ISBN 978-93-84422-56-1

Proceedings of International Conference on Architecture, Structure and Civil Engineering (ICASCE'15)

Antalya (Turkey) Sept. 7-8, 2015 pp. 1-7

Real Probability of Liquefaction, Liquefaction Potential Index Based In CPTU Data for Golem Area, In Albania

Shpresa Gashi1, Neritan Shkodrani

2

1 Department of Geotechnical, Faculty of Civil Engineering, Polytechnic University of Tirana, Tirana, Albania

2 Department of Geotechnical, Faculty of Civil Engineering, Polytechnic University of Tirana, Tirana, Albania.

Abstract: Several authors have studies liquefaction in different approaches like deterministic and probabilistic approach based in situ and laboratory test.

In this article we are going to evaluate the Real Probability of Liquefaction (PR[L]) in Golem area related with the Factor of Safety (FS) computed with the deterministic piezocone penetration test (CPTU)-based model by Juang et al (2008). The real probability of liquefaction is calculated for a given seismic hazard level expressed in terms of acceleration, with the probability of exceeding the same acceleration during the lifespan of engineering structures (usually 50 years). The effects of liquefaction are going to calculate with Liquefaction Potential Index IPL. The computed liquefaction probabilities indicate that during the life span of a structure, the phenomenon may be encountered and it should be taken into consideration during the design stage of the engineering projects.

Keywords: Liquefaction, cone penetration test, probability of liquefaction, liquefaction potential index.

1. Introduction

The simplified procedure for evaluating the liquefaction potential of a soil was created by Seed & Idriss (1971) [1]. Generally speaking, the framework of the analysis with the simplified procedure consists of evaluation of the seismic loading, in the form of the cyclic stress ratio (CSR), and of the soil resistance against liquefaction, in the form of the cyclic resistance ratio (CRR). The liquefaction potential or triggering is then measured with a Factor of Safety (FS=CRR/CSR), in which liquefaction is said to occur if FS<1. In practice, to evaluate CRR correlations with in situ tests data are different tests such as the standard penetration test (SPT), the cone penetration test (CPT), the Becker penetration test (BPT), and shear wave (Vs) measurements. The present study focuses on the use of piezocone penetration tests (CPTU), and thus only CPT-based models proposed by Juang et al (2010) [2] is presented here. For performance-based earthquake engineering (PBEE) design, it is often necessary to determine the probability of liquefaction of a soil at a site in a given exposure time or the Real Probability of Liquefaction (PR [L]). To measure the effect of liquefaction is necessary to calculate Liquefaction Potential Index

IPL. In this study we are going to present the methodology and the result of calculation of FS, (PR[L]) and IPL in the area of Golem where 10 CPTU test where done to conduce this study. The area under study is situated at Golem municipally of Kavaja County, at the central Albanian coast, in Tirana Prefecture. From the seismic point of view this area is located in the Periardiatic Depression, denoted as PL – zone, strongly affected by post– Pliocene compression movements, in direct convergence with Adria microplate. It is characterized by a high seismic activity and according to the seismic hazard map of Albania the second level of hazard represent an earthquake with 10% probability of exceedance in 50 years, or a PGA=0.273g [3]. According to the Albanian earthquakes catalogue, the expected

earthquake surface wave magnitudes (Ms) of the considered area vary from 4.5 – 6.6 [4]. The highest magnitude is

Ms = 6.6 (occurred in year 346 and in the region with coordinates: 41.30 0N; 19.30

0E).The soil behavior type index,

Ic, as defined by Robertson and Wride (1998) [5] is used to identify the soils type and the boundaries of the Ic are indicate in Table 1. The type of soils met in Golem area are silty sands and sandy silt, clay and silty clay.This type of soils are suspected to liquefy when they are in e seismic area and under the ground water table.

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TABLE I: Soil types defined by the Soil Type Index Ic

Soil Type Index Ic Soil behavior tye

Ic 1.30 Gravel sand to dense send

1.31 Ic 2.05 Sands: clean send to silty sand

2.05 Ic 2.60 Sand mixtures: silty sand to sandy silty

2.60 Ic 2.95 Silty mixture: clayey silt to silty clay

2.95 Ic 3.60 Clays: silty clay to clay

Ic 3.60 Organic soils: peats

2. Evaluation of Liquefaction Potential, Based on CPTU Data

To evaluate liquefaction potential essentially we have to compares the cyclic resistance ration (CRR) at a given depth with the earthquake-induced cyclic stress ration (CSR) at the depth from a specified design earthquake. The evaluation procedure can be outlined using empirical equations deduced by different authors (Seed and Idriss, 1971; Olsen, 1997 [6]; Robertson and Wride, 1998; Chen and Juang, 2000 [6], 2006 [7], 2010; Lee et al., 2003 [8]; Yuan et al., 2003 [9]). The determination of CSR, usual is calculate based in methods proposed by Seed and Idriss (1971), in this paper is calculated based in model proposed by Idriss and Boulanger (2006) [10]. For the determination of CRR, different simplified methods have been proposed till today. In this paper the method proposed by Juang (2010) is applied. 2.1 The calculation of CSR after Idriss and Boulanger (2006)

The average uniform cyclic stress ratio (CSR) within a liquefiable layer is defined as

a 1 CSR

7.5 0.65

vo' max

rd

(1)

vo g MSF

Where:

amax is the peak ground surface acceleration generated by the earthquake (g); g is acceleration of gravity;

vo

and ' are the vertical total stress and effective overburden stresses (kPa); r is the depth-dependent shear

v 0 d

stress reduction factor; MSF is a magnitude scaling factor; and K is the overburden correction factor for CSR.

