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.
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.
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.
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.
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.
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
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.