islington substation - transpower new zealand substation revision 0 may 2004 k:\dept_19\1980601...

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Islington Substation SVIP Seismic Hazard Assessment

Transpower Ltd

May 2004

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Islington Substation Revision 0 May 2004 K:\Dept_19\1980601 Transpower SVI seismic\Islington\FINAL REPORT\Islington FinalReport.doc

Islington Substation

Prepared for

Transpower Ltd

Prepared by

Maunsell Limited

47 George Street, Newmarket

PO Box 4241

Auckland

New Zealand

Tel +64 9 379 1200

Fax +64 9 379 1201

[email protected]

May 2004

101980601

© Maunsell Limited 2005 The information contained in this document produced by Maunsell Limited is solely for the use of the Client identified on the cover sheet for the purpose for which it has been prepared and Maunsell Limited undertakes no duty to or accepts any responsibility to any third party who may rely upon this document. All rights reserved. No section or element of this document may be removed from this document, reproduced, electronically stored or transmitted in any form without the written permission of Maunsell Limited.


Quality Information

Document Islington Substation SVIP Seismic Hazard Assessment

Ref 101980601 k:\dept_19\1980601 transpower svi seismic\islington\final report\islington finalreport.doc

Date May 2004

Prepared by Brendon Norrie

Reviewed by David Burns Geoffrey Farquhar

Revision History

Authorised Revision Revision Date Details

Name/Position Signature

0 12/05/2004 Geoffrey Farquhar Geotechnical Engineering Manager


Table of Contents

1.0 Introduction 1 2.0 Scope of Assessment 1 3.0 Geological Setting and Site Description 1 4.0 Drilling Results 2 5.0 Seismic Design Criteria 2 6.0 Geotechnical Considerations 4 7.0 Limitation 5 8.0 References 6

Figures

Figure 1: Location Map

Figure 2: Site Plan

Figure 3: Acceleration Spectra

Appendices

Appendix A: Seismic Hazard Assessment

Appendix B: Drillhole Log

Appendix C: Liquefaction Analysis

Islington Substation Revision 0 May 2004 K:\Dept_19\1980601 Transpower SVI seismic\Islington\FINAL REPORT\Islington FinalReport.doc Page 1

1.0 Introduction

Transpower is investigating the feasibility of terminating 400kV circuits into existing substations as part of the

System Grid Vision Project (SVIP). Upgrading existing substations to accommodate 400 kV circuits will involve

installation of new equipment with significantly larger dimensions and heights than existing substation

equipment.

Maunsell Ltd has been engaged to prepare seismic hazard assessments of three existing substations

(Otahuhu, Whakamaru, and Islington) and one proposed site (Black Point).

This report presents results of the assessment for Islington Substation. It supersedes the Preliminary Seismic

Hazard Assessment prepared in February 2004 that was prepared before drilling investigations could be

undertaken.

2.0 Scope of Assessment

The following geotechnical issues and features associated with the Islington Substation have been assessed,

• Peak ground accelerations for average earthquake return periods of 250, 475, 1,000 and 2,500 years.

• Representative earthquake magnitudes for average return periods of 250, 475, 1,000 and 2,500 years.

• The risk of ground liquefaction and surface fault rupture affecting the substation site.

• Slope stability issues at the substation site.

• Issues affecting construction of the substation upgrade works, maintenance and cost.

The assessment is based on published geological information along with information gained from a 20m

drillhole (DH1) drilled adjacent to the proposed substation upgrade site (Figure 2). The purpose of the drillhole

was to provide data for a preliminary assessment of liquefaction potential.

3.0 Geological Setting and Site Description

Islington Substation is located on the western boundary of Christchurch City (Figure 1), on the Canterbury

Plains.

The plains are a complex of overlapping gravel-dominated alluvial deposits formed over the last 5 million years

from rivers flowing eastwards from the Southern Alps. These alluvial deposits are over 2000m thick in places

and 500 to 1000m thick beneath Islington Substation. In western areas of Christchurch City, the uppermost

20m consists dominantly of well-sorted gravel with minor layers of sand, silt and peat (Springston Formation).

