected blow
count of the SPT (N 1 ) 60 developed by Seed et al. (1985) from empirical
data as recommended by the NCEER for 5%, 15%, and 35% fines in the
sand. However, these are only valid for magnitude 7.5 earthquakes. The
blow count (N 1 ) 60 is the measured blow count N m , corrected to account
for the influencing factors, overburden pressure (C N ), energy ratio (C E ),
borehole diameter (C B ), rod length (C R ), and sampling method (C S ) by
Equation 3.8:
( )
1 60
m
N
E
B
R
S
N
N C C C C C
=
⋅
⋅ ⋅ ⋅
⋅
(3.8)
Suggested ranges of correction factors can be taken from Table 3.6. To
account for the overburden pressure, the C N factor is calculated by using
the effective overburden pressure ′
σ v0 in Equation 3.9 that acted at the time
that the SPT test was carried out,
C
p
N
a
v0
5
= ′
σ
0.
(3.9)
with p a representing the atmospheric pressure of 100 kPa.
Table 3.5 Earthquake magnitudes and number of significant stress cycles
Earthquake magnitude M
Number of significant stress cycles N c
7
10
7.5
20
8
30
Source: After Seed, H.B. and Idriss, I.M., J. SMFD, ASCE, 97(SM9), 1249, 1971.