SEISMIC SITE CLASSIFICATION
Site soil conditions are important in determining Seismic Design Category. Site Class is determined based on the average properties of the soil within 100 feet of the ground surface. Geotechnical engineers use a variety of parameters to characterize the engineering properties of these soils, including general soil classifications as to the type of soil, the number of blows (SPT-N) needed to drive a standard penetration tip 1 foot into the soil, the velocity (Vs ) at which shear waves travel through the material and the shear resistance of the soil (Su ) as measured using standard laboratory test procedures.
The table below lists the six Site Classes recognized by the NEHRP Recommended Seismic Provisions and the engineering parameters used to define them. On many sites, the nature of soils will vary with depth below the surface. The NEHRP Recommended Seismic Provisions permits sites to be categorized as Site Class D unless there is reason to believe that the site should be downgraded to Site Class E or F. Classification of a site conforming to Site Class A, B, or C generally will lead to a more economical structural design than assuming site Class D; however, in order to achieve a higher rated Site Class (A, B, or C) additional testing of the soil stratum is recommended.
Table: Site Class and Soil Types
What is Refraction Mircotremor (ReMI)?
ReMi is a seismic surface wave testing method that is used to aid in seismic site classifications. ReMi method uses ambient noise and surface waves to generate a detailed vertical shear wave velocity (Vs) profile of soil stratums up to 300 feet in depth.
How it works?
The ReMi test setup includes a linear array of multiple equally-spaced geophones established inserted into the soil and connected at one end to a seismograph. The length of the array depends on the depth of investigation; in order to obtain information to 100 feet a length of 300 feet or more is typically required. Once the array geophones are established, the seismograph records both ambient and active noise within the area.
Once the information is collected and interpreted the end product is a one-dimensional column of shear wave velocity variation for each seismic line established at a site.
Why us ReMi testing?
The ReMi method is capable of detecting thin layers and velocity inversions, and is highly reliable and commonly used method for earthquake design and seismic site classification determinations. The ReMi method is particularly effective in noisy environments, which are ideal for shear wave profiling in urban environments where other seismic testing methods are not applicable due to large amounts of ambient noise.