Expressway engineers battled geology

Waikato Expressway engineers faced a massive challenge after finding "global" instability at 13 slip sites in the underlying rock formation through the Mercer hills and 10 swampy zones farther south.

They were up to 20m deep and causing as much as 3m of ground settlement.

That meant 5m of fill had to be laid to build up carriageways by 2m, from total project excavations of about 1.5 million cubic metres of earth and rock.

There may yet be more settlement, although engineers are now confident of having forced out most underlying moisture by laying tonnes of preload rock and earth above vertical fabric "wick" drains.

These are in addition to tens of kilometres of drains bored through the Mercer hills to lower the water table using technology not available to the pioneers who built the original road and rail links between Auckland and the Waikato along a narrow corridor.

A bridge built across the Whangamarino River for $3.3 million early in the project, but left pointing in midair towards the Mercer hills, became a subject of ridicule although a useful lever to extract the extra money needed to push on.

Although Transit insists the structure was always part of a cunning plan to maximise off-road haulage of almost 900,000 cu m of fill from a 35m-deep cut through the hills, the agency has been keen to rename what locals derided as the Bridge to Nowhere.

Transit's initial plan in the 1990s was to try to fit a four-lane highway between the hills and river at Mercer, but early investigations revealed a risk of triggering massive landslides, so the inland route was chosen for the southbound lanes.

Although part of the hill above Mercer had to be excavated to build the interchange there, project manager Jason Shirnack said the associated railway tunnel had been made strong enough to hold up the ground and road-bridge above it.

"There are huge pressures on the tunnel. Because of the global instability in the Mercer area, it has changed from being just a box in the ground to a box holding up the bridge."

A section of the railway line has been moved up to 45m inland in what is a tightly confined design which includes paint-free steel rather than traditional concrete on the road's off-ramp to reduce the structural bulk and allow greater clearance for trucks and trains.

Transit regional manager Chris Allen said the project was the most geotechnically challenging undertaken by the agency in the upper North Island in probably 20 years.