liquefaction assessment of the wairakei/te tumu...

REPORT

Liquefaction assessment of the Wairakei/Te Tumu area

Prepared for Tauranga City Council Prepared by Tonkin & Taylor Ltd Date May 2016 Job Number 851988.v0

Distribution:

Tauranga City Council electronic copy

Tonkin & Taylor Ltd (FILE) electronic copy

Liquefaction assessment of the Wairakei/Te Tumu area Tauranga City Council

Job No: 851988.v0May 2016

Executive Summary

A high level liquefaction assessment of the Wairakei/Te Tumu area has been undertaken by Tonkin + Taylor to inform Tauranga City Council of the liquefaction vulnerability of the land at this site. The increase in liquefaction vulnerability as a result of sea level rise is also assessed. The results presented in this report are indicative and should not be used for plan change, consenting or foundation design purposes.

The liquefaction assessment indicates that the areas have some vulnerability to liquefaction damage. Area 1 (along the coastline) and Area 2 (north of the river) have similar geology. However, the two areas have different liquefaction vulnerabilities. Area 1 is less vulnerable to liquefaction damage at the ground surface because it generally has a thicker non-liquefying crust whereas Area 2 has a thinner non-liquefying crust and is therefore more vulnerable to liquefaction damage. However, both Area 1 and 2 are potentially vulnerable to lateral spreading.

An increase in mean sea level is expected to increase the level of the groundwater table and therefore reduce the thickness of the non-liquefying crust. Increases in sea level of 0.5 and 1.0m have been modelled. The corresponding rise in groundwater level would result in a small increase in liquefaction vulnerability in Area 1, but more significant increase in liquefaction vulnerability in Area 2.

Overall, the preliminary liquefaction assessment of the site suggests the land is likely to be suitable for urban development from a liquefaction perspective. However, a more detailed liquefaction assessment for the 100 year level of earthquake shaking needs to be undertaken to confirm this. The preliminary liquefaction assessment also indicates that specific foundation design and/or ground improvements would likely be required to mitigate the effects of potential liquefaction damage, particularly in Area 2.

Liquefaction assessment of the Wairakei/Te Tumu area Tauranga City Council

Job No: 851988.v0May 2016

Table of contents

1 Introduction 1 1.1 Scope of work 1 1.2 Site description 1

2 Ground conditions 3 2.1 Geology and faulting 3 2.2 Geotechnical investigations 4 2.3 Geotechnical model 4

3 Groundwater 5 3.1 Source data 5 3.2 Modelling methodology 5 3.3 Incorporating sea level rise 5 3.4 Models used for liquefaction assessment 5

4 Seismic shaking hazard 6 4.1 Seismic site subsoil class 6 4.2 Ground shaking hazard 6

5 Liquefaction assessment 7 5.1 Liquefaction process 7 5.2 Liquefaction susceptibility and triggering 8 5.3 Liquefaction consequence 9 5.4 Liquefaction vulnerability at the site 9 5.5 Impact of sea level rise 13

6 Further work 15 7 Conclusions 16 8 Applicability 17

Appendix A Figures

Appendix B Ground Investigation Results

Appendix C Groundwater Modelling

Appendix D Liquefaction Assessment Results

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Tonkin & Taylor Ltd Liquefaction assessment of the Wairakei/Te Tumu area Tauranga City Council

May 2016Job No: 851988.v0

1 Introduction

Tonkin & Taylor Ltd (T+T) has been engaged by Tauranga City Council (TCC) to undertake a high-level, preliminary assessment regarding the liquefaction vulnerability of the Wairakei/Te Tumu area to the east of Papamoa. A secondary objective is to assess the increase in liquefaction vulnerability that may occur due to a shallowing of groundwater levels associated with sea level rise. We understand that the results will be used to aid planning for future development.

This report presents:

The ground conditions and results of the geotechnical investigations undertaken The groundwater models created for the purpose of the liquefaction assessment The seismic shaking hazard for the Wairakei/Te Tumu area The high-level liquefaction assessment results and recommendations.

