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SH20 Manukau Harbour Crossing Project Appendix 3 - Bridge and Structures Report

CER 06/022

State Highway 20 Manukau Harbour Crossing Project Appendix 3 - Bridge and Structures Report May 2006

Opus International Consultants Limited Auckland Office Level 3, The Westhaven 100 Beaumont Street, PO Box 5848, Wellesley Street Auckland, New Zealand Telephone: +64 9 355 9500 Facsimile: +64 9 355 9585 Date: 19 May 2006 Reference: 1-81074.xx Status: Final

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NOTE: Report annotated September 2006 to refer to project updates described in August 2006 Section 92 Response Reports. Refer to notes on following page

SH20 Manukau Harbour Crossing Project Annotation Notes

Reference in text

Note

1 Proposed form of interchange revised August 2006 to “quarter diamond” configuration. Refer to August 2006 Interchange Report and s.92 Response Report to Auckland City Council.

2 Temporary reclamations no longer proposed. Coastal Permit application for temporary reclamation at Onehunga Harbour Road withdrawn August 2006.

3 Extent and area of proposed reclamation revised. Refer August 2006 s.92 Response Report to Auckland Regional Council.

4 Coastal Permit applications for replacement of Old Mangere Bridge were lodged in May 2006 but withdrawn in August 2006 to enable further consideration of design options. General information relating to the proposed replacement bridge as described within this AEE remains relevant. New consent applications will be lodged at a later date as appropriate. The concept design for the replacement bridge will be confirmed at that time.

5 The proposed designation no longer includes the Manukau Cruising Club lease area (refer August 2006 s.92 Response Report to Auckland City Council). The building, carparking area and boat ramp are no longer directly affected by the works.

6 Stormwater management proposals revised. Refer August 2006 s.92 Response Report to Auckland Regional Council.

28 September 2006 P.T.O

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SH20 Manukau Harbour Crossing Project Annotation Notes

Background

Transit’s proposals for the Manukau Harbour Crossing Project were submitted to Auckland City Council, Manukau City Council and the Auckland Regional Council in May 2006.

All three Councils requested further information from Transit, and this information was provided in August 2006. The requests for further information were made in accordance with Section 92 of the Resource Management Act 1991.

Preparation of this additional information has resulted in some revisions to the original proposals, and these are reflected in supplementary documentation dated 28 August 2006. The key revisions made are summarised below.

Key Revisions

• The proposed form of the motorway interchange at Gloucester Park has been revised from a “diamond” to a “quarter diamond” configuration.

• The extent of reclamation proposed within the Manukau Harbour has been reduced – only one area of reclamation is now proposed, this being along Onehunga Harbour Road and Orpheus Drive, east of the Manukau Cruising Club.

• The Manukau Cruising Club and associated parking area is no longer affected by the proposed works.

• Coastal Permit applications relating to demolition of the old Mangere Bridge and replacement with a new pedestrian and cycle bridge have been withdrawn to enable further consideration of design options.

• Stormwater management proposals have been refined to offer an improved degree of treatment prior to discharge to the receiving environment.

Notification Documentation

The documents submitted with the consent applications and Notices of Requirement in May 2006, along with the more recent information prepared in response to requests from the Councils for further information are available in full for viewing as part of the public notification process.

To avoid confusion, the May 2006 documentation has been annotated where appropriate to refer to the recent revisions summarised above and described in the August 2006 reports.

Reports Referred to in Annotations – Full Titles

Title Date Status / revision

August 2006 Interchange Report

Report: SH20 Manukau Harbour Crossing Project - Gloucester Park Interchange: Further Consideration of Alternatives

28 August 2006 Rev A Final

s.92 Response Report to Auckland City Council

Report: Response to Auckland City Council Requests for Further Information under Section 92 of the Resource Management Act 1991

28 August 2006 Final

s.92 Response Report to ARC

Report: Response to Auckland Regional Council Request for Further Information under Section 92 of the Resource Management Act 1991

28 August 2006 Final

SH20 Manukau Harbour Crossing Project – Assessment of Environmental Effects Appendix 3 - Bridge and Structures Report

Page ii Status: Final 19/05/2006

Table of Contents

Executive Summary........................................................................................................................ 1

1 Introduction.............................................................................................................................. 2

1.1 Project Overview ............................................................................................................ 2 1.2 Purpose of this Report.................................................................................................... 2 1.3 Terminology.................................................................................................................... 2 1.4 Report Format ................................................................................................................ 3

2 Existing Mangere Bridge......................................................................................................... 4

2.1 Existing Bridge Structure................................................................................................ 4 2.2 Bridge Side Protection.................................................................................................... 5 2.3 Pedestrian Access.......................................................................................................... 5 2.4 Stormwater Drainage ..................................................................................................... 6

3 Duplicate Bridge ...................................................................................................................... 7

3.1 Introduction..................................................................................................................... 7 3.2 Background .................................................................................................................... 7 3.3 Features Required.......................................................................................................... 9 3.4 Options Considered...................................................................................................... 10 3.5 Structural Form and Span Arrangement....................................................................... 10 3.6 Construction Methodology............................................................................................ 10

4 Old Mangere Bridge and Mangere Footbridge.................................................................... 13

4.1 Old Mangere Bridge ..................................................................................................... 13 4.2 Mangere Footbridge ..................................................................................................... 14

5 Gloucester Park Interchange................................................................................................ 15

5.1 Gloucester Park Interchange Bridge ............................................................................ 15 5.2 GPI Retaining Walls for Interchange Ramps................................................................ 15

6 Other Structures .................................................................................................................... 16

6.1 Walmsley Road Bridge................................................................................................. 16 6.2 Tararata Creek Bridge.................................................................................................. 16 6.3 Walmsley Road Off Ramp Bridge ................................................................................ 17 6.4 Hastie Avenue Footbridge............................................................................................ 17 6.5 Rimu Road Bridge ........................................................................................................ 18 6.6 Beachcroft Avenue Footbridge..................................................................................... 19 6.7 Queenstown Road Bridge Abutment Underpinning...................................................... 20 6.8 Retaining Walls ............................................................................................................ 20


Page iii Status: Final 19/05/2006

Appendices

Appendix A Photographs

Appendix B Drawings

Appendix C Bridge Options Considered


Page 1 Status: Final 19/05/2006

Executive Summary

The SH20 Manukau Harbour Crossing Project (the Project) involves two new structures across the Manukau Harbour for traffic and pedestrian/cycling use respectively, together with two motorway over bridges, one motorway off ramp bridge, one widened motorway stream bridge and two replacement footbridges. Retaining walls will also be required at Queenstown Road, Gloucester Park, Rimu Road and Walmsley Road interchanges.

The duplicate bridge for the motorway across the Manukau Harbour will be located to the east of the existing bridge with a separation of approximately 3.5 metres. It will have a vertical profile generally similar to the existing bridge to ensure aesthetic compatibility given its proximity. The full navigation clearance that was a requirement for the design of the existing Mangere Bridge is no longer required.

