A mid-rise residential project in Rosemont–La Petite-Patrie ran into a snag when the preliminary borehole log suggested a stiff till but the dynamic response didn’t align with the structural model. The architect called us mid-foundation design, and within two days we had a 24-channel landstreamer spread across the site, mapping a sharp velocity inversion that the driller had barely nicked. That’s the reality of Montreal’s subsurface: the glacial till overlying Champlain Sea sediments creates a layered velocity profile that a single borehole or SPT log can misread, and the NBCC 2020 seismic hazard values for eastern Canada don’t forgive a site class mistake. When the bedrock depth jumps from 8 m to 25 m across a single lot—common in the Sud-Ouest and along the escarpment—the only practical way to pin down the seismic site classification is a surface-wave survey that captures the full shear-wave velocity column without interpolation between boreholes.
The NBCC Vs30 boundary between Site Class C and D sits at 360 m/s—a 15 m/s shift can change the design base shear by 25% in Montreal’s seismic context.
Service characteristics in Montreal
Procedure video
Critical ground factors in Montreal
On the national scale, the NBCC 2020 assigns Montreal a relatively moderate seismic hazard, yet the risk profile is dramatically altered by the amplification potential of the Champlain Sea clays, which can reach thicknesses of 40 to 60 meters across the central and eastern parts of the island. When a deep clay basin is assigned to Site Class E (Vs30 below 180 m/s), short-period spectral accelerations can be amplified by a factor of two or more compared to a Class C reference condition, thereby directly increasing the seismic design forces required for the lateral system. A clear example of this was observed on the east end of the island near Pointe-aux-Trembles, where the bedrock drops below 50 meters: a measured Vs30 of 155 m/s pushed the structure into a higher seismic force category. This situation was missed by the original borehole-only investigation, which assumed a stiffer profile based solely on SPT blow counts. The geotechnical team was subsequently forced to retrofit the foundation design by adding shear walls and denser reinforcement—an expensive correction that could have been identified from the start by a pre-design MASW survey. For essential facilities governed by the NBCC importance factor, site class determines not only the base shear but also the foundation type and the allowable bearing pressure under seismic load combinations.
Our services
Every MASW survey we deliver in Montreal includes the raw field records, the dispersion image, the inverted Vs profile, and a formal Vs30 calculation with the NBCC site class designation. This is because a single number without the supporting data does not withstand peer review. The services listed below cover the range of project scales we encounter, from individual lot investigations to corridor-length profiling.
Active-Source MASW Survey
A 24-channel linear spread with a sledgehammer source is processed to yield a 1D Vs profile and Vs30. This setup is suitable for sites where bedrock lies within 30 meters and there is clear line-of-sight. The process includes dispersion curve quality control and multi-mode inversion when higher modes are present.
Combined Active-Passive MASW
To extend the depth of investigation beyond 60 meters, ambient microtremor recording with circular or L-shaped arrays is added. This is recommended for deep Champlain Sea clay basins on the east island and in the St. Lawrence lowlands, where bedrock may exceed 50 meters in depth.
Vs30 Site Classification & NBCC Compliance Report
Formal site class determination is performed per NBCC Table 4.1.8.4.A, including Vs30 calculation, travel-time averaging, and commentary on spectral acceleration implications. The report is sealed by a professional engineer registered in Quebec.
Top questions
What does a MASW survey in the Montreal area typically cost?
For a standard active-source MASW survey on a single lot in the Montreal region, the cost typically ranges from CA$2,300 to CA$4,550. This depends on the array length, the number of measurement locations, and whether passive microtremor recording is needed to reach deeper bedrock. Sites with difficult access, steep terrain along the escarpment, or frozen ground in winter may require additional mobilization time, which influences the final price.
How does the Champlain Sea clay affect the Vs30 measurement?
The Champlain Sea clays that underlie much of central and eastern Montreal are normally consolidated to lightly overconsolidated silts and clays with shear-wave velocities typically between 90 and 180 m/s. This places many sites in the NBCC Site Class D or E boundary zone, where the Vs30 value becomes critical for seismic design. The MASW method is particularly effective in these soft-soil conditions because Rayleigh-wave dispersion is sensitive to the low-velocity layer, and the inversion can resolve the velocity contrast between the clay and the underlying till or bedrock.
Can the MASW survey be performed in winter when the ground is frozen?
Yes, but the frozen surface layer must be accounted for during processing. In Montreal, a high-velocity frozen crust typically 0.8 to 1.8 meters thick introduces a strong velocity gradient that can mask the underlying soil profile in the active-source dispersion curve. We address this by incorporating passive microtremor data that samples below the frost line and by explicitly modeling the frozen layer in the inversion. The result is a Vs30 that reflects the unfrozen soil condition governing the seismic site response, not the temporary winter crust.