Dealing with the discovery of Saxon foundations and negotiating access through a medieval stone ribbed window are some of the challenges facing engineers working on Durham Cathedral.
Durham Cathedral is one of only 29 Unesco World Heritage sites in the UK and one of only 1,031 in the world. It is a stunning example of Norman architecture and engineering. The cathedral replaced a Saxon church on the same site, with work starting in 1093. It took 40 years to build, with its builders pioneering the modern day principle of off-site construction.
Now 922 years old, Durham Cathedral is undergoing a programme of work to modify its interior to make it more accessible. “Open Treasure” is a £10M multi-phase development project and which will install new lifts and create a new connecting structure between the great kitchen and the main cathedral building to form a new exhibition space.
Consultant Patrick Parsons alongside contractor SYL Simpson were appointed to carry out the modifications to the grade I listed structure.
One of the major aspects of the work was to demolish and rebuild an extension built in the 1950s. It links the Monks Dormitory - situated in the cathedral - and the great kitchen.
“As with all old cathedrals, the kitchens were built as separate structures because of the risk of fire, so they were always built as an independent structure with a defined gap,” explains Patrick Parsons project director Mark Turner.
The extension comprised a concrete encased steel structure which spanned directly onto the wall of the cathedral. For the new extension, the 6m space between the buildings had to be reconfigured so step-free access could be installed, and for it to be used as an extension of the specially controlled exhibition space.
“At the time [that the first extension was built] they didn’t seem as bothered as we would be today about simply knocking one or two stones out here and there and just bedding the steel straight onto the wall,” says Turner.
“The walls themselves have more than enough capacity but what you’ve done is irreparable damage to the actual fabric of the building. Now we wouldn’t dream of doing that today, or only as a last resort.”
Conscious to ensure that the new extension did not impose itself on the original stone structure as the previous one had, the consultant came up with a creative solution to support the roof.
“At that time [that the first extension was built] they didn’t seem as bothered as we would be today about simply knocking one or two stones out here ”
Mark Turner, Patrick Parsons
“Initially we were considering using the pocket that had been created for the 1950s structure,” says Turner. “What we actually did, when we had a chance to look at it in more detail and do some intrusive investigation, and work with the architect to agree levels, was to cut off the existing concrete encased steel slightly proud of the building and in effect form a corbel and sit our new structure on top of that corbel.”
The monks’ dormitory from which the extension will be accessed is situated one storey up, on the north west corner of the cloisters. With an extremely impressive high oak trussed roof, the space currently serves as the cathedral library, but it will be transformed into the new exhibition space. At present the only access is up a flight of stairs. But when the new works are completed the stepped entry up to the dormitory will be modified to include a lift.
It was here that the team faced its next challenge.
During the demolition of the existing staircase, the archaeologists discovered the remains of some foundations which were thought to belong to the Saxon church which predated the cathedral. Naturally wanting to preserve the foundations and disturb them as little as possible, the team had to come up with a solution to protect them while installing the lift above.
“The wall that we were adjacent to was one of the walls of the south west tower [of the existing cathedral], and we had to minimise the load on the foundation area,” says Turner.
“One of the things which we started looking into was putting load into the wall of the tower, but the last thing we wanted to do was to damage it. What we ended up doing was a bit of a compromise between avoiding the wall completely and putting a steel [beam] into the existing wall.”
The team was able to isolate the existing foundation, covering it with a layer of sand with a membrane over the top and then casting a reinforced concrete slab above it. However, as the Saxon foundation was not flat - there were stones everywhere at all different levels - the thickness of the slab had to vary so it avoided the ancient construction below. Consequently the slab had to be less than 100mm thick in some places. This was necessary to
ensure the top of the slab enabled the lift to stop level with the floor.
“One of the issues was the lateral stability of the lift and taking out any kind of bending moment which has been generated, and because of the thin slab we couldn’t take out the bending moment where it was only 100mm thick,” says Turner.
“One of the things which we started looking into was putting load into the wall of the tower, but the last thing we wanted to do was to damage it”
Mark Turner, Patrick Parsons
In this case, Patrick Parsons got approval from the cathedral to drill in to the mortar joints of the tower and install small anchor bolts. The bolts were then load tested to then make sure they had enough capacity as they were not being fixed into a solid substrate. This allowed the team to take out the lateral loads which were originally designed to be taken by the foundation for the lift and keep the foundation as thin as possible.
Access into the cathedral for the construction materials also proved to be tricky.
“In the monks’ dormitory there were two routes in but both had relatively small doors,” explains Turner. “The bigger door has a sharp left hand turn at the top of the stairs so trying to get anything of any substantial size in there was a real problem.
In the end, the team carried out an assessment of the sizeable medieval window with stone ribbing at the south end of the dormitory to see if it could be opened up to allow materials to be passed through it. The window was removed and a scaffold tower was built up to it to provide access.
Turner is clearly an enthusiast and has done some research on the building in the archives.
“The cathedral actually has records which date back centuries where the engineers back then put in notes recommending when things should be inspected. It’s fascinating,” he says.
“With any conservation project, you always work on thebasis of minimal intervention and if you do have to do something, you make it reversible.
“The other thing I like about working on old buildings is that you can’t just rely on British Standards or Eurocodes. You do actually have to have a genuine understanding of how that structure is working, not just some idealised version of it.”
The work is due to be finished in October.