Engineering Heritage: Integrating North America's Largest Façade Retention System with Support of Excavation in the United Bldg. Project-Paper

Paper

Pdf Summary

The United Building project in downtown Toronto combines a new 50-storey development with the in-place preservation of two heritage façades: a nine-storey 1928 building and a ten-storey 1961 building. Supporting the record-setting façades above a four-storey excavation required an integrated façade retention system (FRS) and support-of-excavation (SOE) system.<br /><br />Demolition was carefully controlled to protect the façades. Crews retained the buildings’ inner structural bays, used small equipment and strategic saw cuts, and managed debris drops to limit vibration. The SOE used secant piles along two-thirds of the perimeter and micropiles near subway infrastructure and utilities. Larger reinforced piles, king piles, and a network of steel trusses transferred the façade loads to the ground. The existing structure’s estimated 20,000-tonne load was transferred progressively using hydraulic jacks at 175 locations, with strain gauges and other instruments monitoring movement.<br /><br />Excavation proceeded with internal braces, walers, tiebacks, and shotcrete. Design and sequencing were refined collaboratively to address tight site access, sensitive infrastructure, and strict movement limits. Four major operations—demolition, FRS installation, and secant- and micropile drilling—often had to share the constrained site.<br /><br />The FRS and SOE work was completed on time and on budget, with safety and construction quality maintained. Monitoring over 24 months showed movements generally within project criteria; the team found that temperature fluctuations influenced façade deflections and some alert-level exceedances. The authors attribute the project’s success to detailed planning, close coordination, and collaboration among the owner, designers, contractors, inspectors, and suppliers.

Keywords

Toronto

heritage façades

façade retention system

support of excavation

secant piles

micropiles

hydraulic jacks

excavation

structural monitoring

construction sequencing