The Duplicate Bridge will be approximately 21 metres wide with an overall length of 650m between abutments. The proposed span arrangements are likely to be longer than the alternating spans of the existing Mangere Bridge. The new piers are proposed in zones to generally preserve unobstructed the longer span arrangements of the existing bridge. The abutments are located at a similar longitudinal position to the existing bridge’s abutments. The bridge will also span over the Onehunga Harbour Road to the northern abutment. The superstructure is likely to consist of either a post-tensioned concrete box girder or steel box girder. The substructure is likely to be reinforced concrete piers supported by bored piles.

The Old Mangere Bridge, which is currently used as a footbridge across the Harbour, is in very poor condition and beyond its service life. It will be replaced with a 5 metre wide pedestrian/cycle bridge at a similar level to the Old Mangere Bridge.

The Gloucester Park Interchange (GPI) will involve a new bridge over the motorway. A combination of batters and vertical retaining walls are expected at on and off ramps.

The widening of the motorway to the east also necessitates replacement of the following structures:

• Rimu Road / Mahunga Drive Bridge;

• Hastie Avenue Footbridge; and

• Beachcroft Avenue Footbridge.

A new off ramp bridge is required over Tararata Creek, together with widening of the adjacent motorway bridge.



1 Introduction

1.1 Project Overview

The Project will widen SH20 between Queenstown Road and Walmsley Road interchanges from four lanes to six lanes, with bus priority on shoulder lanes where practical. Between Rimu Road and Gloucester Park Interchanges the motorway will be widened from four to eight lanes. The existing Mangere Bridge will be duplicated to the east (i.e. a second bridge will be built) to provide four traffic lanes and a shoulder lane for bus priority in each direction. Pedestrian and cyclist usage will continue to be provided along the line of the Old Mangere Bridge by way of a replacement structure (Mangere Footbridge).

The existing Gloucester Park split interchange will be upgraded to a grade separated diamond arrangement connected to Neilson Street. The Project will require alterations to local streets (Rimu Road/Mahunga Drive, Neilson Street/Gloucester Park Road, Orpheus Drive) and modification to access arrangements to surrounding properties affected by the upgraded interchange at Gloucester Park Road.

The Rimu Road/Mahunga Drive Bridge will be replaced to allow motorway widening. Further south at Tararata Creek, a new off ramp bridge and widened motorway bridge is required.

At the Queenstown Road underpass additional retaining walls and abutments will be required. The Beachcroft Avenue and Hastie Avenue footbridges will also need to be replaced to allow motorway widening.

1.2 Purpose of this Report

This report forms part of a suite of documentation provided in support of planning applications for the Project.

The purpose of this report is to present a summary of the proposed physical works associated with the project. Additional information is provided in the specialist reports, which are included within the other appendices.

1.3 Terminology

Throughout this report reference is made to various structures within the project area, in summary they are generally as follows:

• Mangere Bridge: Refers to the existing high level SH20 motorway bridge crossing the Manukau Harbour;

• Duplicate Bridge: Refers to a new SH20 motorway bridge crossing the Manukau Harbour, proposed as a duplication of the existing box girder Mangere Bridge;

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• Old Mangere Bridge: Refers to the existing low level bridge structure across the Manukau Harbour, now only used by pedestrians and cyclists, which is west of the Mangere Bridge;

• Mangere Footbridge: Refers to the proposed new pedestrian and cycle bridge which is to replace the Old Mangere Bridge;

• Gloucester Park Interchange: Refers to the upgraded interchange including a new bridge and ramp structures at the western end of Neilson Street, also know as Neilson Street Interchange.

1.4 Report Format

The main body of the report covers:

• Proposed structures across the Manukau Harbour;

• GPI structures; and

• Other structures.

Supporting documentation in the appendices includes:

• A series of photographs of existing structures are provided in Appendix A Photographs;

• An index of bridge drawings is provided in Appendix B Bridge Drawings , this lists the bridge drawings contained in Volume 3 Scheme Plans and Drawings;

• A description of duplicate bridge options that were considered is provided in Appendix C Bridge Options Considered.



2 Existing Mangere Bridge

This section provides a description of the existing Mangere Bridge as background information. The only work proposed on the existing structure is collection of surface stormwater runoff.

The widening of the existing Mangere Bridge was included in the consideration of alternatives for the main route option. However, this was not considered feasible as outlined in Appendix C Bridge Options Considered.

2.1 Existing Bridge Structure

The existing SH20 Mangere Bridge is a 644 metre long, 14 span structure, comprising twin prestressed concrete box girders with outer side cantilevers and a central link slab between the box girders. Each box girder is supported on square section, hollow, reinforced concrete column piers and sill beam abutments. Refer to photographs 1 and 2 of Appendix C of this report. The bridge was constructed during the late 1970’s and completed by 1983.

The span lengths are (from the northern abutment):

• 30m–60m-29m-60m-29m-86m-30m-80m-30m-60m-30m-60m-30m-30m.

The roadway is 19.7 metres wide between the faces of the edge barriers and currently carries four lanes of traffic, two lanes in each direction and has footpath and cycle track carried beneath the western cantilever of the deck.

The design of the bridge allowed for the then proposed eastward extension of the Onehunga wharf, beneath and beyond the bridge. For this reason the existing bridge has an elevated deck and two longer than standard centre spans, to provide navigation and operational clearances for the extended wharf. The proposed extension to the wharf has not been carried out.

The box girders are built in to the piers, without movement bearings, so seismic loads are taken by all piers. There are movement bearings at the abutments only. There are hinges/movement joints in the centres of spans 4, 8 and 12, to allow for thermal expansion and contraction of the box girders. Due to the delay in completing construction of the box girders and the use of midspan joints, the deck has sagged below the original longitudinal design profile in the centre of some spans. The alternating pattern of long and short spans was adopted to increase the degree of end fixity in the long spans, particularly those with midspan hinges/movement joints.

Widening the deck of the existing Mangere Bridge is limited by the deck cantilever slab strength and the longitudinal bending capacity of the existing box girders. Even minor



widening within the capacity of the existing box girders would require significant structural works. See Appendix C of this report for further discussion on this issue.

The foundations are reinforced concrete pile caps with reinforced concrete bored piles founded into the bedrock and belled for additional bearing capacity.

The original motorway design provided for future duplication of the motorway bridge, to the east of the first bridge. The northern abutment, which incorporates a tunnel carrying a pedestrian subway through the motorway approach embankment, was constructed double width to accommodate the future duplicate bridge. The southern abutment, which also incorporates a service subway, was only partially constructed for a future duplication.

The carriageway of the Mangere Bridge will be converted to provide four north bound traffic lanes with bus priority on the western shoulder. The existing deck is super-elevated over its entire length and is compatible with the proposed conversion. A cross section illustrating this arrangement is shown in Volume 2 Appendix 1 Detailed Description of Physical Works. The existing central median will be removed and the deck surfacing reinstated.

2.2 Bridge Side Protection

The deck edge traffic barriers are rigid concrete New Jersey Barrier type, with steel top rails. Roadway lighting is by lamps on columns at the outer edges of the deck, behind the barriers.

For the motorway traffic volumes and speed environment at this location, the Transit NZ Bridge Manual requires that the bridge roadway side protection comply with barrier Performance Level 5, or Performance Level 6 if justified by a benefit-cost analysis. A benefit-cost analysis for a Performance Level 6 barrier versus a Performance Level 5 barrier has been carried out. This indicates that the benefit-cost ratio for a TL6 barrier (which meets Performance Level 6) is very low, less than 1.

