comparison of observed and calculated earthquake- …€¦ · comparison of observed and calculated...

Comparison of observed and calculated earthquake-induced settlements at 6 sites in Christchurch, NZ

S. A. Bastani

GMU Geotechnical, Inc., Rancho Santa Margarita California, USA

ABSTRACT: The 2010 and 2011 earthquakes near Christchurch caused widespread liquefaction of the recent sandy fills and young loose sediments in the city and its suburbs. Detailed geotechnical investigations were performed at 6 liquefied sites with various magnitudes of earthquake-induced settlements and deformations. These sites were characterized with a minimum of 4 CPTs and 1 or 2 geotechnical borings with SPTs. This paper presents results of the field and laboratory index tests providing 6 well documented liquefaction case histories. The observed earthquake-induced settlements after the February 22, 2011 earthquake are compared with earthquake-induced settlements estimated utilizing widely used standard of practice empirical relationships.

1 INTRODUCTION

Several simplified empirical relationships have been developed for liquefaction triggering evaluation based on a number of liquefied and non-liquefied case histories from previous earthquakes. High quality liquefaction cases for medium dense sands and high cyclic stress ratio (CSR) are rare. This paper presents 6 high quality case histories (Figure 1) with SPT and CPT measurements and laboratory index tests in the critical layers. The encountered material included loose to dense silty sands, sands, and gravels. The granular soils were interbedded with low plasticity fine grained soils with occasional layers of organic and high plasticity fines. High quality case histories will help in refining the simplified deterministic and probabilistic empirical liquefaction evaluation methods.

This paper primarily focuses on presenting liquefaction case histories, earthquake-induced ground settlements, and comparison of the measured and estimated settlements at the sites. Visual examinations and subsequent field investigations at the sites were performed between December 2011 and April 2012. The field investigations were conducted after the December 23, 2011 aftershocks. Visual observations and communications with the property owners indicated that all sites liquefied during the February 22, 2011 Christchurch Earthquake (Mw = 6.1). The liquefaction triggering of the sites were evaluated with 5 different widely used CPT-based methodologies. A single approach (Zhang et al., 2002) based on Ishihara and Yoshimine (1992) was used to estimate the earthquake-induced settlements with different liquefaction triggering methods.

Figure 1. Site location plan and selected strong motion stations.

2 SITE GEOLOGY

Geology of the city of Christchurch is predominately controlled by fluvial processes. Prior to development, the Christchurch area was underlain by river and floodplain deposits along with deposits associated with swamps, beach dunes, estuaries, and lagoons of Pegasus Bay. The younger deposits (i.e. post-glacial) are about 15 to 40 m thick and overlay the Riccarton Gravel, which is the uppermost gravel of the older (i.e. glacial) deposits and the topmost aquifer (Cubrinovski et al., 2011).

River realignment and stopbank construction has been occurring since the city was established in 1850 which has resulted in abandoned and infilled paleo river channels and former swamps. These areas of young, loose, and soft sediment, along with shallow groundwater, contributed to widespread liquefaction, lateral spreading, and settlement that occurred during the Darfield and Christchurch earthquakes.

3 CASE HISTORIES

High quality data were collected at 6 sites, 5 were in or near the CBD, and 1 (site 4) was located in the Ferrymead area, east of Christchurch. At least 4 CPTs were performed at each site and were complimented with 1 or 2 borings with SPT. The borings were drilled using the Sonic Drilling method. Soil gradation and Atterberg Limits were performed to evaluate susceptibility to liquefaction of the low plasticity fines and wherever soil classifications were in doubt. The boring diameters and hammer efficiency for each site are presented in Table 1. Figures 2 through 7 present the uncorrected CPT tip resistance and friction resistance, measured SPT N-values, moisture contents, Atterberg Limits, % fines, and % 2μ for Sites 1 through 6, respectively. The hammer efficiencies of the automatic hammers were based on the

factory measurements except at Site 3 where a winch and steel cable auto hammer was used. All 6 locations liquefied during the February 22, 2011 earthquake and signs of sand and silt ejecta were observed during the site visit several months after the earthquake. Due to delayed sites visits, evaluation of the volume of sand ejecta after the earthquake could not be accurately done. Based on the observations and laboratory results, the underlying liquefiable sands were generally fine grained sands. The sand layers were generally interfingered with fine grain soils. The fine grain soil layers were generally non- to low-plasticity silts, some rich organic layers, and occasionally high plasticity silts and clays.

