Characterisation of Liquefaction Features in Eastern Christchurch, New Zealand

Transcription

1 Characterisation of Liquefaction Features in Eastern Christchurch, New Zealand Madeline M. Niles University of Canterbury, Department of Geological Sciences, Christchurch, NZ Smith College, Department of Geosciences, Northampton, MA ABSTRACT Analysis of a trench wall in Avonside, Christchurch reveals a complex sequence of liquefaction events from the Canterbury Earthquake Sequence (CES) as well as other sedimentary and anthropogenic depositional units. The CES caused a significant amount of liquefaction in Christchurch, New Zealand, which has cost the government over 1 billion NZD in repairs and buybacks. Trenching in Sullivan Park, located in an inner meander of the Avon River, has revealed evidence for at least three distinct liquefaction inducing events within the trench. These could potentially correspond with the September 2010, February 2011 and June 2011 earthquakes. Cone penetration (CPT) data taken previously within Sullivan Park was compared to a hand auger boring and grain size analysis from a liquefaction feature. Both the CPT and the hand auger identified the liquefiable source to be approximately the same, around 1.5 to 2.32 m below the surface. This helps to confirm the accuracy of CPT testing in the Sullivan Park region. INTRODUCTION Liquefaction from the Canterbury Earthquake Sequence has caused massive amounts of damage in Christchurch, New Zealand and its surrounding suburbs. In order for liquefaction to occur loose, unconsolidated, and saturated sediments must be exposed to cyclic shearing induced from earthquake shaking. The potentially liquefiable sediment must also be topped by a less permeable sediment cap, which allows the buildup of effective pore water pressure. The overburden stress from the overlying material then gets transferred to the pore fluid causing reduced shear strength and sediment failure (Quigley et al 2013). An assortment of variables dictates where, and to what severity, liquefaction occurs, which makes it very complicated to predict and prepare for. There first must be shaking at a high enough peak ground acceleration (PGA) and peak ground velocity for a significant duration of time. This is controlled by the magnitude of the earthquake and the region s proximity to the fault zone. The sediment must also be saturated, which is controlled by local hydrology and the depth of ground water. Grain size also plays a large role in determining the porosity

2 and permeability of the sediment. Fine sands and coarse silts are usually the most prone to liquefying. Other sedimentological properties such as rounding and sorting also impact porosity and permeability thus are somewhat important in determining liquefaction severity. Younger sediments are also more likely to liquefy because they are often looser and less compact, which also impacts permeability (Seed & Idriss, 1982). The CES consists of the September 2010 Darfield Earthquake and its subsequent aftershocks which include 3 earthquakes with M w 6.0; 51 aftershocks between M w 5.9 and 5.0; and 496 aftershocks between M w 4.9 and 4.0 (Geonet, 2014). The Darfield Earthquake occurred on the dextral strike slip Greendale fault on 4 September 2010 and had a moment magnitude (M w ) of 7.1. The Greendale Fault is located approximately 40km west of the Central Business District (CBD) of Christchurch, but still resulted in a significant amount of liquefaction damage despite its distance (Gledhill et al., 2011). The Christchurch earthquake occurred on 22 February 2011 on a blind oblique thrust fault near the Port Hills and had an M w of 6.2. Despite its smaller magnitude, the Christchurch earthquake resulted in higher peak ground accelerations (PGAs) in the city and more severe liquefaction due to its proximal location to Christchurch central business district (Cubrinovski et al. 2011). The June 2011 M w 6.1 and December 2011 M w 5.9 aftershocks also caused some liquefaction in and around the city (Quigley et al. 2013). The February 2011 Christchurch earthquake caused the most amount of liquefaction damage because of the proximity of the fault to the city. Areas to the east of the CBD were particularly affected because they are located closest to the fault source (Cubrinovski et al. 2011). The CES caused a tremendous amount of damage to buildings, infrastructure and lifelines in Christchurch and its suburbs. Liquefaction has caused uneven settlement of land, which has rendered many houses inhabitable. It has also disrupted lifelines such as roads, water pipes and power lines, which are be costly to repair. Liquefaction has also manifested itself in the form of lateral spreading sand dikes, fissures and sand blows. This has caused many areas around Avonside, Kaipoi and Bexley to be red-zoned due to the severity of liquefaction damage. The residents were evacuated and the New Zealand government was required to buy back the abandoned properties. Liquefaction repairs and government buybacks have cost the New Zealand government well over one billion NZD (Quigley et al. 2013). Areas surrounding the Avon River, such as the suburb of Avonside, in eastern Christchurch were particularly susceptible to earthquake liquefaction due to the high concentration of liquefiable sands and silts deposited from the meandering river. The area

