Evaluation of soil liquefaction potential along Tabriz Metro Line 2 based on Idriss-Boulanger and Japanese Highway Bridges methods. Ebrahim Asghari Kaljahi.
Advances in Transportation Geotechnics II – Miura et al. (eds) 2012 Taylor & Francis Group, London, ISBN 978-0-415-62135-9
Evaluation of soil liquefaction potential along Tabriz Metro Line 2 based on Idriss-Boulanger and Japanese Highway Bridges methods Ebrahim Asghari Kaljahi Department of Geology, University of Tabriz, Iran
Mahyar Babazadeh Geotechnical Engineering, Young Researchers Club, Science and Research Branch, Islamic Azad University, East Azarbaijan, Tabriz, Iran
ABSTRACT: Liquefaction phenomenon is one of the most important problems in engineering that occurs in saturated sandy soils during earthquakes. In this phenomenon, pore water pressure increases up to confining pressures. Hence the effective confined pressure becomes zero and the soil doesn’t have any shear resistance, so the soil mass will be unstable and much destructive. In this paper, the liquefaction potential of Tabriz Urban Railway Line 2 (TURL2) according to Standard Penetration Test (SPT) has been evaluated. Evaluation of liquefaction potential based on SPT method is the latest methods offered by Idriss-Boulangr (2008), and is used to calculate the soil liquefaction, and have compared the results with Japanese Highway bridges method. In this research, the data of more than 100 boreholes were evaluated and calculated for designed basic earthquake with magnitude 7.5. The safety factor for liquefaction was evaluated by two methods for several boreholes at different depths and the liquefaction risk potential indexes were determined along TURL2. The analysis results obtained by both methods are compared for 2 boreholes, as sample. The analysis results show that at some horizons along TURL2, liquefaction will happen during strong motions.
1
INTRODUCTION
897
TURKMANESTAN
Tabriz
IRAQ AFGHANISTAN
So called Liquefaction phenomenon, losing the shear strength of soil in the saturated sandy soil with low density, may take place because of the increase in pore water pressure. Increases in pore water pressure can lead to the total loss of soil shear strength. Soils layers that lose their shear strength totally, will act like a thick liquid and will appear with the flow and boiling phenomena. The possibility of occurrence of liquefaction, considering the existence of saturated sandy soils with noticeable thickness in some depths, has been studied. Recently several methods have been presented in order to evaluate the soil liquefaction potential. In this paper the latest method presented by Idriss-d Boulanger (2008) has been used for evaluation of soil liquefaction potential and the results have been compared with the most credible regulation, Japan’s bridge lifeline design regulation. Tabriz city is the one of the biggest cities of Iran with population of about 2,000,000. The location of Tabriz is presented on Iran map in figure 1. Based on Tabriz metro development plan, four metro lines will be constructed following years. The construction of lines 1 with 17km and line 2 with 22km length, have been commenced.
TURL2 is generally extended along west to east direction, from Qaramalek in west to Baseij square in east. Tunnel diameter of this metro line was designed as about 9m and the bottom of tunnel, will located in depth 15 to 30m. Most parts of TURL2 in main part are covered by alluvium deposits. These deposits mostly are consisted of silty sand to sandy silt soils with some fine grained interlayers.
SAUDI ARABIA
Figure 1.
Tabriz location on Iran map.
