Many people might be surprised to learn that a medieval motte-and-bailey castle once stood in the centre of Sheffield, near the confluence of the rivers Sheaf and Don.
But Sheffield Castle was a prominent landmark and an important local stronghold. In fact, it was effectively the birthplace of the city of Sheffield which grew up around the castle and took its name from the river Sheaf.
Despite its prominence, Sheffield Castle was destroyed and completely obliterated by the Parliamentarians during the Civil War.
Virtually no drawings or official records of the castle survive, although archaeological investigations in the 1920s unearthed some of the original foundations.
By then, the site had been completely redeveloped and an indoor market built where the castle once stood. The only reminder of its past history was the name – Castlegate – and a small section of the excavated castle foundations preserved for viewing within the new Castle Market building.
Now the site is being redeveloped once again, this time to create a new public green space. Soon, more of the castle’s remains, including the base of the original gatehouse and part of the moat, will be disinterred along with a stretch of the river Sheaf that has flowed unseen through a concrete culvert for the past 110 years.
Once complete, the Castlegate site will be transformed into a public park area with remains of the 12th century castle on display for the first time since it was demolished in the late 1600s. A 500-people capacity events space will also be created on the site of a former 17th century bowling green, with heritage interpretation boards and public art giving further nods to the site’s historic past.
Contractor Aureos (previously Keltbray Infrastructure Services) was awarded the main contract by Sheffield City Council and moved onto the site of the derelict market in January 2024 to begin phase 1 of the works, clearing existing structures. Phase 2, which is now underway, involves the delicate task of opening and ‘naturalising’ the culverted section of the river.
The concrete culvert, dating back to the early 20th century, 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 concrete walls are up to 900mm thick and sit in trenches excavated into the bedrock beneath the river bed.
The 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.
“The roof slab is one continuous slab of reinforced concrete about 12m wide and 250mm thick,” says Conor Anstey, sub-agent with Aureos. “Removing it has been quite an elaborate process.”
The sequence of demolition involved taking out the roof in 3m segments between the concrete arches. Each segment, measuring 12m x 3m was then subdivided into four 3m x 3m sections. Aureos cored 24mm diameter holes approximately two-thirds of the way through the roof slab and resin-fixed four lifting points to each of the sections.
Then, with the weight transferred to a crane, Aureos cut around the perimeter of the slab, allowing the 3m x 3m section to be lifted clear.
As the 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 four squares, we dropped a prop in to brace the walls,” says 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.”
Although culverted in a concrete channel for several decades, the river Sheaf is still a natural watercourse and considerable care is being taken to prevent unnecessary disturbance or pollution of the river during the works.
Additionally, for health and safety reasons, Aureos is at pains to minimise the amount of work carried out in or above the river itself. To assist this, 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.
“The hydraulic rams were rotated by 180° at the request of Aureos due to restricted access to the valves once the struts were installed,” explains Robert Powell. “We supplied a set of long hydraulic hoses per strut, allowing them to be left connected after installation. This removed the need to re-connect them for adjustment or removal.
“The hoses are secured at ground level and 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,” he adds.
Along one side of the river are existing buildings running continuous with the culvert walls. Consequently, any movement of the walls risks damaging these structures. Real-time load monitoring has therefore been installed to keep an eye on the axial loadings on the Groundforce props and warn of any increase in these loads.
“Our system uses load cells that replace the standard steel pins connecting the prop swivel assemblies to the rest of the prop,” says Powell. “The load pins send a signal to a receiver, providing 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,” he adds.
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 – most likely Corten or a similar ‘weathering’ steel members. Elsewhere, U-shaped pre-cast concrete base-and-wall units will be lowered into the river leaving a clear space above.
Before these go in, up to 1.5m of accumulated silt must be removed from the river bed. But 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.
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