Wall Thickness Design14 August 2006
General
Liquefaction assessment due to seismic forces is a very complex
phenomenon.
Simplified methodology using empirical procedures to evaluate
earthquake-induced liquefaction has been proposed by Seed (Ref.
[1], [2], [3]).
This procedure has been widely used in the industry and has been
verified by field data base.
The procedure compares the cyclic stress ratio (CSR) caused by
seismic acceleration to the cyclic resistance ratio (CRR) of the
soil.
If CSR is greater than CRR, liquefaction may occur at that
area.
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Cyclic Stress Ratio (CSR)
The cyclic stress ratio (CSR), which reflects the stresses exerted
on the soil element by seismic actions is the average cyclic shear
stress (tav) developed on the horizontal surfaces of sand to the
effective overburden pressure (σ’o).
The CSR ratio can be determined conveniently from the following
relationship (Ref. [4]):
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Where
σ’o = Effective overburden pressure at depth under consideration
(kPa)
σo = Total overburden pressure at depth under consideration
(kPa)
amax = Peak Ground seismic acceleration (m/s2)
rd = Stress reduction factor, which decreases from a value of
1 at ground surface to a value of about 0.9 at a depth of
10.7m ( - )
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Cyclic Resistance Ratio (CRR)
Cyclic resistance ratio (CRR) is the ratio of normalized soil shear
resistance (tav) to the effective overburden pressure (σ’o) at a
given depth.
This ratio can be estimated by the following procedure:
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Determine the qc / N60 ratio
Determine the qc / N60 ratio for a given mean grain size D50 from
Figure 1, where qc is the tip resistance (in bars) at a given depth
from CPT, and N60 is the penetration resistance in blows per foot
and corresponds to a transfer of approximately 60% of the
theoretical free-fall hammer energy to the stem.
Figure 1 shows the correlation between the SPT test data and CPT
test data proposed by Robertson (Ref. [5]).
FIGURE 1 – RATIO OF TIP RESISTANCE TIP STANDARD BLOW (qc / N60) VS
MEAN GRAIN SIZE, D50
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2. Determine the N60 for a given qc
Substitute the qc value obtained from the CPT results to the qc /
N60 ratio to determine the N60 value.
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3. Determine the penetration resistance of soil (N1)60
The blow counts N60 is then corrected to the normalized SPT blow
count, (N1)60 where (N1)60 is the penetration resistance the soil
would have under an effective overburden pressure of 1 ton per sq
ft.
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3. Determine the penetration resistance of soil (N1)60
The value of (N1)60 can be determined by multiplying N60 by the
effective stress correction factor, CN (obtained from Figure 2,
(Ref. [4])) as follows:
FIGURE 2 – EFFECTIVE STRESS CORRECTION COEFFICIENT CN VS EFFECTIVE
OVERBURDEN PRESSURE
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4. Determine of the Cyclic Resistance Ratio (CRR)
The cyclic resistance ratio (CRR) at a given depth for a 7.5
earthquake magnitude on Richter scale for silty sands can be
predicted by using the correlation between the field liquefaction
behaviour of sands and the normalized SPT blow count for three
contents of fines (Figure 3, (Ref. [4])).
FIGURE 3 – CYCLIC RESISTANCE RATIO (CRR) VS CORRECTED SPT (N1)60
FOR A EARTHQUAKE MAGNITUDE OF 7.5 ON RITCHER SCALE FOR SILTY
SAND
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General
The CPT results show a wide range of various soil parameters within
the layer of soil concern. These variables include the percentage
of fines, mean grain size, relative density and depth of
soil.
As such, limiting tip resistance for liquefaction to occur for
various soil parameters due to seismic acceleration of 0.23g, for
example, is determined.
These limiting tip resistances are compared with the average tip
resistances from the CPT results and conclusions are drawn for the
liquefaction potential of the soil layer being studied.
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Determination of Cyclic Stress Ratio
Assume that the cyclic stress ratio (CSR) for seismic acceleration
equates to 0.23g.
This CSR ratio corresponds to the CRR ratio and are plotted on
Figure 3 to determine the limiting (N1)60 values at the respective
peak ground acceleration. This is illustrated in Figure 4.
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FIGURE 4 – (CRR = CSR) VS CORRECTED SPT (N1)60 FOR A EARTHQUAKE
MAGNITUDE OF 7.5 ON RITCHER SCALE FOR SILTY SAND FOR SEISMIC
ACCELERATION OF 0.23g
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Determination of Cyclic Stress Ratio (cont’d)
Table 1 presents the corresponding limiting (N1)60 values
determined for seismic acceleration of 0.23g.
TABLE 1 – LIMITING PENETRATION RESISTANCE (N1)60 VALUES AT
LIQUEFACTION
Seismic Acceleration
Determination of qc / N60 ratio
The qc / N60 ratio for a given mean grain size D50 of the soil
layer of interest can be obtained from Figure 1.
Table 2 presents the mean grain size (D50) and their corresponding
qc / N60 ratio for a range of D50 from 0.001mm to 1.0mm.
