What Good Looks Like: Rockfall Protection at Diana Falls
A landslide in 2013 closed State Highway 6 in the Haast Pass, and the installation of a three-attenuator rockfall protection system that followed had to be completed on a slope that was still failing. Doing so safely came down to daily construction decisions made on site and the experience and tenacity of our crew.
In our 'What Good Looks Like Series' we're examining what good looks like across the different types of work we do, from emergency response slope stabilisation and mining ground support to geohazard mitigation and asset integrity maintenance. These aren't showcases of perfect projects with ideal conditions. They look at real work where solutions had to adapt as conditions changed, where collaboration shaped outcomes, and where technical expertise met practical constraints.
Overview
In September 2013, an extreme rainfall event over New Zealand's West Coast - more than 300mm in 24 hours - triggered a landslide at Diana Falls on State Highway 6 in the Haast Pass. The initial failure put around 10,000 cubic metres of material onto the road. Over the following month, as the slip regressed upslope by more than 200 metres, that figure grew past 30,000 cubic metres. The highway, one of only three routes connecting the West Coast to major population centres, was closed for close to three weeks and then reduced to single-lane daylight access, which is how it stayed until construction was nearly complete.
Geovert mobilised in April 2014 as the construction contractor tasked with building a three-attenuator rockfall protection system, developed jointly with designer WSP and barrier specialist Geobrugg, on a slope that was still actively failing. The attenuator concept itself, a modification of conventional rockfall barriers able to manage both large boulder impacts and rainfall-driven debris flows, was sound engineering. Getting it built safely on ground that hadn't stopped moving was a separate challenge again, and one that fell to the Geovert team to solve day by day, on site, for the length of the programme.
Site Conditions and Design Approach
The failure left a work area roughly 300 metres high, up to 50 metres wide, with a face angle between 35 and 42 degrees and a near-vertical 40 metre rock face at its base. The material ranged from 300 tonne boulders, stable enough to stage plant on, through to metres-deep silty gravels that mobilised into debris flows during rain.
The NZ Transport Agency Waka Kotahi, as client, brought together WSP as designer and Geovert and Geobrugg for constructability, material selection and barrier engineering, drawing on drone survey data, scaling and mapping carried out in the months after the initial event. WSP had put forward a range of options, from full realignment through to rockfall shed structures. A three-attenuator system won out because it offered the best balance of cost, buildability and flexibility, and because it could be built top-down with lightweight, portable equipment, progressively reducing risk to the crews below as each barrier went in. It also meant Geovert, Geobrugg and WSP were setting anchor and barrier locations together on site, balancing catch height, self-cleaning slope angle and rock quality in the same conversation rather than handing a fixed design over the fence.
Access and Logistics
Rope-access tracks up both sides of the slip had already been established for inspection and monitoring by the time Geovert's crew arrived in April 2014. The bulk of construction then ran through the Southland and Otago winter with rain, snow, frost, high winds and low visibility, on a site with no nearby cell coverage.
Materials came in by helicopter from a staging area on the Haast side, a five-minute flight from site. Mesh, cable and componentry arrived from Geobrugg in Switzerland, with posts fabricated locally once exact barrier lengths were confirmed. Post lengths varied by up to 2 metres between adjacent positions to follow the terrain. Communication ran on handheld line-of-sight radios between crews and the traffic management team in Wanaka, with a satellite phone kept on site for emergencies.
None of this is unusual for remote New Zealand infrastructure work but what set Diana Falls apart was that the ground itself was an active hazard for the duration of the project.
Hazard Management During Construction
The slip regressed and shed material throughout construction, which meant hazard management wasn't a start-of-project exercise but a daily one. A trigger action response plan, checked against rainfall, temperature, wind and seismic data, determined whether it was safe to work each morning. Two crew members based in Haast crossed both access tracks before dawn to inspect the slope from the bottom up, ahead of any traffic, construction or otherwise, being allowed under the slip.
Spotters were stationed at road level and on the slip itself, watching for movement and using airhorns to raise the alarm, a measure that on more than one occasion halted traffic before rock reached the road. High-risk boulders were fitted with extensometers checked every morning. Sections of the slope were declared no-go zones outright. When Attenuator 1 took a direct impact from a slide before it was finished and needed mesh replacement and cable re-tensioning, it wasn't treated as a failure of the plan but that the plan working as intended, on a site where an unfinished barrier being hit was always a live possibility.
Anchor and Barrier Installation
The barriers needed 6 to 8 metre grouted bar anchors drilled to a consistent standard across ground that ranged from exposed schist bedrock to loose overburden several metres deep. Where bedrock was exposed, anchors went in open-hole with a down-the-hole hammer. Where it wasn't, the crew either relocated the anchor or switched to a top-hammer and hollow-bar grout-flush method to punch through the loose material first. That decision was made anchor by anchor, on the day, by the crew doing the drilling.
The end posts on Attenuator 3, sized up to carry the load of a 40-metre drape tail, weighed over a tonne each, close to the lift limit of the largest helicopters available out of Wanaka and liftable only in near-perfect flying conditions. Part of that same tail was installed directly into an active waterfall on the Haast side of the slip, with mesh panels received by helicopter while the crew worked in the flow. Before the system was signed off, several boulders large enough to exceed the attenuators' design capacity were individually drilled and blasted down to a manageable size, doubling as a live test of the barriers under real impact.
Performance and Legacy
The Diana Falls system has managed multiple rockfall events since completion in late 2014 without unplanned road closures. Regular inspection and maintenance, now carried out by local contractors, has led to changes on the system itself over time, including removing posts on Attenuator 1 that were taking repeated impacts and adjusting the bottom cable on Attenuator 3 to improve debris clearance.
The monitoring data gathered through that inspection programme has also had a wider use. Geobrugg drew on it, in conjunction with Duncan Wyllie of Wyllie & Norrish Rock Engineers, to run over 200 full-scale instrumented field tests over three years, work that showed attenuators behave differently to conventional rockfall barriers, with lower anchor forces and different loading patterns than had been assumed, and that has since fed into attenuator design methodology beyond this one site. Closer to home, the hazard management and access protocols Geovert developed at Diana Falls were carried into the Kaikoura earthquake response between 2017 and 2019, where similar attenuator systems went in under comparable active-slope conditions, though rarely matching the severity of what Diana Falls demanded.
What Good Looks Like
The attenuator concept, developed jointly by Geobrugg and WSP, gave Diana Falls a solution suited to the hazard, but a workable concept and a safely built system are two different things, and the distance between them was covered by construction judgement applied daily, on site, for the length of the project - which drilling method suited this exact patch of ground, whether flying conditions justified lifting a tonne-plus post today, whether the spotter's read on that morning's movement meant the crew should stand down. None of those calls were in the design and were made on site by the people responsible for getting the system built without anyone getting hurt.
As extreme weather events become more frequent, more sites will need protection systems built under conditions like these. Diana Falls is a reminder that the quality of a geotechnical solution isn't decided at the design table alone but by the people who build it on site.