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The route through the gorge is the eastern spoke for the Manawatu freight distribution hub linking to the Hawke’s Bay, Gisborne and Wairarapa regions

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, year in and year out. The howls of Auckland commuters and transport operators would be deafening, and 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, but this time the closure is indefinite as the area is considered unsafe for work. 

Closures of this logistics corridor are regular: from 2004 to the present day, this link road has been closed for around 550 days – and at increasingly frequent intervals. 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 alternate 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 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 geology of the Manawatu Gorge is composed of interbedded, indurated (hardened) greywacke sandstone and weaker, brittle and sheared argillite i.e. mudstone – try saying that rapidly after four beers! These rocks belong to the Esk Head Belt Formation of the Torlesse Supergroup (Triassic age) and are 140–200 million years old (My). The sequence contains thick bands and lenses of chert, limestone, submarine volcanics and red argillite.

Over millions of years, these rocks have been subjected to extreme tension and pressure from the adjacent plate boundary subduction zone. The Manawatu Gorge rocks are now highly deformed, brecciated (broken up) and excessively fragmented. 

There is little cohesion within the formation, apart from the harder sandstone, and the materials are now highly susceptible to slope failure, especially when slopes become too steep.

The angle of bedding and foliation (layering) in the Manawatu Gorge is generally between 55–80 degrees to the east and southeast. This leads to the interbedded harder sandstones forming the more erosion-resistant sandstone bluffs that are visible in most photos of the gorge. 

The weaker connecting argillite bands are easily eroded and highly prone to failure, hence the landslips. There are 18 sandstone bluffs on SH3 as it passes through the Manawatu Gorge. Each bluff is interlinked with a mixture of fractured argillite.

Thick deposits of prehistoric landslide colluvium (rock debris) cover both sides of the Manawatu Gorge. It is thought these materials began to be eroded away from the in-situ bedrock as the ranges emerged from the sea 1.5 My ago as uplift progressed.

The planar surface of the Ruahine and Tararua Ranges represents an old erosion surface. Over the last 5 million years this surface has been overlain with thick marine sediments and terrestrial alluvial gravels. These materials are also a potential source of easily eroded material on the north side of the gorge.

In summary, the flanks of the Manawatu Gorge are composed of prehistoric landslide colluvium interspersed between the harder more erosion-resistant sandstone bluffs. These fractured and sheared materials covering 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. 

Man-made interventions

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 slope instability is directly attributable to the continual road widening and straightening since the road’s inception in 1872.

There have been two major periods of works – in the 1920s–1940s and in the 1960s–1980s – to widen and straighten the road. Each period has preceded a new era in slope instability.

In contrast, the opposite side of the gorge with the railway line has experienced less slope instability. The railway has suffered fewer slope instability movements for two reasons: firstly, the smaller cuts required for the railway have required less of the toe slope support being removed; and secondly, the use of tunnels bypassing the steeper portions of the gorge avoids adjacent toe slope interference. 

Conversely, on SH3 there has been repeated removal of the adjacent toe support on the slopes where they meet the roadway as it traverses the gorge. This has stimulated numerous slope instability episodes as the basal support was removed. Consequently, the road has been regularly inundated with debris.

Excavations in the 1980s trimmed back 15 of the gorge’s 18 strongly jointed sandstone bluffs. This directly caused several small to moderate rock falls (colluvial debris falls). Mesh, rock bolting and drainage stabilisation measures were immediately installed to prevent further erosion of the slope faces.

Mesh, rock bolting and drainage stabilisation measures have been installed in many sections to prevent further erosion of the slope faces

Significant slope failure (necessitating closure of the road) did not occur until 1990 when a 5000 cu m landslide occurred. The slope failure problem and road closures accelerated from 1990 with several large events, with the largest landslide in 2011 measuring 160,000 cu m. These events have become the Achilles heel of the Manawatu Gorge, undermining its operational efficiency.

These 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 line 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.

River actions 

Massey University has dated an aggregational terrace at the western entrance to the Manawatu Gorge. The terrace is thought to have formed at the end of the last glaciation some 12,000–13,000 years ago. It lies approximately 50 m above the current bed of the Manawatu River.

This suggests that 12,000–13,000 years ago the base of the river was 50 m higher than it is today, and indicates the river has down-cut its base at a rate of 4 m per 1000 years since the end of the last glaciation. 

Based on this rate of down-cutting, the river is rapidly (geologically speaking) incising down into the underlying bedrock base. This high rate of down-cutting is potential testament to the fractured and fragmented nature of the underlying bedrock which makes it easier for the river to erode the toe support of the adjacent slope as it seeks to widen its base. 

