In Calgary we see deep excavations every day — from parkades downtown to utility vaults in new communities. The ground here is deceptive. Glacial till looks strong at surface but hides cobbles, boulders, and perched water tables. We have designed shoring systems for pits up to 18 m deep where the till alone would not stand unsupported. Before we touch a design, we always run a classification of soils to confirm the actual stratigraphy. That single step saves weeks of rework later. The city's frost depth of 1.8 m also forces us to consider thermal effects on exposed walls. Every excavation in Calgary requires a site-specific approach.
Glacial till in Calgary has a friction angle of 32 to 38 degrees, but sand lenses can drop that to 28 degrees — we model each layer separately.
Methodology and scope
We follow NBCC 2020 and CSA A23.3 for all deep excavation designs in Calgary. The city's glacial till has a friction angle of 32 to 38 degrees, but the interbedded sand lenses can drop that to 28 degrees. We model each layer separately. For temporary shoring we use soldier piles with timber lagging or secant pile walls. When groundwater is encountered — common near the Bow River — we design dewatering systems with filter layers to prevent piping. We also check bottom heave against the till's cohesion of 5 to 15 kPa. In deeper cuts we incorporate instrumentation geotecnica to monitor wall deflection in real time. That data lets us adjust the shoring before anything moves too far. Every design includes a factor of safety of at least 1.5 against sliding and overturning.
Technical reference image — Calgary
Local considerations
A deep excavation in Calgary can fail in three hours if the shoring is undersized. We have seen a 12-m-deep pit in the southeast — the till stood vertical for two days, then a sand seam gave way. The wall came in 300 mm. No one was hurt, but the schedule lost six weeks. The main risks are groundwater seepage through sand lenses, boulders that deflect soldier piles, and frost heave on exposed walls in winter. We design for these from day one. We also require a pre-excavation survey of adjacent foundations. Settlement of 10 mm can crack a sidewalk slab. We use inclinometers and piezometers on every job deeper than 6 m.
Design of soldier pile walls, secant pile walls, and tieback anchor systems for excavations up to 18 m deep. We size each element using local till parameters and include dewatering layouts when needed.
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Groundwater Control Plans
We design wellpoint systems, deep wells, and filter layers to handle the perched water tables common in Calgary's till. Each plan includes pumping rates, settlement monitoring, and discharge compliance with city bylaws.
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Monitoring and Instrumentation
Installation of inclinometers, piezometers, and settlement points before excavation starts. We provide daily reports during the critical first 5 m of cut and adjust the shoring if readings exceed 50% of allowable deflection.
Applicable standards
NBCC 2020 — National Building Code of Canada, CSA A23.3-19 — Design of Concrete Structures, CSA A23.2-9A / CSA A23.2-9A / CSA A23.2-9A / CSA A23.2-9A / CSA A23.2-9A / ASTM D1586 — Standard Penetration Test (SPT), FHWA-NHI-05-037 — Earth Retaining Structures (reference guide)
Frequently asked questions
What is the typical cost range for geotechnical design of deep excavations in Calgary?
For a standard 8- to 12-m-deep excavation in glacial till, the design and monitoring package ranges between CA$2,670 and CA$12,460. The final cost depends on excavation depth, shoring type, and whether dewatering is required. We provide a fixed-price quote after the initial site visit.
How deep can I excavate in Calgary before needing a shoring design?
Under NBCC 2020 and Calgary's land use bylaw, any excavation deeper than 1.5 m requires a shoring design if it is within 1.5 m of a property line or adjacent structure. For open cuts away from boundaries, the threshold is 3 m. Below 6 m, a full geotechnical design with instrumentation is mandatory regardless of location.
What makes Calgary's glacial till challenging for deep excavations?
The till contains cobbles and boulders up to 600 mm in diameter that can deflect soldier piles during driving. It also has discontinuous sand lenses that carry perched water — we have seen lenses 2 m wide that flow 80 L/min. The till's cohesion drops rapidly when saturated, so we always design for partially drained conditions unless dewatering is verified.
