Liquefaction Report Tonkin and Taylor May 2015

REPORT

Liquefaction Vulnerabilityand Geotechnical AssessmentGuidance for GisborneDistrict Council

Prepared for:

Gisborne District Council

May 2015

Job No: 30214.v1

105 Carlton Gore Road, Newmarket, Auckland 1023 p. +64 355 6000 www.tonkin.co.nz

Distribution:

Gisborne District Council copies

Tonkin Taylor Ltd (FILE)

Liquefaction Vulnerability and Geotechnical Assessment Guidance for Gisborne District Council Gisborne District Council

Job No: 30214.v1May 2015

Table of contents

1 Introduction 4 1.1 Scope 4

2 Geology, geomorphology and groundwater 6 2.1 Poverty Bay Flats 6 2.2 Sponge Bay and Wainui 7

3 Liquefaction description 9 4 Gisborne seismicity 10

4.1 General 10 4.2 Seismic Hazard 11 4.3 Recent liquefaction observations (2007 Gisborne Earthquake) 12

5 Sub-regional liquefaction vulnerability assessment 14 5.1 Existing ground investigation data 14 5.2 Methodology summary 15

5.2.1 Review of existing ground investigation data 15 5.2.2 Liquefaction analyses using existing CPT data 16

5.3 Typical land damage observations and photographs 17 5.4 Liquefaction hazard mapping 18 5.5 Data reliability 20

6 Geotechnical assessment guidance 22 6.1 Low level geotechnical assessment 22 6.2 Moderate and high level geotechnical assessments 22 6.3 Canterbury approach to liquefaction investigations and assessment 23

7 Mitigation 24 8 Suggested further study 25

8.1 Development of geotechnical assessment criteria for land vulnerability categories 25 8.2 Lateral spread consideration 25 8.3 Groundwater model refinement 25

9 Conclusions 26 10 Limitations 27 11 Applicability 28 12 Bibliography 29

Appendix A : Liquefaction description

Appendix B : Liquefaction hazard assessment methodology and CPT triggering analysis output

Appendix C : Typical photographs of land damage

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Tonkin & Taylor Ltd Liquefaction Vulnerability and Geotechnical Assessment Guidance for Gisborne District Council Gisborne District Council

May 2015Job No: 30214.v1

Executive summary

Tonkin & Taylor Ltd (T&T) has been engaged by Gisborne District Council (GDC) to undertake a sub-regional assessment of liquefaction vulnerability in Gisborne. This report describes the geotechnical assessment and analyses used to categorise and map liquefaction vulnerability across Gisborne City, Poverty Bay Flats, Sponge Bay and Wainui.

The primary aim of this study is to provide GDC with a sub-regional assessment and map of land vulnerability to liquefaction. This information will assist GDC to better understand the hazard and appropriate responses, including determining appropriate site specific levels of geotechnical assessment. By ensuring appropriate geotechnical assessment is undertaken in accordance with the relevant mapped vulnerability, Council will be better able to implement building controls to develop greater resilience to liquefaction damage in the Gisborne community. A proactive approach to build resilience prior to a large earthquake event will help to avoid some of the liquefaction related damage observed during the recent Canterbury Earthquake Sequence (2010 – 2011).

Liquefaction is one of a number of natural hazards that can be caused by earthquakes. The Gisborne region is more likely to experience a large earthquake and associated liquefaction than many other areas in New Zealand due to the close proximity of the Hikurangi subduction margin located immediately off the coast. Liquefaction can cause ground surface subsidence due to volumetric densification, the ejection of fluidised sand, topographic re-levelling and lateral spreading, resulting in damage to buildings and infrastructure.

The majority of the study area comprises the Poverty Bay Flats from Gisborne City inland to Te Karaka. This area is characterised by steep hills surrounding a low lying alluvial plain underlain by soft sediments. The flat land at Sponge Bay to the east of Gisborne also comprises loose alluvial deposits. These geologically young, Holocene (10,000 years till present) deposits are much more likely to liquefy than older more consolidated deposits.

The study area has been mapped as one of three land vulnerability categories as shown in Table E1.

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Table E1: Land vulnerability categories for mapping

Land vulnerability category1

Colour Simplified land damage category

Typical land damage and consequent building performance2

Low Yellow None to minor No observed cracks, undulations/deformations at the ground surface

No signs of ejected material at the ground surface No apparent lateral movement No apparent differential settlement

Moderate Orange Minor to moderate

Minor cracking (typically <50mm), buckling and undulations at the ground surface

Minor to moderate quantities of ejected liquefied material at the ground surface (typically <25% of site covered with ejected material)

No apparent lateral movement Differential settlement of buildings causing minor to

moderate structural damage

High Red Moderate to severe

Moderate to large cracks with horizontal and vertical displacement (>50mm), buckling and undulations at the ground surface

Large quantities of ejected liquefied material at the ground surface (typically >25% of site covered with ejected material)

Noticeable ground surface subsidence Lateral spreading Significant total and differential settlement of

buildings causing structural damage

1. For 500 year return period levels of earthquake shaking

2. The land and building damage outlined for each land vulnerability category represents the typical likely behaviour of flat land and light residential dwellings only. Land damage for a particular site may vary due to natural variations and many other factors which effect site performance. Building damage could vary significantly from the typical description as the extent of damage will depend on many building specific variables including the type and shape of the building and foundation stiffness.

The liquefaction hazard map prepared during this study is shown in Figure E-1. A digital copy (GIS shapefile) of the map accompanies this report.

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Figure E-1: The liquefaction vulnerability map prepared for the study area(also shown enlarged as Figure 5-2)

Limitations

The liquefaction assessment undertaken has been mapped at a sub-regional scale and is not to be viewed at scales more detailed than 1:10,000 in urban areas and 1:25,000 in rural areas. A GIS shapefile has been developed primarily based on information obtained from, and analysis of, existing geotechnical investigation data, observations from the 2007 Gisborne earthquake and LiDAR survey derived topography mapped onto a digital aerial photograph base. No liability beyond the client (GDC) is accepted for any of the information presented, as the information provided is only an indication of where potential liquefaction induced land damage at the ground surface may occur at the level of earthquake shaking considered. The vulnerability map would be different for smaller or larger and associated levels of earthquake shaking. It would also be different if climate change effects on groundwater levels was considered.

The map should not be used as a replacement for a site-specific liquefaction assessment of a property. Appropriate geotechnical investigations and an assessment at a property-scale which considers the site topography, geology and depth to groundwater may in some cases indicate that the site can be categorised differently from that shown on the vulnerability map. It must also be appreciated that a site can range from low vulnerability land though to high vulnerability depending upon the property size, topography and underlying geology and the level of earthquake shaking for which the liquefaction is being considered. Properties that straddle two categories shown on the map should be investigated based on the higher vulnerability category.

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1 Introduction

Tonkin & Taylor Ltd (T&T) has been engaged by Gisborne District Council (GDC) to undertake a sub-regional assessment of liquefaction and to prepare a liquefaction vulnerability map for Gisborne City, Poverty Bay Flats and the flat land at Sponge Bay and Wainui. This work has been carried out to assist GDC to better understand the hazard and appropriate responses, including determining appropriate site specific levels of geotechnical assessment.

The Gisborne region is subject to significant seismic activity due to the close proximity of the Hikurangi subduction margin off the east coast of New Zealand. Liquefaction is one of a number of natural hazards that can be caused by a large earthquake and it occurs when loose saturated soils lose strength and stiffness in response to an applied cyclic loading from such as earthquake shaking.

Liquefaction damage during future earthquakes depends on the earthquake magnitude and ground accelerations. This sub-regional level assessment of liquefaction vulnerability has been assessed for a large earthquake. The vulnerability map would be different for smaller or larger levels of earthquake shaking.

This report describes the geology and seismicity of the study area and the sub-regional liquefaction assessment undertaken to develop the liquefaction vulnerability map.

1.1 ScopeThe boundary of the study area is shown in Figure 1-1 and generally follows the base of the steeply sloping hill country surrounding the Poverty Bay Flats. Mapping has been carried out at a scale of 1:10,000 for the existing urban area (Tiles F and G) and 1:25,000 for the surrounding more rural areas (Tiles A to E). The tiles shown on Figure 1-1 represent A2 size sheets at the above mapping scales. The regional liquefaction assessment included the following:

Review of aerial photography, existing liquefaction hazard maps and published geology Review of selected published papers about Gisborne geology, geomorphology and

seismicity Review of published topography maps and bare earth digital elevation model (DEM)

derived from LiDAR survey data supplied by GDC Review of observations and available reports prepared at the time of the 2007 Gisborne

Earthquake Sourcing and collating various existing ground investigation data within the study area

(boreholes, CPT’s, penetrometer tests) held on T&T and Land Development and Exploration Ltd (LDE) job files into a database containing over 1,200 investigations

Review of subsurface information in the database to characterise ground conditions (e.g. soil type, depth to groundwater, etc.)

