Monitoring planimetric displacements in concrete dams

Abstract

Structural safety control of concrete dams is based in the analysis of the response of the structure, characterized by the evolution of several variables representative of its behaviour. Among these variables are absolute and relative displacements of points of the structure and of the foundation, uplift and flow rates in the foundation, and deformations and stresses in the concrete. In particular, the analysis of the displacements is very important, because they reflect the global structural behaviour of the dam. Therefore, monitoring plans of concrete dams usually consider the measurement of displacements of points of the structure and of its foundation. In the more important concrete dams, displacements measurement involves the simultaneous use of different methods, such as rockmeters, pendulums and applied geodesy. For the measurement of planimetric displacements, two methods have long been applied: one uses pendulums, placed inside the structure; the other makes use of applied geodesy. These monitoring methods are complementary, and coexist in many Portuguese large concrete dams since the 1940’s. Pendulums have the advantage of being more precise and, nowadays, can easily be included in automatic data acquisition systems. Geodetic methods can give information not only on the dam but also on the foundation and surrounding terrain. Because they are more expensive, geodetic campaigns are much less frequent, but are very important for the validation of the pendulums readings. In this paper a comparison between these two methods is made using the monitoring data of a Portuguese large concrete dam and some considerations about the LNEC experience on their use are presented.