Explosive justice

A volley of loud explosions rang from the building. Minutes after the dust had settled, the high-reach excavators moved in on one of Christchurch’s last remaining high-rise structures; one that has dominated the city for more than four decades. Toppled to the south of the site, away from the adjacent City Council offices and a busy CBD commuter route, local media reported many witnesses questioning if the implosion had gone to plan as three storeys or more of the building remained.

But any such doubts were quickly dispelled by main demolition contractor, Ceres NZ, and its subcontractor, Controlled Demolition. The latter’s field operations manager Ray Zukowski insisted the implosion had been a success. “The building needed to fall away from Hereford St, with demolition crews keeping that side of the building rigid to pull it away from the road,” he said.

The building was the highest profile structure to be imploded since the Radio Network House demolition, similarly damaged in the earthquakes, three years ago. But there were a number of key – and operational - differences, since RNH was taken down while still in the so-called former ‘Red Zone,’ an area where the public were not admitted. The Police HQ was demolished under strictly controlled access to an otherwise open city centre.

The Police HQ was also brought down with a remarkably small amount of high explosive, Ray said: “Precision was of the essence. Implosion crews charged 185 holes, with just 55 kg (121 lb) of high explosives loaded into the building to bring it down. This was a well-made building... it took a lot of work to get her on the ground.”

Designed in 1968 by the Ministry of Works, as a government building the police HQ was built to more rigorous design requirements than the minimum New Zealand standards of the day. Ceres NZ spokesman Bernie de Vere said that through the Canterbury earthquakes the building performed “very well for its age, was technically repairable but deemed uneconomic to repair due to uncertainty around successfully re-levelling the building.

Purchased in July 2000 by Ngai Tahu Justice Holdings, a subsidiary of Ngai Tahu Property, the latter was faced with demolishing it after considering two traditional methods: soft strip followed by cut-and-crane or implosion.

The second option was chosen. Bernie explained: “The second option was chosen over conventional demolition for a number of reasons. [It afforded] a shorter demolition timeframe, less disruption to neighbours, reduced traffic disruption, improved worker safety (as contractors are not in the building as long) and implosion is the more cost-effective method.”

Originally planned for 30th May at 08:00, with a backup date of 31st May 2015, also at 08:00, the contractor found it was competing for road closures with Christchurch’s Airport Marathon, in which race more than 5,000 runners were due to pass by the building on the allotted day. “The backup time was moved to 17:00, which was mutually acceptable,” said Bernie. “We needed to use the backup day because we had been working through the night earlier in the week removing the three storey podium which surrounded the lower part of the building.”

“This implosion [was] very different from the [Radio Network House]. We [were] no longer working within a CBD red zone. We [had] a large number of existing buildings around us, the river, landmarks and large trees and safety fences [were] set back further.”

Bernie continued: “As is typical of many buildings of its era, the former Christchurch Police Station contained asbestos. We carried out an asbestos removal programme over a 10 month period. All demolition materials [were] handled and disposed of according to industry best-practice. The demolition debris [was] transported using covered trucks and disposed of as per an Environment Canterbury-approved waste management plan.”

The interior of the building was stripped out to expose structural elements to improve post-implosion debris for recycling or disposal and all asbestos was removed. Bernie said that all removal processes were peer reviewed prior to the three story podium that surrounded the tower having been taken down mechanically. Following that, non load-bearing interior partitions and exterior curtain walls were removed on the ground floor and on levels one, two, three, five, seven and nine.

“Using pneumatic rotary percussion drills, columns on the above levels were drilled with a total of 200 horizontal holes 40mm in diameter for subsequent explosive placement. Blasting was designed to ensure the controlled fall of the structure and to soften the structural frame to reduce vibration on impact.”

A double continuous wrap of geotextile fabric and chain link fence was installed as source protection around elements to be blasted and a curtain of similar fence and geotextile fabric was installed around shot floors to contain any materials displaced by explosive detonation. A test blast on two columns on ground level was carried out to determine the minimal amount of explosives required to ensure a successful implosion.

Some 225 non-electric detonators for both in-hole initiation and implosion sequence control were used, along with 2,000 m (6,5617 ft) of detonating cord, and around 60 kg (132 lb) of Orica NZ Senatel magnum explosive 25 mm x 200 mm (1 x 8 inch) cartridges.

To achieve the desired collapse, the majority of the explosives were on the lower blast floors in the building. All charges contained in the bore holes were confined with a stemming material - a combination of sand-filled bags and expandable foam – and no demolition charges were placed outside of a borehole producing an unconfined detonation situation, said Bernie.

A total of eight seismographs were used to monitor ground vibration. “We anticipated that the ground shake would be equivalent to a heavy truck passing by. The heavier particulate matter generated by the implosion [was likely to fall] within 100m of the site, so we imposed an exclusion zone created at 100 m (330 ft) surrounding the building. Historic data indicated that buried constrained utilities under the adjacent streets would not be at risk as a result of the implosion.”

In the event, the building progressively collapsed over a period of eight seconds. Bernie explained: “The rigidity of the structural frame meant the structure rolled onto its side on grade, resulting in a debris pile configuration as high as the building is wide. The majority of the debris impacted within the footprint of the building and the balance well within the demolition site.”

He continued: “It is anticipated that the entire site will be cleared of debris within six weeks of the implosion date. We are taking concrete to Lyttelton Port Company’s reclamation area and we estimate to recover around 600 tonnes of steel. In the event, there has been no debris on streets, and no damage to any underground services or adjacent buildings. The timber and plastic and carpet debris is going to Burwood landfill.”