Months yet before decision on Coromandel’s State Highway 25A road fix

By Tom Taylor of RNZ

The east coast of the Coromandel remains largely cut off from the rest of the country, with access in and out severely impacted by landslides.

A fix for State Highway 25A will not be in place before Christmas, but engineers are drawing closer to a decision on the best path forward.

On Monday, media were allowed a rare glimpse into the cavernous divide that effectively splits the peninsula in two.

A 140m stretch of road is missing from SH25A, which has been closed since cracks appeared on January 27 in the wake of ex-Cyclone Hale.

The site now resembles a quarry, with much of the initial debris cleared from the slip.

But site managers reminded reporters of the dangers that lingered under the surface, instructing them to stick to the paths carved out unless they wanted to sink waist-deep into a quicksand-like substance.

A decision on what would become of this route may take another two months.

Waka Kotahi regional manager of infrastructure delivery Jo Wilton said there were three options on the table.

“Reinstate the road using some form of retaining, a bridge option, and then a bypass, which is where we’d look at going around the slip, up the hill.”

Wilton said engineers were not currently leaning towards any one of these options, with that choice reliant on the outcome of their geotechnical investigations.

She also would not categorically rule out the remote fourth possibility of a tunnel through the mountains. However, the cost of this would likely be a prohibitive factor.

“We can’t say that tunnelling’s completely off the table, but that is quite an expensive option.

“You also need to really look at the geotech to see what you’re hitting when you’re tunnelling, because you can’t actually see when you’re tunnelling - you’re tunnelling blind.”

Parts of SH25A in the Coromandel completely collapsed in January after heavy rain in the North Island. Photo / Waka Kotahi

After a preferred option for the road was selected in May, further design work would need to take place before construction could begin.

It meant the start date could fall into the wetter winter months - less than desirable for a project on such unstable ground.

“A lot of the material we’re finding is kind of clay stuff,” Wilton said.

“When water hits that it goes very much like plasticine and gets very sticky.

“That is quite challenging to work with in winter, so that’s a consideration, whereas there are other methods - say building structures - that can progress in winter, albeit a little bit slowly depending on the weather, because obviously if we get torrential rain, it becomes unsafe to work.”

Wilton said any fix would take nine to 12 months to implement, meaning residents or visitors looking for a solution by Christmas would be disappointed.

The project’s lead geotech engineer, Nathan McKenzie of Beca, said work had been delayed while material was moved to stabilise the site for drill rigs.

His team was now about two-thirds of the way through their investigations.

They had drilled 10 boreholes and were currently working on the 11th and 12th, aiming to find rocks or soil stable enough to support the weight of a new structure without the risk of further slips.

“We’re expecting the geotech investigations to be substantially complete this week, and from then on we’ll be looking to start assessing and basically refining options using that information,” McKenzie said.

He said drilling had uncovered a mixture of material in the ground, from weak slip debris containing a lot of water to areas that were “marginally stable”.

“The residual soils, they are quite variable: some are sandy, some are clay-ey, and then the rock itself is quite variable.

“We’re finding in some areas we’re hitting quite competent rock quite quickly, in other areas it’s very deeply weathered.”

Using the data collected, the team would build a full model of the site, which McKenzie said should provide a clearer picture of what’s needed.

After that, the criteria for a decision would become relatively simple.

“We are looking at cost, we’re looking at something that has to work, something that has to meet design standards, and we’re also looking at something that can be done fast.”

Speed was an important factor for many residents and businesses cut off by the slip and fearing the loss of another summer’s trade.

Second slip

Meanwhile, another slip on the eastern side of the Coromandel between Whangamatā and Tairua was also a cause for concern.

A large chunk taken out of the road had seen this section of State Highway 25 reduced to a single lane open to light vehicles only.

With the rebuild of this road only due to start in April, holidaymakers looking for a relaxing time at the peninsula’s beaches this Easter may face a testing drive to get there.