Before structure planning ever begins, knowledge of a building site’s seismic classification (i.e., is it hard rock or weak clay beneath the proposed structure?) is paramount. A proper foundation is critical to the success of any construction project, from jobsite safety to long-term integrity.

The International Building Code (IBC) or Uniform Building Code (UBC) and National Earthquake Hazards Reduction Program (NEHRP) require knowledge of the soil site class, which ranges from A (hard rock) to F (soft clay). Construction based on an inaccurate site classification can lead to safety and liability problems, while an overly cautious site classification can lead to extraneous construction costs. Proper site classification early in the design stage can save thousands to millions of dollars, depending on a project’s scope.

For commercial or industrial construction, and for structures that will generate large dynamic loads, calculating a site’s seismic shear wave (S-wave) structure is critical, and can be estimated geophysically from the ground surface. In contrast, downhole or crosshole methods are intrusive and sample a very limited area around the borehole(s). Surface-wave methods are largely immune to interference from traffic, wind, and construction. Surface-wave data can provide one-dimensional graphs of S-wave velocity versus depth, 2-D cross sections, or 3-D block models of subsurface S-wave velocity variations. Alternatively, downhole or crosshole seismic methods can be used in laterally restricted spaces where such information is needed.

RETTEW assists with seismic site classification across an array of markets so our clients can design the best foundations for their construction projects.