The result of this continual lateral erosion is the numerous prehistoric landslides documented in the geological record along with precipitating the historically documented events of today.

A gorge of national significance

The Manawatu region is recognised as a ‘logistics centre of gravity’ both nationally and for the lower North Island. The Manawatu hub has evolved because nationwide distribution companies recognise efficiencies and cost savings that can be made by operating from the area.

It has a central location, comparatively cheap land, reliable labour resources, easy access to national road, rail, and air and sea ports. The area has an excellent service industry base, and is within a 160 km radius of around a million people.

These logistics benefits 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 benefits NZ Inc.

The Manawatu Gorge is a short but critical piece of SH3 and in the current government vernacular should perhaps be termed a ‘gorge of national significance’. This short piece of road and associated bridges is the eastern spoke for the Manawatu distribution hub linking to the Hawke’s Bay, Gisborne and Wairarapa regions. 

The NZ government has designated this road ‘strategically important’, but apparently, this is different from a ‘road of national significance’. Perhaps there is some bureaucratic confusion as to the meaning of strategic – perhaps it means the road is significant and of strategic importance, but only in a regional context. 

The importance of this link would appear to be greater than the problem of Arthur’s Pass which, with significantly less traffic and tourists, still managed to get a substantial (and successful) viaduct solution. 

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 the port of Napier. 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 to the gorge

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 potential route options to replace the current alignment. 

All options have an environmental and design compliance timeframe of three to five years with an estimated three to seven-year construction programme. The options are outlined below along with their benefit-to-cost ratios (BCR).
Upon checking the calculations from the report, Corporate Logistics has concerns as to the interpretation of the BCR tables presented in the report and at this time is seeking clarifications as to the costs shown. 

The gorge has already been open for 145 years, so any BCR should logically be calculated on a representative timeline, not the short-term perspective of many New Zealand projects.

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 BCR is estimated at 1.4.

This options shows that 90% of the project’s cost is item 3.4 (cut to waste onsite, including unused ex topsoil) and requires the removal of 52 million cu m of material. It is unknown who will own this resource, but assumes it should logically be the Crown. The extracted material will be low grade; however, it is still envisioned it will have a credit value in the range of at least $1–$2 per cu m. It could be used for fill in other roading projects.

This potential sizable credit has not been addressed in any cited analysis report concerning the BCR calculation. In theory, any applied project credits could generate a more favourable BCR for this option. 

The 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 are retained by the Crown and not by 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.

Construction sums have been estimated at $312 million with a $100 million contingency. Why and what the 32% contingency is for is unclear. If the full contingency is not used, a credit would be applied to the project cost that could potentially improve the BCR. 

A bridge and/or viaduct option design would certainly be a drawcard for the region and attract many domestic and international tourist travellers. The option will, however, still need to consider the shallow-seated regolith (the loose unconsolidated rock that sits atop the bedrock) and bedrock failures that plague the current road. Whether these effects will be greater or less than the current alignment is unclear. A bridge and viaduct option would need to contend with the risk of deep-seated slope failure.

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 for this option.

It is unclear how the construction sums for this option were calculated. The addition of all construction items, as presented in the report, is $74 million. There is, however, a $44 million gap between the sum of the project’s construction costs ($74 million) and the documented total project cost ($118 million).

The Worley Road option is the cheapest out of the four options presented. It has potentially the most favourable BCR, which would increase if there is an error in the construction sums. 

Logistically, however, the route is considered the poorest option of the four presented. 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. 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. Accordingly, there is insufficient research available on this option.

The solution 

The Manawatu Gorge is, in an adaptation of government vernacular, a gorge of national significance and should be accorded such status. The Manawatu River’s action is the principal origin of slope instability in the gorge, with the roading realignments of the past having 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.

The 2012 report suggests a long-term solution will be at least a decade away. The regions of Manawatu and Hawke’s Bay, and the wider NZ Inc, will continue to be logistically impaired while awaiting a solution. It is also recognised that continued road access through the area must be maintained during the reconstruction of whatever option is chosen. 

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?

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 logistically efficient. Once constructed, this new access way would re-establish the efficiency the distribution industry has created dispensing freight from the Manawatu distribution hub to the wider reaches of NZ Inc.

This greenfield option has a projected cost of $300 million, with a potential 52 million cu m stockpile of rock for NZTA roading or other projects. With a potential project credit value at $2 per cu m for the cut-to-waste fill of $100 million, the real cost of this option reduces to $200 million. 

If, however, there is a perceived benefit to future tourism, then a viaduct option may well have merit. 

The government is reportedly releasing a final resolution to the issue in December this year. As the gorge has already been open 145 years, any BCR 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

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