Use of ambient vibration measurements to estimate site fundamental periods at Condado and Isla Verde, PR

Abstract

The knowledge of the natural periods of soil deposits in urban areas can be very valuable for the safe design of structures to avoid resonance conditions. This thesis presents a study aimed to estimate site natural periods of two coastal areas in the cities of San Juan and Carolina in Puerto Rico, named the Condado and Isla Verde districts respectively. The final goal is to prepare isoperiods maps in digital form with the software ArcGIS that can provide in an easy manner the site natural periods superimposed on other relevant information. The Nakamura method was used to obtain the fundamental natural periods of the soil deposits. An ambient vibration survey was conducted to obtain low intensity ground motions (microtremors) required by the method. Spectral ratios were defined in terms of the Power Spectral Density Functions (PSDF) of the horizontal and vertical acceleration time series. Forty six ambient vibration measurements (microtremors) were taken in the Condado and Isla Verde areas. The results obtained with the H/V spectral ratio were compared against other three techniques. The first approach uses the analytical transfer functions of the stratified soil deposits defined with data from downhole tests. This was done to validate the Nakemura’s results with a powerful and robust method. The second technique was based on the use of the continuous wavelet transform applied to the accelerograms in the horizontal directions. This helped to determine the site frequency when the Nakemura’s results were inconclusive. The third technique consisted in the application of the H/V method but using earthquake records from low intensity events recorded in the area by the Puerto Rico Strong Motion Program. This last technique provided mixed results and further studies are warranted.