A five-story mixed-use project on Burke Street hit a snag last fall. The initial seismic site class assumed dense ground, but borings showed deep saprolite over weathered felsic rock—typical of Winston-Salem's Piedmont geology. The structural engineer needed a measured VS30, not an assumption. We mobilized a 48-channel MASW array across the footprint in one morning. The result: a Site Class D profile with a VS30 of 277 m/s, allowing the design team to drop the overconservative Site C assumption and trim lateral system costs. Winston-Salem’s varied terrain—from the downtown grid near 36.0998°N to the ridges west of Silas Creek—makes reliance on default site classes risky. A direct seismic refraction survey can complement MASW when bedrock dips sharply, and for deep liquefaction screening we often partner the data with CPT soundings to resolve thin low-velocity layers.
A measured VS30 in Piedmont saprolite can shift site class one full letter compared to generic assumptions—directly altering seismic design forces.
Scope of work
Key technical characteristics:
- 48-channel linear arrays with 2–5 m geophone spacing for depth resolution down to 30 m
- Dispersion curve picking via wavefield transform and forward-modeled fundamental mode
- VS30 computed per NEHRP guidelines with horizon-by-horizon time-averaging
- Supplemental SPT drilling at array midpoints to ground-truth velocity–N-value correlations in Piedmont residuum
- Site class reporting per IBC 2021 Table 1613.2.3 and ASCE 7-22 Chapter 20
Area-specific notes
Downtown Winston-Salem east of Marshall Street sits on deep alluvium and weathered schist of the Smith River allochthon, routinely yielding VS30 below 250 m/s. Move west toward Buena Vista and the ground changes: thinner residuum over competent granite with VS30 often above 350 m/s. That’s a full site class jump—from D to C—within a two-mile radius. Developers who transfer soil reports between parcels without new geophysics are gambling with the seismic design category. The IBC 2021 ties the design spectral response accelerations directly to site class; a misclassified Site E where Site D is present can inflate foundation costs by 20–30 percent. We have also measured velocity inversions—soft layers beneath stiffer crust—in old fill zones along the Salem Creek corridor. These inversions flatten dispersion curves and can be missed entirely by refraction-only approaches, making high-quality MASW dispersion analysis critical for Winston-Salem’s complex Piedmont stratigraphy.
Standards used
ASCE 7-22 Chapter 20 (Site Classification Procedure for Seismic Design), IBC 2021 Section 1613.2.3 (Site Class Definitions), NEHRP Recommended Provisions (VS30 time-averaging methodology), ASTM D4428/D4428M-14 (Crosshole Seismic Testing — referenced for velocity calibration), ASTM D7400-17 (Downhole Seismic Testing — referenced for supplemental profiling)
Linked services
MASW / VS30 Survey
Active-source 48-channel MASW arrays for IBC site class determination on commercial and institutional projects across Winston-Salem.
Seismic Refraction Tomography
P-wave refraction for bedrock mapping and rippability assessment, ideal where Piedmont rockhead dips sharply beneath residuum.
SPT Drilling with Downhole Seismic
Standard penetration testing paired with downhole velocity measurements for direct VS30 ground-truth at select boring locations.
Liquefaction Screening Package
Combined CPT, SPT, and shear wave velocity data to evaluate cyclic stress ratio and liquefaction potential per NCEER methodology.
Typical parameters
Quick answers
When is MASW required instead of using SPT N-values for site class in Winston-Salem?
IBC 2021 allows site class determination via measured shear wave velocity, SPT N-values, or undrained shear strength. MASW becomes the preferred route when the subsurface contains Piedmont saprolite—partially weathered rock that yields N-values uncorrelated to true stiffness. In downtown Winston-Salem where fill over alluvium is common, VS30 from MASW also avoids the N-value correction ambiguities that arise in silty fine sands.
How much does a typical MASW survey cost for a site in Winston-Salem?
A standard MASW survey with two orthogonal spread lines and a VS30 report typically runs between US$1,460 and US$3,500, depending on array length, number of measurement locations, and whether supplemental refraction or downhole calibration is needed. Sites with steep topography or limited access may fall at the upper end of that range.
What depth does the MASW method investigate for VS30 calculation?
VS30 requires a shear wave velocity profile to a depth of 30 meters. Our 48-channel arrays with 3–5 meter geophone spacing reliably resolve velocity structure to 30–35 meters below grade in most Piedmont conditions. Where bedrock is shallower—common in western Winston-Salem near the Surry thrust sheet—we model the velocity of the underlying half-space using fundamental-mode inversion constraints.