Innovative demolition techniques were adopted by specialist Gnat UK, charged with sensitively deconstructing a concrete multi-celled storage silo located in an historically important eighteenth century Shropshire flax mill.
The combination of using a robotic demolition machine, mounted on a mast climber erected inside each cell, was used to successfully demolish the 500 tonne structure from the inside out, without causing any damage to the surrounding grade-one listed structures just a metre away.
The heavily reinforced concrete silo, containing a honeycomb network of 15 large, rectangular, flax storage cells, was a central structure within the group of up to 220 year old buildings forming the flax mill in Ditherington near Shrewsbury. Built in 1796, the mill complex was originally used to process and spin flax before later conversion into a maltings factory producing malt from barley.
The main grade-one listed, five storey structure boasts international importance as the world’s first iron-framed building and the currently derelict complex is being restored.
The rectangular 21 metre tall 1950s silo, surrounded by several listed buildings, was no longer needed and had to be demolished within its own footprint. Specialist demolition experts Gnat UK were deployed to ensure that client English Heritage’s stringent ‘no external damage’ criteria could be met.
“The silo had to be deconstructed in small pieces, working from within the maze of cells, themselves reinforced by a network of internal concrete columns,” says Nick Turnbull, Managing Director of Gnat UK. “It was impractical to erect internal scaffolding so we came up with the idea of securing access with mast climbers positioned in sequence inside every cell.”
Onto each of the three mast climber platforms Gnat positioned a small 1000kg, yet powerful, robotic DX140 machine to remove the 150 millimetre thick cell walls. Concrete debris was allowed to fall down to ground level through the narrow 400 millimetre gap left between platform perimeter and cell sides.
The 4 metre wide platforms were initially raised 19 meters to deconstruct the silo’s roof. The mast climbers were then slowly lowered inside each cell removing the walls top-down in a carefully controlled sequence to ensure the silo’s continuing stability.
Debris from each cell had to be totally removed from the silo floor area at the base of the silo before the mast climbers could be moved between cells.
“It was a complicated deconstruction procedure, each cell would be reduced in height by 5 meters and we had to repeatedly dismantle and re-erect the mast climbers within the cells,” recalls Nick Turnbull. “But our technique proved significantly quicker, safer and less costly than any conventional method.”
The total 20 metre wide silo was encapsulated by protective sheeting to reduce dust and noise. But existing underground services around the structure prevented the erection of any external cranes or heavy plant.
“Gnat proved a professional and very competent contractor,” said English Heritage Project Manager Andrew Patterson. “They came up with a clever and very successful demolition technique that caused no damage at all to our listed buildings less than 2 metres away.”
“We were working to a very tight timescale,” he added. “Gnat’s relatively fast solution proved invaluable as conventional demolition by hand would undoubtedly have taken much longer.”
The 60 year old concrete silo, encapsulated by protective sheeting to reduce dust and noise. The 21metre tall construction was situated just metres from important listed buildings