Constructing the new Crossrail Arch within the Grade 1 listed Macmillan House necessitated the demolition of the first floor concrete slab where shops and offices had been previously located.
Protecting and maintaining the stability of the historic building fabric within the agreed vibration limits of 3mm/s PPV during the demolition required careful planning and execution.
While the breaking of the slab was within the vibration limit tolerances as predicted, vibration transmitted through the floor with debris impact caused a number of vibration exceedances in the monitors fixed around the building.
The limited floor loadings permitted and the electricity sub-station below offered a restricted spectrum of options for demolition methodology.
With Paul Travis, CSJV Heritage Specialist, a number of mitigation proposals were discussed and a trial using a combination of Fall Arrest bags with polystyrene blocks and plywood sheeting was undertaken to reduce the impact of debris on the slab.
The lightweight polystyrene chips in the bags, coupled with the polystyrene blocks and plywood provided a lightweight, yet robust dampener for falling debris, minimising vibration transfer, while the plywood sheeting provided a smooth surface from which to remove the rubble.
The successful trial created no vibration levels outside the permitted tolerance and allowed the demolition of the slab to continue in a safe and compliant manner, reducing the risk of damage to the historic building fabric or the sub-station.