3 surrounding Sullivan Park has been classified as a red zone and is no longer inhabited due the intensity of the liquefaction. Since liquefaction has been proven to have dire and costly consequences, it is crucial to have a thorough understanding about where it is most likely to occur and the variables that determine this. A variety of methods can be employed to help establish how susceptible sediments are to liquefaction. Cone penetration tests (CPTs) are often used to determine the properties and grain sizes of subsurface sediments, which can also be used to evaluate liquefaction susceptibility. CPTs evaluate liquefaction by inserting a cone into the ground and recording the cone tip resistance (q c ) and the friction ratio (f R ), which determines grain size and likelihood of the sediment liquefying (USGS). Other geotechnical methods include standard penetration tests (SPT), dynamic cone penetration tests (DCPT) and Swedish Weight Sounding (SWS) also evaluate the possibility of sediments liquefying (Quigley et al. 2013). Grain size analysis coupled with analyses on particle shape, hydrologic setting and depositional history can also help glean information about the probability of liquefaction in an area. Previous studies have looked closely at liquefaction in Christchurch after the major earthquakes in the CES. Quigley et al. (2013) looked at a trench in Avonside and documented ten distinct liquefaction events generated from the CES. Other research has looked at Sullivan Park and analysed the modern and paleo-liquefaction features located there (Bastin et al. 2013, a). This study will focus on how grain size and depositional history affects liquefaction. It will also begin to evaluate the accuracy of CPT in prediction liquefaction susceptibility in the region and build upon previous research on Christchurch liquefaction. GEOLOGIC SETTING The majority of Christchurch is built upon marine sands, silts and clays, as well as fluvial gravels and overbank flood deposits (White 2007). The Springston Formation is a prominent geologic unit in Christchurch composed mainly of alluvial gravel and overbank deposits of sand and silt. The overbank deposits are particularly susceptible to liquefaction because they are relatively young, loosely compacted, saturated sediments (Jacka & Murahidy 2011). Areas in the eastern parts of Christchurch, such as Avonside, experienced the greatest liquefaction damage partially due to their proximity to the Avon River (Cubrinovsky & Green 2010). Sullivan Park is located in Avonside within the inner meander of the Avon River (fig. 1). Eastern Christchurch also has a relatively shallow water table, around 1-2 meters, which changes slightly based on tidal regime (Jacka & Murahidy 2011).

4 This ensures that the sediment is saturated, a requirement for liquefaction to occur. The area east of central Christchurch is also very proximal to the fault that caused the Christchurch Earthquake in February of 2011, which resulted in higher PGAs as evaluated by various accelerometers in the area (Bastin et al. 2013, b), thus higher amounts of liquefaction. METHODS Two approximately ten-meter long and 1 to 1.25 meter deep trenches were dug in Sullivan Park, Avonside Christchurch (Fig 1) in February The trench walls were cleaned thoroughly using hand scrapers and trowels. In this study, the western wall of the northern trench (#6) was observed extensively (fig 1). The trench wall was photographed and logged thoroughly, paying close attention to liquefaction features present. Detailed descriptions of liquefaction, sedimentalogical and anthropological units were collected in the field, including colour, sorting and grain size. A hand auger was used to bore a hole into the trench floor that extends approximately 3 m below the ground surface. The sediment was logged and a sample was collected from a potential source for the liquefiable sediment. Twelve other samples were also collected from various key sedimentary, anthropogenic and liquefaction features including feeder dikes and sills and the lateral spreading centre. The grain sizes of these samples were examined in depth. To do this, a small amount of the sediment was mixed with calgon (sodium hexametaphosphate/ sodium polymetaphosphate) using a magnetic stirring device. The sediment was then analysed using a micromeritics Saturn Digitizer II laser-sizer to generate grain size curves and other statistics. Interpreted CPT data was also examined further in relation to the trench log, auger boring, and grain size analysis. McMillan Drilling Services performed the CPT on 9 September They analysed the raw data to determine grain size using the methods of Robertson and Campanella (1983). The susceptibility to liquefaction was also evaluated using methods established by Shibata and Teparaksa (1988). RESULTS The trench at Sullivan Park revealed a complex sequence of depositional, anthropogenic and liquefaction sediments and features. The majority of the trench was composed of alluvial sands and silts deposited by the meandering Avon River. There was a large amount of horizontal and cross laminations in unit one (fig 2), which was composed of dark olive grey fine to medium sands. A mostly continuous coarse silt to fine sand layer