Upload of available cone penetrometer test (CPT) raw data to the T&T database Assessment of the likelihood of liquefaction induced land damage at the ground surface for

each investigation point in the database Analysis of CPT data to calculate internationally recognised quantitative measures of

liquefaction induced land damage. Liquefaction vulnerability parameters calculated include; the Liquefaction Severity Number (LSN), calculated settlement (SVID) and the cumulative thickness of liquefiable material (CTL) at each CPT location for a range of peak ground accelerations including Ultimate Limit State (ULS) and Serviceability Limit State (SLS) at magnitude 7.5.

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A GIS map was set up for the project with layers including; aerial photography, geological maps, LiDAR DEM contours, depth to groundwater, and liquefaction vulnerability estimates from review of existing ground investigation data including map layers showing calculated LSN, SVID and CTL

All of the above map layers were reviewed at a workshop attended by senior staff of T&T and LDE, to ensure the project benefited from local knowledge and ground investigation data held on LDE files. Assimilation and review of information on maps was carried out at the workshop and liquefaction vulnerability map boundaries were determined

Preparation of this report Preparation of an accompanying GIS shapefile (digital copy) of the map for use by GDC in

their GIS system

Figure 1-1: An overview of the study area showing the study boundary and tiles representing an A2 sheetat the scales used for mapping (1:10,000 for urban areas tiles and and 1:25,000 for rural areasTiles to E)

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2 Geology, geomorphology and groundwater

The geomorphology, structure and seismicity of the Gisborne region is controlled by subduction of the Pacific plate beneath the Australian plate at the Hikurangi subduction margin off the east coast of New Zealand.

2.1 Poverty Bay FlatsThe majority of the study area comprises the Poverty Bay Flats from Gisborne/Poverty Bay inland to Te Karaka. This area is characterised by steep hills surrounding a low lying alluvial plain. The Waipaoa River flows through the Poverty Bay Flats, reaching the ocean near the southern end of Poverty Bay.

Figure 2-1: photograph of the Poverty Bay Flats and the surrounding hill country looking west over thecity of Gisborne with the Taruheru river in the foreground (source: Design Arts 2010, Gisborne)

The Holocene (10,000 years ago up to the present) stratigraphy of the Poverty Bay Flats generally comprises fluvial and beach deposits. Fluvial deposits up to 200m thick consist of flood plain silts/mud, river channel gravels and sand deposited as the Waipaoa River migrated across the coastal plain. Estuarine or shallow marine mud and sand is present in localised areas to significant depths. Accumulation of fluvial deposits over the Waipaoa coastal plain occurred behind a migrating sandy shore face and has fed the pro-gradation of the shoreline at Poverty Bay over 12km to reach its present position. Paleo-shorelines indicate an initial rapid shoreline migration (3m/year for the period 7,000 to 5,000 years ago) has been followed by a period of slower migration (2,000 years ago to the present) (M.A. Wolinsky, 2010).

Historically the Waipaoa River migrated broadly over the Poverty flats. Pullar and Penhale (1970) note that around 3,500 years ago the lower river followed a similar alignment to the present day Taruheru River (Figure 2-2A) with the river mouth located at the north eastern end of the present

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day beach shoreline. The Te Arai river was separate from the Waipaoa and flowed south with the river mouth located close to the south eastern end of the modern beach near Muriwai. Since 3.5ka, the Waipaoa migrated west to reach its present position around 1888 and the Te Arai River is now a tributary to the lower Waipaoa.

Upland erosion due to deforestation has driven rapid floodplain aggradation around the modern Waipaoa River. Large floods historically inundated the flats but the river is now relatively fixed as it is contained by engineered levees (Gomez et al., 1998).

Sandy beach deposits are generally interbedded in the fluvial deposits and are associated with historic transgression and regression of the shoreline. The maximum inland transgression is considered to have occurred at the same time as sea levels peaked around 7,000 years ago (Brown, 1995). Figure 2-2B shows a reconstruction of the prograding shoreline from 7,000 years ago to present.

Tilting and differential movement of the more persistent deposits observed in borehole stratigraphy indicates significant historic tectonic deformation, with most of the Poverty Bay Flats having undergone subsidence (Wolinsky et al. 2010).

Groundwater levels across the Poverty Bay Flats are typically shallow and can be within 1m of the ground surface following heavy rainfall and during the wetter months of the year.

Figure 2-2: A) Locations of the present day rivers on the Poverty Bay Flats. B) Reconstruction of thePoverty Bay shoreline progradation from 7000 years ago to present (Source: M.A. Wolinsky et al.)

2.2 Sponge Bay and WainuiResidential development has taken place at Sponge Bay and Wainui to the east of Gisborne. Published geological maps indicate that the low lying flat land located inland of the coast at Sponge Bay comprises Holocene (10,000 years ago till present) silt, sand and gravel river deposits.

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Elevated land nearer the coast comprises sandstone and mudstone sedimentary rock with some allochthonous Melange.

Wainui beach has a narrow strip of gently sloping land along the shoreline which is generally underlain by sandy beach deposits. Steeply sloping sandstone hill country is located immediately west of the shoreline with State Highway 35 and existing residential dwellings located near the coast at the base of the steep slopes.

Groundwater is typically at a shallow depth in the low lying areas at Sponge Bay and near the coast at Wainui.

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3 Liquefaction description

Liquefaction occurs when saturated non-cohesive soils lose strength and stiffness in response to an applied cyclic loading form earthquake shaking. Liquefaction can cause damage to land, buildings and infrastructure. Only some soil types are susceptible to liquefaction and only some earthquakes are strong enough to trigger liquefaction. Geotechnical investigations and assessment can estimate the likelihood of liquefaction occurring and its potential consequence for different structures at a specific site.

A large earthquake in Gisborne is likely to be associated with displacement of the Hikurangi fault. It is expected that such an event could cause widespread liquefaction of the loose to medium dense sands underlying much of Gisborne. In a large event even dense sands can liquefy. Where the depth to the groundwater is shallow, those soil layers can liquefy. Where the non-liquefying crust is thin, water and soil could make its way to the ground surface via the path of least resistance (i.e. through defects in the crust), depositing it on the ground surface as sand and silt ejecta. Likewise, man-made structures such as power poles can rock back and forth during an earthquake creating weaknesses in the non-liquefying crust creating ejecta pathways. Pore water pressures generated from liquefaction can result in uplift of underground services, further creating ejecta pathways.

In areas near streams, lateral movements can occur causing cracking and land deformations. Where sand or silt is ejected to the ground surface, differential settlement often occurs. Liquefaction can also result in the punching failure of some buildings (i.e. they appear to sink into the ground). In general, liquefaction can cause the ground to subside often differentially. Roads and services can suffer significant damage. The extent of liquefaction induced damage will ultimately depend on the intensity and duration of shaking imposed by the earthquake, soil and topographic conditions locally at the site and also the depth to groundwater.

Further detail on the process and effects of liquefaction are given in Appendix A.

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4 Gisborne seismicity

4.1 GeneralThe following historic earthquakes have caused minor to major damage in the Gisborne region:

Table 4.1: Historic earthquakes causing damage in Gisborne

Date Location Magnitude (Mw)

February 1931 Hawke's Bay earthquake (Napier) 7.8

September 1932 Wairoa 6.9

March and May 1947 Gisborne 6.9-7.1

March, 1966 Gisborne 5.6

March 1976 South of Wairoa 5.4

April 1979 Central Hawkes Bay 5.1

March 1984 Raukumara Peninsula 6.4

November 1989 Gisborne 5.0

August 1993 Ormond 6.3

March 1996 Offshore Poverty Bay 6.0

December 2007 Gisborne 6.6

GDC commissioned a review of the National Seismic Hazard Model (NSHM) for Gisborne to estimate ground shaking intensity likely to be experienced for certain return periods. The Modified Mercalli (MM) intensity scale is a seismic scale used for measuring the intensity of earthquake shaking. It is a measure of the felt level of shaking which is different to the Magnitude (Mw) of an earthquake, which is a measure of the energy released. A high energy earthquake at a great depth, which is located far away from the respective property and which is not felt at the ground surface will have a low MM value. Conversely a very shallow earthquake of lesser energy which causes intense shaking at the ground surface has a high MM. The MM estimates developed from the NSHM shown on Figure 4-1.

Figure 4-1: Ground shaking intensity estimates for Gisborne (Source: Pondard, 2012)

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Figure 4-1 shows that when averaged over many years, Gisborne is likely to experience an earthquake having an MM intensity up to 9 in every 500 years and up to 10 every 2500 years (subject to local site conditions). An earthquake having MM up to 8 can be expected to cause widespread liquefaction induced damage with sand boils and water ejection on alluvial plains (such as the Poverty Bay Flats) causing differential settlement, lateral spreading and topographic re-levelling. An earthquake with MM up to 10 is expected to cause widespread and severe liquefaction induced damage with numerous sand boils and water ejection at the ground surface.

The NSHM review states that smaller (MM 6 and 7) earthquakes are estimated to occur every 9 and 35 years respectively and could be expected to result in a few instances of liquefaction expression at the ground surface (small water and sand ejections) on alluvial plains. This is generally consistent with observations made following the 2007 Gisborne Earthquake which had MM intensities ranging from MM8 near Gisborne to MM3 further afield (discussed further in Section 4.3).

