One of the country’s highest roads was recently upgraded and extended by over five kilometres. RICHARD SILCOCK explains how this difficult project was achieved.
THE TASK OF UPGRADING and sealing a 5.56 kilometre section of the Remarkables Ski-field Access Road near Queenstown was given to Fulton Hogan, which was faced with working in an extremely challenging environment, where the pavement has to withstand extreme conditions.
When you are working up to 1280 metres above sea level you would not want to fear heights, or be over-awed and distracted by the inspiring scenery.
Machinery operators had to undergo specialist training and follow strict procedures in relation to safety due to the nature of the steep alpine terrain, extreme climatic conditions, and sheer ‘drop-offs’ along much of the road’s length.
“To manage the risk of the ‘drop-offs’ and where there were no guardrails we installed 1500 metres of hazard flagging prior to commencing work,” says Lachy Sutherland, Fulton Hogan’s project manager for the upgrade.
“This provided visual delineation and identified the edges of the road for our team. During construction a temporary windrow of material was also deposited along ‘drop-off’ edges to further delineate them and help prevent any sliding of equipment off the edge.
“We also refrained from using double drum and three-pointer rollers due to a potential lack of traction in the steep terrain and often icy conditions.”
Another safety innovation was the use of a special truck deck that, via a moving tray, delivered product through the rear tail gate without raising the deck. By attaching various spreaders; aggregate, chip, gritting material and asphalt could be spread without risk of the truck tipping over on the steep gradients and potentially harming the driver and ground crew.
Safety precautions were also taken in keeping the road open for restricted access by those working on the ski field.
Given the financial viability, extreme alpine conditions, availability of materials and the narrow window for construction, special pavement design solutions were devised by the technical team of Fulton Hogan.
“We were able to provide a full design and construction package for the project,” says Lachy.
“To provide the best solution, the site was split into three sections, with each having a different pavement design specification determined by the terrain. This allowed us to construct a road that not only met the environmental conditions, but also the client’s budget expectations.
“At the lower level, which has gradients of up to 17 percent and some tight corners, we utilised 150mm granular overlay with cement stabilisation, asphalt surfacing on the corners and grade 3/5 chipseal on the straight parts.
“For the mid-section, 150mm granular overlay with grade 3/5 chipseal was used as this section is relatively flat and is well drained.
“For the higher section, we used 150mm granular overlay with foamed bitumen stabilisation, asphalt on the corners and grade 3/5 chipseal on the straight parts. This section is the most exposed, has a 21 percent steep gradient, tight corners and is often in a freeze/thaw state.
“In all cases, we used M4-AP40mm aggregate for the overlay which was sourced from the Parkburn Quarry near Cromwell.
“This was selected due to its M4 gradation and high permeability which reduces the risk of freeze/thaw issues found at this attitude.”
Work planning and logistics was also a critical part of this project. The top (highest) section was completed first to alleviate any risk of an early snowfall hampering the construction, as was the lowest section to allow for the establishment of the site compound.
“For the foamed bitumen and cement stabilisation sections we used our Wirtgen WR2500 SK stabilising machine,” says Lachy.
“This was an ideal machine for this job as it allows for the dustless mixing of cement and is fitted with an integrated spreading device directly in front of the milling and mixing drum.”
For cement based stabilisation, cement is mixed with the base course material in the milling machine and then compacted once it is laid, resulting in a far stronger base course layer under the seal.
Foamed bitumen stabilisation is achieved using the same process, however bitumen mixed with water is also added. This increases the strength of the pavement while retaining flexibility and is therefore relatively fatigue resistant. It also reduces freeze/thaw issues.
Application involved a cement truck and a water tanker connected to the front of the Wirtgen by push bars (coupled like a train) which was pushed up hill. This ensured continuous and simultaneous spreading of the cement, bitumen and water (injected to maintain moisture content and regulated by a microprocessor). Using this method up to 800 lineal metres of paving per day was achieved.
Lachy says an advantage of this stabilisation method is that the process is relatively quick, as once the material has been stabilised the surface can be sealed within a couple of days, allowing construction to move on to the next phase.
To ensure optimum compaction was achieved, site tests were done to determine the number of passes required by the rollers to achieve a 98 percent MDD. This information was communicated to the roller operators who completed upwards of 90 passes in any one area, first by vibrating single drum rollers and then by two pneumatic-tyred rollers for the secondary compaction.
The original gravel access road to the Remarkables ski field was officially opened in September 1983. This remedial work and pavement construction was completed over a 15-week period between December 2015 and April this year. It was a continuation of the first four kilometres of sealing that was completed in 2014.
“NZ Ski, in commissioning the upgrade, wanted to reduce the maintenance costs associated with the original gravel road, improve ‘ride-ability’ and make it more attractive to drive for skiers and visitors,” says Lachy.
Fulton Hogan was responsible for the pavement design, initial survey, geotechnical and road construction work. Base Contracting provided some earthworks, culverts and guardrail installation and consultant MWH was appointed to provide peer design review, contractual and financial management, and safety reviews.