Alan Titchall visits the construction site of the highest bridge and longest bridge built on the new Waikato Expressway near Cambridge, and the first in the country to be built to the latest bridge structure standards.
As far as the construction of the Cambridge section of the Waikato Expressway goes the Karapiro Gully viaduct Bridge was one of the easiest bits of the whole project, but it is the bit that everyone wants to see. And for good reason. The 200-metre viaduct has been of interest to contractors around the country and was the highlight of the field trip during the AQA/IoQ conference this year. At 40 metres between the ground and bridge deck at one point, it is also the highest, just three meters shorter in height than the Auckland Harbour Bridge. Other unique features include the use of self-compacting concrete and the fact that, through careful design around the use of 280 tonne crane and steel and concrete, it was relatively inexpensive to build.
Four decades in the making
The $230 million Cambridge section of the Waikato Expressway involves a 16 kilometre, four-lane highway being built between Tamahere and Cambridge. It is the seventh and southern-most section of the Waikato Expressway. The project involves eight bridges, including the viaduct.
Although work started two years ago, the motorway was designated in 1972. The Karapiro Gully Bridge at the southern end of the section (just before the Cambridge interchange) should be finished in December – about seven months ahead of schedule.
Design and site preparation
The steep banks of the Karapiro Gully and its meandering stream made for a challenging construction site.
An access track was built to get everything to the gully floor; a temporary bridge was built to cross the stream; slopes were cut back and temporarily retained to create a safe working space; and brown rock was used to replace weak ground and make it stable enough for the heavy bridge building equipment and to accommodate the existing flood plain.
The site consumed 450,000 tonne of aggregate; 200,000 tonnes of brown rock, and 150,000 cubic metres of sand (for sub-grade improvement loading). Most of this material came from the local Whitehall Winstone Quarry.
Because of the pre-European history in the area, including a pa site on the northern side of the gully, Heritage New Zealand, Ngaati Koroki Kahukura Trust and Ngaati Hauaa have been involved in the project.
A 280 tonne crawler crane, the biggest HEB has in its fleet, did a lot of the work and steered a lot of the ‘thinking’ around the design and construction of the bridge. The bridge beams were lifted into place from the bottom of the gully by the crane with help from a smaller, 150 tonne crane for the more difficult lifts. Special working platforms were built and hung from the beams to allow men to fit them into place using 23,000 weathering steel bolts that were precisely torqued.
The bridge beams are resting on 2.6 metre wide, concrete piers. Each pier is sitting on eight piles rammed into the ground. There are 64 of these piles (eight under each pier pad) that have been driven down up to 62 metres deep on the south side of the gully, and 30 metres deep on the north side.
The ground in the region is typically made up of sand, silt and volcanic ash material (susceptible to water) that has been swept or blown down from the Lake Taupo region during past eruptions.
Such ‘pile’ bridge technology preformed extremely well during the Christchurch earthquakes.
The piles were constructed by driving a 710mm diameter, 12-metre, hollow steel tube into the ground. Each 12-metre section was welded together on site to make up the required length of each pile. The piles were driven into the ground using a large hydraulic impact hammer lifted on top of the pile with the crawler crane. The steel cylinders were then filled with concrete and reinforcing. A total of 2500 metres of steel casing was eventually used for the 64 piles, which took 77 days to complete, or an average of 1.2 days to drive and fill each pile with reinforcing steel and concrete.
It is worth noting that ‘self compacting’ concrete was used, which consolidates under its own weight, which eliminated the need for manual compaction. Around 12,000 tonne of concrete was used, not including pre cast concrete.
Some 1100 tonnes of structural steel makes up the spans holding up a 200 metre long, 23 metre wide bridge deck, which features expansion joints at either end.
Most of the weathering steel came from Napier based company Eastbridge and was sourced from China. The plate thickness ranges from 50mm to 120mm.
The deck features standard precast concrete barriers with galvanised handrails.
Simple and plain it may be, but the Karapiro Gully Bridge has one more outstanding attribute – it must, currently, be the strongest bridge in the country as it was built to the latest ‘technical approval and certification procedures’ required by the NZ Transport Agency for highway structures on state highways’.
You can view the Karapiro Viaduct being built via live webcam at www.karapirogully.co.nz