4.2 Seismic HazardThe National Seismic Hazard Model (NSHM) for New Zealand estimates the magnitude of potential seismic shaking that could be generated by movement along various faults (both mapped and unmapped) across New Zealand. This model was recently updated in 2010 to incorporate over 200 new onshore and offshore fault sources and eleven years of seismicity data collected since the earlier NSHM developed in 2000 (Stirling et al., 2012).

The peak ground acceleration caused by an earthquake is a measure of ground acceleration (‘shaking’) expressed in ‘g,’ the acceleration due to the earth’s gravity (1g=9.81m/s2). PGA can be measured by instruments during an earthquake and will vary depending upon the nature of the underlying ground. The peak ground acceleration (PGA) caused by earthquake shaking is one of the measures of seismic hazard estimated by the NSHM. The PGA generated by the model varies depending upon the return period being considered as this alters the probability of exceedance within a given timeframe. For example, a 475 year return period represents a 10% probability of exceedance in 50 years while a 2500 year return period represents a 2% probability of exceedance in 50 years. Figure 4-2 shows the peak ground acceleration estimated by the NSHM for the 475 and 2500 year return periods for shallow soil sites throughout New Zealand and indicates the relatively high seismic hazard present in the Gisborne region.

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Figure 4-2: Seismic hazard map of peak ground acceleration for return period of 475 and 2500 yearreturn periods (10% and 2% probability of exceedance in 50 years) for shallow soil sites

The NSHM is used to estimate seismic loadings for the design of structures in in accordance with NZS 1170 Part 5:2004 Earthquake actions – New Zealand in order that structures comply or exceed specified operational and life safety performance objectives.

This sub-regional assessment of liquefaction vulnerability has been assessed at a 500 year return period level of earthquake shaking. The vulnerability map would look different for smaller or larger levels of seismic shaking. A 500 year return period level of earthquake shaking has been adopted as it is also the return period used to derive seismic loading for Ultimate Limit State (ULS) design of residential buildings in accordance with NZS 1170 Part 5:2004 Earthquake actions – New Zealand, which considers a 7.5 magnitude (Mw) earthquake. For residential dwellings in Gisborne this corresponds to a PGA of 0.48g for shallow soil sites and 0.4g for deep soil sites.

Accordingly, the vulnerability map and associated land damage categories show what liquefaction damage could look like for a large earthquake. Targeting a 500 year return period of earthquake shaking also allows GDC to assess the relative vulnerability of existing and proposed residential dwellings with respect to the life safety performance objectives of NZS1170.5:2004. This objective at ULS shaking is that while the building may sustain significant damage, occupants must be able to safely evacuate the building.

The 2007 Gisborne Earthquake was the third time since 1966 that Gisborne City has experienced recorded peak ground accelerations exceeding 0.25g (Francois-Holden et al., 2008). Ground damage observations are discussed in Section 4.3.

4.3 Recent liquefaction observations (2007 Gisborne Earthquake)The 6.6 magnitude Gisborne Earthquake occurred on Thursday 20 December 2007 at 8.55pm with shaking lasting 32 seconds. The earthquake was located 50km southeast of Gisborne at a depth of 44km. Measured PGAs exceeded 0.1g at three locations as follows: Central Business District of

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Gisborne City (0.28g), near the middle of the Poverty Bay Flats approximately 10km northwest of Gisborne (0.19g) and Tolaga Bay (0.18g). Shaking intensities on the Modified Mercalli scale ranged from MM8 near Gisborne to MM3 further afield (Francois-Holden et al., 2008).

Ground, building and infrastructure damage occurred as a result of the earthquake. Earthquake related ground damage observed following the earthquake included: landslips, lateral spreading, mud volcanos and sand ejecta at the ground surface (sand boils) as a result of liquefaction of the underlying soils.

The photographs below show sand boils observed on the Poverty Bay Flats following the 2007 earthquake. The sand boils were located around 1.5km inland from the mouth of the Waipaoa River. Liquefaction and associated sand boils such as these can cause damage to buildings. Stronger and/or more prolonged seismic shaking could result in more significant land and building damage due to liquefaction.

Figure 4-3: Liquefaction observed near the Waipaoa River during the 2007 Gisborne Earthquake (source:Mr Derek Birks 2007)

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5 Sub-regional liquefaction vulnerability assessment

The sub-regional liquefaction hazard assessment undertaken has drawn on a number of information sources, observations and analyses to develop a vulnerability map using engineering judgement. The following sections describe the assessment methodology including the data sources used and the analyses undertaken to inform that engineering judgement.

5.1 Existing ground investigation dataThis study has included sourcing and collating existing ground investigation data within the study area (machine boreholes, CPT’s, handaugers and Scala penetrometer tests) held on T&T and LDE job files into a database of over 1,200 geotechnical investigations.

Table 5.1 provides a summary of the existing ground investigations reviewed as part of this study and Figure 5-1 shows the investigation locations.

Table 5.1: Investigation data analyses as part of the liquefaction study

Investigation method Number of investigations

Machine Boreholes 62

Cone Penetrometer Tests (CPT) 430

Handauger boreholes and Scala penetrometer tests 725

Figure 5-1: Map showing the locations of the existing investigations

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5.2 Methodology summaryResistance of a soil to liquefaction is dependent on a combination of various soil properties and shaking intensities. Taking into account all of these factors during a regional wide hazard assessment is neither feasible nor practical. Accordingly, liquefaction hazard mapping can be undertaken using one of a number of simplified methodologies.

Generally, the most basic method of mapping is a ‘geological’ approach where maps are developed primarily on existing published geological maps. Liquefaction hazard map boundaries follow the boundaries of geological units shown on the geological maps. This method has limitations as it does not consider the depth at which liquefaction may occur and the level of shaking required to trigger liquefaction, which is critical for predicting the damaging effects at the ground surface.

More sophisticated mapping methodologies require information about the soil and groundwater conditions and consider specific geotechnical parameters and seismic demand at various return periods in order to calculate liquefaction vulnerability indicators such as the Liquefaction Potential Index (LPI) or the Liquefaction Severity Number (LSN).

Many liquefaction hazard maps prepared for Councils in New Zealand (including GDC’s existing Liquefaction hazard map) are based on a basic ‘geological’ approach. However, we do not consider that the existing published geological maps of Gisborne are sufficient for this purpose as they do not accurately portray the nature and liquefaction potential of the underlying soils nor the damage potential at the ground surface.

Accordingly, more detailed information on ground conditions and the groundwater profile across Gisborne was required to improve the existing liquefaction hazard map. This information was obtained through a review of a number of sources as follows:

Published aerial photography Published topography maps LiDAR DEM’s supplied by GDC A significant number of existing ground investigations describing ground conditions and

groundwater levels across Gisborne

A GIS map was set up for the project with layers including; aerial photography, geological maps, LiDAR DEM contours, depth to groundwater, liquefaction vulnerability estimates from review of existing ground investigation data including calculated LSN, SVID and CTL for CPT data. This mapped information was reviewed at a workshop attended by senior staff of T&T and LDE. Review of information on the map layers was carried out at the workshop and liquefaction vulnerability map boundaries were determined using engineering judgement.

5.2.1 Review of existing ground investigation data

A significant number of existing ground investigations (as described in Section 5.1) were reviewed as part of this study. Typical characteristics which make a soil susceptible to liquefaction are as follows:

1 Loose to medium dense 2 Fine grained and non-cohesive (i.e. not a clay) 3 Fully saturated (i.e. typically below the ground water table)

A database of key ground characteristics at the existing ground investigation locations was developed which included details of the soil composition, stratigraphy (layering), strength and

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groundwater levels. The database includes a record of layers of soil within 15m of the ground surface identified as having characteristics making them vulnerable to liquefaction.

Liquefaction of soil within 10m to 15m of the ground surface has the potential to damage the ground surface if the crust thickness is less than 3m (Ishihara, 1985) for magnitude 7.5 earthquakes with shaking < 0.2g. Thicker layers of liquefying soil at shallow depths generally result in more damaging surface effects especially differential settlement. Liquefaction occurring at significant depth does not typically damage the ground surface as settlement caused by deep liquefaction is generally ‘rafted out’ by the non-liquefying soil crust, which also acts to prevent water, sand and silt from ejecting at the ground surface.

A matrix approach has been used to estimate the damage at the ground surface likely to be caused by the layers of liquefiable soil identified in existing ground investigations. The matrix allows for consideration of the more damaging surface effects of thicker layers of liquefying soil at shallow depths. Further detail about the process used when reviewing existing ground investigation data is included in Appendix B.

