They do it with mirrors

History and topography collide in a tricky road upgrade in the Bay of Plenty, as HUGH DE LACY discovers.

SH36_0.jpgThe northern approach to one of the two single-lane bridges in the Bay of Plenty’s Mangorewa Gorge is so sharp it requires a mirror to aid the traffic negotiating it – and that in itself is an indication of the challenges to its current $4.5 million upgrade.

The work is being undertaken by the Tauranga branch of Downer EDI Works under project manager Bruce McLachlan.

The most direct route between Rotorua and Tauranga, State Highway 36, has seen a huge boost in traffic since it was sealed in 2004 and added to the national highway network in 2005. From barely 500 vehicle movements a day before sealing, traffic volumes have ballooned to over 3500 a day, making the Mangorewa Gorge one of the region’s highest-profile accident spots.

SH36_1.jpgIn a complex project being managed by the Rotorua District Council (RDC), two bridges within 700 metres of each other are being widened to two lanes, and a 1.9 kilometre stretch of the road is being realigned.

Opus International has provided the RDC with the specialist geotechnical, design and supervision advice needed to deliver the work within its tight timeframe.

The work began in May and is scheduled to be finished in March of next year, putting an end, it is hoped, to the series of truck accidents that have repeatedly closed the road: the last one, in 2007, involved a 40-tonne flour-laden curtain-sider that took two days to extricate from the bed of the Mangorewa Stream.

Quite apart from the challenging geography – the gorge is so deep that in winter the sun never reaches the road’s lowest points, and there’s no cellphone coverage – the project incorporates a range of potentially conflicting historical and cultural interests.

SH36_2.jpgThe fact that it cleared all the resource consent hurdles without having to be notified is a vindication of the RDC’s virtually unique role as a localised highway manager to the New Zealand Transport Agency (NZTA), which is funding the project.

A longstanding delegation arrangement exists whereby RDC manages all 215 kilometres of NZTA’s rural and urban roads in the district, and builds approved capital works for the agency.

In getting the project under way, the RDC had to accommodate a range of interests that included no fewer than six iwi for whom the Mangorewa Gorge constitutes both a boundary and a significant historic site, along with the Department of

Conservation which has a number of walking tracks converging on it, and the Bay of Plenty Regional Council.

That this was achieved on a non-notifiable basis shows the value of local connections, according to RDC project manager Kevin Thompson.

SH36_4.jpgIn the end the project will provide not only a much safer two-lane highway for the forestry and Port of Tauranga traffic but also a rest area where the gorge’s history will be highlighted for locals and tourists alike.

One curious aspect of that history is the existence of a concrete tank-trap built by the Home Guard during World War II to thwart Japanese forces rushing south to the tourist mecca of Rotorua, had they launched an invasion of the country through Tauranga.

The surviving half of the tank-trap – the other collapsed into the stream years ago - will find a permanent home at the new rest area, along with the troughs that provided spring-fed water for horses hauling stage-coaches between Rotorua and Tauranga in the days before motorised transport.

So narrow is the gorge that the road has had to be closed on weekdays between 9am and 4pm from May to August, and all work carefully scheduled to ensure it’s open the rest of the time.

SH36_5.jpgThe first part of the project is the replacement of the timber-decked Mangorewa Stream South bridge with what is in effect a huge steel culvert, 11 metres in diameter and bolted to foundations that will be under water, and wide enough to cater for a two-lane carriageway 10 metres wide.

By July this part of the job was nearing completion, and the focus of the work had moved on to widening the rest of the road from as little as 5.2 metres to at least nine metres.

This latter section involves cutting back a vertical bluff of volcanic rock by drilling and blasting.

It’s being achieved by workers from the Hamilton company Avalon Rope Services, who have previous experience in the gorge, abseiling down the cliff-face.

SH36_6.jpgUnder the agreement with the six local iwi, none of the rock from the bluff will leave the gorge.  

Instead 6000 cubic metres of it will be crushed on-site and used as sub-base for the road, with the basecourse and seal being brought in from elsewhere.

The widened carriageway will also require a retaining wall to be built out over the Ohaupara Stream in places upstream from where it converges with the Mangorewa in the gorge.

The third major component of the project is replacing the 5.1 metre-wide Ohaupara Stream bridge, built in the 1970s, to 10 metres – incidentally allowing the removal of the mirror which is the only present means for traffic from both directions to see what’s coming round the corner under the bluff.

The work is being carried out by about 20 on-site staff using a combination of 35-tonne and 50-tonne excavators, the latter adding to the site complexity by being too heavy for either of the bridges being replaced.

SH36_7.jpgA pick attachment on the 50-tonners is used to pull down and break up the rocks after blasting, and tractors and trailers have been found to be the most efficient way of moving the spoil in the confined space.

Once the earthmoving has been completed, the road will be open to traffic fulltime with only stop-go delays.

Given the traffic volumes, a key part of RDC’s and NZTA’s management style has been open public communication, highlighted by a weekly e-mail service to heavy haulage, public transport and emergency services – and indeed any member of the public who wants to receive it – that updates progress and warns of variations to the closure times, backed up by electronic variable message signs at either end of State Highway 36.

On completion the project will be a case of history enshrined in formidable topography. 

Contractor Vol.33  No.7  August 2009
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