Table 1. Borehole diameter and hammer efficiency

Site Boring Size

(in) Hammer

Efficiency (%)Site

Boring Size (in)

Hammer Efficiency (%)

1 3.25 0.94* 4 4 0.85*

2 3.25 0.94* 5 3.54 0.94*

3 3.54 0.60+ 6 3.54 0.94* * Auto trip hammer reported by drill rig manufacturer. + Winch with steel cable with automatic trip hammer, unreliable hammer efficiency.

0.6 0.4 0.2 0Friction (MPa)

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CPT-2

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Z5-1

B-1

B-2

0 20 40 60 80 100Atterberg Limits

0 10 20 30 40 50Moisture Content (%)

B-1 PL

B-1 LL

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B-2 PL

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B-2 Mc

0 20 40 60 80 100%Fines

0 20 40 60 80 100% 2μ

B-1 %Fines

B-2 %Fines

B-1 %2μB-2 %2μ

Drilled Gravel Layer

Figure 2. Site 1 soil profile and laboratory data.


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0 20 40 60 80 100% 2μ

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B-1 %2μ



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B-1 PL

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B-2 PL

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B-1 McB-2 Mc

0 20 40 60 80 100%Fines

0 20 40 60 80 100% 2μ

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Drilled Gravel Layer



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0 20 40 60 80 100% 2μ

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4 GROUND MOTION

For the purpose of this paper, the ground motion stations in the Christchurch area in vicinity of the sites were reviewed. The criterion was to choose free field ground motion stations in the vicinity of the sites with similar subsurface conditions. The peak ground accelerations were estimated based on the measured peak horizontal ground accelerations (PHGA). No strong motion station was available in vicinity of Site 4 at the time of the February 22, 2011 earthquake. Therefore, the PHGA provided at the Canterbury Geotechnical Database was used for Site 4.

A number of strong motion stations were triggered (Figure 1) during the Christchurch February 22, 2011 earthquake using New Zealand GeoNet (2012) database. The maximum of the two horizontal components of recorded accelerations in accordance with the original simplified liquefaction procedures was used in the liquefaction analyses. PHGAs varied from 0.49g (CCCC-NS) to 0.73g (REHS-EW). Low frequency cycles with dampened acceleration amplitudes occurring after liquefaction were observed in all acceleration time histories in the area. PHGAs used for each site are presented on Figure 1.

5 LIQUEFACTION EVALUATION

Five different methods were utilized to evaluate the liquefaction potential at the subject sites. These methods included Robertson and Wride (R&W, 1998), Moss, et al. (2006), Idriss and Boulanger (I&B, 2008), Robertson (2009), and Te Tari Kaupapa Whare Department of Building and Housing (DBH, 2012). DBH (2012) recommended calculating the liquefaction triggering using the Idriss & Boulanger 2008 method,

combined with fines content values using Robertson and Wride (1998) wherever fine contents not measured. The Idriss and Boulanger (2008) method used in this study used either the measured fine contents values (Idriss & Boulanger 2008) or values estimated from the CPT based on the Robertson and Wride (1998) correlation (DBH 2012).

The groundwater table was estimated based on the pore pressure dissipation tests from the CPTs and the measured groundwater depth in the borings at the time of the field investigation. Significant artesian pore water pressure was observed in these areas. The considered groundwater depths varied between 1.0m and 2.7m.

CLiq version 1.7.4.21 software (GeLogismiki, 2006) was used to evaluate the liquefaction potential of the 6 sites. All methods predicted that the sites would liquefy during the February 22, 2011 earthquake.

