02 November 2015

Matt Philp

Engineering Insight: Roads out of Rubble

  • An overview of the Crater Rim Bluffs with a core drilling rig at the summit. Photo: Aurecon.
  • An engineer assessing infrastructure damage at a rockfall site on Sumner Road. Photo: Christchurch City Council.
  • Rockfall damage to Summit Road, following the 22 February 2011 earthquake. Photo: Julian Thomson, GNS Science.

When the walls came down on Sumner Road, a vital lifeline in and out of Christchurch was cut off. Four years on, plans are in place to reopen the road.

It was a “miracle escape”, one of the more high profile experienced in Christchurch on 22 February 2011. A BP fuel tanker heading to Port Lyttelton over Evans Pass narrowly missed being hit by an avalanche of rock from the crater rim bluffs. A photo soon after the earthquake shows the vehicle abandoned, the road ahead blocked by debris, a massive boulder nestled against the rear bumper. The driver ran for his life.

The loss of Sumner Road, that spectacular ribbon of tarseal that descends 2.6 kilometres from above the harbour at Evans Pass to Lyttelton, was a blow to Christchurch in the aftermath of the quake and has been a thorn in the side ever since. Oversized vehicles that previously relied on the route have been forced to make the longer and more difficult journey over Gebbies Pass. Other Port traffic has been able to use the Lyttelton tunnel, but that often necessitates temporarily closing it to the public. More poignantly, a much-loved scenic link between Sumner and Lyttelton has been sundered, and the city has lost an important lifeline in the event of another civil emergency.

Unsurprisingly, the news that the corridor will be reopened has been warmly received. In August, Christchurch City Council went to the market signalling it will soon be looking to procure design and construction expertise, with an expectation of starting work midway through 2016. It’s a huge project, with up to a million cubic metres of rock to be removed. Significant natural features will be affected, notably the dramatic crater rim bluffs, as well as several native species of flora and fauna, some of them endangered. Moreover, much of the work will be undertaken in a hazardous environment, on steep, mobile slopes that are constantly shedding rock. 

Project Manager Lynne Armitage says it’s going to be challenging. “This is not a business-as-usual project, where everything is neatly defined when you start. With an emergency response like this there are a lot more steps to go through. It’s complex.”

Cut off

Before getting into the detail, it’s useful to recall what happened to Sumner Road on 22 February, and how its loss complicated the response in the immediate aftermath of the quake. Peter McDonald, now of the NZ Transport Agency (NZTA), was the Council’s Operations Manager and part of the team co-ordinating the emergency response. He notes the route had only reopened a couple of months earlier, after the September earthquake unleashed rockfall near the top of the road. Work was continuing at Windy Point, where an area of rock had been identified as a risk for the coal facility below. 

In the hours after the second quake struck, the reports coming into the emergency response centre were that Lyttelton was cut off. The route to Governor’s Bay had been hit by rockfall at Rapaki, and there were slips on the north side of the tunnel. Sumner Road was the only alternative route to the city, but the news there was even worse.

“We were aware there’d been a significant rockfall. In fact, it’s amazing no one lost their life.”

“We were aware there’d been a significant rockfall,” Peter says. “In fact, it’s amazing no one lost their life. Our first response was just to close the road and get on with other things. We always work from the point of view of ‘protect life, protect property’, so we started with the city’s lifelines. We needed access to the hospital and the airport.”

Opening a route to the port at Lyttelton was also vital, and a team soon began clearing rockfall from the city end of the tunnel. Barriers were erected at either end of Sumner Road, effectively removing it from the network. The barriers have since been fortified, with Christchurch Earthquake Recovery Authority (CERA) using its powers to declare an exclusion zone. Even so, “thrill seekers” continue to use the road, according to Peter. “Unless you station someone there 24/7, you can’t stop them; but they go at their own risk.

“Sumner Road was always the hazardous and over-dimension route into the city. Immediately post-earthquake, that became an issue, with some of the heavy gear bound for the city unable to come through Lyttelton.” 

The ongoing closure is a burden for the port and the public. “It’s meant that we’ve had to use the tunnel to get fuel supply etcetera into the city. NZTA has had to allow for hazardous goods traffic through there, which requires special arrangements because of a lack of inundation firefighting capacity in the tunnel.”

Meanwhile, over-dimension vehicles are being re-routed through Dyers Pass or Gebbies Pass. “Dyers Pass has some bends and gradient that doesn’t make it ideal for some over-dimension vehicles, plus it goes through a residential area on the city side. Going all the way through Gebbies Pass adds 15 or 20 kilometres, which is a significant extra distance for transporters. What’s more, some areas of that route are narrow; you have to essentially stop traffic at some of the bends because they need the full carriageway,” Peter says. 