Peat layers can reach 3m in thickness (Brown and Webber, 1992).


Driller’s logs for two wells (M35/1034, M35/9022) in the vicinity of the substation describe dense, subrounded

greywacke gravels with some sand to 76m depth. A layer of blue organic clay was encountered between 44

and 51m depth.

The substation is mapped as located on a low terrace adjacent to a historic flood channel of the Waimakariri

River (Figure 2) (Brown and Webber, 1992). Very shallow (<0.5m deep), abandoned braided river channels

can be seen crossing the alluvial surface in a generally west-east direction (Figure 2). The terrace edge

mapped to the north and east of the existing substation was observed on site to be very gentle with a

difference in elevation of less than one metre. This feature has probably been subject to human modification.

4.0 Drilling Results

A single drillhole was drilled on 16 April 2004 by CW Drilling Ltd using a truck-mounted rig and a concentrics

(air percussion) drilling technique. Core was logged by an engineering geologist following the methods and

procedures outlined in the NZ Geomechanics Society ”Guidelines for the Field Description of Soils and Rocks

in Engineering Use”. Standard Penetration Tests (SPT) were performed at regular intervals in the drillhole. A

single standpipe piezometer was installed to allow future measurement of groundwater level.

A log of materials encountered in the drillhole is presented in Appendix B.

The approximate drillhole location is shown on Figure 2. The drillhole location was measured from existing

fixed points by tape measure.

Materials encountered in the drillhole consisted dominantly of dense to very dense sandy gravel and some

gravely sand. The gravels are well-rounded, strong greywacke rock. Fines (silts and clays) were generally

absent except between 8 and 9.5 metres depth where the ground consisted of brown silty, clayey sand. Beds

of peat, although common in the area, were not encountered in the drillhole.

The groundwater level measured in the drillhole during April 2004 was 16.5m below ground level. This level

reflects summer conditions following use of the aquifer for water and irrigation. It is expected that during mid to

late winter, groundwater level would be significantly higher.

5.0 Seismic Design Criteria

A site-specific seismic hazard study for the project was completed by Engineering Geology Ltd. Their report is

presented in Appendix A. Estimated representative earthquake magnitudes from the study are reproduced in

Table 1. Acceleration spectra are presented in Table 2 and on Figure 3.


Table 1 Representative Earthquake Magnitudes.

Average Return

Period

(Years)

Spectral

Period

(sec)

Representative Earthquake Magnitudes (Mw)

250 T�0.5 5.25-6.25 at less than 50km, 6.7-7.2 at 30-50km

T>0.5 6.7-8.1 at greater than 30km



1,000 T�0.5 5.25-6.25 at less than 30km, 6.7-7.2 at 30-50km




Table 2: Recommended horizontal acceleration spectra (5% Damping)

Acceleration Spectra (g) for different Average Return Periods (years) Period

(sec) 250 475 1,000 2,500

0.00 0.10 0.15 0.20 0.25 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.50 2.00 2.50 3.00 3.50 4.00

0.213 0.540 0.598 0.617 0.624 0.627 0.620 0.590 0.540 0.481 0.435 0.397 0.367 0.272 0.203 0.163 0.135 0.099 0.076

0.280 0.740 0.795 0.815 0.823 0.825 0.815 0.775 0.710 0.633 0.573 0.523 0.483 0.358 0.268 0.215 0.178 0.130 0.100

0.364 0.975 1.035 1.058 1.069 1.073 1.060 1.010 0.923 0.822 0.744 0.679 0.627 0.465 0.348 0.280 0.231 0.169 0.130

0.504 1.350 1.440 1.470 1.481 1.485 1.465 1.400 1.278 1.139 1.031 0.941 0.869 0.644 0.482 0.387 0.320 0.234 0.180


The current NZ Loadings Code (NZS 4203:1992) specifies a return period of 475 years for normal structures and 1,000 years for Category 1 structures, which is the relevant classification for the substation. NZS 4203 is being replaced progressively by AS/NZS 1170 and for earthquake loading the relevant parts are AS/NZS 1170.0:2002 Structural Design Actions – General Principles, and BD-006-270 Minimum Design Loads on Structures Part 4: Earthquake Loads and Comments (draft for comment published by Standards NZ). AS 1170 classifies a substation under importance level 4 with a corresponding earthquake return period of 2,500 year and Transpower has confirmed that the 2,500-year earthquake is to be used.

