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Groundforce supports river revival
Contractors redeveloping the old Castlegate Market site in Sheffield are exposing a long-hidden stretch of the River Sheaf with help from specialist hirer Groundforce Shorco.
The city centre site is being redeveloped by main contractor Aureos to create a new public park area, with remains of the 12thCentury castle on display for the first time since it was demolished in the late 1600s.
The project involves the delicate task of opening and ‘naturalising' the culverted section of the river which flows along one side of the site.
The concrete culvert, dating back to the early 20thCentury, is around 90m long and is lined with mass concrete retaining walls topped with a continuous in-situ cast concrete roof slab. The culvert itself is about 4m deep but there is approximately 1.5m of silt lining the river bed.
The 12m-wide, 250mm-thick roof slab not only capped the river but also provided lateral support for the walls of the culvert. Additional support was provided by concrete arches cast at 3m centres along the length of the culvert.
Aureos dismantled the roof in 3m segments which were cut out and lifted clear with a mobile crane. As each roof was broken out, the exposed wall sections were braced using modular hydraulic props supplied by specialist hirer Groundforce Shorco.
"As soon as we'd taken out one section we dropped a prop in to brace the walls," says Aureos sub-agent Conor Anstey. "The first couple were a bit tricky but we quickly got the hang of it and it was pretty seamless after that."
In total, 18 of Groundforce's MP150 hydraulic props have been used to brace the sides of the culvert, using modified versions of the company's Mega Brace as a spreader-beam against the walls, as Groundforce principal engineer Robert Powell explains:
"The Mega Brace is modular and comes in various lengths. The Castlegate scheme used the 1.5m long extensions, which fit nicely between the existing concrete ribs of the culvert structure.
"The beam extensions have fins to the front face, usually used to connect the extensions together to form a longer beam, but the distance between the fins was not sufficient to accept the end bearing plate of the proposed hydraulic struts. We added some welded blocks to the face of the beam, just deep enough for the strut end bearing plate to sit in front of the fins and allow connection to the beam."
Groundforce has rotated the barrels of the MP150 props to allow access to the hydraulic rams from above, rather than from the side in the standard fashion.
"We supplied a set of long hydraulic hoses per strut, allowing them to be left connected after installation," explains Robert Powell. "The hoses are accessible for connecting to the hydraulic pump from the safety of solid ground at the side of the culvert. This removes the need to work from height while providing a method of access over the culvert."
Existing buildings abut the outside wall of the culvert for its entire length and any movement of the walls risks damaging these structures. Real-time remote load monitoring has therefore been installed to provide real-time readings of the axial prop loads which can be accessed from an online portal so the loads can be monitored remotely.
"This gives piece of mind that the props are operating within their design limits and the actual loads are in line with anticipated loads from the geotechnical analysis," says Powell.
All 18 of the Groundforce props are now installed and the roof completely dismantled, exposing the river for the first time in decades. But before the props are removed, permanent support for the culvert walls must be provided.
The plan is to remove the old concrete arches and replace some of these with permanent props. But before these go in, up to 1.5m of accumulated silt must be removed from the river bed.
Once the permanent supports are installed, the river itself will finally be ‘naturalised' with the addition of weirs and fish passes to encourage wildlife to recolonise the river and bring the natural world back into the city centre.
ENDS