5 sloping around 5 to the north interrupts this unit (Fig 2). Other sedimentological fluvial units were composed mostly of fine sands and coarse silts, and occasionally gravels. Also present in the trench was anthropogenic pit fill from a wool factory operated in the area from the 1850 s through the early 1900 s. The factory dumped sheep bones into pits, which can often be found in trenches in the area. This trench had one region of pit fill that was composed of very dark brown moderately well sorted fine sand with sheep bones present at the base of the unit. One piece of coal was also found in the trench, which can also most likely be traced to the wool factory. There was one very distinct lateral spreading feature evident within the southern side of the trench. It was composed of dark grey well-sorted fine sand that grades upwards into very fine sand. The dike itself was around 18 cm at the base and widened slightly as it moved closer towards the surface. The overall spreading centre feature was divided into three separate dikes by coarse silt drapes. The largest of these dikes was to the far right and was around 10 cm wide (fig3.1). The middle dike was narrower, only around 5-10 cm (fig 3.2) and the far left was very narrow, around 2 cm (fig 3.3). The third dike was the only one in the larger feature that extended all the way to the surface. Both the right and centre segments were capped by finer sand which fined upwards further into coarse silt. The feature continued across the trench floor and had a similar appearance on the eastern side of the trench. There were several chunks of topsoil and other units that fell into the spreading centre (fig 3). There were also four other much smaller sand dikes also composed of well-sorted fine sands located in the southern half of the trench. These were subsurface features that did not reach the surface of topsoil and fined upwards into silty sands. As they approached the surface, they occasionally branched out into more sill-like features. The hand auger boring revealed the water table to be 1.77 m from the surface. It also identified a probable source for the liquefied sediment (fig 4). It showed a unit of medium to fine sand that began at 1.66 meters down from the surface that merged into a unit of fine sand to coarse silt that ends at 2.32 meters. This segment from m below the ground surface was likely the source of liquefiable sediment. Grain size analysis from the lateral spreading feature indicated it was composed of mostly fine sand (average of microns in diameter). Similarly, the hand auger sample was composed of fine sand, with a mean sediment size of microns. The CPT data also indicates that the liquefiable sediment source is around 1.5 m to 2.2 m deep (fig 5).

6 DISCUSSION Sequence of liquefaction events The trench in Sullivan Park revealed a complex sequence of liquefaction features, particularly within the southern half of the trench. There was one large lateral spreading feature that dominated the liquefaction evidence present. This feature contains three important silt drapes, which indicate several different liquefaction events formed the larger feature. There are also several pieces of topsoil and nearby sediment that fell into the liquefaction feature, impacting its appearance. Although it is not possible to definitively order these events based on the current information, it is plausible to hypothesise that the largest spreading dike formed during the initial September 2010 event (fig 3.1). According to this logic the middle feature may have formed during the February 2011 earthquake and the final feature could have formed during the June 2011 event. By examining aerial photographs from after the various events, it is evident that the September 2010, February 2011 and June 2011 events created the most liquefaction surface ejecta in Sullivan Park, thus the hypothesis is credible. The pieces of topsoil and other units in the dike most likely fell into it after liquefaction began. The two dikes to the right (fig 3, dike 1 and 2) both stalled before they reached the surface. This was apparent because they were capped by a layer of silt ~5-9 cm thick (fig 3). These events stalled because they did not have enough energy to manifest themselves at the surface. This was also evident in the grain size, which fined upwards. The other four smaller dikes also did not create any surface ejecta and had grain size that fined upwards. Two of these smaller dikes had a branched appearance towards the top, also indicating that they did not have enough energy to reach the surface. Depositional environment Since Sullivan Park is located in the inside bend of the meandering Avon River, the sedimentalogical units located within the trench are all fluvial in origin. Unit one, which had prominent horizontal and cross laminations, is most likely composed of old point bar deposits from the outer bends of the meandering river. This indicates that at one point in time the Avon River flowed through Sullivan Park. Unit one was also sloping slightly in a manner that resembled a channel, which also supports the hypothesis that the river once went through the park. This means that the meander that Sullivan Park is located within has moved outwards, which concurs with the manner that meandering rivers usually evolve.

