Agus Setyo Muntohar – Estimating Ground Settlement Post-Liquefaction Using CPT 1

27-29 May 2010, Jogjakarta, INDONESIA

INTRODUCTION

The Universitas Muhammadiyah Yogyakarta (UMY) campus was constructed on 28 hectare which mostly rested on a sand deposit layer and the ground water table was shallow rela-tively. Theoretically, the site was susceptible to liquefaction during ground-shaking. During earthquake on May 27, 2006, a magnitude Mw 6.3 struck the provinces of Yogyakarta, among those number of seismic damages were re-ported. The damages were identified because of liquefaction phenomena during the ground vibration. Reconnaissance study after the earthquake was found some sand boiling sites

near the Masjid and Library building. This pa-per presents liquefaction evaluation and esti-mation of liquefaction-induced settlement at those building.

Liquefaction-induced ground settlements are essentially vertical deformations of surficial soil layers caused by the densification and compac-tion of loose granular soils following earth-quake loading. Several methods have been proposed to calculate liquefaction-induced ground deformations (Tokimatsu and Seed, 1987; Zhang et al., 2002, Ishihara and Yosemine, 1992). This paper present an esti-mated liquefaction-induces ground settlement by using the method proposed by Zhang et al. (2002). Ground improvement technique by us-

ESTIMATING GROUND SETTLEMENT POST-LIQUEFACTION USING CPT

Agus Setyo Muntohar

Department of Civil Engineering, Universitas Muhammadiyah Yogyakarta, Indonesia

Director of Disaster and Environment Research Institute

E-mail: muntohar@umy.ac.id

ABSTRACT

The Universitas Muhammadiyah Yogyakarta (UMY) campus is constructed on 28 hectare which mostly

rested on a sand deposit layer and the ground water table was shallow relatively. During earthquake

on May 27, 2006, a magnitude 6.3 struck the provinces of Yogyakarta resulted in number of seismic

damages were reported. The damages were identified because of liquefaction phenomena during the

ground vibration. Reconnaissance study after the earthquake was found some sand boiling sites near

the Masjid and Library building. This paper presents liquefaction evaluation and estimation of liquefac-

tion-induced settlement at those building. The liquefaction analysis is based upon force equilibrium

concept using cone penetration test (CPT) method. The ground settlement is calculated from CPT data

which were collected from 9 testing sites at those studied area. The computation shows that the

ground accelerations will make more than 50% of top sand layer liquefied at Library building. In gen-

eral, the estimated ground settlement ranges from 2.5 cm to 13.5 cm at the hazard area. The results

imply that ground improvement should be applied to reduce the effect of liquefaction-induced ground

settlement. After three days of lime-column intallation, the settlement is lower that the tolerable settle-

ment.

Keywords: earthquake, liquefaction, sand, settlement, CPT

2 Enhancing Disaster Prevention and Mitigation @ ICSBE2010 (UII), Jogjakarta, Indonesia, ISBN 978-979-96122-9-8

ing lime-column (LC) is also introduced to re-duce settlement. Trial-field test of LC is pre-sented in this paper.

METHOD OF ANALYSIS

Site Characteristics

Based on the CPT site investigation report, the campus of UMY is covered by 8 m to 10 m thick sandy soil layers. Loose sand layer gen-erally appears at the depth from 3 m to 10 m. The ground water table is at the depth of 0.5 m to 1.0 m. The particle size distribution curves of sites are presented in Figure 1. The average particle diameter D50 of the soil samples range of D50 was between 0.057 mm – 0.841 mm (the mean and variance value was 0.57 mm and 0.054 mm, respectively). Comparing the parti-cle size distribution with the other case histo-ries, it can be concluded from Figure 1 that the studied site is susceptible to liquefaction.

Figure 1 Particle size distribution at the hazard area.

Based on the previous investigation carried out by Muntohar (2009), the critical accelera-tions ranges from 0.34g to 0.69g will generate liquefactions in all depth of loose sand layers in hazard areas.

