Demolition contractor John F Hunt needed help from Japan to complete its latest scheme and is ready to share the technology, subject to market rates. Paul Thompson reports
IMMINENT nuclear war in Europe seems like a paranoid fantasy nowadays, but the fact is that for most of the last 50 years Europeans, both east and west of the Iron Curtain, lived under that fear. Both sides took huge measures to ensure their ruling elite could survive a nuclear strike. Britain was no exception.
Huge underground bunkers with high-strength reinforced concrete walls several metres thick were built throughout London, ready to shelter those in the seats of power in the event of an attack.
With the end of the Cold War, the need for many of these massive underground constructions has gone and the time has come to remove them. But, as demolition contractor John F Hunt found out when trying to break out one of these bunkers at a Sir Robert McAlpine development in central London, that is something easier said than done. The contractor has imported some Japanese know-how to help.
'The concrete is incredibly strong with a minimum strength of 90N/sq mm. It must have been a very hightech mix for 1948, ' says contracts manager Glen Clark.
Because of the strength and robust design, the team had to look at less orthodox methods of demolition, a search that took them to the land of the rising sun and a huge concrete cracker originally designed to help out in the aftermath of the Kobe earthquake.
The massive jaws of the 9 tonne S-90X concrete cracker from NPK had helped to power through steel and concrete following the earthquake in Japan, making fallen buildings safe and dropping the remaining structures.
'There are only three of these cracker attachments in the world at the moment. We brought one of them over and did a few tests on sections of a bridge we had in our yard and it performed brilliantly, ' says Mr Clark.
And so it should. With an optimum cracking force of 575 tonnes and a 2 m jaw opening, the 3.8 m-long monster makes short work of Brazil nuts and reinforced concrete alike.
'The tests we did at the depot revealed that a 1.6 m jaw opening gave the optimum crushing power. It can handle larger dimensions, but we felt it worked best within that range, ' he says.
Boom-mounted on a 55-tonne Liebherr 954 excavator, the optimum jaw opening meant that Mr Clark and his team would first have to spall off 1 m of mass concrete from the top of the bunker roof. Then the NPK cracker could munch away the remainder of the 2.6 m-thick section.
In order to peel back the first metre of the bunker, Hunt's brought in coring specialist Robore Cut to drill through the entire structure both horizontally and vertically so that pipe bursting techniques could be used on the mass concrete.
'The theory was to drill through the slab and the walls until we reached the first layer of mesh reinforcement at about a metre in, ' says Mr Clark, 'then the broken material would be shelled off before the cracker broke away the rest.'
But NATO nuclear bunkers are made of sterner stuff and Mitcham-based Robore Cut found that the high-strength concrete and design of the bunker wore the drill bits out extremely quickly.
The 1948 design had included solid granite setts placed in the concrete to strengthen it against nuclear attack, a structural gimmick that Mr Clark admits 'was a bit of a surprise'.
But the surprise meant that the diamond-tipped drill bits were constantly having to be replaced and the diamond configuration changed. It also led to a rethink on the length of the cores.
'Initially they were to go through the structure but we moved away from long cores and settled on 1.5 metre deep vertical drilling to shell off the outer layer, ' he says.
Even then the strength of the concrete used caused problems with overstressing the bursting rigs, which had to be adapted to cope with the high-strength material.
'Hoses had to be braided to stop them splitting and the rigs were tweaked to cope with the strain.
There were 24 bursting rigs on site at one time, ' according to Mr Clark.
Eventually, though, drilling works were completed with the 200 mm diameter holes passing through the structure and the outer layer of mass concrete removed. With the main structure broken down to the 1.6 metre optimum dimension, the £120,000 cracker was brought in to begin its painstaking task of breaking through the remaining concrete.
Although rebar mesh was present, according to Mr Clark, the volume was surprisingly not that extensive.
'The main volume was in the bottom of the slab, where there was quite a tight mat on the inside of the chamber to absorb the deflection of the slab in case there was a hit, ' he says.
But breaking through reinforced concrete of any strength is a noisy business and when the site is in the legal centre of the capital and actually abuts the offices of eminent barrister Michael Mansfield QC, then noise disturbance must be kept to a minimum.
In an effort to limit noise from the site, Hunt's project team drafted in experts from Applied Acoustic Design who came up with the idea of erecting an enclosure around the entire site.
Initially this 'Acoustic Enclosure' was to be constructed using a 50 mm wood/mineral wool sandwich panel, but difficulties in spreading the loading of the panels forced a rethink.
'Our scaffolding subbie, SHS Scaffolding, was fantastic. The structural design of the enclosure changed four times in all to suit the scheme, but they accommodated every change, ' says Mr Clarke.
Eventually a design with an internal face of 18 mmthick orientated strand board, lined with 100 mm-thick mineral wool, and a 1,500 mm cavity between the outer 18 mm plywood skin was used that dramatically cut noise outside the enclosure by as much as 30 decibels.
'Noise from the site was constantly monitored by Corporation of London Environmental Health. They could not believe how quiet the works were. It also meant we could get a full eight-hour working day in.'
The NPK beast made short work of nibbling through the remaining section of bunker, along with a little help from other pieces of kit, John F Hunt were able to clear the bunker within the 22-week contract period allocated to them.
John F Hunt business development manager Alison Agg confirms the 9-tonne cracker is available for hire, so the next time you need a 2.6 m-thick reinforced concrete cold war nuclear bunker removing, you know where to go.