Spinnaker Drive remains closed at the intersection with SH58 following a slip that deposited a substantial amount of debris on the road.
We understand the frustration over the closure of the lower section of Spinnaker Drive in Whitby, due to a slip that occurred in July 2023.
Proposed remediation option
Mass Block retaining wall
After consideration of all proposed options, we are progressing with the mass block retaining wall option. The retaining wall is proposed to be constructed at the toe of the slip.
Initial modelling shows this solution would stabilise the toe of the slip and allow for realignment of the existing road.
Detailed design
We have progressed the mass block retaining wall into the detailed design stage. The design will be refined to include more detail, which aids us in understanding the costs and construction methodology.
Construction tender
Once we have approved funding for the work, we will place a request for tender onto the Government Electronic Tender Service (GETS). Prospective contractors will have a number of weeks to prepare and submit their tender proposals.
We will then evaluate the tender proposals, award the contract and enter into contract negotiations with the contractor.
Construction
Construction is estimated to take several months, but will be subject to the contractor's programme and any unforeseen circumstances and weather conditions.
Following common engineering practice we considered three scenarios:
- Do nothing – leave as is and close Spinnaker Drive
- Turn Spinnaker Drive into a cul-de-sac and close the entrance off SH 58.
- Do the minimum – what is required to only reopen Spinnaker Drive
- Clearing the debris from the road and repairing road surface to reinstate access to SH 58.
- Reinstate Spinnaker Drive with engineered support option – three options considered:
- Realign Spinnaker Drive
- Realign the road away from the slip to provide greater resilience to the access to Spinnaker Drive.
- Mass Block Gravity Retaining Wall
- Constructing a mass block retaining wall at the toe of the slip to allow the road to be reinstated.
- Bored Pile Retaining Wall
- Construct a bored pile retaining wall to support the toe of the slip and allow the road to be reinstated.
| Positive | Negative | |
|---|---|---|
| Do Nothing Option |
Low cost option Can be implemented quickly |
Would not mitigate against further movement Would require periodic removal of debris Does not reinstate access to Spinnaker Drive off SH58 Access is limited to maintain pipe network |
| Do Minimum |
Can be implemented quickly Restores access to Spinnaker Drive from SH 58 Expect movement from the slip will decrease as vegetation is reestablished |
Recurring closures likely required to allow for debris clean up Higher ongoing maintenance costs Does not mitigate against further movement |
| Realign Spinnaker Drive |
Access to SH 58 off Spinnaker Drive is reinstated The road is more resilient to any ongoing movement |
Does not mitigate against further movement, which may impact the road in the long-term Requires NZTA approval to alter intersection to more like a 'blind corner', which may not be approved |
| Mass Block retaining wall |
Reinstates access to Spinnaker Drive off SH 58 Provides resilience for seismic and high groundwater scenarios Low ongoing maintenance costs, periodic inspections Can be designed for a 50 year design life Wall can be constructed and backfilled in stages, minimising time the temporary cuts to the slop are unsupported |
Higher cost than the options listed above Requires a large volume of debris to be moved off site May require tie back anchors to support and resist overturning and sliding failure |
| Bored Pile retaining wall |
Reinstates access to Spinnaker Drive off SH 58 Provides more resilience for seismic and high groundwater scenarios Can be designed for greater than 50 year design life |
Higher cost than a mass block retaining wall Requires a large volume of debris to be removed off site Will likely require tie back anchors, particularly for high retained heights Temporary cuts to the slope will need to remain stable for a relatively longer period during construction than mass block retaining wall |
Updates
We have considered all concept options and have determined to proceed with the mass block gravity retaining wall to detailed design stage.
We continue to be on track with the concept designs, and detailed design is scheduled to take place between 27 May and completed just before the end of this financial year.
Once detailed design is completed, we will have a better understanding of timeframe and costs, and how best to fund this work.
We have completed all onsite investigations, and the results have been analysed by Pattle Delamore Partners’ (PDP) geotechnical engineers. With this information PDP have developed several concept-level options. PDP have also used these results to evaluate the risk of further slips and instabilities within the vicinity of the slope. They are currently assessing the options and risks to determine a suitable course of action.
Before a final decision is made, we will discuss the proposed option with the affected stakeholders. As part of the decision-making process, we must consider the associated costs, risks, and opportunity to improve our infrastructure.
Once a decision has been made PDP will carry out the detailed design, and we’re aiming to begin this process in May.