The term rd is computed as

ln rd Mw (2)

Where

1.012 1.126sin 5.133 z 11.73 (2.1)

0.106 0.118sen 5.142 z 11.28 (2.2)

Z is the depth of interest (m), and M w is the moment magnitude.

The term MSF is computed as

M w (3) MSF 0.058 6.9exp 1.8

4

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2.2 The calculation of CRR after Juang (2000, 2006, 2010) Cyclic resistance ratio (CRR) in the CPTU-based model is a function of two derived CPTU parameters, the

adjustment cone tip resistance qt1N and the soil behaviour type index I .In the present study the parameter

qt 1N is defined by Robertson (2009 a) [11] and Ic, BJ is defined by Been and Jefferies (1992)

[12]. CRR after Juang 2010 has the following equation:

q c

CRR 0.05 exp A B t 1N (4)

100

Where

A I c , BJ qt 1N 100 14.7 (4.1) (4.1)

B 0.909 I 3 7.47 I 19.28 (4.2)

c , BJ c ,BJ

C 0.059 0.015I 2 (4.3)

c ,BJ

Ic, BJ defined by Been and Jefferies (1992) is

I 3 log Q 1 B 1 2 1.5 1.3

log F

2 (4.4)

c , BJ 10 t q 10 r

qt 1N defined by Robertson (1998) is

q q v 0

P P '

n (4.5)

t 1 N t a a v0

n 0.381I

c , RW

0.05 ' P 0.5

n 1 (4.6)

v 0 a

I c , BW

3.47 log Q 2 1.22 log F 2 (4.7)

10 t 10 R

Where

qc is cone resistance; qt is corrected resistance ( qc 1 a u2 ); a is the area ratio of the cone used (in this

paper, a=0.85); u2 is penetration pore pressure (kPa); Bq is pore pressure parameter = u2 u0 qt v 0 ; u0 is

hydrostatic water pressure (kPa); Qt is normalized cone resistance qt v 0 v' 0 ; R f is friction

ratio f s qc 100% ; Fr is normalised friction ratio f s qt v 0 100% ; Pa= atmospheric pressure (≈100

kPa). 2.3 Calculation of Factor of Safety (FS)

After determine CSR and CRR we can calculate FS which after Lee et al., 2003 is:

FS CRR CSR7.5 (5)

A soil is predicted to liquefy if FS 1.2 (Sonmez, 2003) [13].

3. Probability Calculation

The real probability of liquefaction, PR L, is the probability of liquefaction during the lifespan of an engineering structure for different levels of safety, corresponding to different seismic hazard levels. For a given

hazard seismic level, PR L is calculated by combining the conditional probability of liquefaction for the

corresponding acceleration, PR L PGA a , with the probability of occurrence of the scenario causing the

exceedance of this accerelation PR PGA a.

c, BJ

http://dx.doi.org/10.17758/UR.U0915316 58

P L P L PGA a P PGA a (6)

R

The conditional probability of liquefaction, PR L PGA a, is calculated using the correlation proposed by Juang et al (2006), which represents a direct correlation between the FS obtained in a deterministic way as proposed by Juang and the probability of liquefaction

P L

PGA a 1 with A= 0.74 and B = 5.45 (7)

R

1 FS AB

The seismic hazard curve, is used to determine the probability of exceeding the

acceleration PR PGA aduring the lifespan of the structure (50 years).

After Chen and Juang (2000) the likelihood of liquefaction can be interpreted using the calculated

PR Lvalues in Table 2.

TABLE II: The Classification of Probability of Probability

Probability Description (likelihood of liquefaction)

0.85 PR L 1.00 Almost certain that will liquefy

0.65 PR L 0.85 Very likely

0.35 PR L 0.65 Liquefaction/non-liquefaction is equally

likely

0.15 PR L 0.35 Unlikely

0.00 PR L 0.15 Almost certain that it will not liquefy

It can be seen in Table II that liquefactions will occur only if PR Lgreater than 35% is. The calculation of

PR Lpermits to observe if a layer is susceptible to liquefy during a specific earthquake.

