What do you do with a gorge road and 75 bridges?

Consultant David McGonigal provided an overview at RIMS 2018 of the significant infrastructure challenges in choosing a new route design alternative to the closed Manawatu Gorge.

SH3 CARRIES SOME 7500 vehicles a day and is a national route providing access between the Manawatu and Hawkes Bay and Wairarapa regions.

Not only is it important from a national perspective but it’s also relied on significantly as a regional route. Many of the businesses in Tararua and the Palmerston North area rely on it and there’s a significant amount of commuting that takes place between the Tararua district and the Palmerston North area.

So the big question is – why do we need a replacement? Ever since the route was formed back in the 1800s, it has been susceptible to slips. I would say in the last three to four years, slips have become more prevalent. Because of ongoing instability the gorge has been closed since April last year.

This has impacted significantly on businesses, communities and road users, especially freight operators.

The cost of the closure has been estimated at around $700,000 per week which is quite significant. This is essentially made up of extra fuel costs and travel time costs. When the gorge is closed approximately 60 percent of the traffic has to use the Saddle Road and the remaining traffic the Pahiatua Track. Neither route has been designed, or is appropriate, to carry the state highway traffic and certainly the volumes that we have. The Saddle Road has gradients up to 16 percent so you can imagine the impact that has on trucks doing 10 kilometres per hour for some distance.

There’s also the social issues of traffic having to use residential roads through Ashhurst with noise, particularly during the night when the trucks go through.

So the overall conclusion is that the gorge is no longer a viable option in terms of a long-term and safe route for SH3. So we commenced preparing a detailed business case to identify an alternative to the gorge. In August last year GHD was appointed as the lead consultant to prepare a detailed business case. Essentially it followed a fairly standard process. The first part of a detailed business case is to understand the problems and the benefits of solving those problems.

Suffice that we didn’t spend too long determining the problem – as it was quite clear. We started in August last year and went through a process of identifying a long list of options, public engagement then took place through October and then we refined the long list to a shortlist assessment during November.

The final part is being worked through at the moment and we will have a detailed business case completed in the next couple of weeks [by end of March 2018].

Essentially we identified 18 different options. The historic nature of the issues in the gorge meant there had been many people sitting in their armchairs in the evening trying to come up with solutions for the gorge over the years. So one of the key things that we wanted to do from the outset was to ensure that we captured all these ideas, no matter how imaginative, and put them through the assessment process.

Options included a viaduct right down the middle of the Manawatu River, but it didn’t rank well against the assessment criteria, particularly with environmental issues. Tunnels were the first thing people talked about with any issues in the gorge. We looked at two tunnels. The first was a long tunnel but there were cost and environmental issues. So we looked at a short tunnel.

Another suggested option was just to go straight up and over 300 metres, known as the deep box cut –  with the emphasis on deep – to achieve the gradients required. The amount of fill that would be created would be enough to raise Palmerston North by about two metres.

From the long list we then identified four sure shortlist options to look at in more detail. One to the north was designed to avoid the wind farm [the large Te Apiti Wind Farm is located on 700 hectares on the south side of the Manawatu Gorge]. Another was to widen the Saddle Road.

The geology of the gorge is quite different north and south, and the rock is much harder on the south side so it is much more difficult construction territory, so the north was one of the short options with good gradients – but went through the wind farm.

In the long list there were multiple options to the south. So what we did was we combined the best of all those options because each of those options seemed to have one issue that was a problem.

So we combined all the southern options to come up with what we felt was the best option. 

Geologist checking rock and ground formations along the proposed new route.

The challenge with this one is that to achieve gradients the road became quite long.

We assessed the shortlist options against three categories and objectives – environmental, social, costs and ‘implementability’.

The final preferred option has the lowest overall gradient of an average of 5.8 percent and is the most resilient. Some southern options had the main Wellington fault line running through the area.

The final option also has the lowest overall cost, is the easiest and quickest to construct and performs best against the social and environmental criteria.

Most of the road is on a hill and there will be two lanes plus a crawling lane. It’s possible on the flat on the top the road will be just two lanes.

This option ties back into existing SH3 with a new bridge across the Manawatu River and the railway line. It will then wind its way up a hill and will go between two wind turbines, across the top and head back down to Woodville.

As far as projects go it is certainly going to be a bit of a challenge with construction and the impact of the wind farms, which will be affected significantly with some of the turbines moved along with realigning their underground cabling.

The next stage is where we’ve got the procurement process to go through and consenting property acquisition and then the detail design that we expect to take up to 24 months.

Construction is about four years if everything runs in sequence – that gives us a total of six years. Obviously we will be looking at ways to run things in parallel and hope to reduce that time.

One of the other big challenges is what do you do with the existing gorge, as applying the normal revocation process might not be appropriate.

We have up to 75 bridges in the gorge – if anyone wants a bridge? So there’s going to be a lot of ongoing discussion on what to do with the existing gorge.

This article was first published in Contractor‘s May issue.

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