On the existing Mangere Bridge upgrading the barrier performance level would require strengthening of the deck cantilever slab. The existing barriers on the Mangere Bridge do not fully meet the performance level TL5 standard. While the shape is close to the TL5 standard, the deck connections appear to lack the required capacity. Upgrading the existing side barrier protection is not proposed, but at the design phase stage the costs and benefits of this barrier upgrade may be reviewed.

2.3 Pedestrian Access

The existing footpath and cycle track is carried in a partly enclosed structure beneath the western cantilever of the Mangere Bridge deck. While it provides a very good view of the western Manukau Harbour, this pedestrian access is not favoured by the public because of safety concerns.



Removal of the pedestrian access structure was considered in the assessment of widening the bridge deck. Complete removal of the pedestrian access structure is difficult and costly and is not proposed.

2.4 Stormwater Drainage

The existing stormwater drainage on Mangere Bridge involves a channel on the western edge of the deck. This channel has a series of regularly spaced slots which allows stormwater to discharge to the Harbour below. It is proposed to convey the stormwater collected by the slots via stormwater pipes to each abutment and on to ponds 4 and 5 respectively. Volume 2 Appendix 16 Stormwater Management Assessment of discussed treatment.



3 Duplicate Bridge

3.1 Introduction

The duplicate motorway bridge across the Manukau Harbour will be located to the east of the existing Mangere Bridge with an approximate 3.5 metre separation. It is expected to have a vertical profile similar to the existing bridge to ensure aesthetic compatibility given its proximity. The bridge is likely to be 21 metres wide with an overall length of 650 metres between abutments.

3.2 Background

3.2.1 Navigation and Port Requirements

Consultation with Ports of Auckland and the Harbour Master was carried out in 2003 to establish navigation and requirements upstream of Mangere Bridge. Ports of Auckland advised that they have no plans to extend the Onehunga Wharf eastward and no requirements for shipping access under the new bridge. The Harbour Master advised that existing navigation clearance of the Mangere Bridge is no longer required. There is, therefore no site specific navigation, ship impact, or other shipping requirement to be taken account of in the design of the duplicate bridge. The only vessels passing under the bridge will be small pleasure craft. The navigation clearance requirement for these vessels is limited by the clearance to the proposed new low-level Mangere Footbridge.

3.2.2 Hydraulic Considerations

Data obtained from the Marine Chart (1999 edition) indicates that:

• Tidal currents are at a maximum at flood half-tide, i.e. when the water level is at approximately mean sea level (MSL) with the tide rising;

• The maximum flood tide velocity is 1m/s at a bearing of 118 degrees from the north;

• The maximum (spring) tidal range at the bridge site is 3.4 metres;

• The maximum water depth under the bridge is 7 metres below MSL;

• The top of the Mangere Bridge pier pile caps are at MSL.

At peak tidal flows, with water level close to MSL, the piles and pile caps cause flow resistance. At this water level it is only the existing bridge pile groups for Piers 6 and 7 that sit in the channel and influence flows.

The bridge piers are aligned in the transverse direction at 67 degrees to north. The tidal channel flow angle of 118 degrees to north means that flow approaches the piers at a 50 degree angle. From this angle the pile and pile cap sets of the existing Mangere Bridge



and the duplicate bridge will act independently. This negates any flow shielding between the bridges, and it is also apparent that the combined exposed faces of the foundations of each bridge will present a larger obstacle to tidal flows than the existing bridge alone. This matter is covered in Volume 2 Appendix 15 Coastal Processes Assessment.

3.2.3 Bridge Geometrics

The geometric shape of the bridge deck needs to integrate with the motorway alignment. This is described in Volume 2 Appendix 1 Detailed Description of Physical Works. There is no provision for pedestrian or cyclists on the proposed duplicate bridge. This will be provided by the Mangere Footbridge. Provision for bus lanes on the outer shoulders of both existing and duplicate bridges is proposed.

3.2.4 Aesthetic Considerations

The location of the crossing and the proximity of the duplicate bridge to Mangere Bridge require aesthetic consideration. This is covered in the Volume 2 Appendix 9 Landscape and Visual Assessment.

3.2.5 Foundations

Bedrock at the existing motorway bridge site is Waitemata Group sandstone, at depths ranging from 18 metres to 26 metres, overlain by weaker, softer materials. The bedrock is considered adequate to found the bridge piles on. Details of the ground conditions at the bridge site are given in Volume 2 Appendix 17 Geotechnical Appraisal Report.

The most suitable foundations, both structurally and economically, are expected to be bored piles founded into the bedrock. The piles could be belled at their base if necessary to provide the required bearing capacity.

3.2.6 Abutments

Abutments for a duplicate bridge were originally constructed when the Mangere Bridge was built in the late 1970’s. The duplicate bridge will span over Onehunga Harbour Road to the existing northern abutment location.

The northern abutment structure includes a pedestrian underpass and is expected to be suitable for the duplicate bridge with only minor modifications to fit the proposed new southbound carriageway width.

The southern abutment is not complete and is unlikely to be used. The southern abutment will be located at similar longitudinal position to the existing Mangere Bridge abutment.



3.3 Features Required

The proposed bridge is expected to include the following features:

3.3.1 Bridge Side Protection

For the motorway traffic volumes and speed environment at this location, the Transit Bridge Manual requires that the bridge roadway side protection comply with barrier Performance Level 5, or Performance Level 6 if justified by a benefit-cost analysis. A benefit-cost analysis for a Performance Level 6 barrier versus a Performance Level 5 barrier has been carried out. This indicates that the benefit-cost ratio for a TL6 barrier (which meets Performance Level 6) is very low, less than 1.

A TL5 barrier (meeting Performance Level 5) is proposed for the duplicate bridge. It is expected to be a TL5 concrete edge barrier with a steel top rail. The concrete barrier is typically 820mm high and the total height of the system is usually 1270mm. This edge protection barrier has been suggested to maintain views from the bridge. The concrete section is of similar height to a standard rigid concrete barrier (860mm TL4), which allows comfortable viewing between about 820mm and 1160mm.

3.3.2 Stormwater Collection

Surface runoff from the carriageway will be collected either through a series of cesspits or in an edge channel (as on the existing bridge). The runoff collected will be piped to stormwater treatment ponds at the southern abutment and to the north at Gloucester Park West. Where the bridge is super-elevated (the majority of the length), the cesspits or channel will only be on the western edge of the carriageway. Stormwater treatment is covered under Volume 2 Appendix 16 Stormwater Management Assessment.

3.3.3 Utility Services and Lighting

The lighting for the duplicate bridge will comprise edge mounted steel poles facing the carriageway. The duplicate bridge is likely to be designed to accommodate additional services through the internal boxes. The extent of utility services will be determined during design phase. These are likely to include electrical and fibre optic/communication services for motorway operations.

3.3.4 Maintenance and Access

Expansion joints are likely to be limited to abutments to minimise ongoing maintenance. Maintenance access should be provided to any internal box sections. A maintenance gantry rail and moveable cage should also be provided under each deck overhang to allow maintenance inspections.



The Provision of emergency cross over gates in the medium barrier at each end of the bridge is likely

3.4 Options Considered

A range of scheme options were considered to assess the likely form of the duplicate bridge. These are described in Appendix C of this report and include a range of span arrangements. The proposed duplicate bridge is based on that consideration and is described in the following section.

