Contractor Feature

Contract to BLOW UP Christchurch Police HQ

Beyond economic repair after the devastating earthquakes in Christchurch almost five years ago, a fatally damaged icon of the city’s skyline finally succumbed to demolition. CHRIS WEBB was there as the 13-storey former Police Station was imploded.

IT TOOK JUST 10 SECONDS after a peal of loud explosions rang from the building for one of Christchurch’s best known landmarks, the former Police Station, to come to rest. Minutes after the dust settled, the high-reach excavators moved in on one of the last remaining high-rise buildings due for demolition after the 2010 and 2011 earthquakes.

A legacy of an era when the unthinkable was deemed to deserve rather less design rigour than is the case today, the building had dominated the city for more than four decades. But, it had performed relatively well in the earthquakes and, toppled to the south of the site, away from the adjacent City Council offices and a busy CBD commuter route, still three storeys or more of the building remained to be tackled by more mechanical means.

Felled by main demolition contractor, Ceres NZ, and its subcontractor, Controlled Demolition (CD) at the end of May, both say the implosion went according to plan. CD’s field operations manager Ray Zukowski explains it was supposed to fall away from Hereford Street and the local government offices and other assets there, so that it was essential to keep that side of the building rigid in order for it to fall to the south side of the road. It had done so.

Demolition Police station Contractor
It took just 10 seconds after the peal of explosions for the former Police Station to come to rest

The building was the highest profile structure to be imploded since the Radio Network House demolition, similarly damaged in the earthquakes, three years ago.

There were a number of key – and operational – differences, since Radio Network House was taken down while still in the so-called former ‘Red Zone’, an area where the public were not admitted. The Police Station was demolished under strictly controlled access to an otherwise open city centre.

The building was also brought down with a remarkably small amount of high explosive, Zukowski says.

“Precision was of the essence. Implosion crews charged 185 holes, with just 55 kilograms of high explosives loaded into the building to bring it down. This was a well made building … it took a lot of work to [raze it] to the ground.”

Designed in 1968 by the Ministry of Works as a government building, it was built to more rigorous design requirements than the minimum New Zealand standards of the day. Ceres NZ spokesman Bernie de Vere says 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 traditional methods: ‘soft’ demolition, and cut-and-crane; and ‘soft’ demolition, and implosion.

The second option was chosen.

De Vere explains: “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 May 30 at 8am, with a backup date of May 31, the contractor found itself competing for road closures with Christchurch’s Airport Marathon, in which more than 5000 runners were due to pass by on the allotted day.

“The backup time was moved to 1700, which was mutually acceptable,” says de Vere.

“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.

“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 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.

De Vere says that all removal processes were peer reviewed, prior to the three storey podium, which surrounds 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 2000 metres of detonating cord, and around 60 kilograms of Orica NZ Senatel magnum explosives in 25mm x 200mm 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, says de Vere.

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 100 metres of the site. So we imposed an exclusion zone created at 100 metres 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, says de Vere. “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.

“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.

“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.”

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