For geotechnical investigation of TURL2, more than 100 boreholes were drilled by POR Company and results were presented in a special report. 2
RELATION BETWEEN STANDARD PENETRATION TEST (SPT) AND SOIL LIQUEFACTION
IDRISS-BOULANGER METHOD FOR LIQUEIFACTION EVALUATION
In contrary to previous methods like Seed & Idriss (1971) or Seed et al. (1983), the evaluation of liquefaction potential was based on error and trail (N1)60, Which precise results can be achieved by changing some equations and tables. Furthermore, in this paper the Idriss-Boulanger equation based on SPT number has been used. (N1)60 = NsptŨCNŨCEŨCSŨCRŨCB
(2)
m = 0.748 0 .0768 (
(3)
v
3.1
Standard Penetration Test (SPT) is one of the common field test method in order to determine the resistance of soil against liquefaction phenomenon. The parameters, increasing soil resistance against liquefaction are density, strain value before earthquake, over consolidation ratio, lateral earth pressure, and increases in SPT number. Seed et. al. (1985) studies have been taken for the clean Sand to measure the least ratio of cycle strain which is being expected the occurrence of liquefaction in clean sand with a definite SPT. Having impurity can influence SPT number; therefore it must be calculated in the evaluation of soil resistance regarding liquefaction. If the amount of fine content is less than 5% (FC ≤ 5%) the resistance of soil for liquefaction will not be influenced by fine content but higher percentages of fine content prevents liquefaction. So, it causes increases in required CSR to start liquefaction for the definite (N1)60, and as a result it leads to decrease in potential danger of liquefaction. These parameters and some others are being studied. 3
m
¥P ´ CN = ¦ a µ b 1.7 For Pa = 100 kPa § m a¶
(1)
In the above equation, (N1)60 is the corrected coefficients for SPT, CN is an overburden stress correction factor, CE = ERm/60% and ERm is the measured value of the delivered energy as a percentage of the theoretical free-fall hammer energy, CR is a rod correction factor to account for energy ratios being smaller with shorter rod lengths, CB is a correction coefficient for nonstandard borehole diameters, CS is a correction coefficient for using split spoons with room for liners but with the liners absent, and Nm is the measured SPT blow count. The factors CB and CS are set equal to unity if standard procedures are followed (Idriss and Boulanger, 2010) The value of CN based on Idriss-Boulangr method can be measured by equation 2.
)
1 60
The simplified procedure for estimating cyclic shear stress ratios induced by earthquake ground motions
Seismic demand energy usually is defined on a layer of soil by CSR, Seed and Idriss (1971) simplified procedure is used to estimate the cyclic shear stress ratios (CSR) induced by earthquake ground motions, at a depth z below the ground surface, using the following equation 4. ¥m ´¥a ´ n av 0.65 ¦ v µ ¦ max µ rd (4) m va § m va ¶ § g ¶ In this equation, mv = total vertical stress, mav = effective vertical stress, amax/g = maximum horizontal acceleration ratio at the ground surface, and rd = shear stress reduction factor that accounts for the dynamic response of the soil profile. CSR
3.2
Shear stress reduction factor (rd)
Shear stress reduction factor (rd) has been introduced by Seed and Idriss (1971), as a parameter which accounts for the dynamic response of the soil profile. Idriss (1999), in extending the work of Golesorkhi (1989), performed several hundred parametric site response analyses and concluded that, for the purpose of developing liquefaction evaluation procedures, the parameter rd could be expressed as depth and earthquake magnitude from equation 5a, which is true to 34 meters. The uncertainty in rd increases with such a depth increasing that equation 5a should only be applied for depths less than about 20 m. Liquefaction evaluations at greater depths often involve special conditions for which more detailed analyses can be justified. For these reasons, it is recommended that CSR (or equivalent rd values) at depths greater than about 20 m should be based on site response studies, providing, however, that a high quality response calculation can be completed for the site. In present research the region soil liquefaction has been studied down to 20 meters depth. « ®rd = exp (] ((z) + B(z)M) ® ® ´ ¥ z 5.133µ ¬] ((z)= 1.012 1.126 sin ¦ § 11.73 ¶ ® ® ¥ z ´ 5.142µ ®B(z) = 0.106 + 0.118 sin ¦ § ¶ 11.28
(5a)
If the studied depth is more than 34 meters equation 5b can be used.
898
rd
0.12 e p(0.22M)
(5b)
The relationship between the number modified SPT, (N1)60, and clean sand number (N1)60cs is expressed by clean sand ∆(N1)60. This parameter is based on the percentage of fine content (FC), that has been expressed via equation 6 (Idriss and Boulanger, 2006). $(
)
3.3
¨ · 9.7 exp 1.63
¨ 15.7 · FC 0.01 ©ª FC 0.01 ¸¹ ¸¹ (6) ª
(N1 )60 + (N1 )60
(7)
The cyclic resistance ratio (CRR)
Calculation of soil capacity to liquefaction phenomenon is expressed by CRR. In Idriss and Boulanger (2008) formula, cyclic resistance ratio of soil (CRR) is calculated based on (N1)60cs. In equation 8 the amount of CRR is calculated for magnitude 7.5.
