In spite of the widespread use of displacement measurement instruments, such as pendulum and rockmeters, geodetic surveying methods continue to be used in the displacement monitoring systems because they provide: i) An independent control of the other methods ii) A global relation between the dam, its foundation and the surrounding terrain. 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.Ĭonventional terrestrial geodetic surveying methods, such as precision triangulation, traversing and geometric levelling, have been present at the displacement monitoring systems of the majority of the Portuguese concrete and masonry large dams, since the 1940's. Because they are more expensive, geodetic campaigns are much less frequent, but are very important for the validation of the pendulums readings. Geodetic methods can give in-formation not only on the dam but also on the foundation and surrounding terrain. Pendulums have the advantage of being more precise and, nowadays, can easily be included in automatic data acquisition systems. These monitoring methods are complementary, and coexist in many Portuguese large concrete dams since the 1940's. For the meas-urement of planimetric displacements, two methods have long been applied: one uses pendu-lums, placed inside the structure the other makes use of applied geodesy.
In the more important concrete dams, displacements measurement involves the simultaneous use of different methods, such as rockmeters, pendulums and applied geodesy. Therefore, monitoring plans of concrete dams usually consider the measurement of displacements of points of the structure and of its foundation. In particular, the analysis of the displacements is very important, because they reflect the global structural behaviour of the dam. Among these variables are absolute and relative displacements of points of the struc-ture and of the foundation, uplift and flow rates in the foundation, and deformations and stresses in the concrete. 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 be-haviour.