NBCC 2020 Part 4 requires a site-specific shear wave velocity profile for seismic design in Sault Ste Marie. The city sits on complex overburden atop Precambrian bedrock of the Canadian Shield, where sharp impedance contrasts are common. We run MASW surveys to measure Vs30 directly, not from blow-count correlations. The St. Marys River valley adds a layer of complexity: soft lacustrine clays and silts overlying till, with bedrock depth varying from 2 to over 30 metres across the city. A CPT test often complements the array when we need stratigraphic control, and in areas with suspected loose sands near the river we tie results to a liquefaction assessment per NBCC guidelines. The array geometry adapts to site constraints: 24 or 48 geophones, active source, sometimes passive noise when urban traffic helps the low-frequency range.
A direct Vs30 measurement removes the ±15% uncertainty of N-value correlations in glacial till, a margin that can change the NBCC site class.
Frequently asked questions
What does a MASW / Vs30 survey cost in Sault Ste Marie?
For a standard 2-line MASW survey with active source in the Sault Ste Marie area, budget between CA$2,600 and CA$4,240 depending on array length, number of spreads, and whether passive-source recording is added for deeper profiles. Sites with restricted access or heavy vegetation may require additional line clearing time.
How long does the field work take for a MASW survey?
A two-line survey with a 24-channel array typically takes half a day on site, including setup, calibration shots, and teardown. Adding passive noise recording extends the session by 20–30 minutes per spread. The report with Vs profile and site class is delivered within five to seven business days.
Can MASW be performed in winter in Sault Ste Marie?
Yes, but with a caveat. Frozen ground increases the near-surface shear wave velocity and can bias Vs30 upward by 10–20%. We recommend scheduling surveys between April and October. If a winter survey is unavoidable, we document frost depth and apply a correction factor, though NBCC does not formally recognize adjusted values, so the report notes the condition.
What is the difference between Vs30 from MASW and from SPT correlations?
MASW measures Vs directly from surface-wave dispersion, giving a continuous velocity profile. SPT correlations use empirical relationships to estimate Vs from blow counts. In Sault Ste Marie's glacial tills, those correlations carry ±15% uncertainty, enough to shift a site from Class C to Class D. NBCC 2020 Commentary L recommends direct measurement over correlation when seismic design is critical.
Which NBCC site classes are typical in Sault Ste Marie?
Class C (very dense soil or soft rock, 360 < Vs30 ≤ 760 m/s) is common on the glacial till uplands. Class D (stiff soil, 180 < Vs30 ≤ 360 m/s) appears in the St. Marys River valley where softer lacustrine clays are present. Class E (soft soil, Vs30 < 180 m/s) is rare but found in isolated organic-filled depressions. Class B (rock, 760 < Vs30 ≤ 1500 m/s) occurs where the Precambrian Shield outcrops or lies within 3 m of surface.