ma Pa
1
CRR7.5,1 atm
2 · ¨ ¨ (N ) ¥ (N ) ´ ¸ © © 1 60CS ¦ 1 60CS µ . 14 1 126 § ¶ ¸ © ©ª © ¸ (8) 3 4 · ¸ © ¥ ( N1 )60CS ´ ¥ ( 1 )60CS ´
2 . 8 ¸ ¸ © ¦§ 23.6 µ¶ ¦§ 25.4 µ¶ ¸ ©ª ¹ ¸¹
According to the status of increase in strain or liquefaction potential evaluation based on other than 7.5 magnitude formula CRR is being corrected (Idriss and Boulanger 2003a). There is no need for MSF because the earthquake is magnitude 7.5 in this research. Since the semi-empirical liquefaction correlations are primarily based on data for ground level conditions and effective overburden stresses is in the vicinity of ±100 kPa, Seed recommended that the CRR can be corrected for these effects using the following expression (Idriss and Boulanger 2006): CRRM,Km = CRRM=7.5,1 atm v MST Km 3.4
¥ma´ 1 Cm ln ¦ v µ § Pa ¶ 1 Cm 18.9 2.55 ( N1 )60 Km
2
According to the value of (N1)60 and ∆(N1)60, the value of (N1)60cs is calculated by equation 7. (N1 )60CS
Against to last proposed corrected equations in previous research by Hynes and Olsen (1999), Seed and Harder (1990), this correction is not based on relative density (DR) but according to (N1)60, but according to (N1)60 that can be calculated by this equation:
(9)
Overburden correction factor, km
(10a) (10b)
3.5 Magnitude scaling factor, MSF The magnitude scaling factor (MSF) is used to account for duration effects on the triggering of liquefaction. The MSF relationship was derived by combining a laboratory based relationships between the CRR and the number of equivalent uniform loading cycles, and b-correlations of the number of equivalent uniform loading cycles with earthquake magnitude. The MSF factor is applied to the calculated value of CSR for each case history to convert to a common value of M (conventionally taken as M = 7.5). The MSF for sands was re-evaluated by Idriss (1999), who recommended the following relationship (Idriss and Boulanger, 2010).
MSF = 6.9 exp M 0.058 b 1.8 4 4
(11)
JAPANESE MAIN ROAD BRIDGES DESIGNING REGULATION METHOD
The method in the specifications for Highway bridges (TC4-ISSMGE, 1999) is based on a procedure developed by Iwasaki et al. (1982) termed “simple geotechnical analysis”. The method is an approach in which a soil liquefaction capacity factor, R (CRR) is calculated along with a dynamic load, L (CSR), Induced in a soil element by the seismic motion. The soil liquefaction capacity is calculated by summing of three factors by taking the overburden pressure, the grains size and the fine content into account.
The Km is the overburden correction factor and Km is the static shear stress correction factor. Revised Km relationships are described in more details by Boulanger (2003) and by Idriss and Boulanger (2003a, 2003b) and herein they are not reviewed. When in a layer is mPaa 1, there is no need to correct the soil profile, but if the aforesaid condition is not appointed then the achieved figure from equation 9 by km according to equation 10a must be corrected. By the way in km formula Cm factor is being calculated by equation 10b.
899
R= R1 + R2 + R3 R1 0.0882
N m va 0.7
(12) (13)
0.0 mm D50 0.05 mm « 0.19, 0.02 ® ¥ ´ ® R2 ¬=0.225 log ¦ 0.35 µ , 0.05 0 05 mm mm D50 < 0 < 0.6 mm D 50 § ¶ ® ®= 0.05, 0.6 mm D 2.0 mm 50
(14)
0, 0 D50 40% « 0, R3 ¬ (15) 04FC C 0.16, 40% D550 b 100% 0.04FC Where, ma is the effective over burden pressure in kgf/cm2, D50 is the mean grain size and FC is the fine content. Equations 14 to 16 are applicable for loose and medium sands with relative densities of less than about 60%. The dynamic load induced in soil element is calculated as follows: ¥ a ´ ¥m ´ (16) L ¦ max µ ¦ v µ rd § g ¶ § m va ¶ Whereas mv is the total overburden pressure, mav is the effective overburden pressure, amax is the estimated peak surface acceleration in cm/sec2, g is the acceleration of gravity (980 cm/s2) and rd is stiffness reduction factor. The factor rd depends on depth: rd = 1 − 0.015z
(17)
Where, z is the depth in meter below the ground surface. One of the differences between the Seed-Idriss (which is used in Idriss-Boulanger method) and Iwasaki methods when calculating the cyclic load is the factor 0.65, which in the Seed-Idriss method n yields the average cyclic stress ratio mavva while in the Iwasaki method the load is characterized by peak n cyclic stress ratio mmava x . The blow count value, N, is the value measured with the SPT-equipment most commonly used in Japan which is calculated according to equation 18. The influence of the effective overburden pressure on the N-value is taken in to account since ma is present in the denominator in the expression for R1. N = 0.833 (N1)60
(18)
The values of safety factor in both methods are equal to equation 19, as follows if FS < 1.0 then liquefaction occurrence in the considered depth is probable and if FS > 1.0 it will not be probable. FS = CRR CSR 5
(19)
LIQUEFACTION POTENTIAL INDEX
The Liquefaction Potential Index, IL, has been extended by Iwasaki (1982) for predicting the risk of liquefaction potential. He described that if IL = 0, the risk of liquefaction is very low, if 0 ≤ IL ≤ 5 the risk of liquefaction is low, if 5 ≤ IL ≤ 15 risk of liquefaction is high and if IL > 15 the risk of liquefaction is high too very high. The amount of liquefaction in the studied area can be achieved by equation 20. The value of IL varies between 0 and 100.
