22 C O N T E X T 1 2 4 : M A Y 2 0 1 2 Nigel Sunter Monitoring for Crossrail As Crossrail is tunnelled under London, conservation architects are advising on where movement might cause historic buildings to crack, and how to mitigate the problem. Conservation architects are normally called upon to give advice in relation to cracks that have already appeared in buildings. Their skill is in diagnosing how the cracks might have been caused and to specify measures to repair them or to control them. Purcell MillerTritton has been asked to turn this normal circumstance on its head by predicting where historic buildings might crack in the event of movement taking place, and to propose mitigation measures should these be appropriate. Crossrail will connect mainline rail services from west to east London. Forty-two kilometres of this engineering work will be tunnelled, impacting on two Grade I listed buildings in Soho, partly as a result of deep excavation for the upgrade ofTottenham Court Road underground station, and partly by the digging of the tunnels which run beneath Soho Square.The two properties date from the 18th century, with mid-19th and early 20th century additions, all with very different structural characteristics. The Grade I listed House of St Barnabas on the corner between Greek Street and Soho Square is a typical mid-18th century speculative development. In 1754 it was leased to Richard Beckford (who had made money through sugar plantations in theWest Indies). He fitted it out with lavish rococo plaster and other decoration, and a fine stone cantilever staircase. Backford had use of the house only for a year or so, after which it changed hands several times. In the early years of the 19th century the area became less desirable for fashionable residents of means. In 1811 it was leased to theWestminster Commissioners for Works for Sewers, and it was from this building that Joseph Bazalgette masterminded the new sewage infrastructure for London. In 1861, the lease was taken over by the House of Charity, whose main objective was ‘to afford temporary relief to as many destitute cases as possible, and to have a Christian effect on the poor population’. In support of the latter, a chapel was built on theManette Street frontage in 1862 to the designs of Joseph Clarke, consultant architect to the Charity Commissioners.The House of St Barnabas ceased to be residential in 2006 but continued to provide life skills, structured training and experience in supporting homeless and vulnerable individuals. The construction of the 18th century house is typical of its time, with thick load-bearing brick walls built with soft lime mortar on shallow foundations. A brick vault supports the entrance hall, and floors elsewhere are supported by timber beams in double-joisted construction (characterised by separate floor and ceiling joists). As a result of the engineeringworks, there is predicted to be an overall settlement of up to 70mm. If that drop were consistent and vertical over the whole area, the resultant effect on the building would be likely to be minimal. However, there is a settlement gradient of between 10mm and 70mm across the site. It is this uneven movement which might give rise to damaging cracks. Crossrail will be introducing ‘compensation grouting’ to counteract any differential movement by selectively pumping a cement grout into the strata. It is hoped that by this means cracking will be prevented. In order to monitor the building over and beyond the period of the excavations, various types Left:The Grade I listed House of St Barnabas is a typical mid-18th century speculative development. Crossrail’s engineering works are expected to cause an overall settlement of up to 70mm. Right:The Catholic Church of St Patrick stands on the east side of Soho Square, constructed in 1893 of loadbearing brickwork with a stucco interior.The bricks are mortared with a hard cementitious mix which will cause the walls to act monolithically.
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