Appendix C of this report also summarises consideration of options previously discarded including a new low level bridge and widening of the existing Mangere Bridge.

3.5 Structural Form and Span Arrangement

The duplicate bridge will be of a similar vertical deck profile to the existing one although span lengths and pier locations are likely to be different to the existing Mangere Bridge. The bridge design is currently at the scheme stage of development with design phase still to be undertaken. As a result it is proposed that piers will be located in defined zones with actual pier numbers and locations to be determined during design phase.

These zones relate approximately to the short span lengths of the existing bridge and are shown on drawing 1/69/95/5104/1 in Volume 3 Scheme Plans and Drawings. It is likely that there will be fewer piers than the existing Mangere Bridge. The pile caps supporting the piers are expected to be larger than the existing Mangere Bridge to accommodate more piles at each pier. The pile cap level could be higher than the existing pile caps which are at MSL. This level will be determined during design phase allowing for hydraulic considerations and appropriate construction methods.

A number of options for the duplicate bridge structural form and span arrangement have been assessed. The duplicate bridge superstructure is likely to have either steel or post-tensioned concrete box girders supported on a substructure constructed in reinforced concrete. The details of span lengths, bridge superstructure form, materials selection and the construction method will be decided during the design process.

3.6 Construction Methodology

3.6.1 Bridge Construction Methods

The construction methodology that will be used will be finalised during the design phase. The ranges of possible methods include:

Balance Cantilever Construction

With this method each superstructure span is constructed sequentially, in a series of short sections formed alternately each side of a pier out to midspan. After each half of a span



has been constructed, a single short infill centre section is constructed to complete the span. This construction method can be used with precast concrete segments, cast in-situ concrete and steel superstructures, and is particularly suited to prestressed concrete box girders. This method does not require supporting falsework for the superstructure spans and so minimises the temporary works in the harbour waterway.

Balanced cantilever construction, which was used for construction of the existing motorway bridge superstructure, is well suited to this site as it does not require supporting falsework for the deck spans.

Falsework Supported Construction

This method involves construction of the superstructure on falsework supported on piled foundations. The extent of piled support can be uneconomic. Alternatively, falsework could be secured to completed piers using trusses. This is likely to require two spans of support simultaneously and will be complicated by the horizontal curvature of the bridge. This could be used with cast- insitu concrete or steel superstructures.

Incrementally Launched Push Bridge

This technique involves casting the deck superstructure in segments at one abutment and progressively jacking it and an attached launching nose to travel over sliding bearings on successive piers. This technique requires either a straight alignment or fixed radius curvature. The proposed bridge alignment, which matches that of Mangere Bridge, does not have a fixed curvature so the push bridge is not a suitable technique for this location.

Assessment

If a box girder superstructure were prestressed concrete the sections would be either cast insitu or precast in a yard, delivered to site by barge and lifted into position by a barge-mounted crane. An overhead launching gantry supported on completed piers can also be used to place precast concrete segments or steel box sections. This technique is being used to construct the Waiwera Bridge as part of the ALPURT B2 project. This is illustrated on photograph 8 in Appendix A of this report for a Hong Kong based project.

Concrete for cast-insitu construction would be delivered to each pier position by truck via the access jetties and placed by either pump or crane and skip. This construction method has been used recently for the Upper Harbour Crossing Bridge on SH18 as shown in photographs 6 and 7 Appendix A of this report.

If a concrete superstructure is adopted the selection between precast segmental construction and cast-insitu segmental construction would be made at the design phase. In view of the extended southern spans over the intertidal area that will limit barge access it is possible that cast in-situ concrete or precast concrete installed by overhead gantry may be more economic than installation from a barge.



Steel box girders or plate girders could be prefabricated and erected in long sections using cranes, but this is unlikely to be economic due to the high foundation costs for suitable crane access. Barge mounted cranes can only be used for the central spans. A steel box girder could be constructed by the balanced cantilever method using an overhead launching gantry. The deck would be cast-insitu concrete.

In summary, a steel or prestressed concrete box girder constructed using the balanced cantilever method is a typical solution that would be suited to construction of the duplicate bridge given the proposed bridge length, height over water and the benefits of minimising foundations. The superstructure could be constructed using cast in-situ concrete, precast concrete segments or a steel box section with an insitu concrete deck.

3.6.2 Temporary Works

The majority of piers will require construction within the Manukau Harbour (CMA). It is expected that boring of the permanent foundation piles, construction of the reinforced concrete pile caps and construction of the reinforced concrete pier columns for these piers are likely be carried out from temporary staging. This temporary staging, at each pier position, could be provided by temporary jetties from the foreshore at each end of the bridge site. As for the staging, these temporary jetties could be supported on temporary piles driven from barges. Construction of the two abutments and adjacent piers will be carried out on land.

A temporary reclamation is proposed at the north end of the alignment adjacent to Onehunga Harbour Road. This reclamation will provide access to the temporary staging jetty and will also provide a work area for the contractor. The construction could involve a combination of sheeting piling and batter slopes. Design refinement will occur during design phase as the final construction methodology is confirmed. On completion of construction temporary works, including the piles and reclamation, will be removed. Some of the temporary reclamation will be left in place to create a saline marsh.

3.6.3 Construction Work Areas

The contractor’s main work and establishment area for the duplicate bridge is proposed to take place in an area between Waterfront Road and the foreshore adjacent to the existing Mangere Bridge. This area is required for site offices and facilities, materials delivery and storage, layout and assembly, and storage of construction equipment. Storage of segments and precast beds may be required dependent upon the construction methods adopted. The area south of the new southern abutment will be required for construction access and a possible truss assembly yard. See construction area drawings 1/69/82/5104/93-99 in Volume 3 Scheme Plans and Drawings for location of these areas.



4 Old Mangere Bridge and Mangere Footbridge

4.1 Old Mangere Bridge

The Old Mangere Bridge is at a low level and flat grade and is a direct connection between the Mangere Bridge and Onehunga townships and, for these reasons, it is the preferred pedestrian and cycle crossing over the Manukau Harbour.

The Old Mangere Bridge was completed in 1914. Volume 2 Appendix 11 Archaeology and Heritage Assessment provides an outline of the bridge’s history is provided in Volume 2 Appendix 11 Archaeology and Heritage Assessment. It is a reinforced concrete beam bridge on reinforced concrete piles. It remained in service for vehicular traffic until the new Motorway Bridge was opened in 1983. Refer to photograph 4 in Appendix C of this report.

As a result of the low clearance to the tidal waterway, the Old Mangere Bridge had suffered serious structural deterioration with extensive spalling of concrete and corrosion of reinforcing steel and over the last few years before it was closed to vehicular traffic in 1983, a bailey bridge was put in place over the spans to keep traffic loading off the deck and beams. Since that time it has been reused as a pedestrian/cycle bridge.

There has been ongoing slow but steady deterioration. Many of the original main reinforcement bars in the beams have entirely disappeared due to corrosion. A series of structural repairs have been made over recent years. However it is becoming increasingly difficult to retain structural integrity due to continuing deterioration, and aggravated by additional loading such as ship collision. The deterioration of the reinforced concrete superstructure beams has reached a state at which permanent structural repair is no longer feasible and the safe working life of the structure is extremely limited.

