Since early 2018, when Cyclone Gita hit the country, SH60 across the Takaka Ranges west of Nelson has been reduced to one lane controlled by traffic signals. Due to the severity of damage the remedial work is taking some time. Richard Silcock reports.
When cyclone Gita hit us in February 2018 the heavy rainfall and high winds caused havoc across many parts of the country with many highways and roads severely affected.
SH60 (which traverses the Takaka Hill and ranges west of Nelson) between the townships of Riwaka and Takaka was severely damaged with a considerable number of slips, washouts and pavement degradation.
Restoring the highway was vital to the region as it is the only road to Collingwood and Golden Bay and used daily by the local community, farmers, milk tankers and tourists.
Tasman Journeys and Downer NZ, as the NZTA’s contractors for the maintenance and repair of this highway have been working on the damaged sections over the past 22 months.
Some 10 sites along the highway were repaired in the latter part of 2018 through until mid-last year with work entailing removal of slip material, ground stabilisation, constructing retaining walls and road pavement reinstatement.
This required careful and essential planning prior to the actual works. It included traffic management procedures to keep the highway open to traffic both day and night, albeit down to one lane in places and with some temporary short-term closures, with the traffic controlled through these areas by a series of traffic lights at either end of the work sites.
Work on the last five remaining and bigger sites commenced in January this year with Downers being awarded this contract following a competitive tender.
Likewise, work for these sites has entailed careful planning and according to the NZTA’s manager currently overseeing the project, Chris Robertson, these last five areas of repair all entailed significant amounts of rehabilitation work and pre-construction geotechnical investigation to assess risk and ground stability.
“The design work for these five sites was carried out by a project team comprising our people and BECA geotechical engineers, and has involved drilling boreholes and digging pits to ascertain the earth stability, the rock type (there are large amounts of igneous diorite) and the soil composition,” says Chris.
“While carrying out the remedial work we are also taking the opportunity to ensure the repairs will be sufficiently robust and suited to the local conditions as this will help increase the resilience of this extremely windy and hilly highway during any future storm events.”
The project team completed this preparatory work prior to Christmas 2019 along with careful planning for health and safety requirements, traffic management and construction logistics.
It also included establishing a ‘temporary works’ design and provision for safe working areas for the construction teams, particularly those working below and on the drop side of the ‘live’ highway.
The reconstruction work is taking place simultaneously at all five sites and the traffic light signal system has been moved from the former work sites to cover these sections of the highway under repair with traffic flow alternating in each direction during the construction period.
Downers project manager for this work, Thomas Maw, says there are a number of key challenges associated with this project.
“These include gaining access to the various sites, the steep topography, working in tightly constrained spaces often below road level, keeping the road open to traffic and the safety of our teams involved in the reconstruction.
“The terrain around the project makes construction complex. We are having to cut temporary access tracks from the road level down into the gullies to gain access to the bottom of the gully walls and these tracks need to allow for fully laden 4WD tipper trucks and other specialist equipment.
“In order to keep the traffic flowing and to protect our workers the slip faces are being excavated back to stable ground and then retained using soil nails, steel mesh and shotcreting.
“The weather also continues to pose a risk for the project. Our construction management plans and risk management procedures have incorporated robust controls for water management at each of the five sites and we are keeping a close eye on long-term weather forecasts as we progress through the job.”
At all of the major slip sites the highway level will be reinstated using reinforced earth retaining walls and rock filled facings.
The project teams are working closely with local landowners and users of the highway to help minimise disruptions particularly when hauling some 30,000 tonnes of rock from a quarry in the Riwaka Valley to the construction sites for the MSE walls.
With the construction period likely to extend through winter, special pavement construction will be needed with a temporary seal laid until the weather improves sufficiently to allow for pavement construction to NZTA highway specification.
This surfacing will be undertaken once all the MSE walls are constructed and there is sufficient room to switch traffic between lanes and complete the paving.
To enable the five sites to be worked on simultaneously, specialist teams have been engaged. They include drilling crews for temporary works, earthworks and construction specialists, drainage crews and two paving crews.
“Each site crew has dedicated plant and equipment and at peak construction we will have 10 trucks on the go just to ‘import’ the fill and aggregate for the walls,” says Thomas.
“Most of our team are from the region with a few specialist subcontractors and others brought in from out of town.
“Our guys are all pretty experienced in this sort of work – our earthworks supervisor for instance has over 40 years’ of experience and has most recently been working on the slips along the Kaikoura coastline (SH1) and some other inland roads that were closed by the Kaikoura earthquake.”
The project is expected to be totally completed by early next year, with the highway reinstated to its former two lanes.