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Mellsop, N., Bastin, S., Wotherspoon, L.M. & van Ballegooy, S. (2017) Development of detailed liquefaction case histories from the 1987 Edgecumbe Earthquake Proc. 20th NZGS Geotechnical Symposium. Eds. GJ Alexander & CY Chin, Napier

Development of detailed liquefaction case histories from the 1987 Edgecumbe Earthquake

N Mellsop Tonkin + Taylor, Auckland [email protected] (Corresponding author)

S Bastin QuakeCoRE, University of Canterbury, Christchurch [email protected]

L Wotherspoon University of Auckland, Auckland [email protected]

S van Ballegooy Tonkin + Taylor, Auckland [email protected]

Keywords: liquefaction, case histories, Edgecumbe earthquake

ABSTRACT

Liquefaction and associated lateral spreading during the 1987 Mw 6.5 Edgecumbe earthquake caused severe damage within parts of the Whakatane township. Liquefaction primarily manifested proximal to the Whakatane River in areas underlain by recent fluvial and marine sediments. The development of ground motion intensity and groundwater models for the Whakatane region for the Edgecumbe earthquake enabled CPT based liquefaction assessments to be applied using an extensive CPT dataset. Liquefaction assessments undertaken using the median PGA model and median groundwater model were found to closely correspond with the observed severity of liquefaction manifestation at sites known to have surface manifestation. However, significant over prediction of manifestation severity was evident in the Central Business District (CBD) under these conditions. Sensitivity analyses of the PGA and groundwater models were not able to account for these issues, with reduction in PGA and lowering of water table providing some improvement in areas without manifestation while at the same time providing underestimates in areas with manifestation. Overall, the findings suggest that standard CPT based methods of liquefaction assessment may be causing conservative predictions in the Whakatane CBD; further research is required to examine potential factors behind the inconsistent predictions.

1 INTRODUCTION

Earthquake-induced liquefaction and associated lateral spreading pose a significant hazard to the built environment. It is critical that liquefaction hazards are able to be adequately assessed so that the hazard can be effectively managed and the associated impacts minimized. The simplified liquefaction triggering procedures for assessing liquefaction hazards have been derived from liquefaction case histories collected following large earthquakes (e.g. Idriss and Boulanger, 2008 and Boulanger and Idriss, 2014). Cases where liquefaction is predicted by the simplified procedures, yet was not observed, provide important insights into the limitations of the current assessment methodologies, including their applicability in various soil types.

Mellsop, N., Bastin, S., Wotherspoon, L.M. & van Ballegooy, S. (2017) Development of detailed liquefaction case histories from the 1987 Edgecumbe Earthquake

The 1987 MW 6.5 Edgecumbe earthquake caused localised liquefaction and lateral spreading in parts of the Rangitaki Plains in the Bay of Plenty, New Zealand. Liquefaction and lateral spreading resulted in severe damage to infrastructure and lifelines, including NZ$10 million worth of damage to flood control and drainage schemes within the region (Pender & Robertson, 1987; Christensen, 1995; Dowrick & Rhoades, 1990; Berrill, et al., 2001). The township of Whakatane experienced localized liquefaction and lateral spreading proximal to the Whakatane River, however no liquefaction was reported within the in the Central Business District (CBD).

In this paper, the extents of liquefaction manifestation within Whakatane for the Edgecumbe earthquake as collated from historical photographs, technical reports, and publications are presented. The collated extents are then compared with the predicted liquefaction severity estimated from the extensive CPT dataset collated for the Whakatane township using modelled peak ground accelerations (PGA) and the depth to groundwater at the time of the earthquake. Three peak ground acceleration models and three groundwater models were developed for the Whakatane region to account for the uncertainty in these variables during the earthquake (a median, lower estimate and upper estimate).

