Slip sliding away
After failing during heavy rain in July last year, a notoriously unstable stretch of State Highway 35 north of Gisborne has finally received a solid solution.
State Highway 35 is the only continuous transport route from Gisborne north to East Cape. The highway follows an often tortuous alignment over a mixture of geologically young coastal land, river valleys and terraces, and steep hill country.
Ongoing land movement, earthquakes and frequent high rainfall events continually challenge the integrity and condition of the road.
In July 2008, heavy rain soaked the district, reactivating old land movements above and below the highway on a section north of Ruatoria, known as Kemps Descent. Land movement and bad weather had forced the closure of this section of road on earlier occasions, for up to a week at a time.
But the heavy rain in July created instability below the road, causing drop-outs, as well as movement above the road which encroached on the highway alignment. NZ Transport agency project manager Gordon Hart says about 100 metres of road failed.
Maintenance contractor Fulton Hogan immediately began work to mitigate the effects of the slippage. But it was obvious this stretch of road would always be prone to damage.
“Fulton Hogan carried out temporary work to remove water and to build up the foundation for temporary use, but it was always just a patch job – hectares were on the move,” Gordon told Contractor.
Kemps Descent had caused problems in the past, notably during Cyclone Bola in 1987. Post-Bola investigations identified possible realignment routes to the west and east of the current route, although all route alternatives were faced with the difficulty of traversing historic sites of large-scale land instability. Aerial photographs also highlighted the presence of fault dislocations through this area.
On balance, the western alignment offered the NZTA the lowest future route security risk. In the past, realignment project costs were considered prohibitive, and maintenance of the existing route was favoured. However, these project costs did not appear to incorporate the overall costs (including monetary and social) of ongoing route failures to the wider community.
Soon after the July 2008 event, NZTA regional staff decided to realign the highway on to less unstable land.
And in a feat of good planning and co-operation, three months later in November 2008, work began on site.
The realignment was constructed by Fulton Hogan as an emergency works project under the existing network maintenance contract, with the earthworks subcontracted to Tauranga-based A&R Earthmoving Contractors.
The local community were particularly involved with this project. The new alignment lay across Maori land and a trusting relationship was developed with local iwi. In fact, the new piece of highway, now officially Goldsmith’s realignment, was named after the local Maori family who donated the land.
Gordon Hart says the whole project was carried out very much as an alliance, with all parties involved working closely together to achieve the shared vision. This meant that earthworks could get underway while the design work was still being finalised, and the team was able to secure multiple consents (with conditions) from the Gisborne District Council in a short timeframe.
The collaborative approach allowed the skills of all team members (including geotechnical engineers, machinery operators, consent planners, property professionals and contract managers) to work together to achieve ‘best for project’ solutions at all times – especially when they encountered “surprising” ground conditions.
Those surprising ground conditions are a result of the complex geology in the area – it’s made up of highly fractured and disrupted siltstone and mudstone, overlain in parts by alluvial gravels and ash deposits.
The moisture-sensitive soft rock cut-to-fill material was initially difficult to excavate, but quickly weathered and degraded when exposed.
High groundwater pressures through fractures in the underlying material further complicated construction works, and the presence of at least one seismic shear zone through the site added to the geotechnical complexity and adversely affected batter slope stability.
Gordon says they had up to four geotechnical engineers working on the project at one stage.
The design and construction works sought to achieve a cut-to-fill balance within the site. The principal fill disposal area was on relatively unstable land alongside the new road alignment. The disposal site showed evidence of large-scale historic landslide movement and the Gisborne District Council land conservation staff were very concerned about bout the stability of the fill disposal field. Consequently the design was modified to include new subsoil drainage and two big shear keys to hold the earth.
Given the nature of the earth the contractors were working with, rain quickly made the site unworkable. But in the open nature of the working arrangements, when wet weather made it too slippery for the earthworks crews to move cut-to-waste up the hill to the designated disposal site, after an on-site discussion it was agreed the wet material should cut downhill to a temporary disposal site then re-worked in dryer weather. This kind of proactive decision-making mitigated weather-related delays and allowed works to restart quickly, without having to wait for more formal exchange of notices and instructions.
The Goldsmith’s realignment has achieved a longer-term solution on a stretch of highway that has a lengthy history of route security problems, but which has consistently failed to secure sufficient funding to receive more than just reactive maintenance.
Despite severe testing in July this year, the new alignment section has remained open and in working order.
Both the NZTA and the affected Maori trustees are now keen to see the project model repeated to achieve similar route improvements at unstable sites along State Highway 35, in particular at Busby’s Hill near Tokomaru Bay, which suffered badly over the winter months.
As a sign of a tricky project well executed, Goldsmith’s realignment netted a merit award at this year’s Roading Excellence Awards.
Contractor Vol.33 No.9 October 2009
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