6 EARTHQUAKE-INDUCED SETTLEMENTS

The earthquake-induced settlements were dominated by liquefaction-induced settlements at these sites and the earthquake-induced compaction of the unsaturated soils was expected to be negligible. The five different CPT-based liquefaction triggering methods were combined with the Zhang, et al. (2002) method for estimating earthquake-induced settlements. Earthquake-induced settlements were also estimated using the SPT-based method by Youd, et al. (2001) for liquefaction triggering combined with Ishihara and Yoshimine (1992). The estimated surface settlements calculated by the different methods are summarized in Table 2. The estimated settlements used the data for the full depth of CPTs and borings. In general, Robertson and Wride (1998) and Robertson (2009) liquefaction triggering resulted in relatively similar earthquake-induced settlements due to their similar basis. The estimated settlements based on the SPT had a wider range of over or under estimations relative to the CPT based estimations due the discontinuous nature of the blowcounts and consequently the liquefiable layer thickness. In general, Moss, et al. (2006) triggering method resulted in higher settlements than the other triggering methods. The Canterbury Earthquake Recovery Authority has compiled a comprehensive Google Earth geotechnical database. This database also includes elevation differences that contributed to earthquake-induced settlements for the different Christchurch events. The estimated elevation differences were based on a pre-earthquake airborne LiDAR survey and LiDAR surveys after each earthquake event. Bare earth or terrain models were created by removing points for structures and vegetation that were judged to be higher than 0.5m above the surrounding ground. Then, a Digital Elevation Model (DEM) was developed from each LiDAR set by averaging the ground-return elevations within a 10m radius of each grid point. All of these DEM's used a common 5m grid and used either moving averages or windowed averages. The DEMs were provided in 10cm color intervals. The estimated earthquake settlement at each CPT was normalized with the average of the observed settlement interval by LiDAR at each site in Figure 8 for the February 22, 2011 earthquake. The normalized minimum, average, and maximum settlements are plotted for different liquefaction triggering methods at each site based on the CPT data. Due to similar predications using the Robertson (2009) and Robertson and Wride (1998) methods, only the Robertson (2009) values are

shown in Figure 8. The average predictions were bounded by about 50 percent under prediction at Site 3 to 250 percent over prediction at Site 6. The Robertson (2009) (and R&W 1998) method for liquefaction triggering combined with Zhang, et al. (2002) for liquefaction-induced settlements resulted in the narrowest band for the prediction of earthquake-induced settlements in this exercise.

7 SUMMARY

A large number of high quality soil liquefaction cases have been generated by the 2010-11 earthquakes at Christchurch area. The affected areas were surveyed by LiDAR and subsurface investigations were performed before and after events creating one of the most significant liquefaction databases available to the geotechnical engineering community. This paper presents 6 high quality case histories that have from 4 to 8 CPTs and 1 to 2 borings with SPTs and associated laboratory index tests at each site.

The sites were analyzed using 5 different CPT-based liquefaction triggering methods. All methods resulted in similar stratigraphy for liquefiable soils. However, the calculated earthquake-induced settlements were more variable when the same methodology for liquefaction-induced volumetric strain (Zhang et al., 2002) was used with the different triggering methods. The calculated settlements based on the SPT results resulted in an even wider range of earthquake-induced settlements. On average, the earthquake-induced settlements were under predicted 19% by the Robertson & Wride (1998) and Robertson (2009) triggering methods and were over predicted 39%, 19%, and 26% by Moss, et al. (2006), Idriss and Boulanger (2008), and DBH (2012) triggering methods, respectively. The Robertson & Wride (1998) and Robertson (2009), Moss, et al. (2006), Idriss and Boulanger (2008), and DBH (2012) triggering methods also resulted in normalized earthquake-induced-settlement standard deviations of 40%, 40%, 68%, 67%, and 65%, respectively.