2 LIQUEFACTION OBSERVATIONS

Observations of liquefaction related land damage, including sand ejecta and lateral spreading associated with the Edgecumbe earthquake sequence, were compiled from technical reports, publications, and historical photographs (i.e. Pender & Roberson (1987), Franks et al. (1989) and Jennings et al. (1988)). The records were digitally compiled and presented geospatially for the purpose of this research (Figure 1). The accuracy in location and extent of the observations varies significantly as a function of the method of observations. For example, observations transcribed from aerial photographs onto hand drawn maps are significantly more innacurate than ejecta locations that have been digitised through precise descriptions of where samples have been taken (Such as in Pender & Robertson, (1987)) and near field photographs where landmarks are visible. Because of the above two categories have been chosen to distinguish between the confidence in the location of liquefaction manifestation, these being “Liquefaction Manifestation Confirmed” and “Possible Liquefaction Manifestation”.

The most severe liquefaction manifestation in Whakatane occurred in point-bar deposits within the inside bends of the Whakatane River, particularly the James Street Loop and the Netball Courts, and in paleo-channel deposits along the western side of the Landing Road Bridge (SH2). It is noted that there was no evidence of liquefaction having occurred throughout the Central Business District (CBD). Areas of liquefaction manifestation in Whakatane are summarised in Figure 1.

3 PEAK GROUND ACCELERATION MODEL DEVELOPMENT

The ground motion characteristics of the Edgecumbe earthquake were recorded by four strong motions stations (SMSs) within approximately 100 km of the causative fault plane, however there were no SMSs located within Whakatane. The closest SMS was Matahina Dam, which was approximately 11 km from the fault plane recorded a geometric mean PGA of 0.26g, while the western edge of Whakatane was approximately 9 km from the fault plane.

In order to estimate the Peak Ground Accelerations (PGAs) in Whakatane a number of ground motion models (GMMs) were assessed. From these, the Bradley (2013) GMM was chosen as the recorded PGA was well approximated by the 79th percentile of the Bradley GMM at each SMS location. Using this recorded PGA data, this percentile was then assumed to represent the median PGA for the Edgecumbe earthquake and the inter-event uncertainty was removed. To represent the uncertainty in the PGA only the intra-event standard deviation was used. This approach was

Mellsop, N., Bastin, S., Wotherspoon, L.M. & van Ballegooy, S. (2017) Development of detailed liquefaction case histories from the 1987 Edgecumbe Earthquake taken as in the absence of any recorded data the combined inter- and intra-event uncertainty would result in a wide distribution of potential PGA values in Whakatane. By assigning a modified median representative of this event, the uncertainty reduces and is expected to be more representative of the actual PGA experienced in Whakatane.

The 15th and 85th percentile of the new probability distributions were used to represent upper and lower estimates of the PGA across the region. To provide another assessment of the accuracy of this model, the median PGA models was converted to equivalent MMI using the PGA-MMI relationships of Wald et al. (1999). The resulting MMI values were shown to correlate well with the reported MMI values across Whakatane. From this model a geospatial representation of the PGA across Whakatane was developed, with contours representing the respective PGA estimates presented in Figures 1-3.

4 GROUNDWATER MODEL DEVELOPMENT

Groundwater at the time of the Edgecumbe earthquake was modelled using a kriging interpolation method to develop a groundwater elevation surface between depth to groundwater at monitoring wells and river levels. The groundwater elevation surface was subsequently subtracted from a digital elevation model (DEM) to build a model of groundwater depth. River level data from 1956 to present from monitoring stations was used to derive a river level model at 50 m increments along the river assuming a constant gradient between the recording stations. Using groundwater data available from a range of sources, a correlation was identified indicating that groundwater is governed by the river levels. The groundwater data from more recent investigations were subsequently adjusted to levels expected during the Edgecumbe earthquake based on the river levels at the time of the earthquake. The standard deviation of the correlation between the river levels and groundwater levels was used to create upper and lower depth to groundwater estimates. Two standard deviations were added for the upper estimate of the depth to groundwater, while two standard deviations were subtracted for the lower estimate of the depth to groundwater. A more comprehensive description of the development of the groundwater models is discussed by Mellsop (2017).

