Spectral-Analysis-of-Surface-Waves (SASW) testing is a seismic technique for measuring shear wave velocity profiles at soil and rock sites. The SASW method utilizes a wave source and multiple receivers on the ground surface. The wave source used in SASW testing must apply a vertical force to the ground surface over a wide range of frequencies. High frequencies are used to characterize shallow deposits, while low frequencies are used to characterize deep deposits. Drop weights have been found to be effective sources for generating seismic waves over a broad range of frequencies. This paper describes the design and construction of a mobile drop weight source for SASW testing. The source was designed to generate a wide enough range of varying frequencies such that shear wave velocity profiles can be determined to a depth of at least 100 ft (30 m). The average shear wave velocity in the top 100 ft (30 m) of a soil profile (Vs_(30)) is often used as an index to predict the response of a site to seismic shaking. The source consists of a custom-built trailer with a retractable tripod superstructure. A hoisting system is able to lift a 4000 1b (17.8 kN) drop weight up to 8 ft (2.5 m), and a quick release system to assure free-fall of the drop weight. An inertial measurement is made of the forcing function applied to the ground surface.
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