The author acknowledges that evaluation of finite deformations due to liquefaction is generally a difficult task, and other variables such as sand ejecta and building loads make the estimation of seismic settlements further complicated. It is also understand that the subsurface soil conditions may have been altered due to the aftershocks since the February 22, 2011 earthquake. Since the survey benchmarks had settled due to the earthquake(s) and accurate survey of the total settlements were not available, the estimated earthquake-induced settlements based on LiDAR were the only available measure of the total earthquake-induced settlements at Christchurch.

The estimated earthquake-induced settlements presented in this paper consider the post-earthquake reconsolidation of the liquefied soils and do not consider the effects of sand boils or adjacent building loads. However, the sand boils and building loads generally exacerbate the actual seismic settlements. Therefore, the calculated seismic settlements based on soil reconsolidation should be lower than the actual ones wherever sand boils and/or building loads are present. This paper compared the predicted seismic settlements as generally applied in geotechnical practice with the measured earthquake-induced settlements, which showed that the current estimations correlated reasonable well to the observed values. However, large variations should be expected and addressed by additional field investigations for subsurface conditions such as Christchurch.

Table 2. Estimated EQ-induced settlements using liquefaction triggering methods.

SITE-1

CERA Estimated Range Based on LiDAR: 10-20 cm, USED 15cm for Normalization SITE-4 CERA Estimated Range Based on LiDAR: Not Available

Method/Location CPT-1 CPT-2 CPT-3 CPT-4 CPT-5 Z5-1 B-1 B-2 Method/Location CPT-1 CPT-2 CPT-3 CPT-4 B-1 R&W 1998 15.9 15.4 8.5 12.6 7.8 12.6 R&W 1998 41.3 37.6 34.9 41.49

Robertson 2009 17.2 16.1 8.6 12.7 7.8 12.9 Robertson 2009 43 39.2 35.9 44.6

Moss 2006 28.4 24.2 20.2 23.3 20.1 26.3 Moss 2006 59.8 49.2 48.9 64

DBH 2012 19.2 19.9 13.6 13.7 15 14.3 DBH 2012 59.8 57.8 53.2 62.1

I&B 2008 17 15.9 13.2 11.8 12.5 14.1 I&B 2008 54.3 55.3 48.1 53.7

Y&I 2001 29.4 21.1 Y&I 2001 80

SITE-2 CERA Estimated Range Based on LiDAR: 10-30 cm, USED 20cm for Normalization SITE-5

CERA Estimated Range Based on LiDAR: 20-30 cm, USED 25cm for Normalization

Method/Location CPT-1 CPT-2 CPT-3 CPT-4 B-1 Method/Location CPT-1 CPT-2 CPT-3 CPT-4 CPT-5 CPT-6 CPT-7 CPT-8 B-1 B-2 R&W 1998 15.9 9 11.4 9.3 R&W 1998 5.8 6.1 5.7 9.2 3.5 11.3 8.4 8.2

Robertson 2009 16.5 9.2 11.4 9.4 Robertson 2009 6.4 6.4 6.3 9.4 4 11.8 9.3 8.7

Moss 2006 28.4 16.6 23.2 18.8 Moss 2006 8.2 15.2 11.1 15 6.3 19.7 15.6 13

DBH 2012 24.9 14 17.3 16.1 DBH 2012 11.2 10.7 12.3 16.8 8.8 17.4 18.4 16.5

I&B 2008 23.2 12.5 15.2 13.3 I&B 2008 7.5 8.7 12.9 14.2 6.1 15.1 12.8 13.4

Y&I 2001 21.5 Y&I 2001 4.4 4.2

SITE-3 CERA Estimated Range Based on LiDAR: 0-20 cm, , USED 10cm for Normalization SITE-6

CERA Estimated Range Based on LiDAR: 0-20 cm, USED 10cm for Normalization

Method/Location CPT-1 CPT-2 CPT-3 CPT-4 CPT-6 CPT-7 CPT-8 B-1 B-2 Method/Location CPT-1 CPT-2 CPT-3 CPT-4 B-1 R&W 1998 2.7 5.2 5.6 13.2 12.2 4.6 10.3 R&W 1998 12.4 15.2 12.7 10.7