The purpose of this report is to inform TCC of the liquefaction vulnerability for the areas known as Wairakei (urban growth area) and Te Tumu (future urban area) and the impact sea level rise may have on the liquefaction vulnerability in those areas. The assessment was undertaken at a high-level and is not intended to be used for consenting, foundation design or plan change purposes.

1.1 Scope of work

To undertake a liquefaction assessment at a given site, it is important to have a good understanding of the subsurface ground conditions, groundwater levels and seismic hazard. Therefore, the scope of this project was to:

1 Undertake geotechnical Cone Penetration Test (CPT) investigations to assess the subsurface ground conditions for the area

2 Develop a median groundwater model for the area 3 Undertake a high-level liquefaction assessment including:

a Determine the site’s seismic subsoil class in terms of the NZTA Bridge Manual (third edition)1

b Determine the site’s liquefaction vulnerability and its potential consequences at 25 and 500 years levels of earthquake shaking

4 Repeat the liquefaction assessment assuming 0.5m and 1.0m of sea level rise 5 Prepare this report presenting the results.

1.2 Site description

The study area is outlined in Figure 1 and encompasses approximately 8km² of rural land. It is bordered by Papamoa suburb to the west, the Bay of Plenty to the north and Kaituna River to the south and east. The topography of the study area is variable. The stretch of land along the coastline consists of sand dunes which are undulating and at a higher elevation (ranging from approximately 3m to 9m above sea level) whereas the land along the Kaituna River is relatively flat and at a lower elevation (ranging from approximately 1m to 2.5m above sea level). These areas will be referred to herein as Area 1 and Area 2 respectively.

1 NZ Transport Agency, 2014. “Bridge manual 3rd Edition Amendment 1 (Manual No. SP/M/022).” NZ Transport Agency. Retrieved from www.nzta.govt.nz

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Figure 1 – Map showing the Wairakei/Te Tumu study area.

KEY: Study Area 1 km

AREA 1

AREA 2

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2 Ground conditions

2.1 Geology and faulting

The published geological information2 indicates that the site is mostly underlain by coastal beach deposits to the north, and swamp or alluvial deposits to the south. Both sets of deposits are identified on the geological maps as Holocene-aged. The coastal beach deposits comprise sand and minor gravel of the Tauranga Group. The southern swamp/alluvial deposits run adjacent to the northern bank of the Kaituna River and comprise dark brown to black peat, organic rich mud, silt and sand also of the Tauranga Group.

There are a significant number of active faults within close proximity of the study area. Identified faults are shown in Figure 2 below which has been taken from the National Seismic Model for New Zealand: 2010 Update3. The majority of the active faults in the Bay of Plenty are north of the study area and in the Taupo Rift to the south and southeast. The closest active fault is identified as Fault No. 128 which is about 10km north east of the study area, is capable of a moment magnitude (Mw) of 6.3 and has an average recurrence interval of 1360 years. Larger faults further away in the Waikato basin and Taupo volcanic zone also contribute to the seismic hazard of the area.

Figure 2 – Active faults in the Bay of Plenty and Taupo Rift3. The fault traces are shown as black lines and the

Bay of Plenty and Taupo Rift area is marked by the number ‘2’.

2 Leonard, G.S., Begg, J.G., Wilson, C.J.J. (compilers) 2010. “Geology of the Rotorua area.” Institute of Geological & Nuclear Sciences 1:250 000 geological map 5. 1 sheet + 102 p. Lower Hutt, New Zealand. GNS Science. 3 Stirling, M., et al., 2012. “National Hazard Seismic Model for New Zealand: 2010 Update.” Bulletin of the Seismological Society of America, 102 (4), pp. 1514-1542.

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2.2 Geotechnical investigations

Twenty CPTs were carried out in the Wairakei/Te Tumu area on 24 and 25 February 2016 to a depth of 10m for the purpose of this project. The investigations were evenly distributed across the study area.

The following factors were considered when determining the CPT locations:

Approval from land owners

Ecological areas

Archaeological areas

Location of services (electrical lines, stormwater pipes, etc.)

Ease of the CPT rig access.

The investigation locations are shown in Figure A1 and the CPT results are presented in Appendix B.