5.2.2 Liquefaction analyses using existing CPT data

A review of ground conditions shown on existing investigation logs has been supplemented with a large number of CPT-based liquefaction triggering analyses using the existing CPT data. These analyses calculate a Factor of Safety (FoS) for triggering of liquefaction for each CPT and were undertaken for a range of earthquake shaking scenarios (peak ground accelerations). The CPT based triggering analyses undertaken are discussed further in Appendix B which includes a typical analysis output. Liquefaction vulnerability indicators are used as a proxy for land damage consequence at the ground surface and the following are calculated by the above analyses for each CPT investigation;

Cumulative thickness of liquefiable material (CTL)

Calculated settlement (SV1D)

Liquefaction Severity Number (LSN)

Each of the above parameters provides a quantitative indication of the extent of liquefaction damage expected for the earthquake accelerations used in the analyses. All were considered during land vulnerability categorisation and mapping.

5.2.2.1 Liquefaction severity number (LSN)

LSN is one of the vulnerability indicators that has been calculated using the CPT based triggering analyses described in Section 5.2.2 and considered during preparation of the vulnerability map. The theoretical value of LSN varies from 0 (representing no predicted liquefaction damage) to more than 100 (representing very high predicted liquefaction vulnerability). Very large values of LSN can only be calculated when the groundwater table is very close to the ground surface and the soil layers immediately below the ground surface are susceptible to liquefaction.

The LSN parameter correlates with the damaging effects of liquefaction and is documented in van Ballegooy et al. (2014) where it was found that existing liquefaction vulnerability parameters (e.g. LPI and calculated settlement) did not correlate as well with observed land and foundation damage recorded following the Canterbury Earthquake Sequence (2010-2011).

The key difference between LSN and calculated settlement (SV1D) is that the LSN calculation weights the shallow liquefying soil layers as a key contributor to surface land damage. This inference is supported by observations made during the liquefaction induced land damage mapping undertaken following the Canterbury Earthquake Sequence (2010-2011). During this

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mapping it was observed that shallow liquefying soil layers tended to result in the ejection of liquefied material and various forms of ground surface distortion; cracking and fissuring causing significant differential settlement to residential buildings.

Further detail about the LSN liquefaction vulnerability parameter is available in the T&T ‘Liquefaction Vulnerability Study’ (2013) prepared for Canterbury which is publicly available on the Canterbury Geotechnical Database (CERA, 2015).

As discussed in T&T (2013) and van Ballegooy et al. (2015a & 2015b), LSN parameter is currently the best available indicator of liquefaction induced land damage. This applies to Christchurch and would also apply to Gisborne during future seismic shaking. This is because the geomorphology and ground conditions in Gisborne and particularly the Poverty Bay Flats, are similar to the Eastern Christchurch setting. In addition, this assessment is particularly concerned with potential damage at the ground surface. Accordingly, the ability of LSN to account for the more damaging effects of shallow liquefaction is appropriate and beneficial. When mapped, the calculated values of LSN generally correlate well with our appreciation of liquefaction vulnerability across Gisborne based on our experience having undertaken a number of geotechnical investigations and liquefaction assessments in the area and also with observations made following the 2007 Gisborne earthquake.

Accordingly, the land vulnerability categories selected for this study have been aligned with published LSN ranges correlating to categorised levels of land damage observed in Canterbury as shown in Table 5.2 (Russell et al., 2015).

Table 5.2: LSN ranges for varying levels of land damage in Canterbury

Published LSN range Simplified land damage observation category

Land vulnerability category for Gisborne1

0-15 None to minor Low

16-25 Minor to moderate Moderate

26+ Moderate to severe High


5.3 Typical land damage observations and photographsThe typical land damages observed in Canterbury following the Canterbury Earthquake Sequence (2010-2011) for each ‘land damage category’ are shown in Table 5.3. Appendix C also contains a number of photographs of land damage observed for each category.

The land damage described in Table 5.3 and shown in the photographs in Appendix represents typical behaviour only. Land performance for a particular site may vary due to natural variations and other factors which effect site performance.

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Table 5.3 Typical land damage observations

Simplified land damage category

Typical land damage observations

None to minor No observed cracks, undulations/deformations at the ground surface No signs of ejected material at the ground surface No apparent lateral movement No apparent building damage

Minor to moderate Minor cracking (typically <50mm), buckling and undulations at the ground surface


No apparent lateral movement

Moderate to severe Moderate to large cracks with horizontal and vertical displacement (>50mm), buckling and undulations at the ground surface


Noticeable ground surface subsidence Lateral spreading

5.4 Liquefaction hazard mappingA liquefaction hazard map for Gisborne has been developed based on the sub-regional liquefaction assessment undertaken using the various information sources summarised in Section 5.2, including the review and assessment of over 1,200 existing geotechnical investigations. The vulnerability map zones land within the study area into one of the three land vulnerability categories based on a large earthquake event (see Section 4.2) as summarised in Table 5.4. The map would be different if mapping was undertaken at different levels of earthquake shaking.

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Table 5.4: Land vulnerability categories for mapping

Land vulnerability category1

Colour Simplified land damage category

Typical land damage and consequent building performance2

Low Yellow None to minor No observed cracks, undulations/deformations at the ground surface

No signs of ejected material at the ground surface No apparent lateral movement No apparent differential settlement

Moderate Orange Minor to moderate

Minor cracking (typically <50mm), buckling and undulations at the ground surface


No apparent lateral movement Differential settlement of buildings causing minor to

moderate structural damage

High Red Moderate to severe

Moderate to large cracks with horizontal and vertical displacement (>50mm), buckling and undulations at the ground surface


Noticeable ground surface subsidence Lateral spreading Significant total and differential settlement of

buildings causing structural damage


2. The land and building damage outlined for each land vulnerability category represents the typical likely behaviour of flat land and light residential dwellings only. Land damage for a particular site may vary due to natural variations and many other factors which effect site performance. Building damage could vary significantly from the typical description as the extent of damage will depend on many building specific variables including the type and shape of the building and foundation stiffness.

The liquefaction vulnerability map prepared during this study is shown in Figure 5-2. A digital copy (GIS shapefile) of the map accompanies this report.

The broad sub-regional assessment and mapping undertaken have not allowed for an assessment at an individual property scale. Geotechnical investigations and liquefaction assessment for an individual property which considers the specific site topography, geology and long term depth to groundwater monitoring may in many cases indicate that the site can be categorised differently from that shown on the vulnerability map. It must also be appreciated that a site can range from low vulnerability land though to high vulnerability depending upon the property size, topography and underlying geology. Accordingly, the map should not be used as a replacement for a site specific liquefaction assessment of a property.

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Figure 5-2: Map showing liquefaction vulnerability for the study area

5.5 Data reliabilityExisting geotechnical investigation data relied upon for the hazard map varies in density, quality and depth. Deep investigation data provides better information and a greater degree of confidence in the mapped liquefaction vulnerability. This data is generally concentrated in the existing urban areas. Ground investigation data is typically shallow and sparse over much of the

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rural area meaning the hazard map relies more heavily on other information sources described in this report and is assigned a lower degree of confidence.

Figure 5-3: Map showing the location of deep existing geotechnical investigations concentrated in theurban area.

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6 Geotechnical assessment guidance

It is important that appropriate geotechnical investigations and a liquefaction assessment is carried out for future developments. Preliminary guidance on appropriate geotechnical assessments with respect to the three land vulnerability categories is given in Table 6.1 and Sections 6.1 and 6.2. The guidance given is for the assessment of liquefaction only and additional assessments may be required to investigate other geotechnical concerns such as foundation bearing capacity, settlement or slope stability.

These preliminary recommendations form initial guidance for GDC only and additional work is required to develop them further prior to Council implementation. Drill method, depth and the method of liquefaction analysis would all need to be considered when developing detailed assessment criteria. Complexities introduced by the size and nature of future development will also need to be considered in terms of what is practical and economic. For example, very low importance structures may not warrant deep and costly geotechnical investigations appropriate for higher importance structures.

Table 6.1: Geotechnical assessment recommendations

Land vulnerability Category

Colour Geotechnical Liquefaction Assessment

Preliminary geotechnical assessment recommendations

Low Yellow Low level investigation*

Shallow geotechnical investigations with the engineering assessment likely to focus primarily on other geotechnical concerns

Moderate Orange Moderate level investigation*

Deep geotechnical investigations and quantitative liquefaction analysis may be required.

High Red High level investigation

Deep geotechnical investigations and quantitative liquefaction analysis.

* A low and moderate level investigation is generally only considered appropriate for residential and small commercial development. Lifeline infrastructure, important buildings (e.g. hospitals and other public buildings), large commercial and large scale subdivisions should all be subject to a high level geotechnical investigation.

6.1 Low level geotechnical assessmentLow level geotechnical assessment should address liquefaction, but the key focus is likely to be on other site specific geotechnical concerns. Geotechnical investigations are likely to be scoped in a manner which targets other geotechnical issues such as slope stability, settlement or foundation bearing capacity.

Minimum geotechnical investigation requirements for residential and light commercial buildings in Gisborne are detailed in the GDC guideline ‘Bearing Capacity and Geotechnical Investigation Requirements for Buildings, November 2011’. This guideline requires shallow ground testing using simple hand held equipment to a depth of at least 2.5m. For site specific assessments we consider that a liquefaction assessment based on a site walkover and a review of the ground conditions including ground water levels encountered during this type of investigation should typically be suitable for a low vulnerability site. This would not be sufficient for subdivision or other land use planning purposes.

