A series of large slips that forced the closure of SH1 through the Mangamuka Gorge last July is taking some time to repair. Richard Silcock looks at why work is taking so long.
A section of SH1 through the Mangamuka Gorge (between Kaikohe and Kaitaia) was closed in mid-July last year due to a number of enormous under and over slips, making it impassable for traffic.
The slips, which occured along a 15 kilometre stretch of the highway, were caused by a period of heavy rainfall that severely loosened the unstable ground, resulting in many thousands of cubic metres of earth and vegetation being displaced and either falling 300-metres into the gully and river below or onto the highway itself, completely blocking it.
Of the eight slips, the one at the northern end of the gorge (an under-slip) is the largest, being 100-metres wide and 300-metres long with some 7000 cubic metres of earth displaced.
Kathryn O’Reilly, the Transport Agency project manager overseeing the reinstatement of the highway says the project is proving to be a huge task and has required some complex design and construction methods.
“Our consultant WSP carried out a number of geotechnical drone surveys three days after the event to ascertain and survey the extent of the damage and provide, in conjunction with the contractor, preliminary design plans to the point where re-construction could start,” she says.
Shaun Grieve, design engineer with WSP, says the design work has continued as the reconstruction work progresses and that the design methodology has to be carefully developed due to the ground geology which mitigates conventional stabilisation methods.
The contractor, Fulton Hogan, also had to install a series of drains to divert rainwater away from the various slip sites and then apply hydro-seeding to help prevent any further movement before any work could commence.
For the largest slip, a series of pile supported concrete supporting walls were installed. The first wall was constructed to support a crane, followed by a lower temporary retaining wall which was constructed of 106 reinforced concrete piles drilled to a depth of eight metres interspersed with ground anchors to provide additional stabilisation, and then an upper wall to retain the bank itself.
“With the temporary walls in place, the sub- contractor (CLL) was able to bring in a 55-tonne drilling rig and a crane to drill 49 permanent foundation piles in the centre of the northbound lane of the highway,” says Kathryn O’Reilly.
“The pile casings are 750 millimetres in diameter and have been drilled 16 to 25 metres into the ground and further embedded in 1.5 metres of solid rock. Steel reinforcement cages have then been inserted, the ground water pumped out and concrete progressively pumped in.
“The next step is the construction of a one metre by one point eight concrete capping beam to tie the pile heads together and the installation of 49 steel anchors which will be inserted 24 metres into the slip slope.
“The capping beam formwork will be constructed over the under-slip and supported by the temporary wall and propped off the permanent piles.”
Further complicating the reinstatement of the highway is an ‘old historic slip’ directly above this section of the highway on the inner side which has been found to be unstable.
“This has further hindered work as it requires stabilisation,” says Kathryn.
“A retaining wall will be constructed of 15-metre long ‘H’ beams fixed on concrete piles up to a height of four metres. Pockets will be cut, and soil nails driven into the ground before this old slip face is excavated and back filled concrete retaining panels installed to secure it.
“Now that reconstruction work on this slip is well advanced, work on the next slip and at the southern end of the gorge has begun.
“The logistics of operating multiple sites simultaneously requires another level of planning, almost on a daily basis, given the narrowness of the highway and getting the required plant to the northernmost slip sites.”
Along other sections of the highway the inner bank is also being cut back (retreated) two to four metres to gain some further width for the highway.
Once all the slips have been stabilised, retained and cutbacks completed the repaving and resurfacing of the highway will be done, with the final 50-millimetre asphalt layer being specification AC14.
Fulton Hogan’s acting construction manager for the project, Sarah Whitehorn, who is overseeing the day-to-day construction work, says some of the challenges their teams are contending with are unique due largely to the ground geology, the area’s remoteness, the confined working areas and operating in a native bush conservation environment.
“Due to kauri dieback in the area we had to work with DoC and come up with a solution for carting spoil from our excavation sites to dumps at both ends of the gorge,” says Sarah.
“It necessitated us having to cover the truck loads so that any dieback ‘oospores’ would be contained.
“There is also the logistics of getting the pre-mix concrete to site as the distance from Whangarei is too far to maintain the concrete’s integrity, so we are sourcing it in small batches from a small quarry supplier in Kaitaia.
“Due to the topography of the gorge cell phones don’t work, so we had to have a landline installed to maintain communications, and as the larger trucks are unable to turn due to the narrowness of the highway, most are reversing all the way to the slip work sites from the start of the gorge.”
Sarah adds that the narrowness and confined spaces, along with the machinery at the various work sites, has made it unsafe to allow two lanes of traffic through the gorge.
The inner lane of the highway was temporally opened prior to Christmas and New Year during the work shut-down period to allow light vehicles through under a traffic controlled, one-way system before it was closed again in early January to allow the contractor to complete the piling and carry out further earthworks.
As part of its maintenance contract Fulton Hogan monitored the highway during the shut-down to ensure motorists were kept safe during this period.
It is unlikely, due to the sheer magnitude of the slips and the difficult terrain that the highway will be fully restored to two lanes much before June this year, however, and depending upon the weather, the Transport Agency is hopeful that it will be sooner
“We acknowledge the impact the highway closure has on the local communities, local businesses and the economy and the extra time and cost that is entailed travelling the alternative SH10 route,” says Kathryn O’Reilly.
“We are committing all available resources to get the highway restored and the Fulton Hogan teams are working double 10-hour shifts per day to achieve this as soon as possible.”