2.3 Geotechnical model

Table 1 below presents the generalised soil profile in the upper 10m inferred from the 20 CPTs.

Table 1 – Generalised soil profile in upper 10m.

Layer No.

Unit Depth to top (m)

Depth to bottom (m)

Description CPT tip resistance (MPa)

1 Upper Sand

0 4 Loose to medium dense clean to silty sand. Some evidence of sandy silt close to the Kaituna River.

0 to 12

2 Lower Sand

4 10 Medium dense clean to silty sand. Some evidence of gravelly to dense sand close to the Kaituna River.

5 to 30

Detailed plots showing the CPT tip resistance (qc) and the soil behaviour index (Ic) versus depth and relative to mean sea level (RL) are presented in Figures D1 and D2.

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3 Groundwater

A median groundwater surface was created for the purpose of undertaking the liquefaction assessment. The use of the median model is considered appropriate because using anything other than the median impacts the return period of the liquefaction assessment.4

3.1 Source data

The source data used to create the median groundwater level surface is summarised in Table 2. This data has been referenced against the Moturiki 1953 datum.

Table 2 – Source data used to create the median groundwater surface.

Number of source points used in the model Source of data

Monitoring wells 13 TCC5

River monitoring stations 3 BOPRC6

Mean sea level 1 LINZ7

The location of the monitoring wells and river monitoring stations is shown on Figures A4 and C2 respectively.

3.2 Modelling methodology

The median model is created using the median depth to groundwater record at each of the monitoring wells and the median river level at the different Kaituna River monitoring stations. The median groundwater level surface (Figure A4) was then developed as a 25m by 25m grid by contouring between the points using Inverse Distance to a Power (IDP) method and a series of assumptions were applied in order to create the model. Information about the method and the assumptions is presented in Appendix C.

3.3 Incorporating sea level rise

In order to assess the potential increase in liquefaction vulnerability as a result of sea level rise, two scenarios of sea level rise were applied to the median model (0.5m and 1.0m). An attempt was made to apply damping of the sea level rise along the Kaituna River by considering the tidal influence upstream. However, the effects were minimal given the proximity to the coast. Therefore, it was deemed appropriate to simply apply a 0.5m and 1.0m increase in groundwater directly to each of the monitoring wells. More detail can be found in Appendix C. These changes in groundwater elevation are presented in Figures A6 and A7 respectively.

3.4 Models used for liquefaction assessment

A median depth to groundwater surface was created for the purpose of the liquefaction assessment (Figure A5). This is simply the difference between the LiDAR (Figure A2) and the median groundwater level surface (Figure A4).

Depth to groundwater surfaces created accounting for 0.5m and 1.0m of sea level rise were also created. These can be found in Figures A8 and A9 respectively.

4 Tonkin + Taylor, 2015. “Canterbury Earthquake Sequence: Increased Liquefaction Vulnerability Methodology”. Retrieved from http://www.eqc.govt.nz/ILV-engineering-assessment-methodology 5 Obtained from Tauranga City Council on 1 March 2016 6 Obtained from Bay of Plenty Regional Council on 7 March 2016 7 Obtained from Land Information New Zealand on 4 March 2016

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4 Seismic shaking hazard

4.1 Seismic site subsoil class

The seismic subsoil class in accordance with NZS 1170.5:2004 (Section 3.1.3) for the site is considered to be ‘Class D – Deep or Soft Soil Sites’ due to the large depth to bedrock at the site.

Further investigations and assessment of subsoil class (e.g. deep borehole or microtremor testing) are unlikely to modify the conclusion of Class D.

4.2 Ground shaking hazard

New Zealand Standard, NZS1170.5:2004 Structural Design Actions Part 5 Earthquake Actions, clause 2.1.4 specifies that in order to meet the requirements of the New Zealand Building Code8, design of structures is to allow for two earthquake scenarios:

1 (ULS) “Ultimate limit state for earthquake loading shall provide for... avoidance of collapse of the structural system... or loss of support to parts... damage to non-structural systems necessary for emergency building evacuation that renders them inoperative.”