6.0 Geotechnical Considerations

The following is a brief discussion of the general geological hazards and geotechnical issues that affect, or are

relevant to the proposed Islington upgrade project.

Faulting: The closest active fault to the Islington Substation is the Springbank Fault (Stirling et al, 2000),

greater than 30 km away. The risk, therefore of the substation being affected by surface fault rupture is

negligible.

Liquefaction Hazard: Liquefaction occurs when the cyclic deformations generated by an earthquake cause an

increase of pore water pressures in loose sands and silts. When the pore water pressures equal overburden

pressure, loss of strength occurs (liquefaction) leading to ground deformation, loss of bearing capacity and

lateral spreading. The presence of significant pore water within the soil is essential for liquefaction and hence

material above the ground water level is generally not susceptible to liquefaction.

All low-lying ground in the Christchurch region is potentially susceptible to liquefaction due to silt, fine sand and

peat layers within the alluvium. Literature concerning ground conditions and liquefaction potential in the

Western Christchurch area indicate significant lateral material variability and the importance of gathering site-

specific information (Brown and Webber, 1992; Christensen, 2001; McManus and Berrill, 2001).

Assessment of the susceptibility of a site to liquefaction is generally based on empirical studies of past

liquefaction events. The preliminary assessment of liquefaction potential in this report is based on the results

of field tests combined with a visual assessment of the particle size, plasticity and strength. The principal field

test method used to assess liquefaction potential in this report is the SPT (Standard Penetration Test) Method

recommended by NCEER (Youd et al, 2001).

The SPT method indicates that the gravel-free bed at approximately 8.0- 9.5m depth has a factor of safety

against liquefaction of less than 1 for the 1,000 and 2,500-year earthquake events (Appendix C). SPT results

show all other material encountered in the drillhole to be non-liquefiable due to their high in situ density.


A visual inspection suggests that the clay content of the material between 8.0 and 9.5m is likely high enough to

make this material cohesive and hence non-liquefiable. However a sample of the material for laboratory testing

was unable to be obtained during drilling.

The depth of the water table at the time of drilling (16.5m below ground level) precludes liquefaction of all

material above it, including material between 8.0 and 9.5m. If the water table during winter is found to be

higher than 8m below ground level, there could be a risk of liquefaction of the layer between 8.0 and 9.5m

depth, should the material be susceptible to liquefaction. However, there is a sufficiently thick raft of non-

liquefiable material above this layer to preclude any effects of liquefaction at the ground surface thus the risk of

liquefaction hazard is likely to be very low. Given the lateral variability of ground conditions, the liquefaction

potential for a new substation would need to be assessed for the entire site by investigations spread over the

site.

Slope Stability: Due to the flat-lying ground beneath and around the proposed substation upgrade site and the

small elevation difference across the mapped terrace edge, the substation is not at risk of slope instability.

Foundation Design and Construction Issues: Ground conditions at this site pose no particular problems for

construction of the new substation. Based on existing geotechnical data it is expected that shallow foundations

will be suitable for structures typical of a substation. Specific investigations will be required for design.

7.0 Limitation

This report has been prepared for the particular project described in the brief to us and no responsibility is

accepted for the use of any part of this report in any other context or for any other purposes.

Recommendations and opinions contained in this report are based on a desk study and site observations along

with data from a single drillhole. Inferences about the nature and continuity of ground conditions away from

subsurface investigations are made but cannot be guaranteed.

Should the development proceed, it would be in the interest of all parties that Maunsell be retained to inspect

excavations, fills and foundations, and to verify ground conditions. In any event, Maunsell should be notified if

conditions vary from those described in the report.