On account of the structural deterioration, the use of the Old Mangere Bridge is currently limited to pedestrians and cyclists with areas of the deck fenced off from the public to restrict loading on the worst sections of the structure.

Given the continuing structural deterioration of the Old Mangere Bridge and its limited remaining safe working life, it should be demolished and replaced with a new Pedestrian/Cycle Bridge.

The demolition of Old Mangere Bridge will involve removal of all deck elements and piers. The existing abutments will be left in place unless they need to be demolished or partially demolished in order to construct the new Mangere Footbridge. Piles at pier locations will only be broken back to harbour bed level. Localised use of explosives may need to be used during demolition. The demolition will be planned so that structural members of the old bridge are lifted away as they are demolished. With this process, small fragments of old concrete will be dislodged from the structure and may fall into the Harbour. Any such fragments larger than 25kg will be recovered for proper disposal.

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4.2 Mangere Footbridge

To provide pedestrian and cycling access across the Manukau Harbour a replacement of the Old Mangere Footbridge is proposed. The replacement bridge will be approximately 5 metres wide and 250 metres long and will provide for combined pedestrian and cycle usage. The new footbridge needs to be this width to allow a 3m cycleway and 2m walkway. At that width maintenance vehicle access is also possible. It is likely to have a slightly higher vertical level given the required deeper beams (for longer spans) and to allow for potential sea water level rise.

The superstructure is proposed to have spans of up to 30 metres comprising prestressed concrete beams supported on single bored pile pier columns. The new footbridge would extend between the existing bridge abutments at the ends of the existing causeways.

The existing bridge will be demolished to allow the construction of the replacement Mangere Footbridge. Part of the Old Mangere Bridge may be used to provide construction access during the construction work. The detailed construction methodologies will be developed during design phase. It is however expected that the Old Mangere Bridge will be permanently closed prior to the construction phase for the replacement bridge. The period between closure of the old bridge and opening of the replacement Mangere Footbridge could be up to 1 year in duration. This will effect pedestrian, cycling and recreational fishing access. During this period the suspended walkway on the Mangere Bridge (motorway) will need to be utilised by pedestrians and cyclists.

The existing boat ramp at the end of the Old Mangere Bridge Causeway may also need to be closed during construction. The boat ramp at Orpheus Drive is an alternative launching site. Therefore the construction of the Mangere Footbridge and the reclamation along Orpheus Drive may need to be programmed for different periods to ensure one ramp is always operational.

The Old Mangere Bridge has been struck by cement ships berthing at the Onehunga Wharf on a number of occasions. The collision in November 2002 was particularly damaging. Extensive repairs were completed in July 2003.

The Transit Bridge Manual requirement is that the bridge piers shall either be protected by auxiliary structures designed to absorb energy impact by ship, or the piers should be designed to resist impact from vessels at both normal and extreme operating conditions that could occur during the life of the bridge.

To protect the new Mangere Footbridge from impact from out of control vessels when berthing at the Onehunga Wharf, a protective row of piles across the main waterway is proposed located immediately downstream of the existing Old Mangere Bridge and separate from the new Mangere Footbridge. The piles would consist of 1.5m diameter concrete filled steel piles at 15m centres socketed into the Waitemata Series Sandstone. 15m spacing has been selected so that every second pile will line up with the new

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footbridge piles and will prevent vessels passing between piles. The pile spacings are greater than the Old Mangere Bridge minimum pier spacing of 12.2 metres. It is probable that a major impact would damage and displace the pile such that it required replacement. Damage is considered quite acceptable when a rare extreme event occurs. Structurally, the piles are only required to mean High Water Spring level but would be extended above for safety and visibility reasons. The extent of the piles should not interfere with the operation of the existing boat ramp at the end of the southern causeway and as there is no horizontal member, vertical navigation clearance for small vessels is not an issue.

5 Gloucester Park Interchange

The upgraded interchange will be a standard diamond interchange over SH20 at Gloucester Park Road. The Gloucester Park Bridge over SH20 and mechanically stabilised earth (MSE) retaining walls to support the motorway on and off ramps are required for the interchange.

5.1 Gloucester Park Interchange Bridge

The proposed Gloucester Park Bridge is likely to be two spans of 19.5 metres with a pier in the existing SH20 central median. The new bridge will be approximately 32.4 metres wide by 39 metres long and on a skew angle.

The clearance proposed under the GPI Bridge is 5.4 metres. This is consistent with the Walmsley Road Bridge and proposed Rimu Road Bridge. North of the GPI, a 5.1 metre vertical clearance envelope has been adopted for the SH20 Mt Roskill Extension Project.

The construction of the superstructure will be over live motorway lanes which favours precast concrete beams to minimise construction over the motorway. The proposed bridge superstructure will consist of prestressed concrete beams with 19.5 metre spans supported on bored pile foundations.

The bridge abutments will be formed by a continuation of the motorway MSE on ramp walls under the bridge to align with the motorway off ramps MSE walls. The bridge abutment piles will be contained within the MSE walls to present a continuous wall profile for the motorway in both directions under the bridge.

5.2 GPI Retaining Walls for Interchange Ramps

The GPI ramps involve a substantial amount of earthworks. To minimise the footprint of the interchange both the east facing and west facing on and off ramps will be supported on MSE retaining walls. These retaining walls will be continuous under the Gloucester Park Bridge.

Under the bridge abutment and the north facing on and off ramps the MSE retaining walls are founded on the underlying basalts.

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For the south facing on and off ramps the MSE retaining walls extend over the tuff ring, and lightweight fill construction will be required to ensure foundation stability and minimise differential settlement between the piled bridge structure and the MSE walls founded on the basalt at the edge of the crater ring. The geotechnical aspects of this are outlined in Volume 2 Appendix 17 Geotechnical Appraisal Report.

6 Other Structures

6.1 Walmsley Road Bridge

There is sufficient width and clearance to accommodate the terminating third Southbound lanes. It is not proposed to continue the bus lanes south of Walmsley Road given the adjacent interchange with SH20A. The bridge has, pre-stressed concrete box beams made continuous over the piers so the width available under the existing bridge cannot be significantly increased without demolition and replacement of the existing bridge.

The bridge is in good condition therefore it is intended that the structure will remain.

The structural clearance between the existing motorway median road level and bridge beam soffit is 5.4 metres which is less than the current design standard clearance of 6 metres. This clearance is taken as the limiting clearance for the motorway bridge structures proposed in the section of SH20 north from Walmsley Road to the GPI.

6.2 Tararata Creek Bridge

The existing SH20 Tararata Creek Bridge needs to be widened, with a separate bridge constructed to accommodate the new motorway off ramp. The 4 metre bus shoulder lanes and carriageway widening are accommodated on the Tararata Creek Bridge by shifting the existing centreline approximately 7 metres to the east and widening the bridge on the eastern side.

The existing SH20 Tararata Creek Bridge comprises three spans, overall length of 34 metres. These consist of proprietary, prestressed concrete box beams made continuous over the piers with extensive rock fill and revetment work on the stream banks.

The centreline shift requires the existing deck of the Tararata Creek Bridge to be built up by up to 420mm in order to provide the cross fall on the western carriageway. While some part of this build-up can be made with polystyrene void former it is anticipated that the central bridge beams will have to be strengthened to carry the additional dead load.