2 (SLS) “Serviceability limit states for earthquake loading are to avoid damage to... the structure and non-structural components that would prevent the structure from being used as originally intended without repair after the SLS earthquake...”.

The seismic hazard in terms of peak ground acceleration (PGA) for the area has been assessed based on the NZTA Bridge Manual9. Table 3 presents the return periods for earthquakes with various ‘unweighted’ PGAs with corresponding earthquake magnitudes. The PGAs were determined using building importance level 2 (single storey family residential dwelling)10.

Table 3 – Ground seismic hazard.

NZS 1170.5 Limit State PGA (g) Effective magnitude Meff

Return period (years)

Ultimate limit state (ULS) 0.27 6.0 500

Serviceability limit state (SLS) 0.07 6.0 25 Note: PGA and effective magnitude has been assessed based on Bridge Manual SP/M/022 Third Edition for the following: Building design life 50 years Building importance level 2 (NZS 1170.0:2004, Table 3.2) – single family residential dwellings Return period factor, Ru 1.0 for 500yr; and 0.25 for 25yr return period (NZS 1170.5:2004, Table 3.5) Subsoil class D (Deep soil) – refer Section 4.1 Return period PGA coefficient, C0,1000 0.35 (Bridge Manual Figure 6.1(b)) Site subsoil class factor, f 1.0 (Bridge Manual Section 6.2) PGA C0,1000 x Ru/1.3 x f x g (Bridge Manual Section 6.2) Effective Magnitude, Meff 6.0 for 500yr and 6.0 for 25yr return period (Bridge Manual Table 6.2(d))

8 “Building Regulations 1992”. Schedule 1. Retrieved from http://www.legislation.govt.nz/regulation/public/1992/0150/latest/DLM162570.html 9 As outlined in the latest New Zealand Geotechnical Society (NZGS) and Ministry of Business Innovation & Employment (MBIE) guidelines for Earthquake geotechnical Practice in New Zealand (March 2016). 10 Standards New Zealand, 2004. “NZS1170.5:2004 Structural Design Actions Part 5: Earthquake actions - New Zealand”. Standards New Zealand. Retrieved from https://www.standards.govt.nz/

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5 Liquefaction assessment

5.1 Liquefaction process

It can be readily observed that dry, loose sands and silts contract in volume if shaken. However, if the loose sand is saturated, the soil’s tendency to contract causes the pressure in the water between the sand grains (known as “pore water”) to increase. The increase in pore water pressure causes the soil’s effective grain-to-grain contact stress (known as “effective stress”) to decrease. The soil softens and loses strength as this effective stress is reduced. This process is known as liquefaction.

The elevation in pore water pressure can result in the flow of water in the liquefied soil. This water can collect under a lower permeability soil layer and if this capping layer cracks, can rush to the surface bringing sediment with it. This process causes ground failure and with the removal of water and soil, a reduction in volume and hence subsidence of the ground surface.

The surface manifestation of the liquefaction process is the water, sand and silt ejecta that can be seen flowing up to 2 hours following an earthquake. The path for the ejecta can be a geological discontinuity or a man-made penetration, such as a fence post, which extends down to the liquefying layer to provide a preferential path for the pressurised water. The sand often forms a cone around the ejecta hole. With the dissipation of the excess pore-water pressure, the liquefied soil regains its pre-earthquake strength and stiffness.

The surface expression of liquefaction, water and sand depends on a number of characteristics of the soil and the geological profile. If there is a thick crust of non-liquefiable soil such as a clay, or sand that is too dense to liquefy during the particular level of shaking of the earthquake, then water fountains and sand ejecta may not be seen on the surface. The amount of ground surface subsidence is generally dependent on the density of the sand layers as well as how close the liquefying layers are to the surface. Ground surface subsidence increases with increasing looseness in the soil packing. The ground rarely subsides uniformly resulting in differential settlement of buildings and foundations.

Figure 3 summarises the process of liquefaction with a schematic representation.

Figure 3 – Schematic representation of the process of liquefaction and the manifestation of liquefaction ejecta.