Liquefaction analysis

Calculation, or estimation, of two variables is required for evaluation of liquefaction re-sistance of soils: (1) the seismic demand on a soil layer, expressed in terms of cyclic stress ratio (CSR) to generate liquefaction; and (2)

the capacity of the soil to resist liquefaction, expressed in terms of cyclic resistance ratio (CRR). The approach requires an estimate of the CSR profile caused by a design earth-quake. A simplified method to estimate CSR was also developed by Seed and Idriss (1971) based on the peak ground surface acceleration (amax) at the site. This simplified approach can be summarized as follows:

max0,65' '

av vod

vo vo

aCSR r

g

τ σ

σ σ

= = ⋅ ⋅

(1)

where τav is the average cyclic shear stress; amax is the maximum horizontal acceleration at the ground surface; g = 9.81 m/s

2 is the accel-

eration due to gravity; σvo and σ'vo are the total and effective vertical overburden stresses, re-spectively; and rd is a stress-reduction factor which is dependent on depth. The factor rd can be estimating using the following bi-linear func-tion, which provides a good fit to the average of the suggested range in rd originally proposed by Seed and Idriss [2], that is

1 0,00765 ; if 9,15 m

1,174 0,0267 ; if 9,15 23 md

z zr

z z

− <=

− ≤ ≤ (2)

By using the CPT data, the estimated CRR was calculated from Robertson and Campanella (1985). The flow chart for calcula-tion is shown in Figure 1. The factor of safely against liquefaction (FSL) is defined as:

7.5

L

CRRFS MSF

CSR= ⋅ (3)

where MSF is the Magnitude Scaling Factor to convert the CRR7.5 for M = 7.5 to the equivalent CRR for the design earthquake. The recom-mended MSF is given by:

2.56

174

w

MSFM

= (4)

The equation (4) is based on the NCEER Workshop in 1996 (Youd et al., 2001).

Ground settlement estimation

The ground settlement for level ground can be estimated by using the following equation :

1i

j

v i

i

S zε

=

= ∆∑ (3)

Agus Setyo Muntohar – Estimating Ground Settlement Post-Liquefaction Using CPT 3

Figure 2 CRR estimation from CPT (Modified after Robertson and Campanella, 1983).

where S is the calculated liquefaction-induced

ground settlement at the CPT location; εvi is the postliquefaction volumetric strain for the soil sublayer i; ∆zi is the thickness of the sublayer i; and j is the number of soil sublayers.

The post-liquefaction volumetric strain is es-timated by semi empirical equation. The equa-tion correlates (qc1N)cs and postliquefaction vol-umetric strain (εv) for different FSL. The correlation is shown graphically in Figure 3.

RESULTS AND DISCUSSION

The liquefaction analysis is based upon force equilibrium concept using Robertson and Campanella (1985) method. The ground set-tlement is calculated from cone penetration test data which is collected from 9 testing sites at library buiding. The result of the calculated set-

tlement, for the instance at SB1 point of Library area, is shown in Figure 4. The varation of the estimated ground settlement is shown in Figure 5. For each tested sites, the maximum settle-ment at ground surface is presented in detail in Table 1.

Table 1 Estimated ground settlement Location SB1 SB2 SB3 SB4 SB5 amax (g) 0.43 0.57 0.56 0.27 0.43 S (cm) 2.5 10.5 2.7 11.4 12.2

Location SB6 SB7 SB8 SB9 amax (g) 0.54 0.39 0.42 0.53 S (cm) 4.6 4.7 7.1 13.5

Note: the amax used in this study is the average value at each tested CPT location

Figure 3 Relationship between postliquefaction vol-umetric strain and equivalent normalized CPT tip re-sistance for different factors of safety (Zhang et al.,

2002)

In general, the estimated ground settlement ranges from 2.5 cm to 13.5 cm. The maximum settlement occurs at the surface up to 3 m of sub surface (see Figure 5). According tothe cri-teria of Terzaghi et al. (1996), the allowable settlement of foundation is 2 cm for diffential settlement and 2.5 cm for total settlement. The results imply that ground improvement should be applied to reduce the effect of liquefaction-induced ground settlement.

4 Enhancing Disaster Prevention and Mitigation @ ICSBE2010 (UII), Jogjakarta, Indonesia, ISBN 978-979-96122-9-8

S (cm)

0 5 10 15

qc (MPa)

0 20 40 60

Dep

th,

z (

m)

0

5

10

15

20

FR (%)

0 2 4 6 8

Ic

0 2 4

(qc1N)cs

0 100 200

CRR7.5/CSR

0.0 0.5 1.0

FS

0 1 2

Figure 4 Example plots illustrating the major procedures in estimating liquefaction-induced ground settle-ments using the proposed CPTbased method.

