Controlling rock fall from Nevis Bluff alongside SH6 is a highly specialised undertaking and requires constant monitoring to ensure the highway is not blocked by slips. Richard Silcock reviews how this is being done.
The 140-metre high, 880-metre wide Nevis Bluff is a geographical landmark bordering SH6 midway between Cromwell and Queenstown.
Originally, and at the time of the Central Otago gold rush in the 1860s, what is now a main highway was only a rough track, hewn from the rock for pack horse and cart and used by settlers and prospectors as they forged a way into the rugged Otago hinterland in search of gold.
However, such was the usage of this route, that by 1867 a narrow road was constructed around the bluff face.
Even then Nevis Bluff was prone to slips and numerous large rock falls which often blocked the narrow precipitous road which roughly follows the swirling Kawarau River, some 50-metres below.
The road was eventually sealed around the 1960s and widened in the 1970s and designated a highway, however nothing much else has changed since. Two significant rock falls have occurred, one in 1975 and another in 2000, both of which caused the highway to be closed for several weeks.
John Jarvis, a senior network manager for NZTA based in Dunedin says that this part of SH6 has seen a dramatic increase in the amount of traffic using it, up some 50 percent since 2013.
“With near 5000 vehicles now using this route daily, it is important that we keep the highway open,” he says.
“It is the main route to the tourist mecca of Queenstown from Christchurch and Dunedin as there is no other direct highway. The road via Cardrona from the north is prone to closure during heavy snow falls over winter, while the road via Kingston approaches from the southern end of Lake Wakatipu.”
“The bluff is not unique in terms of rock falls, however the particular geological conditions, location and complexity of rock failures across the bluff face, together with its sheer size, and the potential economic impact it would have on Queenstown, tourism and the local community should it be closed, make it imperative that it is managed appropriately,” he says.
“For this reason the bluff requires constant surveillance to ensure unstable rock does not tumble onto the highway below causing major holdups to traffic and presenting a hazard for motorists.”
While maintenance work was carried out over the ensuing years by the MoWD and subsequent contractors, following an extensive geotechnical investigation and assessment in 2012 by Opus International Consultants (now WSP Opus) on behalf of NZTA, a detailed rock-fall management strategy was compiled which set out a plan for the investigation, monitoring, assessment and mechanisms used for reducing risk.
Under a seven year NOC contract,* which began in 2016 with Aspiring Highways** (a JV comprising Fulton Hogan, WSP Opus, Whitestone Contracting and Base Contracting) this plan is followed with monthly inspections by specialist geotech engineers.
These inspections are usually carried out from a helicopter with visual checks made of known cracks, changes in the shape or form of the face features and known risk areas. These changes are noted and mapped.
Each spring and autumn a more thorough physical inspection is made by engineers who abseil down the bluff face, checking and measuring the amount of rock movement, and where necessary drilling, sluicing, blasting, netting or rock-bolting unstable rock.
Trevor Washington, Fulton Hogan’s operations manager with Aspiring Highways, says last November saw work carried out over a three week period to remove some rock that had been identified during an earlier inspection.
“This required the highway to be closed for short 10-minute periods while we carried out a programme of scaling, heli-sluicing and some minimal rock blasting of several mid-bluff column features which were considered to be vulnerable and likely to fall,” he says.
“We had two teams of three to four guys actually working the face, and several more providing traffic management and oversite control.
“The guys working the face are all qualified abseilers with considerable experience. It‘s pretty skilled work, requiring good judgement, strict adherence to safety precautions and an acquired knowledge on the condition of the bluff.
“Some two-three cubic metres of rock was removed in this instance.
“Sometimes it can be considerably more and the rock is carted away to nearby dumping sites.”
While considered dangerous work, the teams working the bluff have not had any accidents and Trevor says this is testament to the strict safety precautions and experience required.
Rob Bond, a specialist geotech engineer with WSP Opus based in Alexandra and a member of the Aspiring Highways team agrees that the on-going monitoring of Nevis Bluff due to its rock complexity and steepness is an ongoing work in progress.
He says; “The bluff itself presents a number of challenges. It is extremely steep (70-degree slope), has no significant vegetation, is comprised mainly of unstable fractured and brittle Otago schist rock, is bordered by a swiftly flowing river below and is inundated by rabbits – all of which add to the constant erosion.
“While there are differing types of schist found on the bluff, most of it is prone to fracture. It is millions of years old and has been subject to the effects of harsh climatic conditions and seismic movement as the bluff is located close to the Cardrona fault line and displays a number of ‘shear’ zones forming lines of weakness.
“By maintaining visual checks and using computer modelling, along with laser scanning, 3D mapping and seismic profiling we are able to predict likely rock failures and plan a level of work for either stabilisation using nets and rock bolts, or removing high risk ’features’ by sluicing or blasting.
“Despite this we have found that every three to five years we need to carry out a more extensive rock removal programme,” says Rob.
On average approximately $1 million is spent on carrying out this more extensive work. This is regarded as essential expenditure to ensure the safe passage of vehicles and motorists using this section of the highway and to make sure the busy route to Queenstown is not compromised.
While alternative routes have been investigated by NZTA, including construction of a tunnel or rock fall shelters, such alternatives are considered very expensive with estimates in the millions.
The agency is looking however at the feasibility of installing permanent high impact wire fences along the highest risk areas of the bluff.
*This Network Outcomes Contract (NOC) also includes 537 kilometres of the Central Otago highway network.
** Aspiring Highways is a registered trademark brand.