The 2020 National Building Code of Canada (NBCC) places Granby within a moderate seismic hazard zone where site-specific soil amplification can dramatically alter design ground motions. The city sits on the St. Lawrence Lowlands margin, with Pleistocene glacial deposits and Champlain Sea silts overlying Paleozoic Appalachian bedrock. Generic code spectra do not capture how these soft near-surface layers modify seismic waves at a given parcel. A seismic microzonation study maps spatial variations in ground response using field measurements rather than regional proxies. For projects requiring a Site Class determination under NBCC Article 4.1.8.4, we integrate borehole data with MASW and seismic refraction to build a shear-wave velocity profile that reflects actual stratigraphy, not just conservative defaults. The outcome is a design spectrum tailored to the property, often revealing that the default Class C or D assumption overestimates short-period demand or misses amplification at longer periods relevant to mid-rise structures.
A site-specific spectrum in Granby often cuts 15–25% off the short-period base shear compared to the conservative NBCC default, paying for the investigation several times over.
Methodology applied in Granby Quebec
Demonstration video
Risks and considerations in Granby Quebec
A six-storey residential project near the Yamaska River had its structural design nearly finalized assuming NBCC Site Class C. Core samples revealed 14 metres of soft Champlain Sea clay with undrained shear strength below 30 kPa and a plasticity index over 40%. A microzonation study showed the site actually classified as Site Class E, shifting the fundamental period of the soil column toward the building’s own period and doubling the spectral acceleration at 1.0 second. The structural engineer had to redesign the lateral system, adding shear walls and deepening the foundation to reach competent till. The cost of the geophysical survey and response analysis represented less than 0.3% of total construction cost; redesigning after excavation would have been ten times that amount. This scenario repeats across Granby’s low-lying terrain, where the contact between glacial till and overlying lacustrine deposits varies laterally over distances of less than 50 metres.
Our services
Our Granby seismic microzonation studies are delivered as a phased investigation that integrates directly with the geotechnical site investigation. Each phase produces actionable deliverables for the structural and foundation engineer.
Shear-Wave Velocity Profiling
Combined MASW, seismic refraction, and downhole P-S logging to generate a continuous Vs profile to at least 30 metres depth. Directly supports NBCC Site Class determination and site response modeling.
Site Response Analysis
One-dimensional equivalent-linear or nonlinear analysis using DEEPSOIL or equivalent software. Input motions matched to NBCC 2020 uniform hazard spectra for the Granby region, propagated through the measured Vs and modulus degradation model.
Liquefaction Potential Mapping
SPT-based or CPT-based liquefaction triggering analysis following the NCEER/Youd-Idriss framework. Produces contour maps of liquefaction potential index and post-liquefaction settlement across the site.
Frequently asked questions
What does a seismic microzonation study cost for a typical Granby site?
Budget between CA$5,730 for a basic investigation on a small lot with good access, and CA$20,190 for a comprehensive study covering multiple boreholes, geophysical lines, and full nonlinear site response analysis on a larger commercial parcel. The final scope depends on the site area, the number of measurement points required, and whether undisturbed sampling for cyclic laboratory testing is included.
How does microzonation differ from a standard NBCC Site Class determination?
A standard Site Class determination assigns a single letter (A through E) based on the average Vs or N-value in the top 30 metres. Microzonation goes further: it maps how ground response varies spatially across the site and produces a site-specific design spectrum that captures amplification at multiple periods, not just a code-constant acceleration. This matters in Granby where soft clay layers produce strong amplification at periods longer than 0.5 seconds.
When does the NBCC require a site-specific seismic study?
NBCC 2020 Commentary L recommends site-specific response analysis for sites classified as E or F, for buildings taller than 60 metres, and for post-disaster structures. In practice, any project where the default Site Class assumption drives a conservative and uneconomical lateral design is a candidate for microzonation. The cost of the study is almost always recovered through structural material savings.
What geophysical methods do you use in Granby?
MASW (Multichannel Analysis of Surface Waves) is our primary tool for shear-wave velocity profiling because it works well on the flat, accessible terrain typical of Granby. We complement it with downhole seismic in boreholes for depth calibration and seismic refraction where bedrock depth is uncertain. For sites along the Yamaska River, we also run resistivity lines to map water table and saturated sand layers relevant to liquefaction assessment.
Can microzonation reduce my foundation costs?
The reference range for this service in Granby Quebec is CA$5.730 - CA$20.190. The final price depends on the project scope and volume.