EFFETS DE SITE DU BARRAGE EN ENROCHEMENT DENIS-PERRON (SM-3), QUÉBEC

ABSTRACT: In this paper, spectral analysis of strong-motion and ambient noise were carried out at the Denis-Perron (SM-3) dam to evaluate possible site effects and the actual frequency of resonance (FN) of this earth structure. The Denis-Perron Dam is a rockfill embankment dam standing 171 meters high and 378 meters long. It is the highest earthfill dam in Québec. The dam is built in a narrow valley with its left and right abutments resting on a gneissic rock foundation. Three-component digital strong-motion stations were installed on the dam's crest and on bedrock, on the left abutment. The spectral response of the accelerographic data is estimated from a series of three small earthquakes that occurred in 1999 and 2002. The magnitudes Mw of these earthquakes varies between 4.0 and 5.1. The FN was estimated by calculating standard spectral ratios using the bedrock station as the reference site. For all three earthquakes, the spectral amplification curves show a clear peak between 2.0 and 2.2 Hz, in the transverse direction, and between 1.55 and 1.8 Hz, in the longitudinal direction. Amplification ratios range between 5.0 and 18.5 in the transverse directions and between 2.8 and 19.5 in the longitudinal direction. When the site response is evaluated from ambient noise recordings and using the H/V technique of interpretation, the FN is about 2.1 Hz in the transverse direction and 1.8 in the longitudinal direction. Relative ground motions at the two stations sites are presented and discussed. A strong amplification of the vertical motion component was also observed at both station sites.