8.0 References

Brown, L.J. and Webber, J.H. 1992. Geology of the Christchurch Urban Area. IGNS 1:25,000 scale Map.

Christensen, S.A. 2001. Waimakariri District Liquefaction Hazard. Proceedings New Zealand Geotechnical

Society 2001 Symposium, Engineering and Development in Hazardous Terrain.

McManus, K.J., Berrill, J.B. 2001. Soil Liquefaction hazard in Christchurch. Proceedings New Zealand

Geotechnical Society 2001 Symposium, Engineering and Development in Hazardous Terrain.

Stirling, M., McVery, G., Berryman, K., McGinty, P., Villamor, P., Van Dissen, R., Dowrick, D., Cousins, J.,

Sutherland, R. 2000. Probabilistic Seismic Hazard Assessment of New Zealand: New Active Fault Data

Attenuation Relationships and Methods. Institute of Geological and Nuclear Sciences. Client Report 2000/53.

Youd, T.L.. et al, 2001. Liquefaction Resistance of Soils: Summary Report from the 1996 NCEER and 1998

NCEER/NSF Workshops on Evaluation of Liquefaction Resistance of Soils. Jnl of Geotech & Geoenviro Eng,

ASCE, Oct 2001, Vol. 127, No. 10 pp 817-833.

Figures

Figure 1: Location Map


N

10 Km

Figure 2: Site Plan

Approximate position

of proposed 400kV

substation upgrade

Existing Islington Substation

Moffett Street

Roberts Road

Mapped Terrace edge

Drillhole 1

N

200m

Figure 3: Acceleration Spectra

Islington Substation Horizontal Acceleration Spectra (5% damping)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Period, T (s)

Hor

izon

tal S

pect

ral A

ccel

erat

ion

(g)

250 yr475 yr1,000 yr2,500 yr

Appendix A: Seismic Hazard Assessment

File: Copy of 5227 islington gf lh t.doc.doc

Dear Geoffrey,

1.0 INTRODUCTION As requested we have undertaken a seismic hazard study for the Transpower Islington Substation. The requirements for the seismic hazard study were detailed in your fax dated 4 November 2003. The scope of work required the following: • peak ground and response spectra accelerations for 250, 475, 1,000 and 2,500 year

average return periods • representative earthquake magnitudes associated with the above levels of shaking

• report including methodology, results and references.

2.0 SITE LOCATION AND SUSOIL CONDITIONS Islington Substation is located on the Canterbury Plains on the western outskirts of Christchurch. The site is near level. The Canterbury Plains are formed on more than 500m of predominantly gravel alluvium deposited during the latest Tertiary and Quaternary periods (last 5 million years) (Ref.1). Boreholes within 750m of the site confirm the alluvium is dominantly gravel with beds of sandy gravel, sand, silt and minor peat. Most of the gravels beneath the Canterbury Plains are dense. A 20m deep borehole is proposed to be drilled at the site in the near future to confirm the nature of the surficial materials.

3.0 SEISMOTECTONIC HISTORY New Zealand straddles the boundary of the Australian and Pacific tectonic plates, where relative plate motion is between about 35-50mm/year (Ref.2). The majority of the relative

5227

7 July 2005 Maunsell Limited P O Box 4241 AUCKLAND

Attention: Geoffrey Farquhar

RE: TRANSPOWER – ISLINGTON SUBSTATION Seismic Hazard Study

Our Ref: 5227 7 July 2005 Page 2


plate motion is accommodated by the Hikurangi subduction zone in the eastern North Island, the Fiordland subduction zone at the far south western end of the country and by faults of the Axial tectonic belt in the area between the Hikurangi and Fiordland subduction zones. The site is located just east of the Axial tectonic belt in the Canterbury-Chathams platform, an area of stable continental crust that has had few earthquakes in historic time. However, there are a number of active faults in the Axial tectonic belt that are capable of generating strong ground shaking at the site. These include the Ashley, Springbank, Porters Pass and Pegasus faults that are located within 30-50km of the site. These faults are capable of generating up to magnitude Mw7.2 earthquakes and have average recurrence intervals of rupture of between 2,000 and 10,000 years (Ref.4). Further to the west of the site is the Alpine Fault (approximately 125km). This is the largest and most active fault in New Zealand. It has a slip rate of 25mm/year and is capable of generating magnitude Mw8.1 earthquakes with average recurrence interval of 300 years (Ref.4).