6.2 Moderate and high level geotechnical assessmentsWe consider that in most cases, moderate and high level geotechnical assessments should include deep CPT and/or machine borehole investigations and a quantitative liquefaction analysis. In

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many cases it is likely that robust foundations or site ground improvement may be required if the site is confirmed as being vulnerable.

High vulnerability land is typically expected to require more robust foundations and/or more ground improvement than moderately vulnerable land. The engineering analysis undertaken for these two categories is likely to be similar. However, geotechnical investigations are likely to be more extensive (e.g. a greater density of CPT’s or boreholes) for a high level geotechnical assessment. It is important that the level of geotechnical investigations for these land vulnerability categories is assessed by a competent geotechnical engineer on a site by site basis depending on the proposed development.

It should be noted that development works can increase land vulnerability to liquefaction induced damage. Works that could increase liquefaction vulnerability include:

Surcharging the land by filling. Making roads lower than surrounding properties. Increasing groundwater levels and/or piezometric pressures by putting stormwater and/or

effluent waste water onto or into the land. Excavating to create drainage channels or an unsupported free face (see Section 8.2). Penetration of the surface crust.

The effect of development works on liquefaction vulnerability of the site should be considered as part of moderate and high level geotechnical assessments.

6.3 Canterbury approach to liquefaction investigations andassessment

The preliminary geotechnical assessment guidance given in Sections 6.0 to 6.2 of this report is generally consistent with recommendations on the scope of geotechnical investigations required for residential buildings in Canterbury given in the Ministry for Business Innovation and Employment (MBIE 2012) technical guidance titled ‘Repairing and rebuilding houses affected by the Canterbury earthquakes.’ .

Generally this guidance recommends, with respect to liquefaction only, the use of procedures outlined in NZS3604:2011 for areas where future land damage from liquefaction is unlikely based on the observations from the earthquake sequence (MBIE, 2012).

Deep geotechnical investigations are recommended for land where liquefaction damage is possible in future moderate and large earthquakes. The guidance recommends that the scope of the deep geotechnical investigation be determined by a chartered professional geotechnical engineer (CPEng).

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7 Mitigation

The successful management of natural hazards involves a balance between land-use controls, building controls, insurance, and acceptance of risk. The liquefaction vulnerability map prepared is intended to help GDC assess vulnerability to liquefaction induced damage at the ground surface across the study area, enabling Council to implement appropriate building controls, thus promoting resilience to liquefaction in the Gisborne community. Appropriate controls are likely to involve either avoiding areas that, following site specific assessment (at various levels of earthquake shaking), are assessed as being vulnerable to liquefaction damage or appropriately mitigating the effects of liquefaction.

Generally, there are two possible approaches to mitigating liquefaction effects:

1. Accept that liquefaction may occur and design foundations, buildings, services and infrastructure to tolerate the resulting ground surface deformation and other consequences.

2. Improve the ground in such a manner that either liquefaction does not occur or the severity of its effects are reduced. Options which improve land are likely to provide a greater level of community resilience to earthquake events as this option not only provides greater protection to buildings but also to the surrounding land, services, and infrastructure. This is particularly the case where the land improvement is applied on an area-wide basis (e.g. large scale subdivisions) rather than localised land improvement beneath individual buildings. However, this approach may not be applicable in all situations depending on the nature and scale of the development.

Often a combination of the two approaches may be appropriate.

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8 Suggested further study

During this work we have noted additional work items which GDC may consider undertaking to better refine and build on this study.

8.1 Development of geotechnical assessment criteria for landvulnerability categories

The scope of this study does not allow for preparing detailed, prescriptive recommendations or criteria identifying the extent of geotechnical investigations and liquefaction analysis required for each land category for resource or building consent purposes. We recommend this work be undertaken. Further detail on recommended geotechnical investigations and liquefaction analysis criteria could be prepared for GDC if required.

8.2 Lateral spread considerationLateral spreading is the displacement of the ground horizontally with shaking. Associated vertical displacement can also occur. Lateral spreading can occur on sloping or unrestrained ground (ground adjoining a river, foreshore, drainage channel or other free face) as a result of the strength loss of the underlying liquefied soil layers on very gently sloping land (less than 1 in 100). Lateral spreading of a number of metres can occur immediately adjoining a free face, and large cracks (up to 0.5m wide) can appear at a distance of up to 200m from a free face. Topographic re-levelling can also occur.

Lateral or topographic re-levelling has not been considered as part of this study but has been identified as a risk within the current study area and was observed in the 2007 Gisborne Earthquake. Further studies could use the database of investigation information collected in this study along with topographic information to assess the risk of lateral spread across the study area.

8.3 Groundwater model refinementLiquefaction vulnerability is very sensitive to the depth of the groundwater level. Groundwater information available for this study is limited to levels captured on the existing ground investigations at the time the investigation was undertaken. A more robust groundwater model could be developed for the study area with the existing groundwater information already captured. Ideally this would also include installing and monitoring a number of piezometers across the study area to capture ground water level data. Sea level rise and climate change effects could also be considered.

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9 Conclusions

Gisborne is exposed to significant potential seismic shaking and some parts of the region are vulnerable to liquefaction induced damage. This study provides GDC with a sub-regional assessment of liquefaction vulnerability which will assist GDC to better understand the hazard and appropriate responses, including determining appropriate levels of site specific geotechnical investigations and assessment. Suitable site specific assessment of potential liquefaction damage resulting from seismic shaking should enable Council to implement appropriate building controls to develop greater resilience to liquefaction damage in the Gisborne community. Appropriate Land-use controls should also be developed.

The study relies on various information sources including an assessment of over 1,200 existing geotechnical investigations. The liquefaction hazard has been mapped into three land vulnerability categories (low, moderate and high) based on the assessed liquefaction vulnerability across the study area as shown on Figure 5-2. This report also makes preliminary recommendations as to appropriate levels of geotechnical assessment for each land vulnerability category.

The following key conclusions have been drawn:

1 A liquefaction hazard map has been developed using various sources of information and analyses to inform engineering judgement

2 The hazard map shows the relative liquefaction vulnerability at 1 in 500 year levels of earthquake shaking. Different levels of earthquake shaking will produce a different map of liquefaction vulnerability

3 The hazard map provides GDC with a sub-regional assessment of liquefaction vulnerability as a basis for determining appropriate levels of site specific geotechnical investigations and assessment

4 The liquefaction hazard map does not include vulnerability to lateral spreading 5 The geotechnical assessment recommendations made are intended for residential and

small commercial development only. We would expect lifeline infrastructure, important buildings (e.g. hospitals and other public buildings), large commercial and large scale subdivisions to be subject to a high level of site specific geotechnical investigations and assessment

6 The liquefaction assessment carried out has been mapped at a sub-regional scale and is not to be viewed at scales more detailed than 1:10 000 in urban areas and 1:25 000 in rural areas. Accordingly, the map should not be used as a replacement for a site-specific liquefaction assessment of a property.

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10 Limitations

The liquefaction hazard map has been developed primarily based on information obtained from analysis of existing geotechnical investigation data and LiDAR topography mapped onto a digital aerial photograph base. The information presented is only an indication of what we consider to be the potential for liquefaction induced land damage at the ground surface for 500 year return period levels of shaking. We note that site specific assessments should also consider effects at other appropriate return periods of shaking.

Liquefaction damage at the ground surface can be very sensitive to groundwater level. We specifically note that soils across the study area are highly variable and the groundwater information available for this study is limited to levels captured at the time a given investigation was undertaken. There are areas within this study which will have significant seasonal fluctuation in the groundwater level. Accordingly, the map should not be used as a replacement for a site-specific liquefaction assessment of a property. Appropriate geotechnical investigation and assessment at a property-scale which considers the site topography, geology, soil strength and stiffness characteristics and depth to groundwater may in many cases indicate that the site can be classified differently from that shown on the vulnerability map. It must also be appreciated that a site can range from low vulnerability though to high vulnerability depending upon the property size, topography and underlying geology.

Properties that straddle two categories shown on the map should be investigated based on the higher geotechnical assessment level category.

We note that the hazard map prepared shows likely vulnerability to liquefaction induced land damage at the ground surface. Liquefaction is one of a number of forms of earthquake related ground damage including: landslips, rockfall, ground cracking, lateral spread and mud volcanos which are outside the scope of this study.

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12 Bibliography

Boulanger, R. W. and Idriss, I. M. (2014) “CPT and SPT based liquefaction triggering procedures.” Report No. UCD/CGM-14/01, Center for Geotechnical Modelling, Department of Civil and Environmental Engineering, University of California, Davis, CA, 134p.

Brown, L.J., 1995. Holocene shoreline depositional processes at Poverty Bay, a tectonically active area, northeastern North Island, New Zealand. Quat. Int. 26, 21–33.

Fairless, G.J.; Berrill, J.B. 1984: Liquefaction during historic earthquakes in New Zealand. Bulletin of the New Zealand National Society for Earthquake Engineering 17: 280-291.

