Slip lesson in Northland

This article first appeared in Contractor March 2017.

Though not quite of South Island earthquake scale, the Hautapu slip in Northland was nevertheless a tricky repair project. HUGH DE LACY explains.

AS LANDSLIDES GO – and the Kaikoura earthquakes have set a new standard in severity – the slip at Moerewa, 75 kilometres north of Whangarei, was no biggie, but because it was beneath rather than above the road it presented challenges unique to its site.

The slip itself occurred following a tropical storm in mid-2014 that undermined a northbound passing lane running across the face of Turntable Hill.

The passing lane had to be closed, but the slip caused no major traffic impediment other than removing one of the few passing opportunities on a busy stretch of Northland highway leading to the Far North and the Bay of Islands.

But for all that it was a challenging job to fix, especially while keeping the other two lanes open, and, with no short detour route available, it was vital to keep open the transport links between Whangarei and the top end of the island.

The slip initially dropped out near the top of Turntable Hill, forcing the closure of about 150 metres of northbound passing lane, before the second subsidence 50 metres to the south closed the rest of the lane.

Temporary water-filled barriers were installed to keep traffic back from the edge of the slips and they remained in place for the better part of 24 months.

Planning repairs proved so challenging that the tender was not put out by NZTA until the design of the repairs was completed in early April of last year, and it was won by Fulton Hogan’s Northland branch, headed by regional manager Keith Cocking.

The company’s Northland Construction Division, headed by Andy Booth and with Ross Harper as project manager, was given the daunting task of starting the repairs in the middle of last year’s winter.

With a 25 metre drop to the bottom of the hill and little room to manoeuvre, the earthworks methodology was critical to ensuring the safety of the crews working on the site, and also of the travelling public.

Once firm ground had been reached on the northern slip by cutting a track down the face and excavating the unstable material, a 12-tonne tracked rig from Whanganui subcontractor Rock Control drilled 50 holes, each 30 metres deep and 150mm in diameter, for the micro piles that provided the base on which to rebuild the hillside.

Including raking piles at 45 degrees, these anchors were grouted, and rose vertically from the drilling platform where they were connected by a concrete base poured over the top of them.

On this base were then set two rows of gabion baskets which were also tied back to a row of anchors set into the slip-face at 45 degrees.

The rest of the 161 anchors were installed beneath the gabion baskets, and connected in pairs with timber walers to support the rest of the exposed slip.

The slip was then covered in a Terramesh blanket to avoid further erosion.

Extensive improvements to the site’s drainage were also carried out to prevent water getting into the underlying subgrade material which probably caused the slip in the first place.

This work included renewing the drainage culverts across the hill section of the road, subsoil drainage, shot-creting the existing drainage paths and installing new chambers to gather and control the flow of water in the event of a repeat of the downpour that triggered the slip.

From there it was a case of back-filling up to the road-level, followed by standard re-paving and the installation of a guard-rail for increased driver safety.

The secondary slip to the south required a slightly different treatment, though it too was stabilised by anchors over the top section of the slip where the key repair strategy was the installation of a Terramesh basket wall – a gravity-based retaining wall.

With limited space to work in, the secondary slip had to be cleared down to a suitable formation which was found 12 metres below the existing road level.

Heavy rocks were installed to create a flat and solid foundation on which to build the wall.

Complicating this section of the project was the presence of the fibreoptic cable taking high-speed broadband to the top of the country.

The second slip also used a formation of gabion baskets to tie into an existing retaining wall at the southern end.

“It was quite a tricky job, to put it mildly,” Fulton Hogan’s Andy Booth told Contractor.

“Just cutting a track down to the bottom of the slips for the drilling rig was a challenge because we had so little room to work in.

“We worked closely with NZTA, Opus consultants and the local iwi to ensure the best result for the whole community.”

The cost of the project was around $1.5 million, with work beginning in September of last year, two years after the slip was detected.

It was finished just before Christmas, in time for the heavy holiday traffic heading to the vacation spots of Doubtless Bay, the Bay of Islands and the Far North.

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