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5.2 Liquefaction susceptibility and triggering

Liquefaction only occurs in some soil types. These are typically soils which are saturated, non-cohesive, and low to moderate permeability. Soil types which are susceptible to liquefaction are listed below:

Sands and low plasticity/non-plastic silts11 Fine grained low to non-plastic soils with a high moisture content12,13 Young, typically Holocene-aged ( 12,000 years old) deposits

Susceptible soils require a certain level of earthquake shaking in order to trigger liquefaction. Denser soils require more intense and/or longer duration of shaking than those that are less dense.

The trigger level earthquake shaking (in terms of peak ground acceleration (PGA) and magnitude (M)) for each soil layer identified as being susceptible to liquefaction has been assessed by the method proposed by Boulanger and Idriss (2014)14. This method is based on the empirical relationship between the CPT tip resistance (qc) and soil fines content. The trigger PGA and magnitude which has been used is aligned with the site’s assessed seismic shaking hazard as described in Section 4.2.

It is important to note that the Boulanger and Idriss (2014) triggering method is only intended for use for flat land. However, for the analyses presented in this report, the triggering method was applied to sand dunes, in particular in Area 1. This generally underestimates the amount of triggering. We suggest this should be carefully considered when undertaking a more detailed liquefaction assessment.

In both Area 1 and Area 2, all of the soils below the groundwater table are susceptible to liquefaction because they are predominantly loose to medium dense sands. The conclusions of the assessment are presented in Section 6 and the results of the supporting analyses are presented in Appendix D.

11 Bray, J., et al, 2014, “Liquefaction effects on buildings in Central Business District of Christchurch”, Earthquake Spectra, 30 (1), 85-109. 12 Bray J.D. and Sancio R.B., 2006, "Assessment of the liquefaction susceptibility of fine-graded soils", Journal of Geotechnical and Geoenvironmental Engineering, 132 (9), 1165–1177. 13 Boulanger R.W. and Idriss I.M., 2006, "Liquefaction Susceptibility Criteria for Silts and Clays”, Journal of Geotechnical and Geoenvironmental Engineering, 132 (11), 1412–1426. 14 Boulanger, R.W and Idriss, I.M., 2014. “CPT and SPT based liquefaction triggering procedures." Report No. UCD/CGM-14/01, Center for Geotechnical Modeling, Department of Civil and Environmental Engineering, University of California, Davis, CA, 134 pp.

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5.3 Liquefaction consequence

Liquefaction can cause significant damage to land and infrastructure. A summary of potential consequences of liquefaction is provided in Table 4.

Table 4 – Consequences of liquefaction.

Phenomenon Description

Differential settlement A difference in ground settlement between two points which can cause damage to foundations, services and roads.

Sand and water ejected to the surface (sand boils)

This exacerbates differential settlement, can result in damage to paved and other ground surfaces, reduce clearances under buildings, ingress and block buried pipes, etc.

Reduced support to foundations bearing above the liquefied soil

Bearing capacity of the soil could be reduced resulting in subsidence of foundations.

Buoyancy effects Liquefaction can result in upward movement (floatation) of manholes, tanks and other buried vessels being subject to buoyancy effects.

Lateral spread Land above the liquefied soil layer moving either down slope or toward a free edge such as a stream channel. This total and differential lateral movement can cause severe damage to buildings and infrastructure.

5.4 Liquefaction vulnerability at the site

Analyses have been undertaken on the 20 CPTs to assess the potential of the subsoils to earthquake induced liquefaction. From the analyses, the liquefaction potential at the site is summarised as follows:

SLS (1/25yr return period) Liquefaction is not expected to trigger in earthquakes with return periods less than or equal to 25 years.

ULS (1/500yr return period) Liquefaction of the saturated loose to medium dense sand and silty sand is expected. Localised pockets of liquefaction within the gravelly to dense sands and the silty soils is possible.

The consequences of liquefaction damage following a 500 year or greater return period level of earthquake shaking at the site are expected to be similar to those listed in Table 4.

