Overview
For this mixed-use building near Powell Street, the below-grade level sits tight to property lines and within an area of contaminated soils and groundwater. To avoid perpetual treatment of water from perimeter drains, the team selected a tanked basement with a raft slab designed for hydrostatic loads and buoyancy control, paired with anchor shoring at the excavation perimeter.
Subsurface, seismic, and excavation depth
Past and recent investigations show compact to very dense sands over hard sandstone at shallow depth, suitable for conventional founding once verified at the bottom of excavation. Site seismic characterization indicates Site Class C with measured Vs30 supporting code-based design. Planned excavation depth to reach the raft and under-slab drainage layer is on the order of 7 m at the perimeter, which informed shoring type and staging.
Why tanking
Because groundwater on and around the site is contaminated, the basement and slab are waterproofed continuously to grade, the raft is proportioned against uplift, and temporary dewatering is managed through sleeves connected to a clear crushed-gravel drainage layer below the raft. Once the superstructure has enough weight to resist buoyancy, the pumps are removed and the sleeves are sealed. This approach isolates the building from contaminated flows while keeping construction practical.
ESC and water treatment during excavation
The erosion and sediment control plan routes excavation water to a treatment system sized for holding and settling, with metered discharge to the sanitary system under a City waste-discharge permit. Requirements include hourly flow recording, routine street sweeping, protected catch basins, and a monitoring cadence adjusted by season. These measures keep fine sediment and contaminants out of the storm network while excavation proceeds.
Shoring, underpinning, and utilities coordination
Excavation support is shotcrete and anchors, with localized underpinning where neighbouring foundations require support. Along utility corridors, a deadman tieback arrangement and staged anchor rows maintain clearances and control movement. The drawings specify staged cuts, anchor testing and lock-off, removal of near-surface anchors on City property after wall construction, and placement of survey monitoring points along critical faces to confirm performance. Encroachment agreements with the City and adjacent owners are coordinated where anchors or underpinning cross boundaries.
Outcome
By combining a tanked raft with a monitored ESC program and a shoring scheme tailored to neighbours and utilities, the project controls groundwater risks, protects adjacent assets, and delivers a buildable path to subgrade approvals at the design subgrade elevation.
