Designing a safer highway

State Highway 58 is undergoing an extensive safety upgrade in preparation for it becoming a link to the Transmission Gully Motorway. Richard Silcock updates the project.

The $100 million upgrade of SH58 between SH2 in the Hutt Valley and the Transmission Gully Motorway Interchange at Pauatahanui is taking place in two stages.

Once completed, it is designed to become a far safer route for traffic to/from the Hutt connecting with Porirua, the Kapiti Coast and the Transmission Gully Motorway (TGM), once the latter is completed (1).

Due to its narrowness in places and a number of blind sharp bends, SH58 has a history of motor accidents with some 200 crashes over the past 10 years, some with serious and fatal consequences.

The Transport Agency and its consultants looked into ways for improving the highway in regards to safety,  particularly at its intersection with a number of local rural roads. This preliminary work, which was carried out between 2017 and 2018, involved consultation with local communities and other stakeholders including local city councils.

Earlier works (refer Contractor October 2015) widened around a kilometre in the mid-section of the 9.1 kilometre-long highway. An elevated roundabout was also constructed at the intersection with SH2 (known as the Hayward’s Interchange (refer Contractor June 2016) all of which went someway to make the highway safer (2).

Transport Agency principal project manager, Chris Nally says the agency expanded the scope of the works required to achieve greater safety outcomes than those originally proposed.

“We reduced the speed limit from 100km/h to 80km/h, altered the road markings and the signage and installed a median barrier in the central section.

“Design work was then carried out and a works contract for stage one put out for tender.

“Stage one, which is being carried out by Downer commenced in October last year. It covers the eastern end of the highway from the Hayward’s Interchange through to the Mt Cecil Road intersection, a distance of approximately 2.5 kilometres.

“This work has involved substantially cutting the hillside back in a number of places, forming a bench, widening the highway to allow for an uphill crawler lane, taking out or improving tight bends, resealing the road and shoulders and installing median safety barriers. It also included upgrading a water main valve chamber on behalf of Wellington Water.

“Stage two, from Mt Cecil Road to the TGM Interchange, which is being funded through the Governments ‘upgrade programme’ will involve constructing two separate roundabouts at the intersections with local roads, widening and strengthening several bridges, lessening the curvature of the bends and widening the highway (except where it was done earlier), resealing and installing safety median barriers.

“This second stage is expected to commence in the spring of this year following a further tendering process.”

Chris adds that they had initially planned to complete the whole project under one contract, however traffic management of the 15,000 – 17,000 vehicles per day that use the highway, and some difficulties in accessing available aggregate due to other large construction projects being undertaken in the vicinity, required splitting the project into two stages (3).

Rob Sharpe, project manager with Downer, says that despite the Covid-19 level four lockdown period and having to adapt the earthworks to the varying geology of weathered unstable greywacke for the 45-59 degree slopes of the 27-metre high cuttings and four metre wide bench, the project is pretty much back on schedule.

“Preliminary geotech investigations had shown a predominance of solid greywacke, however once earthworks progressed it was discovered there was a lot of variance in the material, with a lot of shattered rock and a seam of argillite” says Rob.

“This was not expected and various design changes were required.

“Around 60,000 cubic metres of earthworks have been completed and the various cuts have required the installation of around 40 soil nails driven to a horizontal depth of five metres. We’ve also installed wick drains as there was a lot of moisture in the soil.

“The vertical cut batters that are not solid rock have also been hydro-seeded as an added stability precaution.

“With the change to the original design spec we’ve had to get new consents, and this also caused a few delays, however we are on track to complete this stage by mid next year.”

Some 15,000 cubic metres of excavated rock and earth has been retained for fill elsewhere in the project, with the rest trucked to clean fill sites in the Hutt Valley.

Paving for the new sections of the highway will be structural high fatigue EME2 asphalt varying in depth of between 140-160mm.

The gradient to the top of Haywards Hill will essentially remain at a 10 percent vertical incline, however the addition of the ‘crawler’ lane will allow other traffic to maintain the speed limit.

The Transport Agency says when the project is completed the journey time between the Haywards and TGM Interchange’s is expected to improve despite the speed limit remaining at 80kph and the that the upgraded highway will provide greater safety for motorists and for the local community.

 

Footnotes:

The Transmission Gully Motorway completion date has been pushed out beyond the end of this year to late 2021 or early 2022. A further $200 million has been granted to the contractor (CPB/HEB JV) by NZTA bringing the total cost of the motorway to near $1 billion.

The Haywards Hill Interchange on SH2 was completed in June 2017 at a cost of $43 million.

Maintaining a supply of approved aggregate to meet the specification has been an issue due to the number of major construction projects in the lower half of the North Island with TGM and the Kapiti Expressway extension to north of Otaki requiring vast amounts.

 

Brief history (Box)

 

The first horse-era track between the Hutt Valley and Pauatahanui was built by pick and shovel in 1871 by a group of settlers and was the only route between the two settlements for some 20-25 years.

The early formation of a road included rough bench cuts and embankments, drains and subsurface structures, however it was not until after WW1 that it was metalled.

During WW2 the Belmont section (now part of Belmont Regional Park) was taken over by the NZ Defence Force as an access route to 62 ammunition storage facilities and was not reopened to the public until the1960’s when it was widened and sealed.

It has since become a main highway connecting Hutt City with Porirua City and beyond.