Vibro compaction of granular soils 69
F
f
q
=
−
⋅
s
c
v 0
(
)
σ
100%
(3.13)
I c is calculated in an iterative process starting with n = 1 as exponent for
clayey soils in Equations 3.11 through 3.13. If the calculated I c value is
above 2.6 the soil is generally considered too cohesive to liquefy. For safety
reasons the NCEER recommends these soils to be sampled and laboratory
tested for confirmation. If the computation arrives at I c values less than
2.6 the soil is more likely granular, and calculations of Q and I c should be
repeated with n = 0.5 for sand. If I c then falls below 2.6, the soil is considered nonplastic and granular and I c can be used to determine the liquefaction potential for sand with fines contents as explained below. However,
should I c be again greater than 2.6, the soil is likely to be cohesive and
plastic and I c should be recalculated with the intermediate value of n = 0.7.
The intermediate I c is then used to establish the liquefaction potential of
this soil, which should also be sampled and tested for verification purposes
according to NCEER recommendations. Soils with a soil behavior index I c
above 2.6 would generally not liquefy but would be rather soft (region 1 in
Figure 3.24) and could therefore suffer considerable deformation during an
earthquake. These soils would also be considered nonliquefiable according
to the so-called Chinese criteria by which liquefaction would only occur if
all three of the following criteria were met (after Seed and Idriss, 1982):
1. The clay content is less than 15%.
2. The liquid limit is less than 35%.
3. The natural moisture content is less than 0.9 times the liquid limit.
Correction of the normalized cone penetration resistance q c,1N of these silty
sands to an equivalent clean sand value (index cs) for determining the liquefaction resistance CRR from Figure 3.23 is by the following relationship:
(
)
,
,
q
K q
c N cs
c
c N
1
1
=
⋅
(3.14)
where K c represents the grain characteristics correction factor, which can
be taken from Figure 3.25 as a function of the soil behavior type index I c
as defined by Equation 3.11.
To correct the CRR 7.5 values, which are only valid for 7.5 magnitude
earthquakes, as taken from Figure 3.22, as a function of normalized corrected SPT blow counts, or from Figure 3.23, for normalized corrected CPT
tip resistances, this value is multiplied by the magnitude scaling factor for
stresses MSF M as proposed by the NCEER and as taken from Figure 3.26,
and as given in Equation 3.15:
CRR
MSF CRR
M
M
=
⋅
7 5
.
(3.15)
F
f
q
=
−
⋅
s
c
v 0
(
)
σ
100%
(3.13)
I c is calculated in an iterative process starting with n = 1 as exponent for
clayey soils in Equations 3.11 through 3.13. If the calculated I c value is
above 2.6 the soil is generally considered too cohesive to liquefy. For safety
reasons the NCEER recommends these soils to be sampled and laboratory
tested for confirmation. If the computation arrives at I c values less than
2.6 the soil is more likely granular, and calculations of Q and I c should be
repeated with n = 0.5 for sand. If I c then falls below 2.6, the soil is considered nonplastic and granular and I c can be used to determine the liquefaction potential for sand with fines contents as explained below. However,
should I c be again greater than 2.6, the soil is likely to be cohesive and
plastic and I c should be recalculated with the intermediate value of n = 0.7.
The intermediate I c is then used to establish the liquefaction potential of
this soil, which should also be sampled and tested for verification purposes
according to NCEER recommendations. Soils with a soil behavior index I c
above 2.6 would generally not liquefy but would be rather soft (region 1 in
Figure 3.24) and could therefore suffer considerable deformation during an
earthquake. These soils would also be considered nonliquefiable according
to the so-called Chinese criteria by which liquefaction would only occur if
all three of the following criteria were met (after Seed and Idriss, 1982):
1. The clay content is less than 15%.
2. The liquid limit is less than 35%.
3. The natural moisture content is less than 0.9 times the liquid limit.
Correction of the normalized cone penetration resistance q c,1N of these silty
sands to an equivalent clean sand value (index cs) for determining the liquefaction resistance CRR from Figure 3.23 is by the following relationship:
(
)
,
,
q
K q
c N cs
c
c N
1
1
=
⋅
(3.14)
where K c represents the grain characteristics correction factor, which can
be taken from Figure 3.25 as a function of the soil behavior type index I c
as defined by Equation 3.11.
To correct the CRR 7.5 values, which are only valid for 7.5 magnitude
earthquakes, as taken from Figure 3.22, as a function of normalized corrected SPT blow counts, or from Figure 3.23, for normalized corrected CPT
tip resistances, this value is multiplied by the magnitude scaling factor for
stresses MSF M as proposed by the NCEER and as taken from Figure 3.26,
and as given in Equation 3.15:
CRR
MSF CRR
M
M
=
⋅
7 5
.
(3.15)