“Vulnerability” is the consequence of liquefaction at the ground surface. It is dependent on the depth to groundwater (i.e. crust thickness) and thickness of liquefiable soils, the slope of the ground surface and the proximity to the river edge. The closer the liquefiable soils are to the ground surface the more vulnerable the land is to damage due to liquefaction. Also, the more sloping the land and the nearer to a river edge the more vulnerable the land is to damage due to lateral spreading.

The vulnerability indicators which have been evaluated for the soils underlying the top 10m to assess the vulnerability of land as a result of liquefaction at this site is summarised in Table 5.

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Table 5 – Liquefaction vulnerability indicators.

Vulnerability Indicator Comments and observations from past events

Depth to groundwater Observations from Christchurch and Japan indicate that the greater the thickness of the non-liquefying crust the less damage is likely to be reflected at the ground surface. Examples of sand boils and damaging differential settlement are very few for sites with a crust thickness greater than 3m15.

Calculated volumetric one dimensional free field settlement (SV1D)16

In Christchurch, land for proposed residential subdivision development is being delineated into technical categories (TC1 to TC3) depending on its expected performance in the event of liquefaction. Calculated free field settlement is being applied as a parameter to be considered in this delineation. While this guideline is not applicable outside of Christchurch, it can be referred to for indicative purposes. ULS calculated settlement of <25mm implies TC1 and >100mm implies TC3 foundation solutions for Christchurch.

Liquefaction Severity Number (LSN)17

LSN is a parameter which characterises the vulnerability of land to damage due to liquefaction for a given level of shaking and a given groundwater level. This parameter has been correlated with evidence of surface ground damage in Christchurch. A higher LSN value indicates a greater likelihood of surface ground damage. LSN of 0-15 indicates a high likelihood of little to no expression of liquefaction at the ground surface whereas LSN of 16 to 25 indicates a high likelihood of minor to moderate expression of liquefaction at the ground surface and LSN greater than 25 indicates a high likelihood of moderate to severe expression of liquefaction at the ground surface.

Lateral spread potential Lateral spreading is the lateral movement of gently to steeply sloping terrain towards a free face. It is caused by earthquake-induced liquefaction.

These have been assessed using the median groundwater model. Our findings regarding liquefaction vulnerability at the site are summarised in Figure A10 and in Tables 6 and 7 below. In summary the site is delineated into two general areas with Area 1 (land adjacent to the coast) having less vulnerability to liquefaction than Area 2 (land adjacent to the Kaituna River). The difference in vulnerability is a function of the crust thickness.

The liquefaction assessment analysis for Area 1 was undertaken on CPTs 1 to 7, 15 and 20. The assessment for Area 2 was undertaken on CPTs 8, 12 and 14 to 18. CPTs 9 to 10 and 19 are considered to be in a transitional zone between the two areas and are not included in the statistics presented in Tables 6 and 7.

15 Ishihara, K., 1985. “Stability of natural soil deposits during earthquakes”. International Conference on Soil Mechanics and Foundation Engineering, San Francisco: 321-376. 16 Zhang, G., Robertson, P. and Brachman, R., 2002. “Estimating liquefaction-induced ground settlements from CPT for level ground”. Canadian Geotechnical Journal, 39(5): 1168-1180. 17 Van Ballegooy, S. et al., 2014. “Assessment of liquefaction-induced land damage for residential Christchurch”. Earthquake Spectra, 30(1): 31-35.

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Table 6 – Summary of liquefaction vulnerability for a ULS (1/500yr) event – Area 1.

Vulnerability Indicator Result General implications from each factor for this site and

residential development

Depth to groundwater

Range: 3.7 - 9.1m Average: 6.4m

Ground surface damage generally not expected as a consequence of liquefaction.

Calculated volumetric one dimensional free field settlement (SV1D)

Range: 0 - 58mm Average: 20mm

Equivalent to TC1 based on average result. TC1 indicates specific foundation design or ground improvement to mitigate liquefaction effects not likely to be required. Some sites may require equivalent to TC2. TC2 indicates specific foundation design or ground improvement required.

