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Home › Seismic › Seismic microzonation

Seismic Microzonation in Repentigny: Site-Specific Ground Response for CSA-Compliant Design

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A common oversight in Repentigny is applying the default NBCC Site Class C to structural designs without verifying the subsurface profile. The St. Lawrence lowlands tell a different story. Much of Repentigny sits on sensitive Champlain Sea clay deposits, interbedded with silts and sands deposited after the last glaciation. When a seismic shear wave propagates upward through these soft deposits, the ground motion at the surface can amplify significantly. We have seen projects where assuming Site Class C instead of a measured Site Class E underestimated the spectral acceleration by over 40%. A CPT test provides continuous tip resistance and sleeve friction data to identify these soft zones, and when paired with shear wave velocity measurements, the site-specific ground response becomes clear. The NBCC 2020 requires this level of scrutiny for structures on potentially liquefiable or sensitive soils.

Defaulting to NBCC Site Class C on Repentigny's Champlain Sea clays can underestimate spectral acceleration by more than 40%—a margin that structural design cannot absorb.

Our service areas

Investigation

Exploratory test pit ›CPT (Cone Penetration Test) ›SPT (Standard Penetration Test) ›
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In-Situ Testing

Field density test (sand cone method) ›Plate load test (PLT) ›Field permeability test (Lefranc/Lugeon) ›
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Laboratory

Grain size analysis (sieve + hydrometer) ›Triaxial test ›Atterberg limits ›
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Our approach and scope

A four-storey institutional building near Boulevard Brien encountered a profile that textbook geology could not predict. The borehole log showed stiff clay till at 18 meters, but a thin, loose sand layer at 6 meters—less than half a meter thick—was completely missed by the standard SPT spacing. That single layer governed the liquefaction assessment. Repentigny’s seismic hazard includes contributions from the Charlevoix Seismic Zone, and the NBCC 2020 uniform hazard spectrum for the region demands a site-specific response analysis when any soil column contains a soft inclusion. We ran a one-dimensional equivalent linear analysis, and the amplification at the surface shifted the peak ground acceleration to a period that matched the building's fundamental mode. The structural engineer adjusted the base shear accordingly. For sites near the L'Assomption River, where groundwater is within 2 meters of grade, the liquefaction assessment becomes inseparable from the microzonation effort—pore pressure buildup in the sand reduces effective stress and increases the site period.
Seismic Microzonation in Repentigny: Site-Specific Ground Response for CSA-Compliant Design
Technical reference — Repentigny

Local geotechnical context

Repentigny's subsurface is dominated by the Champlain Sea basin infill: thick sequences of silty clay with a salt-leached, sensitive crust that can lose more than 80% of its undisturbed strength when remolded. During the 1988 Saguenay earthquake, similar deposits at distances exceeding 350 km showed amplification that exceeded design expectations. The risk here is not just spectral acceleration—it is ground failure. Soft clay layers amplify long-period motion, which resonates with mid-rise structures common along Rue Notre-Dame and the newer developments near the A-40 corridor. A site-specific microzonation study that ignores the degradation of shear modulus with cyclic strain will produce a site period estimate that is too low. That error flows directly into the structural analysis, leading to underestimated inter-story drift and column shear demands. The Champlain clay's sensitivity also raises the possibility of strength loss under cyclic loading, a mechanism that requires laboratory cyclic triaxial testing to quantify.

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Relevant standards

NBCC 2020 (National Building Code of Canada, Division B, Part 4), CSA A23.3-19 (Design of Concrete Structures, seismic provisions), ASTM D4428/D4428M (Crosshole seismic testing), ASTM D7400/D7400M (Downhole seismic testing), ASTM D5311/D5311M (Cyclic triaxial for liquefaction)

Technical data

ParameterTypical value
Vs30 (m/s)Measured via MASW or downhole, classified per NBCC Table 4.1.8.4.A
Site Class (A through E)Determined from Vs30 and undrained shear strength of cohesive layers
Fundamental Site Period, T0 (s)Calculated from 1D wave propagation model
Peak Ground Acceleration, PGA (g)Site-adjusted from NBCC 2020 reference ground motion
Spectral Acceleration Sa(T) (g)Provided for periods 0.2s, 0.5s, 1.0s, and 2.0s
Liquefaction Factor of SafetyPer Seed & Idriss simplified procedure, corrected for fines content
Seismic Settlement (mm)Post-liquefaction volumetric strain integration
Cyclic Stress Ratio (CSR)Calculated at each susceptible layer depth

Questions and answers

When does the NBCC require a site-specific seismic microzonation in Repentigny?

NBCC 2020 requires site-specific ground motion analysis when the site is classified as Site Class E or F, or when the structure is designated as post-disaster or high importance. Repentigny's Champlain Sea clay often triggers Site Class E, making a microzonation mandatory rather than optional for many projects. Additionally, if a site contains liquefiable soils deeper than the standard investigation depth, the code requires a site-specific hazard assessment.

What is the typical cost range for a seismic microzonation study in Repentigny?

The cost for a complete seismic microzonation study in Repentigny, including field shear wave velocity profiling (MASW or downhole), site response analysis, and the final report with design spectra, typically ranges from CA$5,550 to CA$25,670. The variation depends on the depth to bedrock, the complexity of the stratigraphy, and whether time history selection and matching are required for the structural analysis.

How does the sensitive Champlain Sea clay affect the site response analysis?

Champlain Sea clay exhibits a pronounced modulus degradation under cyclic shear strain. Standard modulus reduction curves from the literature can overestimate the stiffness at moderate strain levels. We apply regionally calibrated curves that account for the clay's sensitivity and OCR profile. Failing to do so shifts the site period and can underestimate the spectral acceleration at the building's fundamental period by a margin that the ductility detailing in CSA A23.3 may not cover.

What deliverables do we receive from a microzonation study?

The final report includes the Vs30 determination and NBCC site class, surface acceleration response spectra for the design earthquake (2% in 50 years), acceleration time histories when requested, liquefaction factor of safety profiles, and post-liquefaction settlement estimates. For nonlinear structural analysis, we provide spectrum-compatible time history pairs that match the site-specific spectrum across the period range of interest.

Location and service area

We serve projects in Repentigny and surrounding areas.

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