Taupo Bypass Project: Resourcefully Paving The Way

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TAUPO BYPASS PROJECT

Working around unpredictable geothermal features, local traditions and wet winters have presented some interesting challenges during construction of the East Taupo Arterial, but innovation has been the name of the game for civil contracting firm Fulton Hogan from the start. Since construction of the bypass began in 2008, Project Manager Andrew McRae and his team have built the 16 kilometer bypass, utilizing a variety of both manmade and natural resources to get the job done in a timely, sustainable, community-conscious manner.

DIVIDE AND CONQUER

The project originated in response to Taupo's heavy traffic problem that had been plaguing the popular New Zealand resort town for years. It wasn't until two years ago, though, that construction for the alternate route became a reality. McRae says, "Initially, when the project was put out for tender by the Taupo District Council, it was put out on the basis of a three-year design and construct period. When Fulton Hogan looked at the project, we came up with some ideas that would decouple some particular program constraints from our design. We offered to design the bypass in a two-year period."

In order to successfully manage and deliver the project within that timeframe, McRae and his team split the project into three zones to create manageable sections. The North zone encompassed the bypass' two main structures—a 440 meter bridge that runs over a steam field and the Waikato River Crossing. The other two zones were split up to manage earthworks and accommodate local roads across the alignment.

THE STRUCTURES

The structures are quite unique in design and implementation. The larger of the two bridges crosses the power company, Contact Energy's steam field, which carries steam from wells to a geothermal power station. The purpose of Fulton Hogan's design for the structure was to create one long, elevated bridge that would allow the client to cross over the land rather than buying it. The structure is a 440 meter long steel ladder bridge, made by steel fabricator Eastbridge, constructed without piling due to pumice soils the foundation is made from. Piling was eliminated because piles don't perform that well in pumice, so shallow foundations were created to reduce costs and improve performance.

The Waikato River Bridge is an interesting structure in itself, as its alignment skews the river. Cultural requirements prohibited Fulton Hogan from placing piers in the river, as part of protecting the local Maori people's belief that interfering with the mauri, or natural life force, of the river will destroy its integrity. So the team, along with their subcontractors on the river bridge, Tennix Alliance, extended the span of the bridge from 84 to 100 meters without piers. They elected to use a network arch bridge—the second of its kind in New Zealand. It was constructed on one of the embankments, and with the help of Culham Engineering, was lifted over the river with a number of cranes. McRae adds, "We've set out to make sure the design we've provided overcomes as many contingent issues as possible. We lengthened the span of the bridge and changed its form over the Waikato River to stay out of the water because we knew it was significant to Maori."

WORKING AROUND STEAM, ASH AND PUMICE

The Taupo area is a hotbed of geothermal activity, which presented a variety of unusual dilemmas along the way. When the ground was opened up in one of the geothermal areas, they found that temperatures widely ranged between 30 and 97 degrees centigrade. The team put in geothermal blankets designed to release steam coming from the ground, and dissipate it into the air without causing problems for the pavement that was to be laid.

The Lake Taupo region has experienced a number of eruptions over the past years, and in fact, lies in a caldera from a volcano left thousands of years ago. McRae says, "As a result, we encountered a number of areas where hot gaseous molten rock flow turned old forestland into charcoal. As we cut through the alignment, we encountered rotten trees left from large pumice and airfall deposits. In one particular area, there was fairly significant depth of airfall deposits. The trees had rotted away and left a whole series of tree casts, which became a conduit for water." They had to undercut the area and apply engineering techniques to support the pavement above it.

In fact, the pavement used is innovative, too. The team constructed a foam bitumen stabilized pavement, which gave the client a more durable, longer-life pavement. "We were required to design and construct a 25-year life pavement, but produced a 35-year life pavement that gave the client a better whole of life cost pavement option. From a sustainability perspective, it's smart business for the client and smart business for us." They minimized the amount of imported aggregate required for construction of the road through recompaction and improvement of the pumice subgrades, and have reduced the use of nonreplenishable resources.

The bypass runs through three areas the Department of Conservation has deemed as reservelands. In those areas, Fulton Hogan's prominent design philosophy has been around footprint reduction to minimize disruption. "We've been able to do this quite successfully through utilization of steep batters. The batters on the earthwork cuts are subvertical at 74 degrees, which is quite steep." In one area, they created a steepsided, mechanically stabilized earth embankment reinforced with plastic grids to minimize the footprint at the base of a large gully. "We've used this system that allows us to retain topsoil, so that there's a very steepsided wall that blends into the environment rather than having a large concrete structure," McRae says.

The third reserve presented its own issues. The highly geothermal area contains a number of shifting fissures that bring hot water and steam to the surface. McRae says, "We had to ensure there was no stormwater drainage into these geothermal features. The main reason is that cold water might cause a hydrothermal eruption, which is not safe alongside a highway." The team designed the drainage to keep water out of these features. "We had to take a different approach on the culverts that cross the alignment. Rather than having a single or double large culvert, we had to get three and four smaller culverts to ensure that in high flow conditions, the headponds that sit behind the culverts, don't build up to a point where water could flow back into the geothermal features."

McRae says that the project was largely carried out during the winter for a few reasons. New Zealand's wet winters allowed them to work with the pumice, which requires a lot of moisture to be laid, and the rainfall also significantly reduced the amount of dust that pumice creates. Short winters gave them avery tight timeframe to complete the earthworks, but the team managed to shift 1.2 million cubic metres of pumice over a three month period, which is a significant achievement. McRae says working this quickly couldn't have been done without their subcontractors—Hick Brothers Civil Construction and Seay EarthMovers—who significantly aided with the speediness.

A SUCCESSFUL MISSION

McRae adds, "We set a number of objectives that we wanted to meet for the project. From a safety perspective, it was having zero harm safety targets; from an environmental perspective it was environmental excellence; from a community perspective it was no impact on Taupo. We had an accelerated completion objective and we are currently on target to meet that in September 2010. In terms of an overall project goal, we set out to complete a project that's outstanding in every respect, which I think we accomplished."