Access tracks being made into the SH25A slip area for the geotechnical investigation contractors. Photo / Supplied
Access tracks being made into the SH25A slip area for the geotechnical investigation contractors. Photo / Supplied
Waka Kotahi NZ Transport Agency says geotechnical investigations are progressing well at the site of the huge slip that closed SH25A in the Coromandel in January
Two drilling rigs are on site and if good weather continues, the fieldwork is scheduled to be completed this week.
The surface conditions remain steep and slippery for the work crews.
SH25A was closed at the summit on Friday, January 27, to ensure the safety of road users, when deep cracks formed in the road after ex-Cyclone Hale. The cracked section of the road at the summit then slipped away during the Auckland Anniversary storm event.
Waka Kotahi says the investigations are assessing the ground conditions within the slip area and the hillside above. “Ideally, we want to find stable and strong soils or rock that can support the weight of an embankment or structure without the risk of movement or slipping,” the agency says.
Boreholes are about 50mm in diameter and go to depths of about 20m. Photo / Supplied
The data being collected also helps to determine the amount of excavation needed, slope stability and foundation requirements for the various options being considered. These are all important factors that help determine a solution that can be built quickly and efficiently.
Last month Waka Kotahi’s regional manager of infrastructure delivery, Jo Wilton, said: “There are three main options for reinstating this highway. One — we build a bridge replacing the lost section of the road; two — we create a deviation that bypasses the slip site; three — we make a retaining wall and effectively rebuild the road from the ground up.
“We don’t know which option is most feasible yet. The right solution cannot be established until the geotech work is complete. We know the rebuild must be resilient and as timely as possible. We understand how critical this highway is for Coromandel residents, businesses, and visitors,” Wilton said.
“Boreholes are being drilled to depths of around 20m to help understand what the ground conditions are like, we’re getting data from different methods — boreholes and test pits”, says Waka Kotahi.
Work to rebuild this section of State Highway 25 near Opoutere is due to start in April. Photo / Supplied
“Last week test pits were cut from the sites where we only need to understand the soil conditions at a reasonably shallow depth — e.g. near the side slopes of the slip. They are cut and formed by a digger, are about 2-3m square and are dug to a depth of around 3.5m. They’re good at giving a clear picture of the soil composition.”
The two rigs on site are drilling boreholes, retrieving cores of the soil from greater depths and providing more detailed soil and rock strength information. Boreholes are about 50mm in diameter and enable a core of the soil to be extracted. These are being drilled at the sites of the potential bridge and bypass options.
The agency also reminded drivers to be aware Coromandel roads are not back to usual because many locations along the network are compromised and vulnerable following the storm events this year.
People should be prepared for disruption and delays at one-lane bridges and repair sites and expect unsealed surfaces. Driving to the road conditions is vital to avoid damage to vehicles from the loose chip.
State Highway 25 is now open under stop/go for light vehicles only between Hikuai and Opoutere due to a major underslip. By “light vehicles”, Waka Kotahi means vehicles weighing under 6 tonnes when they’re loaded.
Work to rebuild this section of State Highway 25 near Opoutere is due to start in April.
Other work around the Peninsula
SH25 Opoutere-McBeth underslip: “We’ve looked at what needs to be done to repair this site and the detailed design is underway,” Waka Kotahi says. Work to rebuild this section of the state highway is due to start in April. Two traffic management teams are currently on site, with daytime and night-time traffic management and site monitoring. The road will need to be closed periodically in the coming weeks for work and the agency will work with the community and freight around the timing of this.
SH25A Taparahi slip: Two drilling rigs supported by an earthworks team are working on site.
SH25A western side: Works to cut back the remaining slips that impact the road are underway.
SH25 between Thames and Te Kouma: Vegetation clearing, including the removal of large trunks and root balls, is underway as well as slip removal and drainage reinstatement. This work will continue along this section on a priority basis over the next couple of weeks.
SH25 Coromandel to Tairua: Slip removal and drainage reinstatement are underway on SH25 between Tairua and Materangi Road. This work will continue on a priority basis over the next couple of weeks. Drilling work under stop/go traffic management will continue at a number of sites between Kuaotunu and Coroglen this week.
SH25 Tairua Quarry: The site is open to two lanes under a speed restriction with shoulder and drainage works continuing. Earthworks are completed with more minor works likely to continue.
SH25 Hikuai Settlement Road dump site: The material continues to be moved from stockpile sites on SH25 and SH5A in the Tairua River catchment area to the clean-fill site.
SH25 Whangamatā to Waihī: Work on stabilising a number of slip areas between Whiritoa and Waihī will take place this week.