The existing SH20 Tararata Creek Bridge carries the existing southbound off-ramps to Walmsley Road on a tapered deck section supported by a cast-insitu concrete edge beam. The widening for SH20 requires replacement of this tapered section of the existing bridge with new bridge beams spanning Tararata Creek.

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The widening is expected to be three spans of prestressed concrete Tee beams matching the depth and bending stiffness of the existing proprietary, prestressed concrete box beams. The bridge widening will require extension of the rock fill and revetment in the stream inter-tidal zone. Foundation piles will be initially bored and then driven to found in the deeper competent sandstone base. The widened bridge structure will meet the minimum 600 millimetre clearance over the 100 year average recurrence interval flood.

The new Walmsley Road Off Ramp Bridge is expected to be constructed first and the southbound off-ramp realigned to allow demolition and removal of the tapered deck section of the existing bridge, and construction of the widening required for the six lanes and bus lanes on SH20. The precast prestressed concrete beams for the bridge widening can be lifted off the existing bridge during night time carriageway closures.

6.3 Walmsley Road Off Ramp Bridge

A new, separate bridge is required for the new Walmsley Road southbound off-ramp alignment. The off ramp is a single 3.5 metre lane with 2 metre and 1 metre shoulders. In order to avoid extending the rock fill revetment for the new bridge abutments into the Tararata Creek intertidal zone, a 70 metre long bridge structure is proposed.

The new off ramp bridge over Tararata Creek is likely to be either a four span bridge with the central span pier piles aligned with the pier piles of the existing SH20 Tararata Creek Bridge or a three span bridge with a long central span over the tidal channel section.

A 7.4 metre wide and 70 metres long three span structure using pre-stressed concrete beams up to 30 metres in span supported by single column piers on bored pile foundations, is proposed as the likely structural arrangement. This bridge structure will meet the minimum 600 millimetre clearance over the 100 year average recurrence interval flood.

Construction of the foundations and the reinforced concrete pier columns and cap beams for both the new Walmsley Road Off Ramp bridge and the widening of the existing SH20 Tararata Creek Bridge will be carried out from temporary staging. This temporary staging, to access each pier position, will be provided by temporary jetties from the foreshore at each end of the bridge site. These temporary jetties will be supported on temporary piles driven progressively as the jetty is extended from the creek bank. The temporary jetties will also be used for lifting the precast prestressed concrete beams into position.

6.4 Hastie Avenue Footbridge

The existing Hastie Avenue Footbridge spans SH20 in one span and consequently will need to be replaced for construction of the new southbound carriageway. Construction of a new footbridge creates the opportunity to provide ramps for disabled access over the motorway in place of the existing stairways. The width proposed between handrails over the footbridge is 2 metres.



The new footbridge can provide accessible standard ramps located in a scissor pattern along the motorway berms and back to land on the Hastie Avenue at existing ground level on either side of the motorway. The final layout for the footbridge approach ramps will be determined at the design phase and will depend on the final topography, aesthetic considerations and anticipated pedestrian movements. There is the possibility that the existing stairways can be reused within the scissor ramps for direct access from Hastie Avenue to the footbridge spans over the motorway.

While a single 44 metre span over the motorway is feasible using a steel through truss footbridge, a two span concrete footbridge with handrails is proposed for conformity of appearance with the other bridge structures over SH20. Two 22 metre spans with a central median pier are proposed over the motorway with 10.2 metre spans at a 1 in 12 grade for the scissor ramps. A single pre-stressed beam with the same single tapered column pier supported by a bored pile as used on the Beachcroft Avenue footbridge, is proposed as the likely structural form as this is an aesthetically pleasing appearance.

Whilst the existing footbridge is being replaced, a temporary footbridge will be provided to ensure continued public access. The temporary footbridge is likely to be a single span structure which could be reused for a temporary footbridge at Hastie Avenue (or vice versa).

The existing footbridge span and the eastern pier must be removed for construction of the southbound carriageway unless a temporary eastern span is constructed over the south bound carriageway. When southbound traffic is diverted onto the new carriageway the central median pier for the new footbridge can be constructed and the footbridge bridge beams placed over the motorway during night time carriageway closures.

6.5 Rimu Road Bridge

The existing Rimu Road Bridge has one 25 metre span over the existing SH20 and two 12 metre land spans. The existing bridge beams are precast prestressed concrete inverted T beams with a vertical clearance to the existing carriageway of only 5.16 metres. The bridge pier columns are supported by short piles founded on the underlying basalt. Refer to photograph 5 in Appendix C of this report.

Raising the level of the Rimu Road Bridge is proposed as a vertical clearance of 5.4 metres is required and the lowering of the new motorway carriageway is not practical (as there is insufficient fall for stormwater drainage from the motorway in this vicinity). The existing bridge pier columns and foundations are not expected to have adequate capacity to support two longer spans at a higher level so replacement of the Rimu Road Bridge with a new bridge on the same horizontal alignment and at a higher level is proposed.

The new bridge will be approximately 17.3 metres wide with a 2.4 metre footpath on the southern side and 1.5m shoulders on each side for cycle lanes. The road way will consist of two 3.5m west bound lanes and a 3.5m eastbound lane. It is likely to have a 22 metre



span over the southbound carriageway and a 20 metre span over the northbound carriageway and two 12 metre land spans. The new Rimu Road alignment will be raised by about 500mm to allow the structural depth for the longer spans with a vertical crest curve over motorway centreline. For these spans precast pretensioned beams on pier columns supported by short piles founded on the underlying basalt are expected. The existing eastern pier column foundations can be incorporated into the new central median pier foundation.

In order to keep Rimu Road fully open during construction of the new bridge lanes, a temporary bridge will be built to the north of the existing structure with new on ramp and off ramp intersections. The length required for this temporary bridge will depend on the contractor’s requirements for access along the eastern side of the existing motorway.

Access for construction of the new bridge will be from Rimu Road and Mahunga Drive. When the temporary bridge is open the existing structure will be removed and the new bridge can be constructed with only the western main span requiring work over the live motorway.

The temporary bridge can be removed when the new Rimu Road Bridge and re-grading of Rimu Road is completed. An option, including a new bridge to the south of the existing bridge without the need of a temporary bridge, was also considered but is not preferred given difficulties with the additional land take that would be required.

6.6 Beachcroft Avenue Footbridge

The existing Beachcroft Avenue footbridge spans the existing SH20 carriageways in one 22.5 metre span with two 16 metre spans on the eastern side and a 14m span over the northbound off-ramp and a 12.7 metre land span on the western side. The minimum vertical clearance to the motorway is 5.2 metres. The existing eastern main span pier is in the new southbound carriageway and consequently the footbridge will need to be replaced.

The minimum vertical clearance required for the new footbridge on SH20 between the GPI and the SH20 Mt Roskill extension is 5.4m to match that of Queenstown Road Bridge. Construction of the new footbridge north of the existing footbridge, landing at a higher level on Seacliffe Road, will provide an increased vertical clearance to the motorway without ramps on the western side. A double return ramp will be required to provide an accessible standard access to Beachcroft Avenue from the level required for vertical clearance over the new southbound carriageway. The final layout for the footbridge approach ramps will be determined at the design phase and will depend on the final topography, aesthetic considerations and anticipated pedestrian movements. The width proposed between handrails over the footbridge is 2 metres.