Figure 5 Variation of estimated ground settlement with depth.

A lime-column method was applied for trial-test at the field. The diameter and length of column was designed to 6 inch and 2 m re-spectively. After installation of the lime column, the ground settlement at the trial-test point is estimated to diminish. The estimated ground settlement is shown in Figure 6. Calculation re-sults in Figure 6 show that installation of lime column reduces significantly the ground set-tlement. The settlement is observed, from Fig-ure 6, to decrease in associated with the age of lime column. After one day of installation, the ground settlement decrease to about 1 cm and 2.8 cm for amax = 0.34 g and 0.69 g respective-

ly. In general, after three days of intallation the settlement is lower that the tolerable settle-ment. This result indicates that lime-column technique play a significant role to reduce liq-uefaction-induced ground settlement.

Figure 6 Estimated ground settlement before and af-ter LC intallation (a) amax = 0.34g, (b) amax = 0.69g.

CONCLUDING REMARKS

The computation shows that the ground accel-erations will make more than 50% of top sand layer liquefied at Library building. In general, the estimated ground settlement ranges from 2.5 cm to 13.5 cm at the hazard area. The re-sults imply that ground improvement should be applied to reduce the effect of liquefaction-

Agus Setyo Muntohar – Estimating Ground Settlement Post-Liquefaction Using CPT 5

induced ground settlement. After three days of intallation the settlement is lower that the toler-able settlement. This result indicates that lime-column technique play a significant role to re-duce liquefaction-induced ground settlement.

ACKNOWLEDGMENT

This paper is part of the research funded by Universitas Muhammadiyah Yogyakarta through research grant “Kompetisi Penelitian Dosen” in 2008-2009. Author is thankfull to Ario Muhammad, Setia Dinoor, Ekrar Oktoviar, and Damanhuri for their assistance during the trial-field test.

REFERENCES

Ishihara, K., and Yoshimine, M. (1992). “Evalu-ation of settlements in sand deposits follow-ing liquefaction during earthquakes”. Soils and Foundations, Vol. 32(1): 173–188.

Muntohar, A.S. (2009). “Evaluation of Peak Ground Acceleration Using CPT Data for Liquefaction Potential”. Proceeding of the 4th Annual International Workshop & Expo on Sumatra Tsunami Disaster & Recovery 2009, November 23-25, 2009, Banda Aceh.

Robertson, P. K., and Campanella, R. G. (1985). “Liquefaction potential of sands us-ing the cone penetration test”. Journal of

Geotechnical Engineering, ASCE, Vol. 22(3): 298-307.

Seed, H.B., Idriss, I.M., (1971). "Simplified Procedure for Evaluating Soil Liquefaction Potential". Journal of Soil Mechanics and Foundation Engineering, ASCE, Vol. 97(9):1249-1273.

Terzaghi, K., Peck, R., Mesri, G., (1996). “Soil Mechanics in Engineering Practices”, John Wiley & Sons Inc., New York, USA

Tokimatsu, K., Seed, H.B. (1987). “Evaluation of settlements in sands due to earthquake shaking”. Journal of Geotechnical Engineer-ing, ASCE, Vol. 113(8): 861–879.

Youd, T.L., Idriss, I.M, Andrus, R.D., Arango, I., Castro, G., Christian, J.T., Dobry, R., Finn, W.D.L., Harder, L.F., Hynes, M.E., Ishihara, K., Koester, J.P., Liao, S.S.C., Marcuson, W.F., Martin, G.R., Mitchell, J.K., Moriwaki, Y., Power, M.S., Robertson, P.K., Seed, R.B., Stokoe, K.H. (2001). “Liquefaction re-sistance of soils: summary report from the 1996 NCEER and 1998 NCEER/NSF Work-shops on evaluation of liquefaction re-sistance of soils”. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 127(4): 297 – 313.

Zhang, G., Robertson, P.K. and Brachman, R.W.I. (2002). “Estimating liquefaction-induced ground settlements from CPT for level ground”. Canadian Geotechnical Jour-nal, Vol. 39: 1168-1180.