5 LIQUEFACTION ASSESSMENT

The factor of safety against liquefaction was evaluated with depth at each sounding location using the Boulanger & Idriss (2014) liquefaction triggering methodology to evaluate the likelihood of liquefaction. The soil’s fines content (FC) was estimated using the default Boulanger and Idriss (2014) FC-Ic correlation with the CFC fitting parameter set to zero. The cyclic resistance ratio (CRR) curves for a probability of liquefaction (PL) of 15% were adopted for the liquefaction triggering analyses. Soil layers with Ic values greater than 2.6 were considered plastic in behaviour to liquefy (Robertson and Wride, 1998).

Predicted land damage was subsequently calculated for each CPT using the Liquefaction Severity Number (LSN) (van Ballegooy, et al., 2014) and Liquefaction Potential Index (LPI) (Iwasaki, et al., 1984) liquefaction manifestation severity parameters. Previous studies have shown a good correlation between the LSN and LPI with observed liquefaction manifestation (Juang, et al., 2005a; Juang, et al., 2005b; Toprak & Holzer, 2003; Maurer, et al., 2014; van Ballegooy, et al., 2014). These studies generally find that LSNs greater than 16 and LPIs greater than 5 coincide with minor to moderate liquefaction manifestation, while LSNs and LPIs greater than 26 and 15, respectively align with moderate to severe liquefaction manifestation. It is important to acknowledge that the LPI and LSN are not intended to consider lateral spreading, and therefore may not account for the liquefaction severity observed proximal to the Whakatane River. However, the collated reports indicate sand boils formed in the flat land adjacent to the lateral spread sites and thus should correspond with LSNs and LPIs higher than 16 and 5 respectively.

Mellsop, N., Bastin, S., Wotherspoon, L.M. & van Ballegooy, S. (2017) Development of detailed liquefaction case histories from the 1987 Edgecumbe Earthquake

Figure 1: Map of CPT-based LSN values across Whakatane resulting from the liquefaction assessment undertaken using the median PGA model and median groundwater model.

Figure 2: Map of CPT-based LPI values across Whakatane resulting from the liquefaction assessment undertaken using the median PGA model and median groundwater model.

Mellsop, N., Bastin, S., Wotherspoon, L.M. & van Ballegooy, S. (2017) Development of detailed liquefaction case histories from the 1987 Edgecumbe Earthquake

The resultant LSN and LPI calculated using the median PGA and groundwater models are presented in Figure 1 and Figure 2, respectively. The mapped extent of liquefaction manifestation is summarised in these figures with the contours of the PGA model for each. Each point in these figures corresponds to a single CPT sounding and the corresponding predicted manifestation severity (green=none, orange=minor-moderate, red=moderate-severe). High LSN and LPI values are generally observed within “liquefaction manifestation confirmed” zones and thus correlate well with the observations. Outside of the CBD in areas with no observed manifestation the estimated LPI and LSN values matched the observed values. However, high LSN and LPI values were predicted within the CBD area in which no liquefaction was observed during the Edgecumbe earthquake. Some of this discrepancy in the CBD may be due to incorrect categorisation of the CPT as a consequence of insufficient historical data. However, given the large number of CPTs, it is more likely that the liquefaction assessment is over predicting the manifestation severity.

The lower estimate PGA and groundwater elevation models produced LSN and LPI that better fit the absence of liquefaction manifestation within the CBD, albeit there were still cases, particularly in the south-west of the CBD in which the severity is still over-predicted when using LSN. Additionally, the use of the lower-bound models led to under-prediction of the liquefaction manifestation severity in the areas where manifestation has been confirmed (Landings Rd and Netball Courts). A summary of the LPI values using these two lower estimate models is presented in Figure 3, demonstrating improved correlation with observed manifestation in the CBD, but again underprediction of manifestation severity at Landings Rd and the Netball Courts. The under-prediction of the liquefaction severity at sites where the damage was well-documented indicates that the LSN and LPI produced from the median PGA and groundwater models it likely a better representation of the local conditions in the township at the time of the earthquake.