Francois-Holden, C., Bannister, S., Beavan, J., Cousins, J., Field, B., McCaffrey, R., McVerry, G., Reyners, M., Ristau, J., Samsonov, S., Wallace, L., 2008. The Mw 6.6 Gisborne Earthquake of 2007: Preliminary Records and General Source Characterisation. Bulletin of the New Zealand Society for Earthquake Engineering 41(4), 266-277.

Gomez, Basil, Dennis N. Eden, David H. Peacock, and Edward J. Pinkney. “Floodplain Construction by Recent, Rapid Vertical Accretion: Waipaoa River, New Zealand.” Earth Surface Processes and Landforms 23, no. 5 (1998): 405–13.

Holzer T.L., 2008. Probabilistic liquefaction hazard mapping. Geotechnical Special Publication. (181).

MBIE. (2012). Repairing and rebuilding houses affected by the Canterbury earthquakes. Ministry of Business, Innovation and Employment. December 2012.

Mazengarb, C.: Speden, I.G. (compilers) 2000. Geology of the Raukumara area. Institute of Geological and Nuclear Sciences 1:250 000 geological map 6. 1 sheet and 60 p. Lower Hutt, New Zealand: Institute of Geological and Nuclear Sciences Ltd.

NZS 1170.5:2004 Structural Design Actions – Earthquake Actions. Section 3 – Site Hazard Spectra, Standards New Zealand.

Pondard, N.; 2012. Review and update of earthquake return periods for the Gisborne region, GNS Science Consultancy Report CR 2012/71. 22p.

Pullar, W.A., Penhale, H.R., 1970. Periods of recent in lling of the Gisborne plains basin. N. Z. J. Sci. 13, 410–434.

Russell, J., Van Ballegooy, S., Rogers, N., Jacka, M.E., 2015. The Effect of Subsidence on Liquefaction Vulnerability Following the 2010-2011 Canterbury Earthquake Sequence. Proceedings of the 12th Australia and New Zealand Conference on Geomechanics.

Stirling, M.W., McVerry, G.H., Gerstenberger, M., Litchfield, N.J., Van Dissen, R., Berryman, K.R., Langridge, R.M., Nicol, A., Smith, W.D., Villamor, P., Wallace, L., Clark, K., Reyners, M., Barnes, P., Lamarche, G., Nodder, S., Pettinga, J., Bradley, B., Buxton, R., Rhoades, D., Jacobs, K. 2012. National Seismic Hazard Model for New Zealand: 2010 Update. Bulletin of the Seismological Society of America, 102(4), 1514-1542.

Tonkin & Taylor Ltd., 2013. Liquefaction Vulnerability Study. Report to Earthquake Commission, Tonkin & Taylor ref. 52020.0200/v1.0, prepared by S. van Ballegooy and P. Malan, available at https://canterburygeotechnicaldatabase.projectorbit.com.

Van Ballegooy, S., Malan, P., Lacrosse, V., Jacka, M.E., Cubrinovski, M., Bray, J.D., O’Rourke, T., Crawford, S., Cowan, H., 2014b. Assessment of Liquefaction-Induced Land Damage for Residential Christchurch. Earthquake Spectra, EERI, 30(1), 31 – 55.

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Van Ballegooy, S., Boulanger, R. W., Wentz, R., 2015a. Evaluation of a CPT-based Liquefaction Procedure at Regional Scale. Soil Dynamics and Earthquake Engineering, Special Issue: Liquefaction in New Zealand and Japan. in review

Van Ballegooy, S., Green, R., Lees, J., Wentz, F., Maurer, B., 2015b. Assessment of Various CPT Based Liquefaction Severity Index Frameworks Relative to the Ishihara (1985) H1 – H2 Boundary Curves. Soil Dynamics and Earthquake Engineering, Special Issue: Liquefaction in New Zealand and Japan. in review

Van Ballegooy, S., Lacrosse, V., Simpson, J., and Malan, P., 2015c. Comparison of CPT-based Simplified Liquefaction Assessment Methodologies based on the Canterbury Dataset. Proceedings of the 12th Australia and New Zealand Conference on Geomechanics.

Wolinsky, M.A., Swenson, J.B., Litchfield, N., and McNinch, J.E., 2010. Coastal progradation and sediment partitioning in the Holocene Waipaoa Sedimentary System, New Zealand. Marine Geology, 270(1): 94-107.

Appendix A Liquefaction description


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A1 Seismic Hazards - Liquefaction

Liquefaction is where loose soils below the groundwater level lose strength and stiffness in response to an applied cyclic force, like earthquake shaking. Liquefaction can cause damage to land, buildings and infrastructure. Only some soil types are susceptible to liquefaction and only some earthquakes are strong enough to cause liquefaction. Geotechnical investigations and assessment can estimate the likelihood and consequence making up the risk of liquefaction for a specific site.

A1.1 Process

The process of liquefaction is described by Figure A1 and the commentary below.

Figure A1: Liquefaction process

During Shaking

The support of the overlying ground is transferred from the soil grains to the water between the soil grains. The result is a large increase in water pressure and a loss of soil shear strength (i.e. it becomes like a viscous liquid).

After Shaking

The high water pressures result in water and soil escaping to the surface as sand boils (See Section A1.4.1). The soil grains re-orientate into a denser configuration. This densification in conjunction with the expulsion of soil and water to the surface, usually resulting in differential settlement.

A1.2 Susceptible Soils

Liquefaction only occurs in some soil. Liquefaction susceptible soils are typically:

Sands and non-plastic silts are most susceptible to liquefaction Gravels can liquefy if they have a low permeability matrix or confining layers top and

bottom Clays are too cohesive to liquefy

The distinction between silts that are liquefiable or not are described as either being:

“Sand-like behaviour” and therefore susceptible to liquefaction “Clay-like behaviour” and therefore not susceptible to liquefaction

The NZ Geotechnical Society “Guideline for the identification, assessment and mitigation of liquefaction hazards” (NZGS, 2010) provides further criteria for the assessment of liquefaction susceptible soils. Particular guidance is provided for fine grained soils (silts etc…).

A1.3 Triggering

The intensity and duration of earthquake shaking required to cause (trigger) liquefaction of susceptible soil (Refer Section A1.2) varies depending on the density and fines content of the soil. The likelihood (return period) of earthquake shaking to trigger liquefaction is assessed by considering:

• The local seismic hazard. The likelihood (return period) of earthquakes of various duration (magnitude) and intensity (peak ground acceleration, PGA)

• Field penetration test (CPT and SPT) and fines content results for the soil, and available empirical relationships between these results and the magnitude and PGA to trigger liquefaction.

A1.4 Liquefaction Effects

There is a number of liquefaction effects each of which affect buildings and infrastructure differently. The risk of earthquake induced damage can be accepted, mitigated or avoided.

A1.4.1 Surface Ejection of Soil and Water (Sand Boils)

Liquefied soils often release their water pressures to the surface. This is particularly evident where the crust of non-liquefying soil is relatively thin. This can result in water and soil being ejected to the surface. These are observed as “sand boils” or mini volcanos. This flow of water and soil to the surface can damage floor slabs, pavements and services. Of the effects of liquefaction sand boils is typically the most damaging to residential developments. It results in; uneven subsidence of the ground surface, and damage to buildings, paved surfaces and infrastructure.

Figure A2: Surface ejection of soil and water as sand boils (Christchurch 2011)

A1.4.2 Buoyancy

Liquefaction raised ground water pressures cause buoyancy forces on structures and services. These forces along with the reduced strength of liquefied soils can lead to uplift of pipes, manholes, chambers and swimming pools extending below the groundwater level.


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Figure A3: Manhole uplifted due to buoyancy (Christchurch 2011)

A1.4.3 Bearing Capacity Failure

Liquefaction causes a loss of soil strength and stiffness resulting in reduced support (bearing capacity) to shallow foundations. This can result in subsidence of both shallow and deep foundations if the liquefiable layer is directly beneath the foundation.

A1.4.4 Lateral Spreading

Lateral spreading is the displacement of the ground horizontally with shaking. Associated vertical displacement can also occur. Lateral spreading can occur on sloping or unrestrained ground (ground adjoining a river, foreshore or other free face). It is as a result of ground sliding on a liquefied layer during and possibly after (flow failure) the shaking. Sloping ground only needs to be very gentle for lateral spreading to occur. Lateral spreading of a number of meters can occur immediately adjoining a free face, and can be a number of tens of millimetres at 100m distance. However, in Christchurch where there was gently sloping ground back from the free face, displacements of 100’s of millimetres at 100 metres back from the free face were observed.

Figure A4: Lateral spreading showing horizontal and vertical displacement of land causing damage tostructures. Image is of damage to Kobe Port preserved in the Kobe Port Memorial park following the1995 earthquake (http://www.kevinbertman.co.uk/japan/earthquake.shtml)

A1.4.5 Settlement

As discussed in Section A1.1, settlement can occur from densification of the liquefied soil layer and from expulsion of water and soil to the surface. Settlements of the ground surface are broken down in to two components:

• Total settlement – General overall settlement of the area

• Differential settlement – The difference in settlement between points within the area

• Settlement of a few hundred millimetres can occur depending on the thickness, depth and density, of the liquefied layer. Settlement can cause damage to buildings and infrastructure.

