The Holly Street Development is one of the most well-known in London, as it has experienced significant government and private-sector funding to regenerate what was once one of the most run-down, notorious estates in Hackney.

A few years ago, the first of four 21-story tower blocks was demolished. It was then decided that a second block would be refurbished into modern-day accommodations. With the remaining two blocks yet to be demolished, advice was given to Hackney Borough Council that it was not possible to demolish the units by explosives and that they should be dismantled conventionally. Controlled Demolition Group (CDG), Yorkshire, tendered for the work and was awarded the contract.

After investigating the project in some considerable detail, CDG proposed to the client that explosives could be used, and that the benefits would be significant in terms of safety to the workforce and a shorter demolition schedule. This, in turn, would allow the Council to realize a shorter project-completion timetable and considerable financial savings.

The Council had two major concerns. First, two cast-iron 48" Transco gas mains were situated only 12 meters from one block (and only 1 meter deep), and these served the majority of gas needs throughout the city of London. The second concern was that new 4-story apartments were being constructed some 10-12 meters away. The construction was well advanced, with only the interior fitting-out to be completed.

The new apartments were also owned by Hackney Borough Council, and having worked with CDG on some 15 previous projects in Hackney, they took the view that if CDG said a safe blowdown could be accomplished they would take the chance. They also agreed to underwrite the cost of the additional protection that would be required.

After consulting with Transco and giving vibration and air overpressure predictions based on previous similar projects, both Hackney Council and Transco gave their approval. Controlled Demolition Group was required to install protection to the gas mains to mitigate the risk of impact damage and to reduce vibration levels.

Two different techniques were required to collapse the buildings. One structure had to fall almost vertically to fit between the new apartments, and the other had to be tipped away from the gas mains. This was achieved by using two different delay sequences. A total of 2,000 charges were placed in each building, with each charge being initiated with non-electric shock tube detonators. Non-electric detonators were chosen for their obvious advantages when working in built-up urban areas where the risk of premature initiation from stray currents could be significant.

On the day of the demolition, 1,200 properties were evacuated by the client. Some 80 police were involved in controlling traffic and road closures, and CDG provided 50 sentries to guard the exclusion zone.

At 12:00 noon on Sunday, March 11, 2001 the two blocks were detonated and collapsed exactly as planned, with no damage whatsoever to any of the surrounding properties. Transco immediately carried out checks on the gas mains and confirmed there was no leakage. Field seismographs indicated that the actual vibration and airblast levels generated were well within the predicted limits given at the outset of the project.