9 - NRG - Pipeline Protection - Earthquake
27
NRG ENGINEERING [email protected] The Power to Deliver™ Pipeline Design Training Assessment Of Earthquake–induced Liquefaction 14 August 2006
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NRG - Pipeline Protection - Earthquake
Transcript of 9 - NRG - Pipeline Protection - Earthquake
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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The Power to Deliver™ #*
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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The Power to Deliver™ #*
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.
NRG ENGINEERING
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.
NRG ENGINEERING
(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
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.
NRG ENGINEERING
The Power to Deliver™ #*
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]):
NRG ENGINEERING
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 ( - )
NRG ENGINEERING
The Power to Deliver™ #*
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:
NRG ENGINEERING
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
NRG ENGINEERING
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.
NRG ENGINEERING
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.
NRG ENGINEERING
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
NRG ENGINEERING
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
NRG ENGINEERING
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.
NRG ENGINEERING
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.
NRG ENGINEERING
The Power to Deliver™ #*
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
NRG ENGINEERING
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
The Power to Deliver™ #*
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.
NRG ENGINEERING
The Power to Deliver™ #*
FIGURE 5 – LIMITING TIP RESISTANCE (qc) FOR LIQUEFACTION TO OCCURS FOR 0.23g SEISMIC ACCELERATION, DR = 40 TO 60% AND PERCENTAGE FINES < 5%
NRG ENGINEERING
The Power to Deliver™ #*
FIGURE 6 – LIMITING TIP RESISTANCE (qc) FOR LIQUEFACTION TO OCCURS FOR 0.23g SEISMIC ACCELERATION, DR = 40 TO 60% AND PERCENTAGE FINES = 15%
NRG ENGINEERING
The Power to Deliver™ #*
FIGURE 7 – LIMITING TIP RESISTANCE (qc) FOR LIQUEFACTION TO OCCURS FOR 0.23g SEISMIC ACCELERATION, DR = 40 TO 60% AND PERCENTAGE FINES = 35%
NRG ENGINEERING
The Power to Deliver™ #*
FIGURE 8 – LIMITING TIP RESISTANCE (qc) FOR LIQUEFACTION TO OCCURS FOR 0.23g SEISMIC ACCELERATION, DR = 60 TO 80% AND PERCENTAGE FINES < 5%
NRG ENGINEERING
The Power to Deliver™ #*
FIGURE 9 – LIMITING TIP RESISTANCE (qc) FOR LIQUEFACTION TO OCCURS FOR 0.23g SEISMIC ACCELERATION, DR = 60 TO 80% AND PERCENTAGE FINES = 15%
NRG ENGINEERING
The Power to Deliver™ #*
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.
NRG ENGINEERING
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.
NRG ENGINEERING
(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
