A three-story medical office building near the South Shore Boulevard corridor was stalled for two months. The owner had borings with SPT blow counts but the structural engineer refused to release the foundation design. Why? The IBC Site Class was still a guess. League City sits on a mix of Pleistocene Beaumont Formation clays and recent floodplain deposits along Clear Creek. You can't infer Vs30 from blow counts alone in these layered coastal soils. When the contractor called us, we deployed a 48-channel MASW array across the parking lot, collected Rayleigh wave dispersion over a 100-foot spread, and delivered an inversion profile the next morning. The shear wave velocity data showed a stiff clay layer at 18 feet with Vs exceeding 1,200 ft/s, pushing the site firmly into Class C. The engineer got what they needed, and the foundation went in the following week. For similar soft-soil settings, we often pair the MASW survey with a seismic refraction profile to map the top of the Beaumont clay contact where the velocity contrast is sharpest.
A measured Vs30 of 1,250 ft/s versus an assumed 650 ft/s can drop the seismic base shear by a third. That number shows up on the structural drawings.
Frequently asked questions
How long does a MASW survey take on a typical League City lot?
Fieldwork for one or two spreads on a standard commercial lot takes about half a day. We stake the line, lay out the geophones, and collect multiple shots at each end to stack the dispersion image. You get the preliminary Vs30 value by the next afternoon, and the full report within three business days.
Can MASW work on pavement or inside an existing building?
Yes, with some caveats. On asphalt or concrete we use base plates with a thin sand layer to improve coupling. It works well on parking lots and warehouse slabs. Inside a building, we need at least 50 feet of clear space and we watch for rebar interference in the dispersion image. We have run successful lines inside tilt-wall structures around the Gulf Freeway corridor.
What does a MASW survey cost for a single-family home site in League City?
For a single residential lot, a basic MASW survey with Vs30 classification runs between US$1,530 and US$2,760 depending on the number of spreads and whether we pair it with a test pit for soil description. Custom home builders near the Clear Lake waterfront typically need two lines to capture lateral variability.
How deep can MASW resolve the Vs profile?
The rule of thumb is roughly half the maximum recorded wavelength. With a 240-foot spread and a strong sledgehammer source, we reliably resolve shear wave velocity down to 100 or 110 feet. That covers the typical depth of influence for shallow foundations and most pile groups in the League City area. For deeper targets we switch to a passive-source array or combine MASW with downhole seismic.