Even without those issues in play, it’s vital to reopen the road in case of another major civil emergency, he says: “If anything happens to the tunnel we need that alternative lifeline and that critical over-dimension route.”

Geotechnical hazards

A lifeline, he calls it. Yet, until the last couple of years, there was doubt Sumner Road would even reopen. Lynne says two things gave people pause: could you solve the rock hazard? What would it cost? “The only way to answer that was to drill into the ground and investigate.”

Seen from across the harbour, the landscape of Sumner Road is dominated by the towering crater rim bluffs in the centre, tapering to less severe slopes at Evans Pass and Lyttelton. Geotechnical engineer Ian Wright, the Council’s Principal Hazard Advisor for the Port Hills, describes it as resembling a croissant, with the more complex hazard packed into that middle zone. 

“At the ends of the croissant you have small, discrete outcrops that continue to shed isolated boulders – a rockfall risk. But in that central section, you have 100-plus metres, tier upon tier, of stacked volcanic sequences. With another large quake, or even possibly heavy rain, you’ll see a rockfall swarm, or a cliff collapse, where a large volume of rock will go flying down the hill as a debris avalanche,” Ian says. 

To understand what it was dealing with at Sumner Road, the Council commissioned site-specific investigations by Aurecon, aided by ground condition analysis of the wider area undertaken by GNS Science. Two kinds of rock dominate in the Port Hills: basalt lava and basalt breccia. The former are largely intact lava flows of high mass strength, Ian says.

“When columnar basalt cools, it forms lovely joint patterns. When it breaks it fails along those joints into discrete, hard missiles that tend to stay intact and travel further, whereas the breccias are more like sandstone; when breccia rock hits something it tends to shatter into pieces.”

“It’s no birthday cake with lovely set layers. We drilled three holes, 20 to 50 metres apart, and the geologists could not pick a continuous layer of rock between them.”

If life were simple, these two types of rock would dominate specific areas above Sumner Road and you’d be able to tailor solutions to fit. Unfortunately, the reality is the ground conditions are “a mishmash”, according to Ian. “It’s no birthday cake with lovely set layers. We drilled three holes, 20 to 50 metres apart, and the geologists could not pick a continuous layer between them. Even over short distances the geology up there changes.” 

The earthquakes have added a further complication, according to Chris Massey of GNS: “Because the slopes have been through multiple earthquakes, the condition of the rocks is much worse than before. They are broken and dilated and their mass strength has changed. The result is we are getting much larger volumes of debris coming off the slopes than before the quakes.

It’s a mess, in other words. Ian says at both ends of Sumner Road – towards Evans Pass and near Lyttelton – it will be possible to tackle the rockfall risk at source. “We’ve mapped the individual problem boulders that are sitting as part of an outcrop. For those boulders we will either move them into an area of low slope so they can’t take off, or we will deconstruct them using low velocity explosives – ‘rock pop’ them.”

However, the central bluff area is far more problematic. “I’ve looked at it from a helicopter and there’s a great amount of cracking there and a huge volume of rock. You couldn’t get anybody up there to knock off individual rocks, or bolt them back into the face. Some of those rocks are the size of buses.”

Thinking big

Instead, a plan began to form around the end of 2013 to treat the bluffs as a one-rock mass. “We have to think big. It sounds drastic, but the idea is that we are just going to take that entire hazard away – bulk earthworks.”

First, however, an agreed scope of works, funding and consents needed to be nailed down. Enter the NZTA, which has committed to funding the reinstatement of Christchurch’s council-owned roads at a rate of 83 per cent of eligible works.

Initial council estimates for reinstating the entire Lyttelton/Sumner Corridor, which includes remediating several sites on the Sumner side currently protected by shipping containers, came to $108 million. But that eye-watering figure included major road realignment on the city-side, which further investigation has determined won’t be required. The budget agreed between the NZTA and council now stands at $60–80 million, with $40–60 million of that earmarked for Sumner Road, which is – by far – the largest and most difficult of the corridor projects.

That figure is predicated on the goal of reinstating Sumner Road as it was pre-quake. There is no budget for widening the road, for example. That said, small improvements aren’t out of the question, according to Mark Yaxley, NZTA Regional Planning and Investment Manager, Canterbury and West Coast. 

“On the route down to Lyttelton where they have to build retaining walls, clearly you’ll build it so you can get a bit of widening if you can. But the intention is to restore functionality.”