Figure 3: Map of CPT-based LPI values across Whakatane resulting from the liquefaction assessment undertaken using the lower estimate PGA model and lower estimate groundwater model.

Trenching work conducted at sites within the CBD in which liquefaction was predicted but where manifestation was not observed revealed stratigraphy that did not contain liquefaction features,

Mellsop, N., Bastin, S., Wotherspoon, L.M. & van Ballegooy, S. (2017) Development of detailed liquefaction case histories from the 1987 Edgecumbe Earthquake further confirming that liquefaction was unlikely to have triggered at these sites. Results of the trenching work are presented by Bastin et al. (2017) and indicate that some of the inconsistencies at these false positive sites may result from the inability of the CPT to discern fine scale inter- layering of sands and silts, and/or the presence of pumice which are not accounted for in the simplified methodologies. These factors, along with other potential mitigating factors such as the difference between measured and actual ground-water depths due to partial saturation are the focus of more in-depth assessment of a selection of case study sites summarised by Mellsop (2017).

6 CONCLUSIONS

A back calculated liquefaction assessment has been undertaken for Whakatane during the Edgecumbe earthquake using a large Cone Penetration Test (CPT) dataset and standard CPT- based liquefaction assessment methods. By performing the liquefaction assessment with varying PGA and groundwater models, the influence of these variables on predicted liquefaction manifestation severity has been considered. Liquefaction assessments undertaken using the median PGA model and median groundwater model were found to correlate well with the severity of liquefaction manifestation at those sites that are known to have surface manifestation. However, under these conditions significant over-prediction of manifestation severity was evident in the Central Business District (CBD).

Using the lower estimate PGA and groundwater models, LSN values accurately captured the liquefaction manifestation severity at sites with surface manifestation, but again resulted in over- prediction in the CBD. LPI results from these lower estimate models under-predicted liquefaction manifestation during the Edgecumbe earthquake where manifestation was observed, and still resulted in some minor over prediction in the CBD. Overall, the findings suggest that standard CPT based methods of liquefaction assessment may be producing conservative estimates of liquefaction manifestation severity in the Whakatane CBD, and further research is needed to improve these approaches.

7 ACKNOWLEDGEMENTS

We thank the Whakatane District Council for providing access to the study sites and for the supply of historical data and data related to the development of the groundwater model. We thank local engineering firms for the supply of historical geotechnical investigation information. This work was funded by QuakeCoRE – New Zealand Centre for Earthquake Resilience, with additional support from the Ministry of Business, Innovation and Employment (MBIE) and the Earthquake Commission. This is QuakeCoRE paper number 0200.

REFERENCES

Bastin, S., van Ballegooy, S., Mellsop, N. & Wotherspoon, L. (2017) Liquefaction case histories from Whakatane, New Zealand as a result of the 1987 Edgecumbe Earthquake -– Insights from an extensive CPT dataset and paleo-liquefaction trenching. PBD III Earthquake Geotechnical Engineering. Vancouver, CA.

Berrill, J., Christensen, S., Kennan, R. & Pettinga, J. (2001) Case study of lateral spreading forces on a piled foundation. Geotechnique. 51(6), pp. 501-517.

Bradley, B. A. (2013) A New Zealand-Specific Pseudospectral Acceleration Ground-Motion Prediction Equation for Active Shallow Crustal Earthquakes Based on Foreign

Mellsop, N., Bastin, S., Wotherspoon, L.M. & van Ballegooy, S. (2017) Development of detailed liquefaction case histories from the 1987 Edgecumbe Earthquake

Models. Bulletin of the Seismological Society of America. 103(3), pp. 1801- 1822.

