The NZ Transport Agency will not be proceeding with plans to build a replacement bridge at the Gates of Haast as its construction will not guarantee route security along this section of State Highway 6.
Three years of investigations have found that a new bridge cannot be engineered to withstand a major landslide or another significant event in the area, says the Transport Agency’s Canterbury/West Coast Highway Manager Colin Knaggs.
“Landslides, slips and washouts are a reality along State Highway 6 because of the nature of the terrain on which the highway has been built.”
He says the Transport Agency has spent almost three years working on a design option for a new bridge, the best option for the site considered to be a cable stayed bridge which would cost an estimated $35 million to build.
“The high cost and the low benefits would make it extremely difficult to justify construction funding.
“Even if the Transport Agency was to build this new bridge, it could not guarantee the bridge would not be damaged if the Haast landslide was activated.”
Mr Knaggs says following the completion of the draft Bridge Design Statement, concerns were raised about the on-going risks associated with the new bridge.
“The Transport Agency recognises the Gates of Haast is a unique site, not least because of the long detour (more than 930kms) and that any prolonged closure of the highway would have a significant impact on the region.
“An overall route strategy for SH6 through Haast Pass is considered the best solution, with the Transport Agency reviewing the risk profile of the area.”
He says a management plan is being developed to monitor the landslide, in particular at the toe or base, to minimise the risks to road users.
“We will also undertake a $1 million preventative maintenance programme to install sub-soil drains in the hillside above the road to reduce slope failure and provide greater land stability at the site.”
Mr Knaggs says the monitoring programme will also review any future road failures through the area and any issues with road reinstatement with the view of building a piled retaining wall at the toe of the landslide, if needed to help protect the highway.
“If the road is closed by a landslide, we are confident of being able to re-open it to single-lane traffic at the site within two to seven days, depending on the extent of slope failure.
“In the event of complete slope failure, above and below the road, it is estimated the road would be re-opened within 21 days. An event of this nature has an assessed frequency of every 500 years.”
The Transport Agency recognises the importance of the highway as a lifeline route for the West Coast and its role in the economic development and prosperity of the region.
“Funding will always be made available to undertake preventative maintenance on the highway and keep it open,” he says.
The Haast landslide is located just to the north of the Gates of Haast bridge and was last activated in late 1970s.
For more information please contact:
Media Manager - Christchurch
NZ Transport Agency
T: 03 964 2885
M: 021 427 442