We continue to be on track for the development of the range of options by the end of March. We are now looking at completion of the onsite investigations by mid-March due to a scheduling delay. The information gathered from these investigations will validate the options that are being prepared, and allow us to understand the associated risks and costs.
Once the investigations are complete, the options will be presented to the project team, and then to Council.
PDP will be undertaking further onsite investigations, which will feed into the development of a range of options. They are currently arranging for contractors to carry out the investigations. The aim is to have all major onsite investigations complete by the end of February.
Once the investigations are complete, the options will be presented to the project team, and then to Council.
December
The gap analysis report has been completed, which is a review of all available information and investigations carried out, to identify if further investigation is required.
PDP are now working on their recommendation for further investigation.
November
The contract for the investigation and remediation design for the Spinnaker Drive and Pukerua Beach Road slips has been formally awarded to Pattle Delamore Partners (PDP).
The first phase of the project has kicked off, which involves an analysis of all the technical information currently held for both slips and a meeting with affected Spinnaker Drive residents. It is anticipated this initial phase will extend into early next year.
October
The tender for investigation and remediation design for Spinnaker Drive closed 15 October 2024, and our Transport team will be evaluating the proposals.
September
The tender for the investigation and remediation design, has been placed onto the Government Electronic Tender Service. Going out to a competitive public tender is required as per NZTA (Waka Kotahi) and Council procurement policy. The tender process enables us to ensure a value-for-money supplier is chosen to deliver the work.
The process of awarding the tender will take about two months, as it involves evaluating all tenders and then contract negotiations.
Frequently asked questions
We are being prudent with public money and public safety. Given there is alternative access, it would be negligent on our part to rush to reopen the road before a permanent solution is constructed.
The size of the slip is quite significant, ranging from 8 to 10 metres in height, resulting in a substantial amount of earth that has moved. We are also still unsure of the cause of the slip.
Geotechnical experts have advised us if we move any of the material without a permanent solution in place, more of the hill could come down. This would further endanger road users and possibly impact on private dwellings.
As our top priority is safety, we secured the area and closed the road.
What caused the slip?
We have not discovered a clear conclusive cause of the slip. Wellington Water undertook extensive inspections, which included lab testing of ground water at the base of and within the slip, which concluded Council’s current water, wastewater, and stormwater pipes were not the source of water that affected the slip.
Can the Council close the road?
The Local Government Act 1974 allows for councils to close a road under section 342.
What is the remediation process?
As Council is the downhill receiver of the slip material, we are responsible for the work required to reopen the road.
The contract has been formally awarded.
At a high-level the stages of work are:
- Initial works – includes the gap analysis, risk assessment of boundaries for neighbouring properties, and determine if any further investigation(s) are required.
- Option Assessment – carry out any further investigation(s), and a number of different solutions will be provided to the project team to review. We are on track for completion in March 2025.
- Council decision – to confirm preferred solution by Council (date TBC).
- Preferred Option Design – timeframe will be updated after the decision has been made by Council.
Once we have that design, we will undertake a further competitive public tender for an appropriate construction company to build the permanent repair.
Going out to a competitive public tender is required as per NZTA (Waka Kotahi) and Council procurement policy. The tender process enables us to ensure a value-for-money supplier is chosen to deliver the work.
When spending ratepayer money, it is important to ensure the permanent solution is sensible and the right one – with the goal to do it once and do it right.
Why can’t the road be temporarily reopened?
We have looked at what it would take to temporarily reopen Spinnaker Drive, to allow some vehicle access prior to permanent retaining works being undertaken. Unfortunately, the costs involved are prohibitive compared to a full final repair.
Temporarily opening two lanes would require significant earthworks, and retaining works, which could cost as much as two-thirds of the expected permanent solution.
Opening one lane only would require less earthworks but the associated active traffic management is estimated to cost between $500 – $1000 a day, depending on whether traffic flowed just one way (downhill) or both ways (minimising queuing issues on SH58).
Therefore, given there remains a viable alternative route, temporary solutions have been deemed an unwise use of ratepayer money.
Have people been speeding through the detour route?
We have carried out a speed survey on Bosun Terrace, following complaints that vehicles were speeding through this detour route.
The driver feedback sign was installed at 26/28 Bosun Terrace facing Leeward Drive from 10 - 18 April 2024, and at 34/36 Bosun Terrace facing Postgate Drive from 18 April – 1 May 2024.
We found that an average of only 2% of drivers per day were driving over the speed limit of 50km/h.
If residents are concerned about a speeding vehicle, it should be reported to police.
