Project Description

A two storey commercial development was built with a single basement level at Elev. 90.98 m. Thirty three boreholes, three test pits and detailed soils laboratory testing showed approximately 3 to 4 m of compact to dense sand, underlain by approximately 21 m of stiff to hard clayey silt till and/or clay, 3 to 7 m of wet sand with cobbles and boulders, followed by red, weathered to sound, Queenston shale.

Short term groundwater level observations in sealed standpipe piezometers showed perched water levels in the upper sand and subartesian pressure from a deep water bearing, gravel and cobble layer.

Braced open cut excavations comprised steel soldier piles and timber lagging with internal rakers in nonsettlement sensitive roadway areas, and a continuous caisson wall alongside existing settlement sensitive buildings. Straight shafted, reinforced concrete caissons were advanced with multiple steel safety liners to approximately 30 m depth using an allowable end bearing capacity of 2.5 MPa (25 tsf) plus a shaft resistance of 0.6 MPa (90 psi) for rock socketed caissons. Deep well dewatering and tremie concreting were carried out. Low strain integrity testing was used to confirm no significant concrete shaft defects and sound end bearing piles into shale bedrock.

Localized shallow footings were constructed on the upper loose to compact native sand using 190 kPa (4000 psf) soil bearing pressure subsequent to a supervised soil densification and field testing program.

Geotechnical site inspection and laboratory materials testing included foundations, temporary shoring, imported aggregates and excavated native soils, soil backfill compaction, vibration monitoring, subsurface drainage, concrete cylinders, structural steel and roofing.