In February I was fortunate enough to be allowed access, with other members of the IStructE History Study Group, to the spectacular vaults of the former Clerkenwell House of Detention. The site is being developed by Sans Walk, a small property development company.
Andrew Smith who arranged the visit is the consulting structural engineer, and is collaborating with Bill Harvey on the assessment of the complex of arches and vaults which are the most interesting aspect of the site.
The House of Detention was a Middlesex County remand prison serving the City’s Courts as well as Clerkenwell Court House, built and progressively extended and altered from 1774/75 up to 1853/57 or so.
The original plan was cruciform, with a wing across its base, and was of four storeys including a semi-basement surrounded by the customary areas. It is this basement that survives and it was the focus of the visit. It was used for the reception of prisoners, medical examination and baths as well as kitchens. The basement therefore had a considerable variety of rooms, from halved cells to large rooms, whose structure necessarily accommodated the more regular layout above. The larger spaces provide the most spectacular areas today.
During the Second World War part of the basement was altered to form a bomb shelter, entailing the insertion of many walls to subdivide the large rooms, to wholly or partially close off openings and to block direct blast routes from one division to another. More recently its principal use was as a chamber of horrors, but that ceased in the late 1990s.
None of the basement fabric survives from before the major rebuilding and extension of 1774 to 1775, but a great deal of the brickwork is from this date. This is of high quality, of plum stock bricks in a lime mortar that probably contains a pozzolan, most probably trass, which was used for both walls and vaults.
Work between 1816 and 1818 also used plum stock bricks and an effectively identical mortar, but some rooms were spanned by York stone slabs, and its brick arches were laid with plum stocks in Roman cement – however, their skewbacks were not laid in Roman cement, so these arches may well have been cut into 1774 to
Where larger rooms were required in the east and west wings, the walls above were carried on both transverse and longitudinal arches springing from granite monolith skewbacks carried on granite monolith octagonal columns.
In the central spine, the walls above were carried on cast iron beams – inverted Y beams, asymmetric I beams cast with a single-sided inverted Y, asymmetric I beams and fish-belly beams. The last of these appear to have been used where they were inserted over openings cut in earlier walls.
Work in 1845 appears to have introduced yellow stock bricks laid in Portland cement, but they may have been used earlier than this, and the final 1853 work was also carried out in Portland cement and included moulded Gault clay bricks – presumably from Arlesey.
The floors are largely of granite slabs, but some areas are of York stone and there are a few slabs of slate, that are mostly laid direct onto the ground. There are drains beneath all the former areas and the base of the rainwater pipes that drained into them survive in places.
The final 1853 work has a raised floor of York stone slabs carried on sleeper walls to allow heating pipes to be run beneath.
As a first step towards bringing the building back into use, hopefully as offices for which change of use consent has been gained, the Second World War walls have largely been removed, and the pockets cut to bond them into the original work and other damage is being made good.
- Bill Harvey and Andrew Smith are currently assessing the structure’s capacity to carry the fill and car park use. Reassuringly, the structure shows no signs of distress whatever from such loads.