The Increasing Role of Surface-Wave Measurements in Site Characterization

Ken Stokoe

University of Texas

Date & Time
Building 3, Rambo Auditorium
Alan Yong

Geotechnical engineers and geophysicists are faced with the problem of characterizing geological materials for site-specific design in the built environment. When the design or analysis includes determining the dynamic response of important infrastructure and critical facilities due to dynamic loading such as earthquake shaking, seismically-based measurements in the field are essential for direct characterization of material stiffnesses, particularly shear wave velocities. These measurement techniques were generally adapted from exploration geophysics, required boreholes, and involved small-strain body waves. Today, field seismic measurements are widely used and are evolving because: (1) the measurements have a strong theoretical basis, (2) they can be performed in the field and laboratory, thus forming an important link between these measurements, and (3) in recent developments in the field testing involving surface waves, they are noninvasive which makes them very cost effective. Active-source, surface-wave methods are used today over depths ranging from a few meters to more than 500 m, and passive surface-wave methods are used for even greater depths. Examples of shear-wave velocity (VS) profiles evaluated using Rayleigh-type surface waves are presented. The active-source, Spectral-Analysis-of-Surface-Waves (SASW) method is used to illustrate the measurements. VS profiles have been performed in materials ranging from uncemented soil to unweather rock. Some interesting observations are: (1) the coefficients of variation (COVs) in the VS profiles are generally less than 0.15 over sites with surface areas of 50 km2 or more as long as material types are not laterally mixed, (2) the stiffness of rock near the ground surface is generally overestimated, and (3) when values of VS measured in the field and laboratory are compared, biases in VS at soil sites versus rock sites exhibit opposite trends.

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