London Bridge has always been a mainline railway terminal into which trains run on viaducts from the counties to the South-east of the city. To be at the same level as the trains, the passenger concourse is raised above the neighbouring streets on a complex of vaulted spaces. These are of traditional, heavy, brickwork construction in semicircular barrel forms, some of which intersect at right angles to create large floor areas roofed by groined vaulting bearing on freestanding piers.
The bold structural forms of these spaces, which have been used predominantly for bonded warehousing, give them an imposing appearance. Rather than demolish and clear away the masonry to make way for a new installation, the existing structure was modified to create major passenger access ways to the new Underground station and between it and the mainline concourse.
The warehousing was served by three public roads that run below the station, linking Tooley Street on the north side to St Thomas Street on the south. The most westerly of these, Joiner Street, has been closed for incorporation of the space into the new station. As trafficked and storage areas they were never required to be habitable or highquality spaces and basic civil engineering construction in mass brickwork was therefore considered appropriate. No barrier to moisture is normally incorporated in such construction, and there was no particular concern about staining caused by the inevitable percolation of moisture from above.
A new look for the oldbrickwork
New access and communication routes within the station complex and a new Underground station have required intricate planning and implementation.
However, it has proved economical and practical to form new openings in the existing brickwork, remove walls and effect alternative support for some of the vaulting to create attractive and exciting new facilities while maintaining support of the existing mainline station above.
An important part of converting the old brick structure was extensive cleaning. During 160 or so years of service, percolation of moisture from above has led to leaching of lime and salts down through the masonry, and this has produced extensive staining and encrustation on the surface of the brickwork. Some surfaces have, from time to time, been decorated with lime distemper, but otherwise there has been little if any maintenance work. The brickwork in Stainer and Weston Streets, the roadways under the station that remain open, still shows the considerable accumulation of lime encrustation, leached deposits from the masonry, hydrocarbon and other airborne pollutants, dirt and grime that has built up on the surface.
Two principal methods of cleaning might be considered appropriate - chemical cleaning and abrasive blasting.
The latter was chosen because it was considered that it offers more control.
The pressure of the blasting must be regulated very carefully to avoid excessive scouring of the mortar and damage to the surface of the bricks. Developments in equipment, abrasives and blasting techniques have improved considerably within the last decade. Harsh blast cleaning that results in the removal of the surface of bricks along with stains and blemishes should rightfully be regarded as crude, insensitive and inept.
Specialist contractors were invited to survey the brickwork and tender for its cleaning, repair and modification.
Part of the negotiations included execution of trials to demonstrate proposed techniques and their results.
Requirements of Health and Safety legislation govern many techniques used in cleaning masonry. In addition, at London Bridge the presence of a tunnelling access point for the Jubilee Line Extension adjacent to the areas to be cleaned imposed additional restrictions - no dust or fumes could be produced that might be drawn down into the tunnel. Blasting with water-borne abrasives rather than with dry ones is generally preferable, because the latter generates excessive dust - a risk to health. However, large quantities of water are used with the former method and that would also have been unacceptable. As a compromise, blasting was done with a minimum-water slurry of fine olivine abrasive. This removes dirt, but clings to the brickwork, necessitating a rinse with clean water. Clinging slurry also obscures the work and repeated blasting is often necessary. All run-off had to be collected in tanks and removed.
At each end of the escalator hall (below the mainline station concourse) new reinforced-concrete beams were inserted to support sections of the vaulting. Shoring needles were inserted through the vaulting just above the new support level and the brickwork below removed. Once the new beams were in place, new sections of brickwork were inserted to resettle the vaulting on the new support. For aesthetic reasons, face brickwork in these areas was rebuilt, using reclaimed bricks jointed in natural hydraulic lime mortar to match the original. Where unaltered, the cleaned brickwork was carefully repointed.
The realistic appreciation of the results of cleaning is very important.
The original brickwork at London Bridge was never intended to provide dry interiors and fine finishes and some dampness and staining as a result of percolation from above had been tolerated. It would be unreasonable to expect cleaning to result in pristine brickwork that would henceforth maintain a fine and unblemished appearance.
Happily the supervising architect understood properly the rugged, workaday nature of the structure and his expectation that the cleaned brickwork should have a 'romantic' character has been interpreted very well indeed.