4.0 METHODOLOGY

4.1. Ground Shaking

Estimates of seismic hazard for the site have been based on seismic hazard estimates contained in the draft Australian/New Zealand Standard (AS/NZS1170.4). This document includes design earthquake response spectra for throughout New Zealand (Ref.3). The response spectra were calculated by the Institute of Geological and Nuclear Sciences (GNS) using a probabilistic seismic hazard model (Ref’s.4 and 5). The seismic-source component of the model incorporates 305 active faults and a grid of distributed seismicity sources with parameters estimated from the catalogue of historical earthquakes. This has been used in conjunction with attenuation models for crustal earthquakes and for subduction zone earthquakes that have been modified from overseas models to better fit New Zealand strong-motion earthquake data (Ref.6). The hazard analysis used a ‘magnitude weighting’ approach, with earthquakes of magnitude less than 7.5 given lower weighting. This was done to recognise that damage-potential increases with magnitude for a given amplitude of motion because duration of shaking generally increases with magnitude.

Acceleration spectra for sites throughout New Zealand can be obtained from AS/NZS1170.4, but they are dependent on site subsoil conditions. Subsoil conditions at the site fall within the definition of site subsoil Class D in AS/NZS1170.4. Class D is for deep soil sites. AS/NZS1170.4 defines the depth of soil that must be exceeded for sites to fall within Class D. The depth depends on the nature and strength of soil. The site easily falls within the prescribed limits.

Spectra are obtained by the product of four factors as described below:

C (T) =

Ch(T) Z R N (T,D)

where Ch (T) = the spectral shape factor which is dependent on site subsoil conditions

Z = hazard factor, which depends on where the site is located R = return period factor N (T,D) = near-fault factor

For the Islington site the spectral shape factor for subsoil Class D has been adopted. The Z factor is 0.25. The appropriate R factors have been selected to provide spectra

Our Ref: 5227 7 July 2005 Page 3


for return periods of 250, 475, 1,000 and 2,500 years. They are 0.76, 1.0, 1.3 and 1.8 respectively. The spectra obtained from AS/NZS1170.4 are truncated above a spectral period of 0.56 seconds. The justification given for this in AS/NZS1170.4 is that the resulting spectral shape is much closer to recorded shapes from large-magnitude earthquakes that are appropriate for consideration for ultimate loading conditions for the more seismic parts of New Zealand. However, there are few spectra from large earthquakes in New Zealand and so we do not recommend this for the Islington substation site. We recommend increasing the 0.2 second spectral acceleration by 10 percent to obtain more realistic spectra.

4.2. Representative Earthquake Magnitudes

Representative earthquake magnitudes have been based on review of the seismic sources contributing to seismic hazard at the site. This indicates that at short periods (T�0.5 seconds) the hazard comes from a combination of randomly located moderate sized earthquakes (M=5.25-6.25) within 50km of the site and magnitude M=6.7-7.2 earthquakes associated with faults located at the western edge of the Canterbury Plains (includes Ashley, Springbank, Porters Pass and Torlesse Faults). At longer periods the hazard comes from these same faults as well as the Alpine Fault located further to the west.

5.0 RESULTS AND RECOMMENDATIONS

5.1. Peak Ground and Spectral Accelerations The recommended horizontal acceleration spectra (5% damping) derived using the methodology described in Section 4 are shown in Figure 1 and summarised in Table 1. The peak horizontal ground acceleration corresponds to the zero period spectral acceleration value in Table 1. We recommend that if vertical spectra are required they be taken equal to two-thirds of corresponding horizontal values.

5.2. Representative Earthquake Magnitudes

The recommended representative earthquake magnitudes for different return periods are summarised in Table 2.