TABLE 2 – QC / N60 RATIO
Mean grain Size, D50 (mm)
qc / N60
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Determination of CN Value
The effective overburden pressure (σ’o) for soil depth of interest
or a range, for example, from 1m to 5m, is determined and the
corresponding effective stress correction factor, CN are obtained
from Figure 2.
TABLE 5.3 – CN VALUES FOR SOIL DEPTH 1M TO 5M
Soil Depth (m)
CN for Dr = 40 to 60%
CN for Dr = 60 to 80%
1
0.2
1.6
1.6
2
0.4
1.6
1.6
3
0.6
1.6
1.6
4
0.8
1.55
1.53
5
1.0
1.35
1.36
Determination of N60 Value and qc Value
With the given CN and (N1)60, the N60 term can be determine from
the penetration resistance of soil relationship.
The N60 term is then used to determine the various limiting tip
resistance value, qc, for liquefaction to occur for various depth
(1m to 5m), mean grain size (0.001 to 1mm), percentage fines (<
5% to 35%) and relative density (40 to 60% and 60% to 80%) and are
illustrated in Figures 5 to 10.
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FIGURE 5 – LIMITING TIP RESISTANCE (qc) FOR LIQUEFACTION TO OCCURS
FOR 0.23g SEISMIC ACCELERATION, DR = 40 TO 60% AND PERCENTAGE FINES
< 5%
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FIGURE 6 – LIMITING TIP RESISTANCE (qc) FOR LIQUEFACTION TO OCCURS
FOR 0.23g SEISMIC ACCELERATION, DR = 40 TO 60% AND PERCENTAGE FINES
= 15%
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FIGURE 7 – LIMITING TIP RESISTANCE (qc) FOR LIQUEFACTION TO OCCURS
FOR 0.23g SEISMIC ACCELERATION, DR = 40 TO 60% AND PERCENTAGE FINES
= 35%
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FIGURE 8 – LIMITING TIP RESISTANCE (qc) FOR LIQUEFACTION TO OCCURS
FOR 0.23g SEISMIC ACCELERATION, DR = 60 TO 80% AND PERCENTAGE FINES
< 5%
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FIGURE 9 – LIMITING TIP RESISTANCE (qc) FOR LIQUEFACTION TO OCCURS
FOR 0.23g SEISMIC ACCELERATION, DR = 60 TO 80% AND PERCENTAGE FINES
= 15%
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FIGURE 10 – LIMITING TIP RESISTANCE (qc) FOR LIQUEFACTION TO OCCURS
FOR 0.23g SEISMIC ACCELERATION, DR = 60 TO 80% AND PERCENTAGE FINES
< 35%
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Determination of Soil Liquefaction Regions
With the soil relative density, Dr, and percentage of fines of the
soil in the region of interest, the appropriate graph in Figure 5
to 10 is selected.
A comparison of the soil tip resistance can then be made against
the limiting soil tip resistance value, qc, for liquefaction to
occur in the appropriate graph, based on the mean grain size of the
soil, to determine the propensity of the soil region of interest to
liquefy during earthquake.
Soil liquefaction would most likely occur if the soil tip
resistance is lower than the limiting value.
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REFERENCES
Seed, H. B. and Idriss, I. M (1971), ‘Simplified Procedure for
Evaluating Soil Liquefaction Potential,’ Journal of the Soil
Mechanics and Foundations Division, ASCE, Vol. 97, No. SM9,
September, 1971.
Seed, H. B. (1979), ‘Soil Liquefaction and Cyclic Mobility
Evaluation for Level Ground during earthquakes,’ Journal of the
Geotechnical Engineering Division, Vol. 5 No. GT2
Seed, H. B, Idriss, I.M. and Arango, I. (1983), ‘Evaluation of
Liquefaction Potential using Field Performance Data,’ Journal of
Geotechnical Engineering , Vol. 109, No. 3, March, 1983.
Seed, H. B., Tokimatsu, K., Harder, L.F. and Riley M. Chung (1985),
‘Influence of SPT Procedures in Soil Liquefaction Resistance
Evaluations, ‘Journal of Geotechnical Engineering, Vol. 111, No.
12, December, 1985.
Robertson, P.K. And Campanella, R. G. (1985), ‘Liquefaction
Potential of Sands using CPT,’ Journal of Geotechnical Engineering,
Vol. 111, No. 3, 1985.
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(CSR) and (N
Ritcher Scale
For 0.23g
c
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
0.0010.0100.1001.000
Soil Depth = 4m
Soil Depth = 5m
c
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
0.0010.0100.1001.000
Soil Depth = 4m
Soil Depth = 5m
c
0.00
2.00
4.00
6.00
8.00
10.00
12.00
0.0010.0100.1001.000
Soil Depth = 4m
Soil Depth = 5m
c
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
0.0010.0100.1001.000
Soil Depth = 4m
Soil Depth = 5m
c
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
0.0010.0100.1001.000
Soil Depth = 4m
Soil Depth = 5m
c
0.00
2.00
4.00
6.00
8.00
10.00
12.00
0.0010.0100.1001.000
Soil Depth = 4m
Soil Depth = 5m