To design a structure or other engineered system that is surrounded by, embedded in, or comprised of the earth around it, the engineer needs to know the physical characteristics of the site. Firsthand knowledge can be gained through digging or drilling. The engineer will sometimes choose to augment this approach with a noninvasive investigation technique to infer information about the remainder of the site that was not accessed directly. If the site lies beneath a body of water, direct investigation becomes more difficult, and engineer must rely increasingly on indirect investigation techniques to characterize it.
One technique for indirect site investigation is the Spectral-Analysis-of-Surface-Waves, or SASW, method. In the SASW method, the variation in shear wave velocity of the earth with depth is determined using seismic surface waves. This information has many uses in geotechnical site investigation and characterization. A key application involves design of structural foundations to withstand dynamic load such as those caused by earthquakes or vibrating machinery (see, e.g., Richart et al., 1970). Under dynamic loading conditions, the most important soil properties are the shear modulus and material damping characteristics. The maximum value of shear modulus, which occurs at small strains (strains less than 0.001%), can be determined from the shear wave velocity measurements.
The shear wave velocity of a material is also used to infer in situ conditions such as density, stress state, and soil cementation (Stokoe et al., 1989), and as an index property to evaluate sampling disturbance, classification. In the latter respect, knowledge of the shear wave velocity profile of the site is valuable in virtually any geotechnical site investigation.
This project has addressed experimental aspects of applying the SASW method to characterize geotechnical sites offshore. The SASW testing technique is presented in the context of seismic site investigation methods.
Related Publications: Luke, B.A., Stokoe, K.H., Wright, S.G., Roesset, J.M., and Lee, B., “Experimental Investigation of Surface Wave Velocities in Simulated Ocean Bottoms,” Proceedings, Offshore Technology Conference, Houston, Texas, OTC Paper No. 7112, pp. 287-297, May 1993.
Piccoli, S., Luke, B.A. and Stokoe, K.H., II. “In Situ Shear Wave Velocities From SCPT and SASW Measurement,” Proceedings, 10th European Conference on Earthquake Engineering, Vol.1, 28 August – 2 September 1994, Vienna, Austria, pp.337-342.
B.A. Luke and K.H. Stokoe, II, “Application of the SASW Method Underwater,” Journal of Geotechnical and Geoenvironmental Engineering, American Society of Civil Engineers, Vol. 124 No. 6, June, 1998, pp. 523-531.