7 The majority of the other depositional units are composed of varying amounts of very fine sands, fine sands and occasionally medium sands and coarse silts that were also deposited from the Avon River. The most likely fluvial environment responsible for the origin of these sediments are overbank deposits. These originated from flooding of the river channel, which led to the deposition of sandy natural levees and siltier floodplain deposits. The gravel which is seen at one point in the trench can probably be traced back to higher energy flood channels which are normally composed of gravel in a matrix of sand. The depositional units mapped within the trench are very chaotic and occasionally lenticular. This could possibly be because of high energy flooding disrupting previously deposited sediments. The overbank deposits could have also been disrupted further from earthquake shaking. Auger, CPT and grain size comparisons The hand auger boring, CPT data and grain size comparison all indicate a source of liquefiable sediment that is approximately the same from around 1.5 to 2.32 m. The grain sizes collected from the auger and from the large lateral spreading features are approximately the same. The hand auger boring indicated that the liquefiable source is around 1.66 to 2.32 m deep (fig 4). This depth is corroborated by the CPT data which estimated the liquefiable source to be 1.5 to 2.2 deep (fig 5). The fact that both the hand auger and the CPT identified nearly identical sources for the liquefiable material indicates that CPTs are a reasonably accurate way to evaluate the liquefaction susceptibility of a region. Big picture Sullivan Park is located within an inner meander of the Avon River, in a topographically lower region (fig 6). The location of the trench corresponds with the area that had the most severe lateral spreading, with lateral movement greater than 1.2m (fig 6). This could potentially be because the Avon River used to flow through the northern part of the trench, so the southern part would have been directly in the inside meander bend. A similar pattern can be observed in another old paleo-channel present in the area (fig 6). The worst liquefaction occurred directly inside the meander bend, which also has lower topography today (fig 6). The location of Sullivan Park in an inner meander bend fits well into the overall pattern of where the most extreme liquefaction would be expected to be found.

8 CONCLUSIONS In Sullivan Park, at least three distinct events occurred to induce the liquefaction seen the trench s main lateral spreading centre. These potentially align with the September 2010, February 2011 and July 2011 earthquakes. Future research could look more into this idea by examining liquefaction events seen in other areas. Cone penetration test data, the hand auger boring and grain size analysis of the main lateral spreading feature all corroborate the idea that the liquefiable layer can be found ~1.5 to 2.32 m below the surface. This suggests that CPT data is a relatively accurate way to evaluate liquefaction susceptibility. More work analysing a larger sample of CPTs in respect to actual observed liquefaction could validate this idea further. Finally, the liquefaction seen in Sullivan Park fits well into the larger geologic setting of the Avonside area. It confirms the concept that areas within the inside meander bends or paleo-channels are particularly prone to liquefaction due to sediment size, loose compaction, youth of sediment and high saturation. Further work could be done to examine paleo-liquefaction from events that occurred prior to the CES. Paleo-liquefaction can be seen elsewhere in Sullivan Park, but is not present in this trench. ACKNOWLEDGMENTS Thank you to Josh Borella for his guidance and support through this process. I would also like to my field team Grayson Carlile and Katherine Kuklewicz for helping with logging and providing general support. Thank you to Sarah Bastin for providing guidance and allowing us to access Sullivan Park trenches and data. Thank you to McKinnon Main for providing his trench log and general support. Thank you to Grayson Carlile also for access to aerial photographs displaying liquefaction ejecta and to Erin Markey for helping edit. Finally, thank you to Sam Hampton and Darren Gravley and to Frontiers Abroad for providing this opportunity.