As elsewhere in Christchurch, there’s been a strong drive to find efficiencies. Spoil will be trucked to the nearby quarry at Gollan’s Bay, but any reuseable red rock will be set aside to rebuild historic retaining walls at Lyttelton and Sumner.

As well, there’s an understanding the road, which is likely to be designated a no-stopping zone, can’t be made entirely risk-free, says Mark. “It’s about doing enough to reduce the risk while getting the road open to a suitable standard. There are a huge range of options, from a minimum to some serious bulk earthworks. We’ve worked with the Council to find the sweet spot in the middle.” 

He concedes no one knows exactly how much it will end up costing – hence that $40–60 million range. “As the Council has collected more geotechnical information, it’s been able to improve its design assumptions. We’ve agreed a staged solution with the Council that allows us to make decisions as work progresses. But it’s a pretty tough environment to model a perfect solution.”

What about consenting? Ordinarily, this kind of project would have to be publicly notified; there would be a long hearings process, possibly followed by an Environment Court hearing. By consenting it under order in council, the Christchurch City Council has been able to fast-track the process, with a commissioner making the final decision. Of 141 public submissions on the plan, 95 were in favour.

But there are some complications. The crater rim bluffs are a nesting site for an at-risk bird species, the New Zealand pipit, so it’s been agreed major work won’t take place during the nesting season. More problematically, the area is also home to three at-risk species of lizard: the Canterbury Gecko, the Jewelled Gecko and the Common Skink.

The lizards’ presence requires the council to obtain a wildlife permit from the Department of Conservation (DOC), which administers the land above Sumner Road. The Department’s Partnerships Ranger, Rachel Brown, says the department has adopted a collaborative approach, agreeing a plan with the council to relocate 200 lizards from the bluffs to predator-free Riccarton Bush, where it’s hoped they’ll establish a replacement community. In addition, there are plans for habitat enhancement at an important lizard site along Port Hills.

Engineering solutions

So this is where the Sumner Road project now stands: consented, funded, and with an engineering solution in mind. Dealing with the rockfall hazard is the priority before any work starts on the road itself, says Lynne Armitage. Blasting and “scaling” – removing the relatively easy surface material – will be favoured at the edges. But for the central bluffs, the solution will be major earthworks – cutting the slope back and “benching” the rock.

“It’s got to be a top-down approach. The top of the bluffs is 100 metres above the 130-metre-high talus slope below it, which is three times higher than your average office building, so you can’t reach it from the ground and you don’t want to be working under those hazards. So the idea will be to zig-zag down the slope from an existing track off Summit Road, cutting a series of roads, or benches.”

“It’s got to be a top-down approach. The top of the bluffs is 100 metres above the 130-metre-high talus slope below it, which is three times higher than your average office building.”

The finer detail will be worked out by contractors, with the council looking to procure services before year’s end. Lynne describes the solution as “standard engineering and earthworks”, adding mining companies and others familiar with mitigating rockfall hazards are the most likely tenderers.

“We’re not being too prescriptive,” she says. “We’ll be sharing all our investigative work, our concepts, how we’ve assessed the risk, etcetera, but it’s the contractors who will bring the real expertise to the table. We know what we want to do, but the contractors will know the best way to do it.”

Once the rockfall hazard has been tackled, the road itself will be repaired. Lynne says in many places the damage is surface-only and relatively easily repaired. But there are a few “hotspots” where half the road has disappeared that will require significant reconstruction. “You can either clip on a solution, or we may need to cut back some rock on the inner side of the road. We don’t know yet. We’re about to start a ground investigation to see what competent ground we have left.”

“We are, however, acutely aware of the need to return a certain level of service to the road, because it is a key over-dimension freight route from the port. We can’t change the nature of the road, or make it narrower. We have to get back what we had.”

Or maybe something better. By its very nature, the work mitigating rockfall hazard should make Sumner Road more capable of withstanding a future disaster. Wearing his old emergency response hat, Peter stresses the importance in post-quake Christchurch of the “R” word -– resilience. “If there ever is another event we want to be able to have that route open within a couple of days,” he says.

Hopefully it will never be tested. 

To prioritise future investigations, each mass movement has been categorised based on the nature of the hazard and the consequence of the hazard occurring. Class I (high priority) hazards could potentially result in loss of life and severe damage to dwellings and/or critical infrastructure, which may lead to the loss of services for many people. Class II (intermediate priority) hazards have the potential to affect critical infrastructure, as well as severely damaging dwellings. Class III (low priority) hazards have the potential to cause only minor damage to dwellings and local infrastructure.
Roads out of Rubble