When the National Building Code of Canada (NBCC 2020) requires a seismic site class for a new development in Windsor, the MASW method provides a non-invasive path to obtaining the necessary VS30 value. Unlike borehole logging or crosshole tests that require drilling through the city's complex overburden of glacial till and lacustrine clay, a multi-channel analysis of surface waves maps shear wave velocity across the upper 30 metres without disturbing the site. This is particularly useful on the tight urban lots of Walkerville or the industrial plots near the Detroit River, where access is constrained and the subsurface often conceals buried fill from a century of manufacturing history. The data feeds directly into the seismic hazard calculations that govern structural design forces under Division B of the Ontario Building Code, and our field team runs both active-source and passive-source arrays to capture velocity dispersion from the near-surface weathered zone down to the stiff clay till that defines Windsor's deeper stratigraphy.
A correctly inverted VS30 profile in Windsor's clay plain can shift a site from Class D to Class C, directly reducing the seismic base shear coefficient and the structural cost.
Our approach and scope
Windsor sits at roughly 190 metres above sea level on a flat lake plain, yet the soil column beneath that plain is anything but uniform. In the Little River corridor and parts of South Windsor, shallow water tables within 2 metres of grade can reduce the effective stress in silty sands, which directly lowers measured VS values unless the survey geometry accounts for pore pressure effects. The July 2023 magnitude 2.5 tremor near Amherstburg, while minor, reminded engineers that the Western Quebec Seismic Zone extends influence into Essex County, making accurate site classification a non-negotiable step. For sites where the MASW dispersion curve suggests borderline NEHRP Class D or E conditions, we often recommend pairing the survey with a
CPT test to correlate tip resistance with velocity, or with a
seismic refraction line that maps the depth to the high-velocity glacial till contact along the same alignment. Our processing workflow picks fundamental-mode dispersion manually before running a solid Monte Carlo inversion, avoiding the aliasing errors that automated picking algorithms can introduce in layered clay profiles typical of the St. Clair clay plain.
Local considerations
The eastern sections of Windsor, particularly around the former Ford City and Tecumseh boundary, are underlain by up to 15 metres of soft, compressible silty clay that plots firmly in NBCC Site Class E territory when VS30 drops below 180 m/s. A site classified as Class E under the 2020 NBCC carries a substantially higher spectral acceleration multiplier than the Class C or D values assumed in preliminary design, and discovering this after structural drawings are complete forces either a costly redesign or an expensive ground improvement campaign. We have seen cases where a single MASW line, poorly oriented relative to the buried river channels that cut through the clay plain, missed a low-velocity lens that a parallel line with stone columns spacing considerations would have flagged. The risk is not just regulatory—it is physical—because underestimating site amplification on soft Windsor clay can lead to resonance with the 1-2 Hz natural frequency typical of mid-rise residential buildings that dominate the city's new infill construction.
Relevant standards
NBCC 2020 (National Building Code of Canada), Division B, Part 4, ASTM D7400-19: Standard Test Methods for Downhole Seismic Testing (applicable principles for shear wave velocity), ASTM D5777-18: Standard Guide for Using the Seismic Refraction Method (geophone array setup), Ontario Building Code (O.Reg. 332/12), Article 4.1.8, CSA A23.3-19: Design of Concrete Structures (seismic provisions referencing site class)
Frequently asked questions
What does a typical MASW survey cost on a residential lot in Windsor?
For a standard active-source MASW line on a single-family or small multi-unit lot within Windsor city limits, the cost generally ranges from CA$2,250 to CA$3,980 depending on the array length, number of shot points, and whether passive data collection is needed to reach the full 30-metre depth. Sites with poor access or heavy traffic noise may require additional processing time.
How long does it take to get the VS30 report after the field survey?
Fieldwork for a single MASW line typically completes in 2 to 3 hours. The dispersion analysis, Monte Carlo inversion, and report preparation are usually finished within 4 to 5 business days. Combined active-passive surveys with longer arrays may add 1 to 2 days to the processing timeline.
Can MASW work on paved surfaces or inside existing buildings?
MASW requires coupling geophones to the ground surface, which is feasible on asphalt or concrete pavement by drilling small anchor holes or using base plates with coupling gel. Inside existing buildings, the method is generally not practical due to reflections from foundations and walls that contaminate the dispersion image, though we can sometimes run a passive array in an adjacent parking area.
How does Windsor's clay geology affect the MASW results?
The thick sequence of glacial till and glaciolacustrine clay beneath Windsor produces a gradual velocity increase with depth rather than sharp contrasts, which is actually favorable for MASW because it generates a clean, interpretable fundamental-mode dispersion curve. The challenge comes from the very low shear wave velocities in the upper 5 to 10 metres, which require low-frequency geophones and careful source coupling to avoid near-field effects in the inversion.
