West Valley City grew fast. Orchards and farmland flipped to subdivisions and tilt-up commercial in two generations. That pace left gaps in the geotechnical record. A lot of the subsurface here is Lake Bonneville sediment, but the stiffness varies block by block. The 2020 Magna quake reminded everyone that the Wasatch Fault is active. Our MASW survey fills the data gap. It gives the structural engineer a measured VS30 value and an IBC site class, not an assumed one. That number changes foundation design, seismic base shear, and whether you need a liquefaction assessment before the footing is drawn. We run the survey, process the dispersion curve, and deliver the shear wave velocity profile in four business days.
A measured VS30 in West Valley City often drops the site class one letter compared to the default code assumption — and that changes the seismic design category.
Local geotechnical context
We use a Geometrics Geode seismograph with a spread of two dozen geophones. The array is 69 meters long for a 30-meter target depth. On a West Valley City commercial lot, we lay the line parallel to the proposed building footprint. No drilling, no cuttings, no traffic disruption. The hammer hits a steel plate, the geophones catch the Rayleigh wave, and the software stacks the shots. The risk in skipping this survey is a site class assumption that does not hold. Default site class D is common in the IBC, but a stiff profile in Hunter might qualify as C. That reduces base shear and saves steel. The opposite is worse: assuming C and discovering soft clay at 12 meters during excavation. Then the foundation is locked, and the structural engineer has to retrofit the lateral system.
Common questions
What is the cost of a MASW survey in West Valley City?
A standard active MASW survey with one array and VS30 classification runs US$1,860 to US$3,200 depending on site access, array length, and whether we combine it with a boring. The price includes field work, dispersion analysis, inversion, and the signed report.
How does MASW determine the IBC site class?
The array records Rayleigh wave propagation. We extract the dispersion curve, invert it to a shear wave velocity profile, and compute the average VS30 in meters per second. That value maps directly to Site Class A, B, C, D, or E per IBC Table 1613.2.3.
Can you run the survey on a paved lot in West Valley City?
Yes. We use bolt-down geophones on asphalt or concrete. The sledgehammer source works on pavement. The data quality is often better than on soil because the coupling is cleaner. We do not core or cut the surface.
What is the difference between active and passive MASW?
Active MASW uses a sledgehammer or weight drop and reaches 25 to 40 meters. Passive MASW uses ambient noise and goes deeper, up to 100 meters. For IBC site classification, active is standard. We add passive only when the structural engineer needs a deeper profile for basin effects or site response modeling.
