Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Dr. Hesham El Naggar

Abstract

A pilot seismic microzonation of the Greater Toronto Area (GTA) is used to establish the conditions and limitations of geophysical methods for site response investigations in city conditions. Maps of fundamental soil resonant frequencies, amplifications at these frequencies and interpolated average shear wave velocity of top 30 m of soil profile (VS- 30) used in soil classification were compared to the maps of drift thickness and surficial geology for the GTA. The non-applicability of the interpolated VS-30 map for site classification between measured test points is indicated. It is also shown that the soil response cannot be estimated properly using VS-30 values only. In order to enhance the capability of the horizontal-to-vertical-spectral-ratio (HVSR) method to resolve the fundamental soil resonances, a procedure and a computer program were developed for separation of ambient vibrations from nearby traffic as well as distant sources using the recorded waveforms before calculating the HVSR. A portable seismic station was developed for field HVSR waveforms recordings. It was also used for identification of building vibration modes. The influence of building vibrations on the HVSR result was investigated considering a benchmark building before construction started and after its completion. This influence is expressed as suppression or split-up of HVSR resonance if the building and soil resonances are close. This effect spreads out to distances comparable to the maximum dimension of the building. The experimentally obtained building resonant frequency at first vibration mode was found to be significantly higher than that calculated using empirical equations proposed by building codes, while the damping factor was less than the prescribed value. Additionally, the concept of using the HVSR inside a building to identify its resonances was examined using recorded waveforms, but the results did not confirm applicability of the HVSR for this purpose. The limitations and initial conditions that are necessary for successful implementation of refracted shear wave seismic profiling (SH-profiling) and multi-channel-analysis-ofsurface- waves (MASW) methods for application in urban areas are discussed. The problems with interlaying low velocity soil layer are pointed out. The soil response functions obtained from the microzonation studies using low intensity seismic sources differ from the response during an earthquake. An approach to estimate the changes of soil response in relation with expected Peak Ground Velocity (PGV) and Intensity of Modified Mercalli Scale (IMM) is proposed. The results were found to be in agreement with strong motion data from the epicentral area of a strong earthquake. It was concluded that the results from seismic microzonation studies should be considered in conjunction with models that simulate the change in dynamic characteristics of soil and buildings during expected earthquake events.

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