In Christchurch damage as a result of settlement was relatively small compared to that attributed to sand boils (refer A1.4.1) and loss of support to foundations (refer A1.4.3).


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Appendix B Liquefaction hazard assessment methodologyand CPT triggering analysis output

B1 Review of existing ground investigation data

Investigation data has been assessed using a matrix developed to prioritise the more damaging surface effects of thicker layers of liquefying soil at shallow depths. The matrix shown in Figure B1 assigns a ‘consequence rating’ to each investigation point based on the location and thickness of the liquefiable soil layers identified during the review of the existing investigation logs. The thickness of the liquefiable layer is recorded as a percentage of liquefiable material within the relevant ‘location layer’ as shown in Figure B1. For example, a liquefiable soil layer 500mm thick at 0.5m deep represents 50% of the relevant 0-1m location layer and therefore has a consequence rating of 80.

As the consequence rating increases, so does the risk of liquefaction induced land damage at the ground surface (differential settlements and sand boils etc…). The matrix applies rating increments (2 to 10) in a manner which weights the consequence rating values given by the matrix to be higher for thicker liquefiable soil layers located at shallow depths. The consequence rating for a borehole identified to have multiple layers of liquefiable material is the sum of the values determined for each layer from the matrix. For example, following the matrix shown in Figure B1, if a borehole with three liquefiable soil layers each 500mm thick at 0.5m, 2m and 4m below the ground surface has a consequence rating of 172 (50% at 0-1m, 25% at 2-4m and 12.5% at 4-8m =100+48+24=172). This example is presented graphically in Figure B2.

Thickness Less than 5%

Less than 10%

Less than 20%

Less than 50%

Less than 100%

Location Rating 2 4 6 8 10

0-1m 10 20 40 60 80 100

1-2m 8 16 32 48 64 80

2-4m 6 12 24 36 48 60

4-8m 4 8 16 24 32 40

>8m 2 4 8 12 16 20

Figure B1: The assessment matrix used for the study

Thickness rating

Location rating

Consequence rating

10 10 100

6 8 48

4 6 24

Total 172

Figure B2: Diagram showing the decreasing consequence rating for deeper liquefiable soil layers


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Consequence rating value ranges have been determined for the land vulnerability categories (and the associated LSN ranges) discussed in Section 5.3 of this report and are shown in Table B1. These ranges were assigned based on a comparison of the calculated ‘consequence rating’ values with the calculated LSN values for existing CPT investigations. Consequence ratings for all investigation points were mapped and reviewed to ensure that they generally correlate well with our appreciation of liquefaction vulnerability across Gisborne based on our experience having undertaken a number of geotechnical investigations and liquefaction assessments in the area and observations following the 2007 Gisborne earthquake.

Table B1: Liquefaction vulnerability parameter ranges for varying levels of land damage

Published LSN range Simplified land damage observation category

Consequence Rating range

Land vulnerability category for Gisborne

0-15 None to minor 0-60 Low (yellow)

16-25 Minor to moderate 61-140 Moderate (orange)

26+ Moderate to severe 141 -300 High (red)

B2 Quantitative assessment using existing CPT data

Quantitative analysis of CPT data within the study area was undertaken to calculate internationally recognised liquefaction vulnerability indicators. A CPT based liquefaction triggering analysis was undertaken in accordance with the method outlined in Boulanger and Idriss, (2014) which calculates a Factor of Safety (FoS) of triggering of liquefaction for each layer of a CPT data point. Analyses were undertaken for each CPT location for a range of peak ground accelerations including Ultimate Limit State (ULS, 0.4g) and Serviceability Limit State (SLS, 0.1g) at an earthquake magnitude of 7.5. Analyses were also undertaken for 0.2g, 0.3g and 0.5g loadings.

Liquefaction vulnerability indicators calculated in the above analyses included calculated settlements (SVID), the cumulative thickness of liquefiable material (CTL) and the Liquefaction Severity Number (LSN). The CTL parameter represents the sum of the thickness of all layers within the CPT that have a calculated FoS < 1.0 (i.e. they are likely to liquefy at the given level of shaking). LSN represents the more damaging effects of shallow liquefaction as it incorporates a depth weighted function. A typical CPT liquefaction triggering analysis output follows and includes ‘Liquefaction Assessment Notes’ which give additional detail about the analyses undertaken.

(Assumed pre-drill values)

CPT Name Database ID Investigation Date Event and PGA Magnitude PGA (g)

GWD (m)

Trigger Method Settlement Method Pre-drill Depth (m)

Qc (MPa) Fs (MPa) ɣ (kN/m³)

INPUT CPT1p 52068 19/10/2010 User Specified 7.5 0.4 1.4 BI2014 ZRB 0 2 0.01 18

Exceedance Probability S - Calculated Settlement (mm) CTL - Cumulative Thickness of Liquefaction (m)

LPI - Liquefaction Potential Index LSN - Liquefaction Severity Number CT - Crust Thickness (m) LPI Ishihara

OUTPUT 15% 44 2 7 10 1.5 6

50% 41 1.8 6 9 1.5 4

85% 37 1.5 5 7 6.2 3

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CLIENT, PROJECT

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Environmental and Engineering consultants

TITLE

Gisborne City CouncilGisborne Liquefaction Mapping

CPT Liquefaction Analysis

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30214

Gisborne

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1. Sensitive, fine grained 6. Sands - clean sand to silty sand

2. Organic soils - peats 7. Gravelly sand to dense sand

3. Clays - silty clay to clay 8. Very stiff sand to clayey sand *

4. Silt mixtures - clayey silt to silty clay 9. Very stiff, fine grained *

5. Sand mixtures - silty sand to sandy silt

*Heavily overconsolidated or cemented

CPT-based soil behavior type classification chart by Robertson (1990)

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Gisborne

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V1.3

(Assumed pre-drill values)

CPT Name ID Investigation Date

Event and PGA Magnitude PGA (g) GWD (m) Trigger Method Settlement Method Pre-drill Depth (m) Qc (MPa) Fs (MPa) ɣ (kN/m³)

CPT1p 52068 19/10/2010 User Specified 7.5 0.4 Varies BI2014 ZRB 0 2 0.01 18

Thicker lines represent the 50% probability of exceedence case and the thinner lines to the left and right of the thicker lines represent the 85% and 15% probability of exceedance cases respectively.

3 of 7 pages

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Tonkin & Taylor


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LOCATION

30214

Gisborne

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CPT ID 52068

CPT Name CPT1p

PGA a= 0.4g

Magnitude Mw= 7.5

Depth to groundwater 1.4m

Predrill depth 0m

Assumed predrill tip resistance and skin friction Qc= 2MPa & Fs= 0.01MPa

Trigger method Boulanger & Idriss (2014)

Settlement method ZRB

Probability of Liquefaction PL%= 0%

CFC 0

Total depth of CPT 15

Maximum depth of analysis

RL

The inputs listed in Table 1.1-1 below have been adopted for the liquefaction analysis.

Table 1.1-1 Summary of inputs for liquefaction analysis

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Liquefaction Assessment Notes

1.1 Liquefaction triggering analysis (Boulanger & Idriss, 2014)

This CPT based triggering analysis calculates a factor of safety (FOS) against liquefaction for each CPT data point. The factor of safety comparison uses the principles of the ‘simplified procedure’ developed by Seed & Idriss (1971), as set out in the Boulanger & Idriss (2014) publication, was used for this analysis. The simplified procedure compares the seismic demand imposed by an earthquake (referred to as the Cyclic Stress Ratio, or CSR) with the resistance of the soil to liquefaction (referred to as the Cyclic Resistance Ration, or CRR).

The CSR varies with depth and is calculated as a function of the peak ground acceleration and earthquake magnitude. The CRR is a function of the normalised tip resistance of the CPT, corrected for fines content. The fines content in this calculation is based either on laboratory test results, or the apparent fines content assessed by Boulanger & Idriss (2014).

The CPT can be used to assess the behaviour of the soil, and whether it responds as a fine grained or coarse grained material. Robertson & Wride (1998) presented formulae to calculate a single index, the Ic, to represent the behaviour of the soils. The higher the Ic, the more fine grained the material behaviour is. The Ic is based on the normalised cone tip resistance and skin friction. For this calculation, the normalised values are based on the iterated results presented by Robertson & Wride (1998). Where the calculated Ic exceeds 2.6 the soil is assessed to be too fine grained to liquefy.

1.2 Assumptions

The following assumptions have been made in the liquefaction analyses:

1. The material is a standard material consistent with the empirical liquefaction databases. In other words, material is not welded, cemented or pumiceous in a manner that affects the penetration test results.

