An amazing video shot from a light aircraft on Saturday shows the sheer enormity of the task facing Waka Kotahi to repair the Kopu to Hikuai Road, State Highway 25a on the Coromandel Peninsula. Video / Philip Hart
Rock was found quickly in some areas of the slip site on Coromandel’s State Highway 25A whereas the soil was wet and soft in others, geotechnical investigations have discovered.
The investigations are now finished and Waka Kotahi NZ Transport Agency Waikato BoP is analysing the soil samples to help it decide on the best way to restore the highway.
“After drilling 14 boreholes and five test pits we found the material above and in the slip was variable,” the agency said on Facebook.
“Our assessment will take all this data into account while also considering factors such as cost, resource, and environmental impacts to determine the best repair option. We’re hoping to have a decision on a permanent fix in May.
“We’ll still be busy working at the site, with drainage material being installed this week to protect the area while the final option is determined.”
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.
Geotechnical investigations on the State Highway 25A slip are finished. Photo / Waka Kotahi NZ Transport Agency
Waka Kotahi regional manager of infrastructure delivery Jo Wilton said the options are retaining the road, building a bridge or building a bypass by going up the hill around the slip.
Wilton said engineers were not leaning towards any one option, with the choice reliant on the outcome of geotechnical investigations.
She also would not categorically rule out the remote 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 you’re tunnelling blind.”
After a preferred option for the road was selected next month, more design work would be needed 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, meaning residents or visitors looking for a solution by Christmas would be disappointed.
