A mind-boggling lack of foundations and a previously undiscovered floor were not the only challenges at this historic city centre site.
Project: Sheffield Cathedral Gateway Project
Client: Sheffield Cathedral chapter
Contract value: £2.9m
Region: Yorkshire & the Humber
Main contractor: William Anelay
M&E subcontractor: Quartzelec
Start date: February 2013
Completion date: April 2014
Built as a medieval parish church, Sheffield Cathedral has seen construction works in every century since the 1400s. York-based contractor William Anelay was appointed earlier this year to bring the Grade I-listed building into the 21st century.
“We wanted to make the building fit for the modern age,” says Gateway Project Steering Group chairman Canon Simon Cowling. “That included two things: full accessibility and a commitment to make the building’s heritage available to people in the city and region.”
The major refurbishment project involves new underfloor heating, reflooring, new lighting, adaptable seating and an accessible entrance.
“The previous main entrance involved steps and went into an area where the light was baffled by stonework – it did not draw you in,” Canon Cowling says. “There was a fully accessible entrance but it was tucked around the corner, which was unacceptable to us.”
Underfloor heating installed in the 1960s was beginning to corrode and fail, and a lack of thermostatic control meant it left the space too cold or far too warm.
The challenge for William Anelay was protecting the immense history in the building while adapting it to suit the future.
Before it could begin working in the cathedral it undertook a 10-week period of protecting and screening off sensitive areas.
“The first job we did on site was screened off the nave,” says senior site manager John Hutton. “This was to protect visitors from the construction works and to prevent dust and noise escaping. We put a Perspex section in the hoarding to allow people to see what is going on.”
Furniture was placed into storage and sculptures on the walls protected. These included two Tudor memorials – to the fourth and sixth earls of Shrewsbury – along with marble busts of former vicars of Sheffield and memorials of people whose remains were interred in the churchyard.
“We were very unlucky. We think perhaps some of the work in the 1930s was not finished”
John Hutton, William Anelay
The contractor used timber frames with compressible insulation to stop any machinery or accidents damaging the historically valuable artefacts – work that had to be carried out extremely carefully to avoid damage occurring during the protection phase.
Once this was done, the contractor could begin digging up the floor to get at the heating system.
“The plan was to cut the floor into 1 sq m chunks like a chess board,” Mr Hutton says. “We had been told that the floor was about 300 mm thick, so using a road saw with a 350 mm cutting blade, each piece would then come loose and we would lift them out into a dumper.”
Floor beneath the floor
The cutting phase went to plan, leaving a lattice of incisions, but when the contractor went to lift the pieces, it found it was only removing half the depth of the floor. Another layer remained untouched below.
“The consultants had information from the 1960s that suggested a 300 mm slab was used,” Mr Hutton says. “But it did not say this had been put down on top of the floor from the 1930s.”
“They just dug out what they thought was a suitable base and away they went”
John Hutton, William Anelay
Incredibly, tests carried out before the sawing operation had hit some of the rare areas where there was no earlier slab – seemingly proving the theory it was only 300 mm deep.
“We were very unlucky,” Mr Hutton says. “We think perhaps some of the work in the 1930s was not finished.”
Once the reality was established, it was more cost -efficient to remove the top layer as planned and then re-cut the lower layer using the same saw.
“Had we known it was thicker we would have used a bigger saw but those are very expensive. As it was we just repeated the operation used for the top layer.”
An excavator with a stone magnet attachment was used to pull up the slab once it was cut, and this was placed on to a dumper and taken away as waste.
The entire process of removing the floor took three weeks longer than expected, which ate significantly into the five-week buffer the contractor had built up by completing early works ahead of time.
Site workers carried out a reduced level dig to 560 mm below the final floor level to create room for the new heating system. Further excavation provided trenches for services.
Stood on nothing but soil
But at this point the second unforeseen obstacle appeared. “We found there was little or no foundation under the building,” Mr Hutton says. “It was just built on soil.