Yours sincerely ENGINEERING GEOLOGY LTD T Matuschka Encl: Tables 1 and 2 Figure 1


TABLE 1. Recommended Horizontal Acceleration Spectra (5% Damping) for


Average Return Period (years) Period (sec) 250 475 1,000 2,500 0.00 0.213 0.280 0.364 0.504 0.10 0.540 0.740 0.975 1.350 0.15 0.598 0.795 1.035 1.440 0.20 0.617 0.815 1.058 1.470 0.25 0.624 0.823 1.069 1.481 0.30 0.627 0.825 1.073 1.485 0.40 0.620 0.815 1.060 1.465 0.50 0.590 0.775 1.010 1.400 0.60 0.540 0.710 0.923 1.278 0.70 0.481 0.633 0.822 1.139 0.80 0.435 0.573 0.744 1.031 0.90 0.397 0.523 0.679 0.941 1.00 0.367 0.483 0.627 0.869 1.50 0.272 0.358 0.465 0.644 2.00 0.203 0.268 0.348 0.482 2.50 0.163 0.215 0.280 0.387 3.00 0.135 0.178 0.231 0.320 3.50 0.099 0.130 0.169 0.234 4.00 0.076 0.100 0.130 0.180


TABLE 2. Representative Earthquake Magnitudes

Return Period (years)

Spectral Period (sec)

Representative Earthquake Magnitudes










References

1. Brown, L.J. and Weeber, J.H. (1992) ‘Geology of the Christchurch Urban Area’, Institute of Geological & Nuclear Sciences Geological Map 1, Scale 1:25,000.

2. DeMets, C.J., Gordon, R.G., Argus, D.F. and Stein, S. (1994) ‘Current Plate Motions’,

Geophysical Journal International, Vol.101, pp425-478.

3. Standards New Zealand (2003) ‘Draft AS/NZS1170.4 Standard, Structural Design Actions. Part 4 Earthquake Actions’.

4. Stirling, M., McVerry, G., Berryman, K., McGinty, P., Villamor. P., Van Dissen, R.,

Dowrick, D., Cousins, J. and Sutherland, R., (2000) ‘Probabilistic Seismic Hazard Assessment of New Zealand: New Active Fault Data, Seismicity Data, Attenuation Relationships and Methods’, Client report 2000/53, prepared for Earthquake Commission Research Foundation, Institute of Geological and Nuclear Sciences Ltd, Gracefield, New Zealand.

5. Stirling, M.W., (2000) ‘New National Probabilistic Seismic Hazard Maps for New

Zealand’, Paper 2362, Proceedings 12th World Conference on Earthquake Engineering, Auckland, New Zealand.

6. McVerry, G.H., Zhao, J.X., Abrahamson, N.A. and Somerville, P.G., (2000) ‘Crustal

and Subduction Zone Attenuation Relations for New Zealand Earthquakes’, Paper 1834, Proceedings 12th World Conference on Earthquake Engineering, Auckland, New Zealand.

Figure 1: Islington Substation Horizontal Acceleration Spectra (5% damping)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Period, T (s)

Hor

izon

tal S

pect

ral A

ccel

erat

ion

(g)

250 yr475 yr1,000 yr2,500 yr

Appendix B: Drillhole Log

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Appendix C: Liquefaction Analysis

LIQUEFACTION POTENTIAL BY FACTOR OF SAFETYDH1

0

5

10

15

20

25

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Factor of Safety Against Liquefaction

DE

PTH

(m)

2,500 year event Liquefiable (USBR)

Non-liquefiable (USBR) GWL

7/07/2005Islington SVIP

LIQUEFACTION POTENTIAL BY (N1)60

0

5

10

15

20

25

0 10 20 30 40 50SPT (N1)60

DE

PTH

(m)

DH1 475 year 1000 year

2,500 year event Liquefiable (USBR) Non-liquefiable(USBR)

GWL

Islington SVIP 7/07/2005

islington substation - transpower new zealand substation revision 0 may 2004 k:\dept_19\1980601...

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