2. The pore pressure profile is hydrostatic unless specified in Table 1.1-1

1.3 Calculated volumetric strain and settlements (Zhang, Robertson & Brachman)

Soils that have liquefaction potential densify if subjected to cyclic shearing. This calculated volumetric densification has been investigated by Ishihara & Yoshimine (1992) and Zhang et al. (2002). Ishihara & Yoshimine (1992) presented a series of curves that calculate the volumetric densification of sand samples based on their relative density and factor of safety against liquefaction. The curves are based on samples of sand tested in a laboratory at different densities and undergoing cycles of shear strains. The factor of safety is based on the simplified method originally presented by Seed and Idriss (1971). The curves were updated by Zhang et al. (2002) and included the normalisation of CPT values in the correlations.

The outputs of the calculation are volumetric strains, which can be integrated through layers of potentially liquefiable materials to estimate settlements of the ground.

1.4 Calculated liquefaction vulnerability indicators

1.4.1 Crust thickness

This is the thickness of the non-liquefiable crust at the top of the CPT. It is at least the depth to saturated soil beneath the groundwater table, and can be deeper.

1.4.2 Cumulative Thickness of Liquefaction (CTL)

This parameter represents the sum of all layers within the CPT that have a calculated FoS < 1.0

1.4.3 Calculated settlement

The settlements have been calculated by integrating the calculated volumetric strains with depth. The volumetric strains are calculated using the Boulanger & Idriss (2014) and with the normalised CPT tip resistance based on Zhang, Robertson & Brachman (2002) (ZRB) (Recommended):

Note that the calculated settlements often do not correlate well with observed settlements but are better understood as a proxy for the likelihood of damage at the ground surface. As the calculated settlement increases, so does the risk of adverse effects at the ground surface.

1.4.4 Liquefaction Potential Index (LPI)

The vulnerability of sites to liquefaction was considered by Iwasaki et al. (1978) and Iwasaki et al. (1982). Iwasaki’s Liquefaction Potential Index (LPI) is presented as a measure of the vulnerability of sites to liquefaction effects. The LPI is defined as:

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Where W(z) = 10 – 0.5z, F1 = 1-FOS for FOS<1.0, F1 = 0 for FOS>1.0 and z is the depth below the ground surface. FS is the factor of safety calculated using the Boulanger & Idriss (2014) method.

The paper notes that the LPI values range from 0 to 100, with the following published indicators of liquefaction induced damage:

LPI range Damage

LPI = 0 Liquefaction risk is very low

0 < LPI ≤ 5 Liquefaction risk is low

5 < LPI ≤ 15 Liquefaction risk is high

LPI >15 Liquefaction risk is very high

Some researchers have subsequently updated the calculation and interpretation based on field mapping during earthquakes.

1.4.5 Liquefaction Severity Number (LSN)

The Liquefaction Severity Number (LSN) represents the more damaging effects of shallow liquefaction on residential land and foundations. The formula used to calculate LSN is presented below. LSN considers volumetric densification strain within layers as a proxy for severity of liquefaction.

Where εv is the calculated volumetric densification strain using Boulanger & Idriss (2014), and z is the depth to the layer of interest.

As the LSN increases, so does the risk of severe effects on the land and residential dwelling. In general, the following levels are reasonable for a typical groundwater table and using Boulanger & Idriss (2014) triggering:

LSN Effects

0-5 Negligible to Minor, no major effects expected

5-20 Minor, generally consistent with acceptable performance under SLS conditions (i.e. little settlement or permanent dwelling damage). Ejection of material can be expected at the ground surface, but likely to be localised in nature.

20-40 Moderate, liquefaction evidence possible and lateral spreading possible if free faces present. Generally consistent with acceptable performance under ULS conditions (i.e. settlement)

>40 Severe, high risk of substantial damage to the site and/or dwelling

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2.0 References

Seed, H.B. & Idriss, I.M. (1971). Simplified procedure for evaluating soil liquefaction potential: Proceeding of the American Society of Civil Engineers, Journal of the Soil Mechanics and Foundations Division, v. 93, no. SM9, 1249-1273

Robertson, P.K. & Wride, C.E. (1998). Evaluating cyclic liquefaction potential using the cone penetration test, Canadian Geotechnical Journal, 35:442 – 459

Ishihara, K. & Yoshimine, M. (1992). Evaluation of settlements in sand deposits following liquefaction during earthquakes, Soils and Foundations, 32 (1), 173–188

Iwasaki, T., Tatsuoka, F., Tokida, K.-i. and Yasuda, S. (1978). A practical method for assessing soil liquefaction potential based on case studies at various sites in Japan, in Proceedings, 2nd International Conference on Microzonation, San Francisco, pp. 885–896.

Iwasaki, T., Arakawa, T. & Tokida, K. (1982). Simplified Procedures for Assessing Soil Liquefaction During Earthquakes Proc. Conference on Soil Dynamics and Earthquake Engineering. Southampton, 925-939

Boulanger, R.W. & Idriss, I.M. (2014). CPT and SPT Liquefaction Triggering Procedures, University of California

Zhang, G., Robertson, P. K. & Brachman, R. W. I. (2002). Estimating liquefaction-induced ground settlements from CPT for level ground, Canadian Geotechnical Journal, 39, 1168–80

Some other useful publications:

Bray J.D. & Sancio, R.B. (2006). Assessment of the liquefaction susceptibility of fine grained soils ASCE Journal of Geotechnical and Geoenvironmental Engineering, Vol. 132(9), 1165-1177

Department of Building and Housing (2011) Revised guidance on repairing and rebuilding houses affected by the Canterbury earthquake sequence, November 2011

Department of Building and Housing (2012) Interim guidance for repairing and rebuilding foundations in Technical Category 3, Appendix C to the Guidance Document. 27 April 2012

Earthquake Commission Act 1993

Youd, T.L. & Garris, C.T. (1995). Liquefaction-induced ground surface disruption ASCE Journal of Geotechnical Engineering 121 (11), 805 – 809

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Appendix C Typical photographs of land damage

Typical photographs of none to minor land damage

LSN Ranges 0 - 10 Moderate to severe

2.1

None to minor land damage



Even lawns and undamaged pavers

Undamaged concrete driveway and kerbing

View of Aston Drive with little damages

Undamaged asphalt driveway

Aerial shot of Broomfield area showing no liquefaction ejec-ta on the roads or Broomfield Common

View of Centaurus Road with little damage

2.2




Flat lawn

Slightly undulating lawn

Slightly undulating lawn and driveway

Undulating lawn

Undulating lawn

2.3

Aerial photo of North New Brighton showing little damage

Typical photographs of minor to moderate land damage

3.1

Minor to moderate land damage



Undulating lawns and liquefaction ejecta

3.2

Piles of sand along the footpath

Undulations on road with ponding water Liquefaction ejecta pile on road

Isolated area of liquefaction ejecta on lawn Isolated area of liquefaction ejecta on lawn




3.3

Liquefaction ejecta around washing line pole Liquefaction ejecta on lawn

Foundation damage Undulating paving slabs

Liquefaction ejecta on lawn Foundation damage




3.4

Foundation damage Liquefaction ejecta on lawn

Liquefaction ejecta on lawn Liquefaction ejecta in planter

Liquefaction ejecta in planter with undulating pavers Foundation damage




3.5

Liquefaction ejecta at a New Brighton Road junction Aerial photo of liquefaction ejecta at Cashmere High School

Liquefaction ejecta on lawn Liquefaction ejecta on lawn

Liquefaction ejecta on lawn Isolated area of liquefaction ejecta




3.6

Damage to asphalt Liquefaction ejecta on lawn

Liquefaction ejecta in piles on road Undulating paving bricks

Foundation damage Isolated area of liquefaction ejecta




3.7

Foundation damage Liquefaction ejecta on lawn

Large area of liquefaction ejecta on lawn Liquefaction ejecta in many places on lawn

Foundation and brickwork damage Liquefaction ejecta and brickwork damage




3.8

Liquefaction ejecta in many areas on lawn Undulations in lawn

Liquefaction ejecta covering driveway Undulations in paving area

Liquefaction ejecta on lawn Undulations in lawn, liquefaction ejecta removed




3.9

Foundation damage

Liquefaction ejecta on lawn

Liquefaction ejecta alongside culvert

Foundation damage

Liquefaction ejecta, dwelling tilting Liquefaction ejecta, dwelling tilting

Typical photographs of moderate to severe land damage

4.1

4.2

Moderate to severe land damage



Large patch of liquefaction ejecta Foundation damage

Liquefaction ejecta piles along Androssan Street

Liquefaction ejecta around property

Liquefaction ejecta piles along Avonside Drive

Foundation damage




4.3

Aerial photo showing liquefaction ejecta on Seabreeze Close

Liquefaction ejecta in turning circle of Seabreeze Close

Outside of dwelling showing level liquefaction ejecta reached on brickwork and windows

Large amounts of liquefaction ejecta

Large amounts of liquefaction ejecta and tilting dwelling Large amounts of liquefaction ejecta around property




4.4




Large amounts of liquefaction ejecta inside dwelling

Close up photo of road damage seen in aerial photo of Woolston

s) Foundation damage, tilting dwelling


Job No: 30214.v1May 2015

Liquefaction Report Tonkin and Taylor May 2015

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Transcript of Liquefaction Report Tonkin and Taylor May 2015