No timeframe for SH25A fix amid concerns NZ might have to import expertise

Wednesday, 1 February 2023

Aerial video of the slip on State Highway 25A between Kopu and Hikuai on the Coromandel Peninsula. Video first published January 31 2023.

The restoration of the Coromandel’s major arterial route will be ”complicated” and the capacity to undertake the work might have to come from overseas, according to an expert.

And with engineers still unable to properly assess it due to ongoing weather conditions, no one is willing to put a timeframe on when the vital road might reopen.

The “huge” landslide between Kopu and Hikuai on State Highway 25A on the Coromandel Peninsula is roughly 40m wide and in some places up to 10m deep.

The slip was the conclusion of weeks of speculation that cracks in the road, caused by sodden earth after Cyclone Hale, may lead to the entire carriageway giving way.

Dr Arezoo Rahimi, an expert in slope repair technology at the University of Auckland, says that there are two feasible options for the restoration of the road; a “redesign” or “repair”.

Both are contingent on the geology of the slip and the ability to get machinery and equipment to the site.

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Inspections at the summit of State Highway 25A (SH25A) on Monday show the remainder of the road has collapsed into the gully, as a result of heavy rain which continues to fall in the Coromandel region. (Image supplied by Waka Kotahi)

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The initial assessment of the size and nature of the slip will be crucial, she says. Finding where the slip begins and ends on the hillside is the first step upon which the success of the repair would hinge.

Dr Arezoo Rahimi says that any fix will be complicated.

“These kinds of assessments, to come to the conclusion about what was the main reason, that will provide us with proper adoption to repair or redesign… How extensive it is, how deep it is. Do we have any historical slip surfaces there? Where is the groundwater table? We have to see all that before we decide on the proper methods.”

Beyond the size and scale of the landslide, it will also be essential to analyse the soil structure to establish what about it made the area liable to slip.

The initial cracks on SH25A caused after heavy rain lashed the peninsula during cyclone Hale.

“In New Zealand it is good to consider all geotechnical properties that play a part. For instance, we have slopes that are safe when they are dry and there are many of them, but once it rains some of them will fail… We call them [dry soils] unsaturated soil.”

The ability of dry soil to take advantage of capillary action, which keeps soil together through the effect of the suction of water rising to the surface, is undermined after heavy rain. This principle, Rahimi says, is often ill-considered in New Zealand road engineering.

“When you have rain that capillary action will be gone and that additional sheer strength will be gone… This theory is not usually considered in the design. I’m pretty sure it’s not considered here.”

Once the appropriate course of action is determined, Rahimi says that a mixture of “back filling” and “slope stabilisation” methods like anchor rods, or shoring treatments could be used.

The wash-out at Motu River Waka Kotahi says will provide experience to crews fixing SH25A.

Rahimi says that regardless of the methods chosen, drainage ought to play a crucial role on the road now infamous for its soggy conditions. Existing drainage is sometimes clogged, if not built far enough into hillsides or simply too small in diameter to move water off the road.

“Having proper drainage, whether it’s horizontal drains whether it’s trenched drains. Whatever system of drainage, it has to be proper and also consider the soil properties and the rainfall characteristics.”

Rahimi says that by looking at the images of the slip material and the substrate beneath the road, the soil could be difficult to work with.

“I can see soil that is quite crumbly, I would say. Sometimes they call it collapsable soil… it seems like fine particle soil mixed with some coarse grain… That could have interesting properties to consider in designs.”

There might not be enough engineers to go around all the slips too.

The issue with professional capacity comes down to the number of rain-derived landslips, Rahimi says, and some additional capacity may have to be imported or developed quickly.

“I will assume we need to develop that capacity, we should develop it.”

A spokesperson for Waka Kotahi told Stuff that it could yet be “a week” before the slip is deemed safe enough for engineers to approach and take samples and measurements.

The spokesperson said that the agency has experience dealing with washouts, like the one on State Highway 35 beside the Motu River. In the case of the SH35 however, some portion of the road remained from which crews could stabilise the slipped away section.

This is not the case between Kopu and Hikuai.

In the meantime, Rahimi says temporary bridge structures made of steel or concrete could be investigated to restore the flow of traffic over the essential route.