Services

Ōtaki Gorge Road closure

Background | Timeline | By the numbers| Media updates | Drone footage of the slip site | On site pictures

Road access to private property and popular recreational facilities within the Tararua Forest Park is now restored after a seven-month closure.

Kāpiti Coast District Council has completed extensive earthworks at the slip-prone Blue Bluff site on Ōtaki Gorge Road, meaning the public can access Ōtaki Forks again.

The road was closed for safety reasons in mid-December 2015 due to concerns over the stability of the hillside following a series of slips.

Group Manager Infrastructure Services Sean Mallon says Council worked with a team of experts to look at what could be done long-term to help stabilise the bank and restore access to the area.

More than 70,000m3 (as a loose measure) of debris and slip material has been removed, which is enough material to fill Paraparaumu’s Coastland Aquatic Centre 25-metre pool 47 times.

“It’s been massive job and we’re delighted to get the road opened in time for the second week of the school holidays. It’s great news for Ōtaki and we know local businesses will be pleased to see visitors to the popular tourist attraction coming back into our district,” Mr Mallon says.

Work has included the cutting of wide steps into the slip face and reducing the angle of the slope to minimise the risk of debris blocking the road.

“There’s no guarantee slips may not happen again, but this work is all part of the on-going management of the slope that’ll help reduce the risk of future road closures,” Mr Mallon says.

“We’ll continue to keep an eye on the active area and are establishing a monitoring and cleaning programme that’ll help ensure safe access to the Tararua Forest Park from Ōtaki.”

Two residents were affected by the road closure, a private property owned by the Campbells, and a Department of Conservation in-residence caretaker, Steve Elgar.

“For the Campbells and Steve it’s been a long seven months and we’re grateful for their patience and cooperation while we’ve done this work,” Mr Mallon says.

Work started has taken more than three months and has an estimated price tag of $500,000 - almost half covered by a contribution from the NZ Transport Agency’s emergency works fund.

Background

Ōtaki Gorge Road was closed at Blue Bluff in mid-December 2015 due to concerns over the stability of the hillside in the area. Contractors clearing debris from the bank, after a small slip at the time, had noticed deep fissures 100 metres above the road which showed the land was likely to be moving. The road was closed immediately to protect public safety.

In mid-February 2016, a fresh slip in the area occurred following heavy rain with a large amount of rocks and soil coming off the bank and big areas of debris hanging above the road. Council then worked with a team of experts – geologists, surveyors and consultant engineers – to assess the site, monitor various points on the slip face, and look at what could be done long-term to help stabilise the bank and restore access to the area. 

Once the best option to do this was confirmed, work commenced in early April. It has spanned more than three months and has been a mammoth task. As well as extensive earthworks, the work has included building wide steps into the slip face and reducing the angle of the slope to minimise the risk of debris blocking the road in the future.

The final piece of work once material is all removed is to get some barriers transported to site to act as a catch fence. The road will then be graded, grass cut, weed sprayed, drainage systems checked and reopened mid-July.

The work overall comes with an estimated price tag of $500,000 and almost half of that has been covered by a contribution from the NZ Transport Agency’s emergency works fund.

This work is part of the on-going management of the slip-prone Blue Bluff slope, it is to help reduce the risk of road closures in the future and ensure on-going, safe access to the Tararua Forest Park from Ōtaki.

Timeline

July-August 2015

A reasonable sized slip occurred at Blue Bluff due to the severe weather event   and 5,500m3 of material was removed off site

September-December 2015

A further estimated 2,500m3 was removed from site

December 2015

Further remedial work was carried out and an estimated 1,000m3 was removed when major   cracks were discovered 100 metre above the road.

February 2016

Another slip occurred on site and an estimated 5,000m3 was cleared.

April 2016

Contractors started major earthworks on site

 

July 2016

Road access restored

By the numbers

  • The slip occurred at Blue Bluff, which is an area on Ōtaki Gorge Road situated about 13 kilometres inland from State Highway 1.
  • Ōtaki Forks is a popular recreational area, which is 3km past the slip site.
  • Blue Bluff is clearly an active area that Council has had to carefully manage over the year, with remedial work in the area being regularly required since 2002.
  • Two residents were affected by the road closure, a private property and a Department of Conservation in-residence caretaker.
  • Council engaged the services of specialist consultants, including surveyors, geologists and engineers to provide expert advice, and the design completed was:
    • Top batter slope 33m in height, laid back at an angle of 45 degrees
    • Three subsequent batter slopes 25m in height laid back at an angle of 53 degrees
    • 3 benches at 7m wide each
    • Road width of approximately 8m, with a barrier placed 2m from the toe of the last batter slope to act as a debris catch fence, so road width around 5m
  • GPS technology was employed to ensure a build to design approach
  • In total 55,000m3 of material (solid measure) or 70,000m3 (loose measure,) has been removed from site, this is approximately 100,000 tonne of material
  • Material removed would equate to 9,300 truckloads (in a 7.5m3 truck), which is enough to fill up the Coastlands Aquatic Centre 25-metre pool in Paraparaumu 47 times – this is based upon the main pool only, where 70,000m3 is 70,000,000 litres (the pool is 1,500,000 litres).
  • Equipment used included 1 x 36 tonne excavator, 3 x 40 tonne dump trucks, 2 x 20 tonne excavators, 1 x 15 tonne excavator, 1 x 21 tonne super reach excavator, 1 x 20 tonne bulldozer.
    • The 36 tonne excavator has a 4m3 bucket
    • The 40 tonne dump trucks can hold 23.5m3 each load
    • The super reach excavator has a reach of up to 18m
  • Whilst there is no guarantee that future slips may not occur, completing the 7m-wide benches will catch any slip material and a monitoring and cleaning programme will be established.

Media updates

Work to begin on clearing Blue Bluff slip (18 Mar 2016)

Council urges people to stay clear of Blue Bluff slip (22 Feb 2016)

Fresh slip at Blue Bluff, Ōtaki Gorge Road (20 Feb 2016)

Ōtaki Gorge Road closed due to slip (22 Dec 2015 - 27 Jan 2016)

Ōtaki Gorge Road closed due to unstable banks (14 - 29 Aug 2015)

Drone footage of the slip site

Taken January 2016

On site pictures

Visiting Ōtaki

Vehicle access to Ōtaki Forks and Tararua Forest Park from the Kāpiti side may be shut off at the moment, but there are lots of other things to see and do in the area. 

Tramping in the Tararuas - photo by Mark Coote

Historic Maori buildings at St Mary's - photo by Mark Coote

Fruit picking - photo by Mark Coote

Cycling along the river - photo by Mark Coote

The lookout at the estuary - photo by Mark Coote