MASW / VS30 Testing in Chilliwack — Shear Wave Velocity for Seismic Site Class

We still see projects in Chilliwack where the geotechnical report skips direct shear wave measurement and relies on assumed site class based on SPT blow counts alone. That shortcut can push a building into a more conservative seismic design category than necessary — or worse, miss a soft soil condition that should have been caught. The 2015 NBCC puts real weight on measured Vs30, and for good reason. When we run a MASW survey on a Chilliwack site, we are not guessing at amplification factors. We measure Rayleigh wave dispersion, invert for a 1D shear wave velocity profile, and calculate Vs30 directly. For sites near the Vedder River with complex post-glacial sediments, that measured profile often tells a different story from what borehole blow counts suggest on their own.

A measured Vs30 of 195 m/s and one of 175 m/s are only 20 m/s apart — but they land you on opposite sides of the Site Class C/D boundary in the NBCC.

Our approach and scope

Chilliwack sits at the eastern edge of the Fraser Valley, where the soil profile shifts from thick alluvial silts near the Fraser River to compact glacial till against the Cascade foothills. That contrast means a Vs30 value that works for a lot in Greendale may be completely wrong for a site up toward Promontory. We run the survey with a 24-channel seismograph and 4.5 Hz geophones, using a sledgehammer source for shallow resolution and a weight drop when we need deeper penetration past 30 meters. The dispersion curve gets picked carefully — fundamental mode only, no skipping on low-frequency data. After inversion we cross-check against any available borehole logs or SPT drilling data. If the site has been logged with CPT, we can also compare the small-strain shear modulus from MASW with the cone resistance profile. That kind of ground-truthing matters when the NBCC site class boundary sits right at 180 m/s or 360 m/s and a 15 m/s difference changes your seismic base shear by 20 percent.

Site-specific factors

Two lots on Chilliwack Mountain can sit 300 meters apart and have completely different seismic response. One sits on shallow bedrock with a Vs30 north of 760 m/s — Site Class B, low amplification. The other sits over a buried paleochannel filled with soft organic silt and loose sand, where Vs30 drops below 180 m/s and the site class lands in D or even E. If the structural engineer designs both buildings to the same base shear, one is overbuilt and the other is under-designed. The 2015 NBCC site coefficients for Site Class D and E amplify short-period spectral acceleration by 30 to 70 percent compared to Class C. That difference flows straight into lateral force-resisting system costs. We have seen developers in Chilliwack save six figures on structural framing just by measuring Vs30 and proving the site qualifies for a better class than the default assumption. The flip side matters more: not measuring it and missing a soft soil condition is a liability no one wants to carry after an earthquake.

Technical data

Parameter	Typical value
Standard	ASTM D4428 / D7400 (crosshole/seismic, adapted for surface wave)
Line length	46 m or 69 m depending on target depth
Receiver spacing	1.0 m or 1.5 m (24-channel spread)
Source	Sledgehammer on aluminum plate (shallow); accelerated weight drop (deep)
Frequency range	Typically 5–50 Hz for Vs30; lower for deeper bedrock mapping
Vs30 calculation method	Travel-time average per NBCC 2015, Section 4.1.8.4
Reporting	Dispersion curve, 1D Vs profile, Vs30 value, NBCC site class

Complementary services

Standard Vs30 Survey

One or two MASW lines per lot, 30-meter target depth, sledgehammer source. Suitable for single-family residential, townhouse blocks, and small commercial buildings. Report includes dispersion analysis, 1D Vs profile, and NBCC site class letter. Typical turnaround: 3 business days.

Deep Vs Profiling

Longer spread (69 m or 92 m) with weight-drop source for penetration to 50–100 m depth. Used when site class requires Vs30 to 30 m but we also need bedrock depth for foundation decisions. Common on larger mid-rise projects in Chilliwack where piles may be considered.

2D Vs Cross-Section Mapping

Multiple parallel MASW lines with lateral interpolation to map shear wave velocity across the site footprint. Useful for subdivisions and commercial pads where soil variability is expected. Helps the structural team decide if one site class covers the whole property or if zones need different design spectra.

Combined MASW + SPT/CPT Package

We run MASW alongside intrusive testing on the same day. The shear wave velocity profile gets calibrated against SPT N-values or CPT tip resistance. This combination satisfies both the NBCC site class requirement and the foundation bearing capacity analysis in a single coordinated field program.

Frequently asked questions

What does a MASW / Vs30 test in Chilliwack cost?

For a standard single-line MASW survey with Vs30 calculation and NBCC site classification, budget between CA$2,080 and CA$3,830 depending on line length, access conditions, and how many lines are needed. A typical single-family lot with one 46-meter line runs at the lower end. Larger commercial sites needing two or three lines with weight-drop source and 2D mapping land at the upper end. We provide a fixed-price quote after reviewing the site location and project scope — no surprises after the field work.

How long does a MASW survey take on site?

Field work for a single MASW line takes about 60 to 90 minutes, including layout, multiple shot stacks at each end, and a quick in-field quality check of the dispersion image. If we are running two or three lines on the same lot, the crew is typically off site in half a day. The processing and reporting — dispersion picking, inversion, Vs30 calculation, and the signed report — takes two to three business days after the field work.

Will a MASW test help me get a better site class for my Chilliwack project?

Often yes, and that is exactly why many structural engineers in the Fraser Valley now require a measured Vs30 instead of relying on the NBCC default assumptions. If your site has stiff glacial till or shallow bedrock — common on the slopes around Chilliwack Mountain and Promontory — the measured Vs30 can bump the site class from the default D up to C or even B. That reduces the seismic design forces and can cut structural costs noticeably. The report gives you the measured number and the corresponding NBCC site class letter, backed by raw field data the building official can review.