While a single 42 metre span over the motorway is feasible using a steel through truss footbridge, a four span concrete footbridge with handrails is more likely for conformity of



appearance with the other bridge structures over SH20. This is likely to involve 19.7 metre and 22.6 metre spans with a central median pier over the motorway with a 22.4 metre span over the northbound off-ramp. Additional spans include a 17.4 metre land span and 10.2 metre spans at a 1 in 12 grade for the double return ramp to Beachcroft Avenue on the eastern side. A single prestressed beam for each span, with the same single tapered column piers supported by bored piles as used on the original Beachcroft Avenue footbridge, is proposed as this has an aesthetically pleasing appearance.

Whilst the existing footbridge is being replaced, a temporary footbridge will be provided to ensure public access. The temporary bridge is likely to be a single span structure which could be reused for a temporary footbridge at Beachcroft Avenue (or vice versa). The existing footbridge span and the eastern pier must be removed for construction of the southbound carriageway. When southbound traffic is diverted onto the new carriageway the central median pier for the new footbridge can be constructed and the footbridge bridge beams placed over the motorway during night time carriageway closures.

6.7 Queenstown Road Bridge Abutment Underpinning

The existing bridge structure provides sufficient width for three 3.5 metre lanes in each direction. There is insufficient available width to accommodate bus shoulder lanes under Queenstown Road Bridge. The bus shoulder lanes terminate at the south of the bridge. The vertical clearance to the northbound lanes exceeds 6 metres and the minimum vertical clearance to the southbound shoulder lane is 5.4 metres. The 5.4 metre clearance is consistent with clearance proposed elsewhere on this project. Photograph 3 in Appendix C of this report illustrates the existing bridge.

The existing bridge abutments are supported on Reinforced Earth walls founded on Waitemata sandstone. The founding level for the southbound side abutment is close to motorway level but the founding level for the northbound side abutment is 2 metres above the widened motorway level. Consequently the existing abutment Reinforced Earth walls need to be underpinned with insitu drilled piles. No other alterations to the existing structure will be required.

Underpinning the existing bridge abutment Reinforced Earth walls with reinforced concrete cantilever drilled piles is proposed. On the northbound side the underpinning wall will extend 280m to support the batter up to the off-ramp. On the southbound side the underpinning wall will only be required for the extent of the abutment wall.

Construction of the underpinning walls will require closure of the existing motorway shoulders and may require reduced lane widths under the Queenstown Road bridge.

6.8 Retaining Walls

Retaining walls are required as part of the works along SH20. These chainages at the retaining walls are as follows.



• 680 – 900: To support the cut created by the eastern side of the Tararata Creek off ramp, a piled retaining wall is required along sections of SH20;

• 2080: Additional retaining walls may be required for realignment of the road at Mahunga Drive;

• 3440 – 3940: MSE retaining walls required at Gloucester Park and Orpheus Drive to support fill and bridge abutments;

• 4920 – 5240: Due to the cuts for the Queenstown Off ramp and SH20 widening, two retaining walls would be required from 4920 to 5240. Additional low retaining walls may be required either side of Queenstown on ramp;

• 4300 – 4560: MSE retaining wall along east side of Onehunga Bay Reserve;

• 5195 – 5225: Concrete soldier pile along east side of SH20;

• 5260 – 5420: MSE retaining wall by creek in Hillsborough gully.



APPENDIX A

Photographs



Photograph 1 : Existing Mangere Bridge looking south from the suspended walkway

Photograph 2 : Existing Mangere Bridge looking north on line of the proposed Duplicate Bridge



Photograph 3 : Queenstown Road Bridge looking east

Photograph 4 : Old Mangere Bridge looking south from Mangere Bridge walkway



Photograph 5 : Rimu Road / Mahunga Drive Bridge viewed from off ramp looking south



Photograph 6 : SH 18 Upper Harbour Bridge Duplication Construction showing the temporary jetty



Photograph 7 : SH 18 Upper Harbour Bridge Duplication Construction Site

Photograph 8 : Typical Launching Gantry, Shenzhen Western Corridor, Hong Kong. Courtesy Uwe Baier



APPENDIX B Bridge Drawings



Appendix B: Bridge Drawings Index

The following bridge and structural drawings are provided in Volume 3 Scheme Plans and Drawings.

Drawing No Description

1/69/95/5104/1 Manukau Harbour Crossing – Plan & Typical Sections

1/69/95/5104/2 Manukau Harbour Crossing – Bridge Options

1/69/86/5104/1 Tararata Creek Bridge – Widening

1/69/86/5104/2 Tararata Creek Bridge – Typical Cross Sections

1/69/86/5104/3 Walmsley Road Off Ramp Bridge

1/69/87/5104/1 Hastie Avenue Replacement Footbridge

1/69/88/5104/1 Rimu Road Replacement Bridge

1/69/89/5104/1 Gloucester Park Bridge - General Layout

1/69/89/5104/2 Gloucester Park Bridge - Elevations

1/69/89/5104/3 Gloucester Park Bridge - Cross Sections

1/69/91/5104/1 Beachcroft Avenue Replacement Footbridge – General Layout

1/69/92/5104/1 Queenstown Road Bridge

1/69/93/5104/1 Mangere Footbridge



APPENDIX C Bridge Options Considered



This appendix outlines the option assessment used to define the likely form of the proposed bridge. The other options that were previously discarded are also discussed. As these other bridge options were not considered practicable the options considered in detail are therefore based on provision of a new, structurally separate, high-level duplicate bridge alongside the Mangere motorway bridge.

C1 Options for a Duplicate Bridge

The new bridge would be a duplicate of the existing Mangere Bridge on the eastern side as was provided for in the original design of the Mangere Bridge and approach motorway formations. The existing Mangere Bridge median barrier will be removed to allow it to carry the northbound traffic and a new, structurally separate, high-level Duplicate Bridge constructed alongside the Mangere motorway bridge to carry southbound traffic.

The Duplicate Bridge will cross the Manukau Harbour at the same level as the existing Mangere Bridge with the approach motorway formations duplicated to the east of the existing motorway to both the north and south.

C1.1 Structural forms for a duplicate bridge

Structural forms suitable for a bridge of this length, width, height, over water and requiring deep foundations are:

1 Single or multiple cell prestressed concrete box girder superstructure on reinforced concrete piers and pile foundations. This is the structural form of the existing Mangere Motorway Bridge;

2 Single or multiple prestressed concrete beam and slab deck superstructure on reinforced concrete piers and pile foundations;

3 Multiple cell steel box girder superstructure on reinforced concrete piers and pile foundations;

4 Multiple steel plate girder and reinforced concrete slab deck superstructure on reinforced concrete piers and pile foundations;

5 Multiple steel truss and reinforced concrete slab deck superstructure on reinforced concrete piers and pile foundations.



For structural form 2, concrete beams and deck slab, the spans are limited to less than 40m and so would require a large number of piers. Due to the depth (approximately 33 metres) to founding bedrock, long foundation piles are required, making piers costly. Structural form 2 is not likely to be a cost-effective option for this site given the number of piers that would be required.