LSN Range: 0 - 7 Average: 3


Lateral spread potential

Possible

LiDAR data indicates site is undulating with numerous free edges. Lateral spreading is unlikely to occur, but slumping of the dunes is possible. Foundation design or ground improvement likely to be required to mitigate.


Vulnerability Indicator Result General implications from each factor for this site and

residential development



Minor ground surface damage including sand boils and settlement expected in a ULS event. Foundation design or ground improvement required to mitigate.



Equivalent to TC2 based on average result. TC2 indicates specific foundation design or ground improvement required.

LSN

Range: 3 - 47 Average: 14

Chance of minor damage but generally, ground surface damage not expected as a consequence of liquefaction.


Highly likely

LiDAR data indicates site is gently sloping towards the river. Land above the liquefied soil layers likely to slope down towards a free edge, in this case the river. Foundation design or ground improvement required to mitigate.

Most of the land in Area 1 has a thick crust (i.e. large depth to groundwater), low calculated free field settlements and low LSN. While ejection of sand to the ground surface as a result of liquefaction is unlikely in this area, lateral spreading towards the coast and slumping of the dunes is likely to occur at 500 year or greater return period levels of earthquake shaking (see Figure 4).

Area 2 is generally lower lying. This area has a thinner crust, higher calculated free field settlements and higher LSN. Sand ejecta and differential settlement is likely in these areas at 500 year or greater return period levels of earthquake shaking. Lateral spreading towards the river is also expected to occur.

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Figure 4 – Example of a cross section indicating where slumping and lateral spreading can occur.

The preliminary liquefaction assessment presented in this report focuses on the liquefaction vulnerability for the 25 and 500 year levels of earthquake shaking. This is in line with the Building Code and NZS 1170.5 requirements for residential property structural performance. When undertaking assessment for any application for subdivision consent, a detailed liquefaction assessment is required for the 100 year level of earthquake shaking. This is a requirement of the Resource Management Act 199118. Having assessed the liquefaction vulnerability for the 25 and 500 year levels of earthquake shaking, it is possible to gain an appreciation of what the liquefaction vulnerability of the site is likely to be at 100 year level of earthquake shaking. The site is not expected to liquefy for the 25 year level of earthquake shaking and experience only moderate liquefaction-related damage for the 500 year levels of earthquake shaking. As such the liquefaction vulnerability of the site for the 100 year level of earthquake shaking is likely to be low. The more detailed liquefaction assessment for the 100 year level of earthquake shaking, as recommended above, must be undertaken to confirm this low likelihood.

18 “Resource Management Act 1991”. Retrieved from http://www.legislation.govt.nz/act/public/1991/0069/latest/DLM230265.html

liquefiable material non-liquefiable material groundwater table ground surface direction of lateral spread direction of slumping

KEY:

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5.5 Impact of sea level rise

The liquefaction assessment at 25 year and 500 years return period levels of earthquake shaking were repeated to account for a 0.5m and 1.0m rise in sea level. The impact of the sea level rise on liquefaction vulnerability in the Wairakei/Te Tumu area is presented in Appendix D and summarised in Table 8 below.

Table 8 – Summary of impact of sea level rise on liquefaction vulnerability at SLS and ULS.

SLS (1/25yr Return Period) ULS (1/500yr Return Period)

Area 1 Negligible impact. Liquefaction is not expected to trigger at 25yr levels of earthquake shaking.

Negligible impact. In most instances, the depth to groundwater is still greater than 3.5m so ground surface damage is generally not expected.

Area 2 Negligible impact. Liquefaction is not expected to trigger at 25yr levels of earthquake shaking.

Moderate impact. The depth to groundwater is generally less than 3.5m and shallower as a result of sea level rise so ground surface damage is expected to increase.

The vulnerability indicators presented in Table 5 have been re-evaluated to account for a 0.5m and 1.0m rise in sea level. The findings are presented in Figures A11 and A12 and summarised in Table 9 for Area 1 and Table 10 for Area 2.


Vulnerability Indicator

With 0.5m SLR

With 1.0m SLR

General implications from each factor for this site and residential development








Equivalent to TC1 based on average result. TC1 indicates specific foundation design or ground improvement not likely to be required. Some sites may require equivalent to TC2. TC2 indicates specific foundation design or ground improvement required.





