“After exhaustive searches of every building in the city, the operation turned to recovery and the deconstruction of buildings began. Many buildings had to be carefully pulled apart rather than simply smashed to the ground.”
- Carl Devereux, Regional Director - New Zealand, Aurecon
Complex challenges face communities in the aftermath of a devastating natural disaster. Where do you start when hundreds of kilometres of water and sewer mains as well as roads need addressing, and social housing and major community facilities need rebuilding?
Christchurch also faced a huge number of rockfalls from multiple exposed bluffs which had tumbled down the hillsides; as well as cliffs which had collapsed and an imminent threat of boulders coming loose in outlying areas.
Close to 8 000 properties were deemed to be so badly damaged that they were considered unlikely to be rebuilt. As Christchurch looked at how to recover, a complex series of deconstruction and geotechnical work around the city centre and Port Hills, as well as the rebuild of public infrastructure, took priority. The assessment and repair of horizontal infrastructure such as roads, and sewage and water pipes was prioritised to the worst-affected areas that impacted on the most people.
One of the most urgent requirements was building assessments as people could not safely return to their homes and workplaces until full damage reports were completed. Aurecon engineers flew in from across the world to support the Christchurch City Council (CCC) and the Stronger Christchurch Infrastructure Rebuild Team (SCIRT) to undertake detailed engineering evaluations of buildings, rockfalls, and public infrastructure. It was a process that took two years.
Four senior Aurecon engineers were also seconded into the Canterbury Earthquake Recovery Authority (CERA) to advise on deconstruction and geotechnical issues. This included helping to manage demolitions within the CBD’s red zone for many of the city’s tallest buildings, which were on the verge of collapse. These engineers helped ensure swift, controlled, safe and coordinated demolition. Uncontrolled demolition was considered an ‘extreme’ risk that could have led to further loss of life.
The Christchurch City Council show the reality of the aftermath in a video.
All in all, there were more than 1 200 commercial building demolitions in the CBD alone, including more than 140 tall buildings. CERA’s Significant Buildings Unit (SBU) demolition framework ultimately rewrote the rules for tall building demolitions, including navigating how to safely complete a fast track demolition programme in a highly competitive demolition tender market. The true success of this project can be measured in the safety record of the completed demolitions – despite several major aftershocks, none of the demolitions resulted in serious injuries or fatalities.
In particular, the Hotel Grand Chancellor and the Copthorne Hotel were both described by international experts as the most complex demolitions they had ever seen in a modern city. The Hotel Grand Chancellor was the highest priority for demolition. Only two options were considered: implosion or a combination of cut and crane for the upper levels and high reach demolition for the lower levels. The latter was chosen and required close oversight by SBU engineers.
The legacy of this work is the SBU methodology for assessing high rise building damage and prioritising buildings for urgent demolition. Global demolition contractors now also have a better understanding of how to demolish earthquake damaged structures, while Christchurch was able to start to move forward.
"The question of whether or not to demolish a building is one of the toughest decisions an engineer has to make . . ."
Aurecon’s Carl Devereux walks through the Hotel Grand Chancellor in the aftermath destruction.
The situation in the Port Hills suburb was unique due to architecturally designed properties, which utilised construction material not common in the NZ industry and requiring differing deconstruction techniques. As these sites were in residential areas, demolition works had the potential to affect the general public and the impact on the only road in and out of Sumner.
Working closely with CERA, the Aurecon Geotechnical Team led by Jan, applied a Safety in Design approach, which promotes a zero harm culture and innovating to achieve this. Demolition solutions included cranes, helicopters and remote-controlled machinery. Other innovations included collaborating with the University of Canterbury for the use of an unmanned aerial vehicle (drone) to collect information from high-risk areas using high-definition cameras and advanced GIS field data capture systems.
In the hours following the February 2011 quake, it became apparent that landslides and tumbling boulders were a major hazard in Port Hills. 714 properties were red-zoned because the risk of further earthquake-triggered rockfalls was high.
Rapid response teams were mobilised to respond quickly after major aftershocks to inspect the region for further damage and rockfall. The Port Hills Geotechnical Group (PHGG), which comprised six consultancies managing nine sectors, was established to mitigate geotechnical risks, including rockfall, cliff collapse, landslides, and large retaining wall failures.
Aurecon’s Camilla Gibbons, together with her team of ten, were responsible for the rockfall, landslide, and cliff collapse work in two key sectors of Port Hills – one, which covered approximately one-third of the area; and the second, which displayed the most severe geotechnical damage.
Despite being under significant pressure to reopen some local croquet pitches, Camilla maintained they would not be opened until an inspection of the cliffs had been carried out. During a June aftershock, the cliff did fail and over half of the croquet pitches were inundated with slip material and large boulders.
Christchurch suffered from thousands of aftershocks and Camilla continued to inspect the most vulnerable areas in Port Hills after the earthquake in June 2011 which, from a rockfall perspective, was more damaging than the February quake. New sizeable failures and cracking occurred after a relatively minor 5.6 magnitude earthquake in June 2011 and she immediately ordered a partial closure of the main road to the Sumner township. The speed of her action was lauded when a larger 6.3 magnitude quake hit an hour later causing catastrophic failure of the 75-metre high cliff face. 15-metre thick sections collapsed on to the portion of the road that she had just closed to traffic. Thanks to her action, nobody was hurt and no vehicles were damaged.
Critical to a flourishing future city is the safety of its children. At the time of the 2011 quake, there were 215 schools across the region educating more than 76 000 students.
With 90 per cent of its buildings framed in timber, the New Zealand Ministry of Education decided to commission destructive testing of two standard types of timber-framed school buildings in the aftermath. These buildings performed very well in the earthquakes, with no major structural damage caused by ground shaking. Building damage, if any, was caused by liquefaction rather than shaking.
In late 2012, senior Aurecon structural engineering staff joined the New Zealand Ministry of Education’s Engineering Strategy Group (ESG) to provide technical leadership during the destructive testing process.
While it was widely accepted that timber frames have a relatively low risk of sustaining potentially life-safety hazard damage during earthquakes, engineers had previously given overly conservative results when determining earthquake resilience.
The tests confirmed that timber-framed buildings have a strength and resilience significantly greater than previously calculated and, in many cases, in excess of 100 per cent of the New Building Standard. They also indicated that a factor of two can conservatively be applied to the calculated strength of timber-framed buildings.
Today, the Ministry’s approach to assessing earthquake resilience of all school buildings uses the new calculation tools developed by Aurecon engineers and their ESG colleagues. This is saving hundreds of millions of dollars, which can now be spent ensuring classrooms are fit for 21st century teaching and learning in support of the future leaders of Christchurch – its children.
Here is the video of the earthquake resilience testing of timberframed school buildings.