Steel superstructure forms 3 and 4 are technically suitable. Experience on previous similar projects has shown that, overall, concrete superstructures have been more economic. However the advantage is small and dependent on market conditions at the time of construction.

A steel truss form would be very different in appearance to the existing bridge and the much greater structural depth would be visually very intrusive. Therefore this form was not considered further.

C1.2 Span Arrangement Options for a duplicate bridge

(a) Option 1 Fourteen Spans – Existing Arrangement

This option would have fourteen spans with the pier positions aligned with the existing Mangere Bridge piers (refer Figure C-1). With the south abutment adjacent to the existing Mangere Bridge south abutment, the overall length of the duplicate bridge is 650 metres. The fourteen spans would be arranged in 30-60-30-60-30-86-30-84-30-60-30-60-30-30 metre lengths starting from the northern end, to align the piers with those of the existing motorway bridge.

The superstructure would be a box girder, either prestressed concrete or steel, with side cantilevers to give the required deck width. The box girder depth could vary from 2 metres at midspan to approximately 4.5 metres at the piers for the longest spans. It would be continuous the full length of the bridge, and built-in at the centre six piers to resist longitudinal loads, with movement bearings at the abutments, three northern piers and four southern piers. There are likely to be movement joints at the two abutments only.

Spans of this length, particularly of prestressed concrete, are suited to balanced cantilever construction that does not require ground-supported falsework. The existing motorway bridge was constructed using this method.

The piers would be expected to be twin, square section, hollow, reinforced concrete columns supported on reinforced concrete pile caps. The foundations would be reinforced concrete bored piles founded into bedrock.



Preliminary assessment has indicated that the existing north abutment is dimensionally suitable and has the necessary structural capacity to support the new bridge. It is therefore proposed that it be used. As the existing southern abutment was only partially built, a new reinforced concrete abutment is likely to be constructed adjacent to the existing bridge abutment. As cross access will be possible beneath the box girders in front of the abutment, it is not necessary to extend the service subway in the existing abutment to the new abutment. This will be considered further during design phase.

b) Option 2: Eight Spans – Longer Length Spans

This option has longer spans than option 1 with eight spans and seven piers (refer). As for Option 1, the overall length of the duplicate bridge is 650 metres. With this option the new piers would coincide with the short span lengths of the existing bridge, so generally preserving the longer span arrangements of the existing Mangere Bridge.

The superstructure is a varying depth continuous box girder, concrete or steel, as for Option 1, with a depth at the piers of approximately 5 metres. The first two piers from each abutment would likely have movement bearings, the box girder being built-in at the centre three piers, which resist the longitudinal loads. The movement joints are provided at the abutments only.

As for Option 1, balanced cantilever construction would be most suitable for this option. The abutments, piers and foundations are similar to Option 1.

c) Option 3: Nine Spans – Medium Length Spans

This option has seven spans of medium length with six piers (refer Figure C-1).

As for options 1 and 2 the overall length of the duplicate bridge is 650 metres. The pier positions do not line up with those of the existing bridge, but are generally adjacent to the piers for the short 30 metres spans.

The superstructure proposed is a varying depth continuous box girder as for Option 1, with a depth at the piers of approximately 4 metres. The first two piers from each end have movement bearings, the box girder being built-in at the centre four piers, to resist the longitudinal loads. The movement joints are at the abutments only.

As for Option 1, balanced cantilever construction would be most suitable for this option. The abutments, piers and foundations are similar to Options 1 and 2.



Figure C-1 : Duplicate Bridge – Arrangements Considered



C1.3 Structural Form

The bridge design is currently at the scheme stage of development with design phase still to be undertaken. As a result it is proposed that piers will be located in defined zones with actual pier numbers and locations to be determined during design phase.

These zones relate approximately to the short span lengths of the existing bridge. It is likely that there will be fewer piers than the existing Mangere Bridge. The pile caps supporting the piers are expected to be larger than the existing Mangere Bridge to accommodate more piles at each pier. The pile cap level could be higher than the existing pile caps which are at mean sea level. This level will be determined during design phase and allowing for appropriate construction methods.

It is considered that either a prestressed concrete box girder or steel box girder would be suitable superstructure solutions, with the substructure expected to be constructed in reinforced concrete.

C2 Other Options Previously Dismissed

The following options were considered and dismissed during earlier scheme development.

C2.1 New Bridge at Low Level

A new bridge could be constructed at the minimum height above the estuary sea level, thus reducing pier heights, approach embankment heights and, therefore, costs. However, the approach embankments for the new bridge would be at a much lower level than the immediately adjacent approach embankments of the existing Mangere motorway bridge. The height difference between the embankments would require either the construction of high retaining walls for the existing bridge approaches, or constructing the new bridge on a different alignment so the new approaches are well clear of the existing approaches.

Retaining walls of the necessary height would require costly piled foundations, extending approximately 33m to bedrock. Also construction of the walls would require major excavation of the existing approach embankments and consequent closure of at least one traffic lane during construction.

Construction on a new alignment would need to be sufficiently far from the existing Mangere Bridge to avoid high retaining walls at the abutments. It would probably go further beyond the existing highway corridor and designation boundaries of the proposed works. From a road geometry perspective the vertical alignment of the motorway is constrained by the need to maintain sight distances over the crest of the vertical curve and to maintain the vertical clearance to Onehunga Harbour Road, an Auckland City principal overweight and over-dimension route connecting to the port. This prevents the vertical alignment of the motorway from returning to a low level until half way across the harbour. There are unlikely to be construction cost savings and the operating cost savings for a bridge on this



alignment will be small. The appearance of the lower alignment is likely to be less attractive particularly when viewed against the existing bridge.

For the above reasons, this option was not considered practicable and was not considered further.

C2.2 Widening the Existing Mangere Bridge

Widening of the existing four lane motorway bridge would need to be of sufficient capacity to carry an additional four traffic lanes, effectively doubling the width of the bridge. Widening of this magnitude would require the construction of one or two new box girders alongside the existing box girders. An analysis has shown that the existing box girders do not have the capacity for the additional loads that would be imposed if the deck cantilevers only were widened.

The width of the extensions beyond the existing structure would be such that new pier columns would be required to support them. These columns would then require extending the pier foundation pile caps and constructing new foundation piles, as the increase in load is beyond the capacity of the existing piles.

Widening the existing motorway bridge has major structural problems, particularly in linking the extensions to the existing bridge so that they act as a single structure under seismic loads. Due to settlement effects, some of the self-weight and traffic loads from widening would be imposed on the existing bridge foundations rather than the extension foundations. Increasing the loads on the existing foundations is undesirable as it reduces their design safety factor.

The magnitude of the extensions required, with new box girders, pier columns, pile caps and piles would therefore be similar to that of a new, structurally independent bridge. The cost of widening the existing bridge would be greater than that of a new, separate bridge, as the necessary structural connections between new and existing structures would be expensive.

Due to the delay in completing the existing motorway bridge and the use of intermediate movement joints in the centres of three spans, the deck has sagged below the original design motorway profile at the centre of some spans. Extending the existing structure would require this deficient deck profile to be matched on the extension, which is undesirable.

For these reasons, the widening of the existing Mangere Bridge was not considered practical.

sh20 manukau harbour crossing project appendix 3 - bridge and … · 2010. 1. 11. · • hastie...

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