9 REFERENCES a. Bastin, S., Quigley, M.C., Bassett, K., Green, R.A., Modern and paleo-liquefaction features in eastern Christchurch and strategies for locating per-historic liquefaction. b. Bastin, S., Reid, C.M., Quigley, M.C. & Basset, K.N., Earthquake impacts on soft sediments in Eastern Christchurch. Field Trip Guides, Geosciences 2013 Conference, Christchurch, New Zealand. Geoscience Society of New Zealand Miscellaneous Publication, 136B, 21 p. Cubrinovski, M. & Green, R.A. (eds.), Geotechnical Reconnaissance of the 2010 Darfield (Canterbury) Earthquake, (contributing authors in alphabetical order: J. Allen S.Ashford, E. Bowman, B. Bradley, B. Cox, M. Cubrinovski, R. Green, T. Hutchinso n,kavazanjian, R. Orense, M. Pender, M. Quigley, and L. Wotherspoon), Bulletin of the New Zealand Society for Earthquake Engineering, 43(4): Cubrinovski, M., Bradley, B., Wotherspoon, L., Green, R., Bray, J., Wood, C., Pender, M., Allen, J., Bradshaw, A., Rix, G., Taylor, M., Robinson, K., Henderson, D., Giorgini, S., Mal, K., Winkley, A., Zupan, J., O Rourke, T., DePascale, G., & Wells, D., Geotechnical aspects of the 22 February 2011 Christchurch earthquake, Bulletin of the New Zealand Society for Earthquake Engineering. Vol 44, No.4. Geonet, Aftershocks. info.geonet.org.nz/display/home/aftershocks Gledhill, K., Ristau, J., Reyners, M., Fry, B., & Holden, C., The Darfield (Canterbury, New Zealand) M w 7.1 earthquake of September 2010: a preliminary seismological report. Seismological Research Letters. Volume 82, Number 3 Jacka, M.E., & Murahidy, K.M., Observations and characterization of land damage due to liquefaction and lateral spreading. Auckland, New Zealand: 9 th Pacific conference on Earthquake Engineering. Building and Earthquake Resilient Society Paper 41 Quigley, M.C., Bastin, S., & Bradley, B.A., Recurrent liquefaction in Christchurch, New Zealand, during the Canterbury earthquake sequence, Geology, doi: /g Robertson, P.K. & Campanella, R.G., Interpretation of cone penetration tests. Canadian Geotechnical Journal. Vol. 20 pp Seed, H.B. & Idriss, I.M., Ground motions and soil liquefaction during earthquakes, Monograph Series, Earthquake Engineering Research Institute, Berkeley, CA. Shibata, T., Teparaksa, W., Evaluation of liquefaction potentials of soils using cone penetration tests. Soils and Foundations. 28 (2) pp USGS, Conte Penetration Testing. From earthquake.usgs.gov/research/cpt/ White, P.A Geologic model of the Christchurch Formation and Springston Formation. GNS Science Consultancy Report 2007/117/ Environment Canterbury technical report.

10 319 FIGURES AND CAPTION Avonside and Sullivan Park CBD FIGURE 1

11 322 S N Figure Topsoil Moderately well sorted fine to medium sand. Horizontal and cross laminations. Fine sand to coarse silt. Pit fill. Moderately well sorted fine sand. Lamb bones at base Well sorted fine to very fine sand. Moderately well sorted very fine sand to course silt Moderately well sorted fine to medium sand Moderately well sorted fine to very fine sand S ilt Well sorted fine sand. Moderately sorted fine to very fine sand. Well sorted fine-medium sand. Fines upwards into silty sand. Well sorted fine sands. Fines upwards into silty sand Well sorted fine to medium sand matrix with 5-10% 1-3 mm oblong gravels Course silt to fine sand. Fines upwards Silt drapes

12 355 Figure 3

13 Depth (m) Depth (m) Cone Tip Resistance q c (MPa) LIQ. SUSCEP TIBILITY Figure Topsoil Pit fill Fine sand with horizontal laminations and cross beds Fine sand to coarse silt channel bed 1.0 Fine sand with horizontal laminations and cross beds Fine sand to silt 1.5 water table Mottled fine sand Fine to medium sand LIQUEFIABLE SOURCE Fine sand and silt LIQUEFIABLE SOURCE Pebbles and gravel in silt-fine sand matrix Blue gray silt Gravel with silt- fine sand matrix Figure 5

14 Elevation (m) High (4.92) Low (0.48) A Sullivan Park B C 365 Figure 6 FIGURE CAPTIONS Figure 1: A) Map showing Sullivan Park and Avonside in respect to Christchurch. B) Location of Sullivan Park with respect to the Avon River. C) Location of trenches within the park. Yellow indicates trench wall logged (trench #6) Figure 2: Trench log and legend showing mapped liquefaction features, as well as sedimentary and anthropogenic units. Boxed area shows key lateral spreading center and is shown larger in figure 3. Figure 3: Detailed image of main lateral spreading center found within the trench. Numbers represent possible order of liquefaction events. Figure 4: CPT data from borehole within Sullivan Park showing cone tip resistance in relation to depth. CPT identifies the area with highest liquefaction susceptibility to be between ~1.5 to 2.2 m below the surface (indicated by red box). Figure 5: Hand auger borehole log from within trench. Units located ~1.66 to 2.32 m below the surface are composed of medium fine sand to silt and are likely the liquefiable source. This corresponds approximately with CPT data and grain size analysis of liquefaction features. 390

15 Figure 6:a) DEM image showing elevation of Avonside region. b) Map showing horizontal lateral spreading along paleo-channel in the area. Old paleo-bank corresponds with more lateral spreading. c) Sullivan Park and inner meander lateral spreading map. Adapted from Bastin et al