Boulanger, R. W. & Idriss, I. M. (2014) CPT and SPT Based Liquefaction Triggering Procedures. Report No. UCD/CGM-14/01, Davis, California: Center for Geotechnical Modeling.

Christensen, S. A. (1995) Liquefaction of Cohesionless Soils in the March 2, 1987 Edgecumbe Earthquake, Bay of Plenty, New Zealand and Other Earthquakes. Christchurch: University of Canterbury.

Dowrick, D. J. (1988) Edgecumbe Earthquake - Some Notes on its Source, Ground Motions, and Damage in Relation to Safety. Bulletin of the New Zealand Society for Earthquake Engineering. 21(3), pp. 198-203.

Dowrick, D. & Rhoades, D. (1990) Damage ratios for domestic buildings in the 1987 Edgecumbe earthquake. Bulletin of the New Zealand National Society for Earthquake Engineering. 23(2), pp. 137-149.

Franks, C. A., (1989) Ground damage and seismic response resulting from the 1987 Edgcumbe Earthquake, New Zealand. New Zealand Journal of Geology and Geophysics. Volume 32, pp. 135-144.

Idriss, I. M. & Boulanger, R. W. (2008) Soil liquefaction during earthquakes. Monograph MNO- 12, Oakland, CA.

Iwasaki, T., Arakawa, T. & Tokida, K. (1984) Simplified Procedures for assessing soil liquefaction during earthquakes. International Journal of Soil Dynamics and Earthquake Engineering. 3(1), pp. 49-58.

Jennings, D. N., Edwards, M. C. & Franks, C. R. (1988) Some Observations of Sand Liquefaction in the 2 March 1987 Edgecumbe (New Zealand) Earthquake. Sydney.

Juang, C. H., Yang, S. H., Yuan, H. & Fang, S. Y. (2005a) Liquefaction in the Chi-Chi earthquake - effect of fines and capping non-liquefiable layers. Soil and Foundations. Volume 45, pp. 89-101.

Juang, C. H. et al. (2005b) Estimating severity of liquefaction-induced damage near foundations. Soil Dynamics and Earthquakes. Volume 25, pp. 403-411.

Maurer, B. W., Green, R. A., Cubrinovski, M. & Bradley, B. A. (2014) Evaluation of the Liquefaction Potential Index for Assessing Liquefaction Hazard in Christchurch, New Zealand. ASCE Journal of Geotechnical and Geoenvironmental Engineering. 140(7).

Mellsop, N. (2017) Liquefaction case histories from the 1987 Edgcumbe earthquake – Insights from an extensive CPT dataset, direct push cross hole shear and compression wave velocity (VsVp) testing and paleo-liquefaction trenching. ME Thesis, University of Auckland.

Pender, M. J. & Robertson, T. W. (1987) Edgecumbe Earthquake: Reconnaissance Report. Bulletin of the New Zealand Society for Earthquake Engineering. 20(3), pp. 201-249.

Mellsop, N., Bastin, S., Wotherspoon, L.M. & van Ballegooy, S. (2017) Development of detailed liquefaction case histories from the 1987 Edgecumbe Earthquake

Toprak, S. & Holzer, T. L. (2003) Liquefaction potential index: field assessment. Journal of Geotechincal and Geoenvironmental Engineering. 129(4), pp. 315-322. van Ballegooy, S. et al. (2014) Assessment of Liquefaction-Induced Land Damage for Residential Christchurch. Earthquake Spectra. 30(1), pp. 31-55.

Wald, D. J., Quitoriano, V., Heaton, T. H. & Kanamori, H. (1999) Relationship between Peak Ground Acceleration, Peak Ground Velocity, and Modified Mercalli Intensity in Calafornia. Earthquake Spectra. 15(3), pp. 557-564.