20
IL
(20)
° F v W ( z ) v dz 0
On the above equation, F is defined as an index. If FS ≤ 1.0, then F = 1 – FS and if FS > 1.0 then F = 0. In this equation W(z) is a weight function based on the depth for estimating the ratio of soil liquefaction that is being used in different depths. Z is the layer depth in which the liquefaction potential is being evaluated in it. 6
LIQUIFACTION ANALYSIS IN BOREHOLES
For geotechnical investigation of TURL2, more than 100 boreholes with depth 25 to 45m were drilled by POR Co. (2010). This investigation showed that there are some silty sand to sandy silt sediments in the main part of TURL2 (km 3+00 to 10+00) those is susceptible for liquefaction. These sediments are deposited by Mehran River. Groundwater depth is varies between 5 to 20 m in this area. Liquefaction hazard in 2 boreholes as samples of more than 100 boreholes, have analyzed based on Idriss-Boulanger (2008) and Japanese Highway bridges (TC4-ISSMGE, 1999) methods and compared each other and has been brought of the result in figures 2 and 3. Boreholes specifications are shown in Table 1. It should be noted in these analyses, maximum acceleration intended for the Tabriz city, 0.35g (Babazadeh, 2012) The following graphs seismic force required initiating liquefaction by “Load” and soil resistance to liquefaction phenomenon with “Resistance” is shown which are in order of the concepts CSR and CRR. According to these graphs, in which the soil
Table 1.
Boreholes Specifications.
Boring No.
BH-19
C2B1
Location
Qarahaghaj St.
Akhuni St.—Station C2
Ground water depth
8m
8m
Geotechnical Depth USCS specifications (m)
900
2 4 6 8 10 12 14 16 18 20
GW-SM SM SM SM SM SM CL-SM SM SM SM
NSPT Depth USCS (m)
NSPT
12 19 18 15 14 19 33 37 41 43
8 12 16 15 15 15 21 22 25 28
2 4 6 8 10 12 14 16 18 20
ML ML ML-SM SM ML ML ML ML ML ML
Figure 2. Comparison of results in the borehole C2B1: a. NSPT Correction; b. Liquefaction Load and Resistance condition; c. Safety Factor; d. Liquefaction Index.
Figure 3. Comparison of results in the borehole BH-19: a. NSPT Correction; b. Liquefaction Load and Resistance condition; c. Safety Factor, d. Liquefaction Index.
901
resistance to liquefaction under seismic force, it is the point where safety factor (FS) was less than one and increases the index of liquefaction (IL). 7
CONCLUSION
This research has been carried out to evaluation of liquefaction phenomenon along the TURL2 route, based on the Standard Penetration Test (SPT) results, by the latest method in liquefaction potential evaluation (Idriss and Boulanger, 2008) and comparing these results with Japanese Highway Bridges method. After general liquefaction evaluation for TURL2, two sample boreholes have been concluded among all boreholes that have been investigated, and the results have been showed in 4 graphs for both methods. With comparison of the achieved results from both methods, it can be shown that the occurrence of liquefaction is possible for some parts during strong motions, based on the two mentioned methods, but they do not have the same results. According to the results, it was found that Japanese Highway bridges method determines liquefied sandy soils and may cause the parts which don’t have the capability of liquefaction experientially determines liquefied. By comparison of the used data by Idriss and Boulanger (2008) for their studies and developing of their method, with TURL2 data, it can be finding out that the Idriss and Boulanger evaluation method is suite for liquefaction evaluation in TURL2. Also Idriss and Boulanger method is based on (N1)60 and error and trial process, the results can be near to the reality. The liquefaction potential of more than 100 boreholes along TURL2 with acceleration 0.35g for earthquake magnitude 7.5 was studied. Practically in the layers that NSPT is more than 30, liquefaction has not been observed. According to the Figures 2 and 3, in which resistance factor (CRR) is less than load (CSR) is equal the most liquefaction hazard, furthermore FS end in less than 1.0. Considering the liquefaction risk analysis with Iwasaki et al. (1982) method, the mostly liquefaction risk takes place with high degree in lower depths and near to ground in the length of the TURL2 investigations. Results show that at some horizons along TURL2, liquefaction will happen during strong motions. Based on the obtained results by the current methods that the soil type is silty Sand to sandy Silt such as Qaramalek area, Abbasi Street, Ghods Street, Shariati Street and Akhuni Cross so in these areas the liquefaction risk is high. According to geology studies and geotechnical investigations, Mehran river has an important role in deposition of liquefiable soils.
