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Officially opened in November 1999, the Otira Viaduct in Arthur’s Pass replaced the narrow winding road that was prone to avalanches, slips and closures – might this be an option for the Manawatu Gorge?

The Achilles heel of State Highway 3 – By Giancarlo (John) Hannan

News that the Manawatu Gorge road will be closed indefinitely has raised a storm of protest from residents and businesses in the lower North Island. Why is the route so unstable and what are the options going forward?

Imagine if the southern approach to the Auckland Harbour Bridge was regularly blocked by rock falls from the adjacent bluffs and traffic had to divert through the upper harbour, and the best that the roading authorities could do is clear the road. The politicians would be scrambling to be seen as the first to demand answers.

A parallel type of situation currently exists in the lower North Island where a short but critical link of road on State Highway 3 (SH3) through the Manawatu Gorge is closed again due to landslides. 

Closures of this logistics corridor are regular: from 2004 to August 2017 this link road has been closed for around 550 days. Thousands of commuters and hundreds of businesses incur extra costs and inconvenience every day this link is closed, yet it has taken 13 years for the government to seriously explore a solution. The alternative routes of the Saddle Road and Pahiatua Track are both substandard and not viable long-term solutions.

Corporate Logistics has calculated that conservatively these closures cost around $100,000 per day to the regional economy. This equates to around $55 million additional cost in the last 13 years. In addition, over $30 million has been spent by the NZ Transport Agency (Transit NZ prior to 2008) on emergency work and repairs during this period. Conservatively, a further $15 million has been spent in upgrades or repairing the damage to the alternative routes.

In short, over $100 million has been jointly incurred by road users and the NZTA/Transit NZ following periodic closures of SH3 through the Manawatu Gorge since 2004, and this cost continues to rise as time ticks on.

The cause of failure – geological speak

The Manawatu Gorge opened in 1872, so it is a very old link road of economic significance. The problems with the rock falls and landslides have become more prevalent since the major road widening and straightening initiatives when the slopes of the gorge became a lot steeper. Recently, heavier vehicles may have exacerbated the problem, but this is anecdotal.

The sides of the Manawatu Gorge are composed of prehistoric landslide colluvium (rock debris – mainly comprising argillite) interspersed between the harder more erosion-resistant sandstone bluffs. These fractured and sheared materials mantling the sides of the gorge are the source of slope instability. It is difficult to mechanically bond these loose and unconsolidated materials, so any remedial slope stabilisation measures are heavily reliant on the harder sandstone bluffs. 

The cause of repeated slope failure of SH3 through the Manawatu Gorge is well known by geologists and geotechnical slope engineers. By far the greatest cause of the slope instability is directly attributable to the continual road widening and straightening since the road’s inception in 1872.

Recent failures are related to the aggressive trimming of the harder sandstone bluffs earlier in the 1980s where the toe of the slope was undercut to recess the line of the slope away from the road. This provided a ‘safe’ area to accommodate small rock falls following prolonged high-intensity rainfall.

The overall negative effect of this work, however, was two-fold. Firstly, it remobilised the toes of the ancient landslide deposits that cover the side of the gorge; secondly, it destabilised the already fractured, weakened and landslide-prone argillite rock. 

The upshot? An ongoing problem for the future.

A gorge of national significance (GoNS)

The Manawatu region is recognised as a ‘logistics centre of gravity’ both nationally and for the lower North Island. The logistics benefits from operating in this region result in faster response times and lower cost-to-serve models, especially for the lower North Island region. This also promotes business efficiency across the country and fundamentally NZ Inc.

This short piece of road and its bridges form the eastern spoke for the Manawatu distribution hub link to the Hawke’s Bay, Gisborne and Wairarapa regions. The government has designated this road ‘strategically important’, but apparently this is different from a ‘road of national significance’ (RoNS). 

Beyond the disruption to domestic operators, the regional exporters and importers who rely on the Manawatu Gorge link are now impeded when shipping product through Napier Port. They are further limited since the 2016 Kaikoura earthquake which also closed CentrePort in Wellington, effectively choking the southern spoke of the Manawatu hub. In addition, thousands of tourists also use the route each year.

Alternative route options

In 2012 the NZTA commissioned a confidential report that has recently been made public, detailing the issues connected with the closure of SH3 through the Manawatu Gorge. This report presented four route options to replace the current alignment. 
Option A

Option A is a greenfields option and traverses the Tararua Range south of the existing gorge route. It is 5910 m in length and has a projected cost of $309 million. Its benefit-to-cost ratio (BCR) is given at 1.4 (where the benefits outweigh the costs by a factor of 1.4).

This greenfields option appears a viable solution. Such approaches are well utilised in other parts of the world. Based on the data presented in the 2012 report, it is suggested the option should cost under $300 million, provided any credits from the removal of the estimated 52 million cu m of low-grade material are retained by the Crown and not the contractor, to be later on-sold back to the Crown.

Option B
Option B is a bridging and viaduct option. It is 6670 m in length and follows the same corridor as the existing SH3 passageway. The project is expected to cost $412 million and has an anticipated BCR of 0.9.

A bridge and/or viaduct option design – much like that of the Otira Viaduct in Arthur’s Pass – would certainly be a drawcard for the region and attract many domestic and international tourist travellers. The option will, however, still need to consider both deep-seated and shallow-seated regolith (the loose unconsolidated rock that sits atop the bedrock) and bedrock failures. 

Option C
Option C is the Worley Road route north of the Manawatu Gorge. It is 10,500 m in length with a documented project cost of $118 million. BCR is predicted at 1.5.

The Worley Road option is the cheapest out of the four options presented and has potentially the most favourable BCR. Logistically, however, the route is considered the poorest option of the four. The government spends millions nationwide improving and reducing the length of freight connections. Adding a further needless 5 km onto the freight passageway between Manawatu and Hawke’s Bay does not make sense.

Option D
Option D is a tunnel option with a length of 5380 m. The construction sum is $1.8 billion and largely composed of $300,000 per metre cost for the construction of a four-lane tunnel. The BCR for the project is 0.2.

Firstly, economically this is the most expensive solution. Secondly, at least two major active faults would need to be crossed. Thirdly, the propensity for catastrophic deep-seated landslides in this area has not been sufficiently addressed in any of the publicly available reports.

The solution 

The Manawatu River’s action is the principal origin of slope instability in the gorge, with the roading realignments of the past having just accelerated this natural process. The planners of the future need to recognise it is always problematic to fight nature and find a better option to link the Manawatu and Hawke’s Bay regions.

From a geological perspective, it appears that the greenfields project of taking a multi-lane direct line over the Southern Tararua Range is possible and also logistically efficient. Once constructed, this new access would re-establish the efficiency the distribution industry has created dispensing freight from the Manawatu distribution hub to the wider reaches of NZ Inc. 

The government is reportedly releasing a final resolution to the issue in December 2017. As the gorge has already been open 145 years, any cost-benefit calculation should consider a project benefit of at least that time horizon. 

Procrastination has caused $100 million to have already been consumed in the 13 years since 2004. Extrapolating this value along with the longevity of the gorge road would indicate a significant investment is justified in restoring the viability of one of New Zealand’s most important road links.

Giancarlo (John) Hannan is a senior analyst at Corporate Logistics, a specialist logistics, supply chain and market research consultancy based in Palmerston North; he trained as a geologist with Massey University; for further information, visit www.corplogistics.co.nz

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