“The same building today would require thousands and thousands of cubic metres of concrete – but they just dug out what they thought was a suitable base and away they went.”
“We called the architect and he came with the engineer – they were shocked”
John Hutton, William Anelay
It became apparent that roof-supporting columns in the nave of the cathedral had very little foundation below them. With loose soil and stone below the column and machinery working in the area, there was a risk of a roof collapse.
“We called the architect and he came with the engineer – they were shocked,” Mr Hutton says.
Machinery movements were immediately stopped for a week and a half as a precautionary measure as emergency works swung into action to stabilise the supporting columns.
Timber shutters formed around the base of each column were filled with concrete to create blocks covering about 1,500 sq mm to hold the columns in place.
By this point all slack in the programme had been taken up, leaving the contractor on a very tight schedule. The project has to be completed in April next year to give the cathedral time to prepare for a major celebration of the centenary of the Diocese of Sheffield early in the following June.
The floor has now received a layer of concrete underlain with hardcore, followed by installation of a damp-proof membrane. Then the steel reinforcement mesh will be fitted, followed by the concrete floor slab.
The new underfloor heating – a rigid insulation board – will be installed between the floor slab and the floor finish.
“The heating system is basically a plastic piping loop, which you screed over and then lay down the limestone paving,” Mr Hutton explains. “The client has specified Ponton L4 paving from Peterborough – it’s very high quality.”
“We have to get the material in early or it just does not happen”
John Hutton, William Anelay
Historic plaques cut out of the existing floor by a stonemason have gone offsite for conservation. Gaps will be cut in the new floor for these to be bonded back in at the same places.
Furniture including a decorative 3 m by 7 m timber frame will go back in, as will the pulpit and font.
A two-for-one boiler upgrade
Two Remeha Gas 310 Eco Pro boilers, wheeled into the boiler house, will replace the cathedral’s existing inefficient boiler. New pipework will be installed to connect these boilers to existing flow and return channels in the cathedral undercroft, which will transfer hot water to the new heating system.
Sound and lighting systems are being installed, with the roof used to house cables, minimalising interventions with historic materials.
External works include taking up paving, cobbles and pollards and putting in a more accessible approach to the cathedral.
Part of the 1960s-built extension to the cathedral will be painstakingly taken down and rebuilt with more glazing and a roof light to form the welcoming main entrance.
If all goes to plan, this mix of modernisation and historic protection work will be completed to programme in early April 2014. “Fingers crossed there are not too many more surprises in store,” Mr Hutton says.
Sitting in the heart of Sheffield city centre next to a main road and a tram stop, the cathedral is well placed to attract worshippers – but not for a construction project.
“The biggest problem we have is access,” Mr Hutton says. “South of the site are tram tracks; to the east is a pedestrian area; north is the cathedral itself. Access is from the west, which is a very tight one-way street with a lot of public parking and congestion.”
Deliveries are often planned for between 6am and 7am to make sure they are completed before the area gets busy and difficult to use. “We have to get the material in early or it just does not happen,” Mr Hutton says.
Space is also at a premium within the cathedral where small doorways and regular columns make the space tight for vehicles. “It is incredibly challenging for the drivers, but this is what they do all the time,” he says.
Working on a Grade I-listed cathedral in a bustling city centre has required the involvement of a number of parties.
The church has its own detailed planning process outside of secular control. “It is actually more rigorous in some ways,” Canon Cowling says. As part of this there was an archaeological assessment.
Local businesses were informed about the project by both the cathedral and the main contractor, and public viewing points have been established from the street and from within the building. The project architect has been kept closely informed of developments throughout the scheme.
As well as the minimal foundations, there have been other discoveries from digging deep below the 15th century place of worship.
“A number of bones have been found but they are, in the jargon, disarticulated – not part of complete skeletons – so not burials,” Canon Cowling says.
“They are bagged carefully by the contractor and at the end of the project will be given a dignified interment.”