Spinnaker Drive slip & road closure

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.

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.

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.

The tender for investigation and remediation design for Spinnaker Drive closed 15 October 2024, and our Transport team will be evaluating the proposals.

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.

There are varying complexities with all slips and the main complexity with this slip is ‘where did the water come from?’ and the fact that the second movement increased the height of the slip face from approximately 2–3 metres high to 8–10 metres high.

Subsequently Council received further geotechnical advice to not touch the slip until more information was known and a robust final solution was designed.

Since the slip Wellington Water have undertaken extensive three waters testing and investigations but no clear, conclusive, cause for the water source that triggered the slip has been identified. Council has not rushed the resolution of this particular slip as it understands the causative issues, addresses competing infrastructure remediation project demands across the city, and there are viable alternative options for road users affected by the closure (which is not always the case).

Council is going out to tender, which follows NZTA and Council procurement policy, for the design of a retaining structure required to the road to be reopened safely. The tender process enables Council 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.

Spinnaker Drive remains closed at the intersection with SH58 following a slip in July 2023.

The information below outlines what we've done so far and what our next steps are as we work towards the reopening of the road as soon as practical, subject to competing activities across the city.

Designing a final solution

We are working through the process to carry out a competitive public tender to engage a supplier to deliver the final geotechnical design work required for a permanent repair. We expect to have this out to market next few months.

Once we have that design, we will undertake a further competitive public tender for an appropriate construction company to implement the permanent repair.

This combined process is expected to take at least six to eight months.

Speed assessment on Bosun Tce

We have carried out a speed survey on Bosun Tce, following complaints that vehicles were speeding through this detour route.

It found that an average of just 2% of drivers per day were driving over the speed limit of 50km/h.

The driver feedback sign was installed at #26/28 Bosun Terrace facing Leeward Dr from 10 - 18 April, and at #34/36 Bosun Terrace facing Postgate Drive from 18 April – 1 May.

If residents are concerned about a speeding vehicle, it should be reported to police.

Warning sign replacement

You may have noticed that the two portable electronic variable message signs (VMS) on the SH58 approaches to Spinnaker Road, advising of the closure ahead, have been replaced with fixed static signs.

The aim of this is to minimise ongoing costs, now that it is clear the closure is likely to be in place for longer.

Content already on the website.

Testing and investigations

One of our priorities has been to understand why the land slipped, so we can deal with any issues, such as water run-off, in the design of a final solution.

Wellington Water has carried out extensive testing and investigations, and has so far concluded Council’s water, wastewater or stormwater pipes are not the source of the water affecting the slip.

We’ve also carried out extensive geotechnical testing and investigations which have included drilling to understand ground conditions, and a Light Detection and Ranging (LiDAR) survey to map the surface of the slip in 3D.

Why is the road still closed?

The slip site is a bit like an iceberg; from the road you only get a small glimpse of what’s at play, the risk doesn’t look too bad, and it appears easy to fix. Unfortunately, behind it lies a large volume of material. The slip face scarp ranges from approximately 2-3 metres high to 8-10 metres high (so a significant amount of earth has moved).

Our top priority is safety, and we’ve been advised by geotechnical experts not to modify the base until we have more information and an agreed robust solution ready to construct. As a result, we have closed the road and secured the area.

We understand the time to address the slip is frustrating some residents, but we are mindful (especially in this economic climate) of making sensible decisions when spending ratepayer money. We also want to make sure that the permanent solution we land on is the right one – with the goal to do it once and do it right.

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 and $1000 a day – depending on whether traffic flowed just one way (downhill) or both ways (minimising queuing issues on SH58).

As a result, given there remains a viable alternative route, temporary solutions have been deemed an unwise use of public money.We remain focused on reopening Spinnaker Drive as soon as practical, subject to competing activities across the city.

Currently, we are working through the process to carry out a competitive public tender to engage a supplier to deliver the final geotechnical design work required for a permanent repair.

We also replaced the two portable electronic variable message signs (VMS) on SH58 that advise of the closure ahead with fixed static signs. The aim of this is to minimise ongoing costs, now that it is clear the closure is likely to be in place for longer.

Spinnaker Drive remains closed at the intersection with SH58 following a slip in July 2023.

We are working through the process to carry out a public tender to engage a company able to deliver the final geotechnical testing and design work required for a permanent repair. We are aiming have this out to market in the next few months.

Once we have that design, we will undertake a further public tender for an appropriate construction company to implement the permanent repair.

We have also carried out a speed survey on Bosun Terrace, following complaints that vehicles were speeding through this detour route. It found that an average of 2 per cent of drivers, per day, were driving over the speed limit of 50km/h. The results showed 85% of vehicles were travelling under the speed limit. The 85th percentile speed counted was 44.8km/h.

Average vehicles travelling over the speed limit was around 2% of vehicles, with an average speed of 53.3km/h. If residents are concerned about a speeding vehicle, it should be reported to police.