4. Liquefaction Potential Index (IPL)

The liquefaction potential index (IPL) is used to evaluate the ground failure risk. It severity categories were

proposed originally by Iwasaki et al. (1982) [14] and modified by Sonmez in 2003.Te calculation is used the FS (Equation 5) and a depth weighting function W(z) (Equation 8.2). In this way the contribution of the soil liquefaction at different depths to the failure of the ground is estimated. Sonmez (2003) proposed the change of the threshold value of FS between non-liquefiable and liquefiable layers from 1.0 (Iwasaki et al., 1982) to 1.2 and suggested following equations:

20

IPL F (z ) W (z ) dz where FS CRR CSR (8)

0

F (z ) 1 FS for FS 0

F (z ) 2 106 exp(18.427 FS ) for 0.95<FS 1.2 (8.1)

F (z ) 0 for FS 1.2

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qc (MPa) Ic (MPa) FS P[L|PGA=a]% IPL %

0 4 8 12 16 1 2 3 4 0 0.6 1.2 1.8 2.4 3 3.6 0 35 70 105140175 0 15 30 45 60

0 0 0 0 0

1 GW=1.7 m

1

1

1

1

2 2 2 2 2

3 3 3 3 3

4 4 4 4 4

5 5 5 5 5

6 6 6 6 6

7 7 7 7 7

8 8 8 8 8

9 9 9 9 9

10 10 10 10 10

11 11 11 11 11

12 12 12 12 12

13 13 13 13 13

14 14 14 14 14

Cone tip resistance Soil Type Index Factor of Safety Real Probability of Liquefaction

(Mpa ) Liquefaction Potential Index

(Robertson and Wride, 1998) ( Lee et al., 2003) (Juang et al., 2006) (Sonmez, 2003)

Fig. 2: Liquefaction evaluation for Golem area, BH.2

qc (MPa) Ic (MPa) FS P[L|PGA=a]% IPL %

0 4 8 12 16 1 2 3 4 0 0.6 1.2 1.8 2.4 3 3.6 0 35 70 105140175 0 15 30 45 60

0 0 0 0 0

1 GW=1.2 m 1

1 1

2 2 2 2 2

3 3 3 3 3

4 4 4 4 4

5 5 5 5 5

6 6 6 6 6

7 7 7 7 7

8 8 8 8 8

9 9 9 9 9

10 10 10 10 10

11 11 11 11 11

Cone tip resistance Soil Type Index Factor of Safety Real Probability of Liquefaction

(Mpa ) Liquefacti Potential Index (Robertson and Wride, 1998) ( Lee et al., 2003) (Juang et al., 2006) (Sonmez, 2003)

Fig. 2: Liquefaction evaluation for Golem area, BH.8

http://dx.doi.org/10.17758/UR.U0915316 60

W (z ) 10 0.5 z , for 0 z 20m

W (z ) 0, for z 20m (8.2)

z depth (m)

To interpret the obtained values of IPL (Equation 8), a classification proposed by Iwasaki (1982) and

modified by Sonmez (2003) is used. In Table 3 liquefaction potential categories are presented. TABLE III: Liquefaction potential classification proposed by Sonmes (2003)

Liquefaction Potential Index IPL Liquefaction potential category 0 Non-liquefiable (based on FS 1.2 )

0 IPL 2 Low

2 IPL 5 Moderate

5 IPL 15 High

IPL 15 Very high

5. Results

The detailed lithological profiles based on the soil Behavior Type Index Ic for two represented borehole for

Golem area are shown in Fig. 1 and Fig.2. The dashed line green, in Fig.1 and Fig.2, corresponded to a value of Ic 2.8 .This value represents the uppermost limit below which the liquefaction assessment is necessary, Yuang

et al., 2003.In Fig.1 where Ic 2.8 are two intervals to calculate about the liquefaction potential. The first

interval is 0.72-7.97 m depth with 1.31 Ic 2.05 (Sandy to silty sand) and the second interval 7.97-12.78 m

depth with 2.05 Ic 2.60 (Silty sand & sandy silty). In Fig.2 almost in all penetration depth Ic 2.8 . There are

two interval where1.31 Ic 2.05 (Sandy to silty sand).The first one is 0.0-6.46m depth, and the second one is

7.14-10 m depth. The interval where 2.05 Ic 2.60 (Silty sand & sandy silty) is 6.46-7.14 m depth. Factor of Safety, FS<1.2(the dashed line red) is for two interval. In Fig.1 in the interval 8.5-12.7 m depth and the second interval is 3.8-8.2 m depth in Fig.2.

The conditional probability of liquefaction PR L PGA a show that in generally the susceptible layers is

liquefiable. According to the value in Table 2, in Fig.1 for the interval 8.5-12.7 where P[ L PGA a] 100 the

liquefaction is “almost certain that will liquefy”. In the Fig.2 P[ L PGA a] 100 P[ L PGA a] 100 the

liquefaction is “almost certain that will liquefy” for the interval of depth 3.8-8 m depth. Liquefaction potential index that evaluate the ground failure risk has a value >15, that means “very high risk” in the same interval for both Fig1, and Fig.2 where FS<1.2.

6. Conclusions

The Factor of Safety, the conditional probability value obtained for the given seismic hazard level indicate that in generally the liquefaction is almost certain to happen. The liquefaction probabilities attempt such values requiring to consideration of the phenomenon during the design of engineering projects especially because this is an important area of truism of the sea coast line in Albanian. Since this type of soils are met along the coast line of Adriatic Sea a study like this is very important in a large scale.

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Geotechnical. Engineering Division, ASCE, 97 (9), 1249-1273.(Periodical style)

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