REFERENCES Babazadeh, M. 2012. Geotechnical data analysis for evaluation the potential of soil liquefaction based on “Idriss-Boulanger” method. Case Study: Tabriz urban railway line 2, M Sc Thesis, Islamic Azad University, Zanjan Branch. Boulanger, R.W. 2003. Relating Ka to relative state parameter index. J. Geotech Geoenviron Eng, ASCE; 129 (8): 770–3. Golesorkhi, R. 1989. Factors influencing the computational determination of earthquake-induced shear stresses in sandy soils, PhD thesis, University of California, Berkeley, 395 pp. Hynes, M.E. & Olsen, R.S. 1999. Influence of confining stress on liquefaction resistance, in Physics and Mechanics of Soil Liquefaction, P. V. Lade and J. Yamamuro (eds.), pp. 145–151. Idriss, I.M. & Boulanger, R.W. 2008. Soil Liquefaction during Earthquakes, Earthquake Engineering Research Institute, Oakland, CA, 235 pp. Idriss, I.M. & Boulanger, R.W. 2010. SPT-based liquefaction triggering procedures. Report UCD/CGM-10/02, Center for Geotechnical Modeling, University of California, Davis, CA, 259 pp Idriss, I.M. & Boulanger, R.W. 2006. Semi-empirical procedures for evaluating liquefaction potential during earthquakes. J. of Soil Dynamics and Earthquake Engineering, Elsevier, 26, 115–130 Idriss, I.M. & Boulanger R.W., 2003a. Estimating Ka for use in evaluating cyclic resistance of sloping ground. Proc. 8th US-Japan Workshop on Earthquake Resistant Design of Lifeline Facilities and Countermeasures against Liquefaction, Hamada, O’Rourke, and Bardet, eds., Report MCEER-03-0003, MCEER, SUNY Buffalo, N.Y., 449–468. Idriss, I.M & Boulanger R.W. 2003b. Relating Ka and Ks to SPT Blow Count and to CPT Tip Resistance for Use in Evaluating Liquefaction Potential. Proc. of the Dam Safety Conference, ASDSO, Minneapolis. Idriss, I. M. 1999. An update to the Seed-Idriss simplified procedure for evaluating liquefaction potential, Proc., TRB Workshop on New Approaches to Liquefaction, January, Publication No. FHWA-RD-99-165, Federal Highway Administration. Iwasaki, T., Arakawa, T. & Tokida, K. 1982. Simplified Procedures for Assessing Soil Liquefaction during Earthquakes. Proc. Conference on Soil Dynamics and Earthquake Engineering. Southampton, 925–939. POR Co. 2010. Geotechnical investigation report of Tabriz urban railway line 2 (TURL2). Seed, R. B. & Harder, J. L. F. 1990. SPT-based Analysis of Cyclic Pore Pressure Generation and Undrained Residual Strength: Proc., H.B. Seed Memorial Symp., Vol. 2, BiTech Publishing, Vancouver, B. C., Canada, 351–376. Seed, H.B. & Idriss, I.M. 1971. Simplified procedure for evaluating soil liquefaction potential. J. Soil Mech Found Div., ASCE; 97: 1249–73. Seed, H.B. 1983. Earthquake resistant design of earth dams. Proc. Symposium on Seismic Design of Embankments and Caverns, Pennsylvania, ASCE, N.Y., pp. 41–64. TC4-ISSMGE, 1999. Manual for Zonation on Seismic Geotechnical Hazards; Revised edition, Technical Committee for Earthquake Geotechnical Engineering (TC4) of the International Society of Soil Mechanics and Geotechnical Engineering (ISSMGE)
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