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UBC Theses and Dissertations

Insitu measurement of dynamic soil properties with emphasis on damping Stewart, William Patrick


Measurements of the shear wave velocity (Vs) of near-surface soils by downhole and crosshole techniques have become fairly common, including the use of the seismic cone penetration test (SCPT) both at UBC and commercially. A full trace (typically in the order of 400ms long) of the received signal is normally recorded at selected depths, but traditionally only one point is used to determine Vs,. This research is to determine if, these records could provide, at minimal cost, further information on soil properties. Initially alternate methods of Vs calculation were investigated, but the main thrust of this research was use of the amplitude information in the signals to determine bow-strain damping. A variety of equipment; including three source types (mechanical swing hammer, Buffalo gun, and drop weight), three types of receivers (accelerometer, geophone, and bender), and the use of two cones; has been investigated and used for SCPTrs at several sites. The nature of the measured signals in both the time and frequency domain has been investigated and the importance of windowing to isolate the shear wave from the balance of the signal has been clearly demonstrated. The cross-over method of velocity determination has been most commonly used at UBC. Two other methods (cross-correlation and phase of cross-spectrum) have been developed and compared. The recommended approach is the phase of the cross-spectrum method applied to windowed signals. Five methods of damping calculation have been considered in some detail. Three of the methods (rise time, attenuation coefficient, and spectral ratio slope{SRS)) were available in the literature, and the other two methods (modified SHAKE and damping spiral) were developed as part of this research. The most general is the damping spiral method, and it can be shown that the SRS method is a special case of the damping spiral approach. The SRS method is applied simultaneously for several depths within a soil layer, eliminates geometric corrections and was found to be the most accurate approach. Attempts were made to evaluate damping from actual earthquake records, both local (Pender Island earthquake, using SHAKE) and foreign (Eotung array, Taiwan, using the SRS method), but met with little success. Specific recommendations have been developed for all three facets of the measurement and calculation of damping. It has been shown that these recommendations lead to results that are repeatable and that are consistent with both laboratory and published values, for both shear wave velocities and damping.

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