A six-storey mixed-use project on Glen Drive hit a snag in permitting when the geotechnical review flagged insufficient shear wave data. The site sat on a complex glacial till overburden typical of the Coquitlam River basin, where VS profiles can shift dramatically within 20 metres horizontally. We mobilized a 48-channel MASW array with 4.5 Hz geophones and recorded surface wave dispersion across two orthogonal spreads, resolving a VS30 of 310 m/s that placed the site firmly in NEHRP Class D. In Coquitlam, where the NBCC 2020 seismic hazard values for the Cascadia subduction zone demand accurate site amplification factors, surface wave testing isn't optional box-checking. It's the direct measurement that determines whether your structural design forces multiply by 1.3 or 1.8. For deeper velocity contrasts beneath thick alluvium, we often pair the MASW spread with a seismic refraction line to constrain the P-wave model simultaneously, which tightens the inversion and reduces ambiguity in the top 30 metres.
A 50 m/s difference in VS30 can shift the NBCC site class boundary and alter the seismic design base shear by 20% or more.
Local considerations
The most common mistake we encounter on Coquitlam sites is substituting SPT blow counts for VS30 using empirical correlations published for Holocene sands in California, which systematically overestimate stiffness in the local glacial till matrix. A contractor on Austin Avenue ran N60-based VS30 estimates of 380 m/s and designed for Site Class C, but our direct MASW measurement returned 275 m/s, forcing a redesign of the lateral force-resisting system after footings were already poured. The error was hidden in a thin, highly-weathered silt layer at 5 metres depth that the SPT sampler missed but the surface wave dispersion clearly resolved as a low-velocity waveguide. In the Coquitlam River corridor, where post-glacial lacustrine clays interbed with granular outwash, the VS profile often contains sharp impedance contrasts that violate the gradual-stiffness assumption embedded in most N-value conversion charts. Direct shear wave measurement eliminates that blind spot.
Quick answers
What depth does the MASW test reach in Coquitlam soil conditions?
With active-source sledgehammer shots on a 48-channel spread, we reliably resolve VS profiles to 30–35 metres in Coquitlam's glacial till and outwash deposits. When we incorporate passive-source ambient noise recording, the array can resolve velocity structure to 50–80 metres, which is useful for identifying deep soft layers that affect site period calculations. The practical depth limit depends on the stiffness contrast and the maximum geophone offset: our standard 115-metre spread provides a half-space resolution wavelength of roughly 35–40 metres for the fundamental Rayleigh mode.
How does the NBCC 2020 use VS30 for seismic design?
The NBCC 2020 classifies sites from A (hard rock, VS30 > 1500 m/s) through E (soft soil, VS30 < 150 m/s) based on the average shear wave velocity in the top 30 metres. The site class then determines the short-period and long-period amplification factors Fa and Fv that multiply the reference spectral accelerations. In Coquitlam, where mapped PGA values for the 2% in 50-year hazard reach 0.4–0.5 g, moving from Site Class C to Class D can increase the design spectral acceleration at 0.2 seconds by 30–40%, so accurate VS30 measurement has a direct impact on structural costs.
How long does a typical MASW survey take, and what site access do you need?
A single MASW spread with 24–48 geophones takes about 60–90 minutes of field time, including array layout, multiple shot gathers, and passive recording. For most Coquitlam residential or commercial lots, we need a reasonably flat 50–100 metre strip free of buried utilities for the geophone line. We coordinate with BC One Call for utility clearance before planting spikes, and we can work on paved surfaces using base plates or on grass with standard spikes. The processed report with VS profile and site class is typically delivered within 3–4 business days after the field session.
What does MASW testing cost for a Coquitlam building site?
