Mansion House London: Risk assessment and protection

Settlements occurring during the first phase of tunneling for the extension of the Docklands Light Railway in London caused concern for damage to the Mansion House during subsequent phases of the work. The building is a landmark structure and no further damage could be accepted. Tunneling was halted until a satisfactory protection program was in place. Various means of protecting the structure were investigated, ranging from full structural underpinning to a least-risk methodology based on the observational method. This paper describes the background to the development and implementation of the observational method to successfully protect the building and complete construction. This required appropriate contingency measures to be identified and planned in advance. Of the contingencies involving preventive or strengthening works, a system of structural ties through the building was selected as most appropriate, least risky, and most economical. Trigger levels for building response were established. Extensive instrumentation was installed and the building was fully monitored during the subsequent phases of tunneling.