Geotechnical Design of Deep Excavations in Sault Ste. Marie

A developer broke ground for a 10-storey residential tower near Sault Ste. Marie's downtown core, adjacent to a century-old brick building on Queen Street East. The initial test pits revealed a chaotic sequence of glaciolacustrine silts over dense basal till, with groundwater appearing at just 2.8 metres depth. That scenario—tight urban site, sensitive neighbouring structures, and water-charged overburden—defines the challenge of deep excavation design in this city. Our laboratory team took those disturbed samples and ran a full suite of index and strength tests, feeding directly into the finite element model that sized the shoring system. In Sault Ste. Marie, where the Lake Superior clay belt and Precambrian bedrock control every cut, a CPT test provides the continuous stratigraphic profile that SPT alone cannot resolve, especially when identifying thin drainage layers that can destabilize a vertical face.

In Sault Ste. Marie's glaciolacustrine soils, we don't design for average conditions—we design for the spring thaw when groundwater peaks and apparent cohesion vanishes.

How we work

The freeze-thaw cycle in Sault Ste. Marie, with winter lows reaching minus 30 degrees Celsius, forces a design approach that goes beyond standard earth pressure theory. Frost penetration in the silty overburden can temporarily increase apparent cohesion, only to release it catastrophically during spring melt. We incorporate this seasonal behaviour into our staged excavation analysis, specifying insulation blankets or heated enclosures for critical winter cuts. The presence of varved clays—thin alternating layers of silt and clay deposited in glacial Lake Algonquin—creates anisotropic drainage that we quantify through in-situ permeability testing before finalizing the dewatering plan. Our structural back-analysis always cross-references the NBCC 2020 seismic provisions, given the moderate seismicity of the region, to ensure that temporary tieback loads account for peak ground acceleration. The design package includes detailed shop drawings for soldier pile and lagging, secant pile walls, or diaphragm walls, depending on the proximity to the St. Marys River and the resulting hydrostatic pressure.

Local considerations

NBCC 2020 Part 4 and CSA A23.3 mandate a design that prevents collapse and limits deformation to protect adjacent infrastructure—a requirement made particularly stringent in Sault Ste. Marie given the inventory of heritage masonry structures near the canal district. The biggest technical risk we see is basal heave in excavations that bottom out in soft glaciolacustrine clay; the reduced passive resistance at the toe can trigger a sudden inward movement of the entire shoring wall. We run undrained and drained stability checks at every stage, using modified Bishop or Spencer methods when the failure surface extends under neighbouring footings. A secondary risk is piping erosion at the interface between the granular water-bearing layer and the underlying silt, a condition we mitigate through filter criteria applied to the jet grout or deep well screens. Uncontrolled dewatering in Sault Ste. Marie has historically caused settlement up to 80 millimetres in nearby buildings—our instrumentation plan mandates piezometers and inclinometers read daily during active pumping.

Technical parameters

Parameter	Typical value
Maximum excavation depth analyzed	>25 m
Design groundwater level	1.5–3.5 m below grade (seasonal)
Soil unit weight range (overburden)	18.5–21.2 kN/m³
Undrained shear strength (varved clay)	35–70 kPa
NBCC seismic category (Sault Ste. Marie)	Sa(0.2) = 0.18 g
Typical shoring type	Soldier pile & lagging, secant pile
Frost penetration depth for design	1.8 m

Other technical services

Subsurface Investigation and Lab Testing Program

We design the borehole and test pit layout to target the glacial stratigraphy specific to the Sault Ste. Marie area, running consolidated-undrained triaxial tests on undisturbed Shelby tube samples to capture the stress-strain response of the varved clay.

Shoring Wall Structural Design

We produce stamped calculations and construction drawings for cantilever, anchored, or internally braced systems, detailing waler sizes, tieback preload forces, and soldier pile embedment depths to resist both active earth pressure and seismic increment.

Dewatering and Groundwater Control Design

Using field permeability data, we model the radius of influence for deep wells or wellpoints, specify pump capacities, and design sump and discharge systems that comply with the Sault Ste. Marie Region Conservation Authority requirements.

Construction-Phase Instrumentation and Monitoring Plan

We define threshold values for lateral deflection and vertical settlement, install inclinometer casings behind the shoring, and provide weekly interpretation of readings to trigger contingency measures if movements approach 75% of the design limit.

Reference standards

NBCC 2020 (National Building Code of Canada) – Part 4 Structural Design, CSA A23.3 – Design of Concrete Structures, ASTM D2487 – Standard Practice for Classification of Soils, ASTM D1586 – Standard Test Method for SPT and Split-Barrel Sampling, Canadian Foundation Engineering Manual (CFEM) 4th Edition

Frequently asked questions

How much does a deep excavation design cost for a project in Sault Ste. Marie?

For a typical urban excavation in Sault Ste. Marie, the geotechnical design package—covering subsurface investigation planning, laboratory testing, shoring wall calculations, dewatering design, and sealed construction drawings—ranges from CA$3,040 to CA$11,830. The final cost depends on excavation depth, number of retained sides, proximity to the St. Marys River, and the complexity of the ground profile. A 6-metre cut with soldier piles will fall at the lower end; a 15-metre secant pile wall adjacent to a heritage structure requires substantially more analysis.

What laboratory tests are essential for designing a shoring system in Sault Ste. Marie's soils?

We consider consolidated-undrained (CU) triaxial tests with pore pressure measurement as the minimum for determining the effective stress strength envelope of the glaciolacustrine clays. Paired with one-dimensional consolidation tests to obtain the compression index and preconsolidation pressure, these allow us to model the undrained behaviour during the excavation and the drained settlement behind the wall. Atterberg limits and particle size distribution classify the material per ASTM D2487, while falling-head permeability tests on undisturbed samples refine the dewatering model.

What differentiates deep excavation design in Sault Ste. Marie from other Ontario cities?

The combination of varved glaciolacustrine deposits from glacial Lake Algonquin, the shallow depth to Precambrian bedrock across much of the city, and the extreme seasonal groundwater fluctuation sets Sault Ste. Marie apart. The bedrock surface is often irregular, creating pockets of trapped water that surcharge the shoring. Additionally, the winter construction season requires frost-protection measures and a design that accounts for the temporary strength gain and subsequent thaw weakening of the upper 1.8 metres of soil—a cycle less pronounced in southern Ontario cities.