Robertson 2009 2.7 6.2 6.9 15.5 14.6 5.9 12.8 Robertson 2009 13.4 16 13.3 12.3

Moss 2006 6.5 9.4 10.3 23.3 19.9 10 16.7 Moss 2006 22.5 26.9 26.2 19.6

DBH 2012 4.2 11.2 12.8 23.6 24 12.9 20.5 DBH 2012 22 26 20.6 20.8

I&B 2008 4.3 11.8 13.4 24.7 26.5 13.1 22.4 I&B 2008 20.5 24.5 19.9 18.8

Y&I 2001 40.9* 58.5* Y&I 2001 40

* Auto Trip Hammer with Steel wire pulley

Note: R&W (1998): Robertson & Wride (1998), I&B (2008): Idriss & Boulanger (2008), Y&I (2001): Youd & Idriss (2001).

0 1 2 3 4 5 6

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Liquefaction Triggering Method:Moss et al. (2006)

Idriss and Boulager (2008)

Robertson (2009)

DBH (2012)

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Figure 8. Comparison of liquefaction triggering methods compared to observed settlements

8 ACKNOWLEDGMENT

The author would like to thank Hushmand Associates, Inc. for providing the data presented in this paper and Professor Robertson and Mr. Gregory Silver for reviewing this manuscript.

9 REFERENCES

Department of Building and Housing, Te Tari Kaupapa Whare, 2012. Interim guidance for repairing and rebuilding foundations in Technical Category 3, 106 pp.

CLiq, v.1.7.4.14, 2012, Software for Liquefaction Potential Evaluation using Cone Penetration Tests and Standard Penetration Tests, Prepared by GeoLogismiki Geotechnical Software.

Cubrinovski, M., Green, R. A., and Wotherspoon, L., 2011, Geotechnical Reconnaissance of the 2011 Christchurch, New Zealand Earthquake, 184 pp.

GeoNet, 2012. The Official Source of Geological Hazard Information for New Zealand, http://geonet.org.nz/.

Idriss, I. M., and Boulanger, R. W., 2008. Soil Liquefaction during Earthquakes, Earthquake Engineering Research Institute MNO-12, 237 pp.

Ishihara, K. and Yoshimine, M., 1992, Evaluation of Settlements in Sand Deposits Following Liquefaction during Earthquakes, Soils and Foundations, Vol. 32, No. 1, pp: 173-188.

Moss, R. E. S., Seed, R. B., Kayen, R. E., Stewart, J. P., Der Kiureghian, A., Cetin, K. O., CPT-Based Probabilistic and Deterministic Assessment of In Situ Seismic Soil Liquefaction Potential, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 132, No. 8, August 1, 2006.

Robertson, P. K., and Wride, C. E., 1998, [better to reference the main Can Geot Journal paper in 1998] Cyclic liquefaction and its Evaluation based on the SPT and CPT, in Proceeding of the NCEER Workshop on Evaluation of Liquefaction Resistance of Soils, pp. 41-87.

Robertson, P. K. (2009). Interpretation of Cone Penetration Tests-a Unified Approach, in Canadian Geotechnical Journal, Vol. 46, No. 11, pp: 1337-1355.

Seed, H. B. and Idriss, I. M. (1971). Simplified Procedures for Evaluating Soil Liquefaction Potential, J. Geotech. Engrg. Div., ASCE 97(9), pp. 1249-1273.

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., and Stokoe, K. H., 2001. Liquefaction Resistance of Soils: Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshop on Evaluation of Liquefaction Resistance of Soils, Journal of Geotechnical and Geoenvironmental Engineering, vol. 127, No. 10: pp. 817-833.

Zhang, G., Robertson, P.K., Brachman, R., 2002. Estimating Liquefaction Induced Ground Settlements from the CPT, Canadian Geotechnical Journal, 39: pp. 1168-1180.

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