Seismic Site Class Determination Using Shear


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Dear David,

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(ESP). Each quarter, ESP produces an informational article about a particular topic that we feel may be of value to our clientele. We hope you find the material informative and we welcome the opportunity to assist you with any support that you may need. All articles presented are also available on our website

Seismic Site Class Determination Using Shear Wave Velocities

In the past decade, most states, including North Carolina, South Carolina, and Virginia have adopted the International Building Code (IBC), which requires that seismic design be performed for all new building construction (except single and two-family homes). The seismic design process includes the determination of the Site Class, which ranges from A (hard rock) to E (soft soil profile), with Site Class F reserved for very problematic soils. Most building sites are assigned Site Class C (very dense soil and soft rock) or D (stiff soil profile). The lower the Site

Class, the more bracing and anchoring is required within the building components, resulting in increased costs of design and construction.

While the surface wave method measures a larger volume, often to 100 feet depth

The SPT sampler stops on rock



Site Class can be determined using the Standard Penetration Testing (SPT) method, where “blow counts” required to drive a standard sampling tool one foot while drilling are counted (Nvalues). However, since the sampling tool cannot sample rock, only the N-values collected above rock can be used to calculate the Site Class. Also, it has been shown by others that the Nvalue analysis outlined in the building code often underestimates the strength of Silty Soils, very common in the Piedmont and Mountains of the Carolinas and Virginia. The limitations of the

SPT method may yield a lower Site Class determination for seismic design as compared to other available methods

Fortunately, the IBC also allows the use of shear wave velocity data to calculate Site Class.

Properly obtained, shear wave velocities can provide more representative data for evaluating the strength of soil and rock in contrast to using the SPT method. In addition, use of the shear wave velocity method for IBC Site Class determination is not limited to the depth of the soil profile.

Use of shear wave velocity data can often yield a more favorable Site Class determination by including the higher velocities of the rock profile.

There are several methods to obtain shear wave velocity data. Typically, the most cost-effective method is by using one of the surface wave testing techniques (such as MASW, SASW, or ReMi). The basic method consists of laying out a line of geophones (ground motion sensors) connected to an engineering seismograph and impacting a metal plate on the ground surface with a sledgehammer. Once the surface wave data are obtained, the data are analyzed to produce a model of the shear wave velocities of the soil and rock. As with any testing method recognized in industry standards with specifications, the analysis and determination process must be performed by experienced and trained/certified professionals.

In many cases, the use of the shear wave velocity method has provided a higher Site Class for building sites than that obtained from the SPT method. The cost of a half-day geophysical test is relatively small compared to the potential savings that can be achieved by using a higher Site Class, such as “C” instead of “D”.

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