Possible

Possible

LiDAR data indicates site is undulating with numerous free edges. Lateral spreading is unlikely to occur, but slumping of the dunes is possible. Foundation design or ground improvement likely to be required to mitigate.

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Vulnerability Indicator

With 0.5m SLR

With 1.0m SLR

General implications from each factor for this site and residential development




Minor ground surface damage including sand boils and settlement expected in a ULS event. Foundation design or ground improvement required to mitigate.




Equivalent to TC2 based on average result. TC2 indicates specific foundation design or ground improvement required.



Chance of minor to moderate expression of liquefaction at the ground surface (e.g. sand boils and undulations).


Highly likely

Highly likely

LiDAR data indicates site is gently sloping towards the river. Land above the liquefied soil layers likely to slope down towards a free edge, in this case the river. Foundation design or ground improvement required to mitigate.

Sea level rise of up to 1.0m is expected to have minimal impact on the liquefaction vulnerability of Area 1. Despite a metre of rise in sea level, Area 1 still has a thick non-liquefiable crust (>3.5m) and therefore low calculated free field settlement and LSN values. Conversely, the liquefaction vulnerability of Area 2 is expected to increase with a rise in sea level. A metre rise in sea level is expected to reduce the crust thickness in Area 2 by about 0.8m and increase calculated free field settlement and LSN values under ULS levels of earthquake shaking, resulting in minor to moderate chance of liquefaction expression at the ground surface.

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6 Further work

If the Wairakei/Te Tumu area is going to require a plan change for residential purposes, we recommend that TCC undertake the following works:

A more detailed liquefaction assessment for the 100 year level of earthquake shaking to inform subdivision consenting and plan change.

More detailed geotechnical investigations to inform foundation design and building consent applications.

Reassess the liquefaction vulnerability using design profiles prior to or following any earthworks design or liquefaction mitigations works (i.e. changes to the soil properties and/or site topography are likely to impact the liquefaction vulnerability of the site).

A more detailed lateral spreading assessment using available empirical methods to get a better understanding of the likely extent of lateral displacements at different levels of earthquake shaking.

Continue to closely monitor groundwater levels in existing monitoring wells particularly if earthworks are expected to take place. This would allow for more accurate future liquefaction assessments.

Install additional monitoring wells in the western part of the study area to gain a better understanding of groundwater levels in that area. This would also allow for more accurate future liquefaction assessments.

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May 2016Job No: 851988.v0

7 Conclusions

The preliminary high-level liquefaction assessment of the Wairakei/Te Tumu area suggests the land is likely to be suitable for urban development purposes. However, a more detailed liquefaction assessment for the 100 year level of earthquake shaking needs to be undertaken to confirm this. The preliminary assessments also suggests some of the land (in particular Area 2) is likely to require specific foundation design and/or ground improvement to mitigate the potential effects of liquefaction damage due to the presence of liquefiable soils below the ground surface. A list of recommended further work has been listed in Section 6.

Whereas areas 1 and 2 have similar soil profiles in the upper 10m, the two areas have different vulnerabilities to liquefaction. Area 1 is less vulnerable to liquefaction damage at the ground surface because it generally has a greater crust thickness whereas Area 2 has a thinner crust and is therefore more vulnerable.

Liquefaction is not expected to trigger at 25 year return period levels of earthquake shaking. At 500 year return period levels of earthquake shaking, liquefaction is expected at depth in Area 1 resulting in possible slumping of the sand dunes and little or no ground surface expression of liquefaction. In Area 2, liquefaction is expected close to the ground surface, likely resulting in ground surface expression of liquefaction (e.g. sand boils). Lateral spreading is also likely to occur in Area 2 for the 500 year level of earthquake shaking.

Sea level rise will increase the site’s vulnerability to liquefaction damage, particularly in Area 2 since the groundwater table is already close to the ground surface. The increase of sea level in this area will reduce the non-liquefying crust thickness, increasing the vulnerability of the site to liquefaction.

liquefaction assessment of the wairakei/te tumu...

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