Monitoring vibrations in large dams

Abstract

According to the general guidelines of the International Commission on Large Dams (ICOLD) regarding structural safety and health monitoring, the performance of dams under operational/ambient vibrations and during seismic events must be evaluated. This is particularly important considering that dams are civil engineering structures of great relevance for populations which entail a high potential risk1. Therefore, Seismic and Structural Health Monitoring (SSHM) is a topic that has gained importance and thus the installation of monitoring systems for continuously measuring vibrations has been proposed for most of the new large dams, to evaluate their behaviour since the early stages of their service life, and for some of the older dams, built several decades ago, in which deterioration problems (e.g. swelling reactions) may have evolved over the years. In this scope, the present paper focuses on studying the dynamic behaviour of large dams over time, including identification of modal parameters (natural frequencies, modal damping ratios and mode shapes) and seismic response analysis, based on measured accelerations time histories. A comparison between experimental results obtained from continuous vibrations monitoring data and numerical modelling results from 3DFE models is presented. The goal is to emphasize the interest of the combined use of recorded data from continuous dynamic monitoring systems and of numerical modelling software for SSHM of dams. The case studies are two large arch dams that have been under continuous dynamic monitoring over the last ten years: Cabril dam (132 m high), the highest Portuguese dam, and Cahora Bassa dam (170 m high), located in Mozambique, one of the highest dams in Africa. The monitoring systems installed in both dams have similar schemes and were designed to continuously record acceleration time series in several points located in the dam body (upper part) and near the dam-foundation interface, using uniaxial and triaxial accelerometers. Specific software was developed for analysing monitoring data, including automatic modal identification and automatic detection of seismic events. The main experimental outputs are presented for Cabril dam and Cahora Bassa dam, with emphasis on the evolution of natural frequencies over time, on vibration mode shapes for various water levels, and, finally, on the measured response in accelerations during seismic events. The numerical results are computed using a 3DFEM program, based on a solid-fluid coupled formulation to simulate the dam-reservoir-foundation system, considering the dam-water dynamic interaction and the propagation of pressure waves throughout the reservoir.