Working within the confines of a Listed Building environment, Innovative demolition techniques were adopted by specialist Gnat UK, when charged with sensitively deconstructing a reinforced concrete multi-celled storage silo located in a historically significant eighteenth-century Shropshire flax mill.
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 1950’s silo, surrounded by several listed buildings, was no longer needed and had to be demolished within its footprint.
The heavily reinforced silo contained a honeycomb network of 15 sizeable rectangular flax storage cells and was a central structure within the group of buildings (some up to 220 years old) forming the flax mill in Ditherington near Shrewsbury. The mill complex, built in 1796, was initially used to process and spin flax before later conversion into a maltings factory producing malt from barley.
Gnat’s Husqvarna robotic demolition machines, mounted on mast climbers erected inside each cell, were used to successfully demolish the 500-tonne structure from the inside out in order to avoid damage to the surrounding grade-one listed structures - situated just a metre away!
Specialist demolition experts from Gnat UK deployed to ensure that 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 yet powerful 1000kg Husqvarna DX140 Robot to remove the 150-millimetre thick cell walls. Concrete debris was allowed to fall to ground level through the narrow 400-millimetre gap left between the 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 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 dismantle and re-erect the mast climbers within the cells repeatedly,” recalls Nick Turnbull. “But our technique proved significantly quicker, safer and less costly than any conventional method.” The full 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 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.”