Coromandel slip: Embankment to support State Highway 25a had fill tipped into it in the 1960s

By Phil Pennington for RNZ

Analysis of a massive slip that split a Coromandel highway shows the road design of decades ago was not up to extreme weather in the current climate.

A huge crane and a piling rig have been trucked in to begin work on a $30 million to $40m, 125m-long steel bridge over the 110m wide slip on State Highway 25a.

An engineering analysis for Waka Kotahi found the underlying soils were strong enough to prevent a deeper and larger slip occurring.

State Highway 25A, Kopu to Hikuai Rd on the Coromandel Peninsula. Photo / Waka Kotahi

What failed at Taparahi was fill that had been tipped in to form an embankment to support the road back in the 1960s.

Cracks up to 3cm wide across both lanes on top of the embankment were spotted on January 16 this year.

Engineers tried to save the road by preventing water ponding on and seeping down into the embankment, but it did not work, the 160-page report by engineers Beca said.

“The failure appears to have been largely confined to placed fill material used to form an embankment to support SH25a.

“The failure of the embankment appears likely to have been associated with the extreme rainfall that occurred in the preceding months, sufficient to allow water pressures to build up within the embankment to levels greater than would have been allowed for when the embankment was constructed in the 1960s.”

January had seven times more rain than normal near the slip, or 1400mm, though at no point was the rain especially intense.

Investigations after the slip, which occurred in two phases, on January 28 and February 1, located the original ground surface in many spots, “indicating that the in situ soils had sufficient strength to prevent a deeper and larger failure from occurring”.

The first of 15 bridge beams being fabricated by teams working around the clock in Napier. Photo / Waka Kotahi

The land around the slip had been fairly stable since the slip.

“Recent wet weather did not trigger instability in the steep natural slopes above the failed embankment or in the in situ materials supporting the embankment.”

The engineers looked into but had found no way of temporarily reinstating the road while the permanent repair was being done, it said.

How to fix the highway

The report detailed five fix options, of which Waka Kotahi chose the second cheapest and fastest, a steel bridge with a precast concrete deck.

This was costed at $23m to $28m and forecast to take 11 to 13 months to build.

The agency has promised to build it faster, by next March. It expected to get a building consent exemption from Thames Coromandel District Council, the July 17 report said.

Though engineers agreed that liquefaction was unlikely, nonetheless, any new build had to cope with “poor” conditions and “marginal slope stability” at the slip site itself.

“All remediation options developed have considered slope stability and include stabilisation measures to meet required design standards.

“Care will be needed during remediation to avoid undercutting at the base of these slopes” above the slip.

Realigning the whole section was ruled out, by cost (up to $54m) and time (up to three years), but also wildlife laws and lack of anywhere to dispose of spoil.

To the north was land managed by DoC, and to the south Māori freehold land, which would have taken several months to try to acquire, with no guarantee whether areas needed a change of alignment or other roadworks.

An embankment or a retaining option posed greater risks around ground conditions than building a bridge.

The bridge would have long spans of 25-37m to cut down on how many piers were needed, “and thus reduce safety and construction risk”.

The bridge’s 15 big steel beams were being made in Hawke’s Bay. “The team there are working around the clock, seven days a week,” an NZTA update said.

- RNZ