Long-term Behavior of a Spanish Arch Dam

PREFACE

In this paper, the long-term behavior of an arch dam for the last 36 years is outlined. It is based on dam safety management tasks; namely safety reviews, surveillance and monitoring carried out during construction, first impoundment and regular operation of the dam and its reservoir.

The dam was constructed in 1974/75 and has a maximum height of 102 m over foundation, a crest length of 302 m and a reservoir capacity of 110 hm³

The monitoring system consist of the following:

The design of the monitoring system was tailored to the specific characteristics of the dam and its foundations, aiming at implementing a robust, efficient and useful solution.

DAM CONSTRUCTION

Control and monitoring of dam behavior started in the early stage of the construction, in 1974. Since the beginning, the installation of all instrumentation sensors was made very accurately and all details, constants, formulas, material characteristics and readings were systematically documented by OFITECO personnel.

Apart from the control of concrete temperature, joint movements before, during and after the injection of vertical joints was monitored. In addition, concrete deformation was controlled with 1D and 2D extensometer groups, providing essential data for the calculation of absolute stress and strain conditions in the dam.  If this data would not have been recorded during the construction phase, only relative stress and strain conditions could have been estimated afterwards.

Control of joint injection pressure and deformations, both in internal and external joint meters.

FIRST IMPOUNDMENT

A detailed monitoring program was set out in order to control thermal, deformational and hydraulic variables during all stages of first filling.

Periodical reports were submitted with a detailed evaluation and interpretation of monitoring data. Temporal evolution of variables was shown and compared in hand made charts, and some specific results were presented in drawings.

Tensional state due to reservoir level raise (19 m) and thermal variation (winter-summer) during first impoundment

DAM AND RESERVOIR OPERATION

In order to carry on with dam behavior control after first impoundment, reports and safety reviews were periodically prepared so that anomalies in dam behavior or monitoring system performance have always been foreseen. In the beginning of the nineties, OFITECO developed the first version of DAMDATA which allowed management and visualization of large quantities of monitoring data and efficiently helped to improve the quality of safety reports.

Absolute horizontal deformations registered with direct and inverted pendulums at different elevations. Graphics prepared with previous version of DAMDATA by Ofiteco.

In addition to the visualization of timeline charts of monitoring data in diagrams, various drawings showing hydraulic, deformational and thermal scenarios have been updated over the last decades. In these drawings, key aspects are portrayed and data is interpreted.

Deformation of vertical joints in August 2008

Uplift pressure at different dates compared with theoretical laws.

Moreover, a statistical analysis of specific sensors has periodically been carried done with AUSMODEL. This software tool, which was also developed by OFITECO for the analysis of monitoring data, quantifies the influence of external variables like temperature and reservoir level on control variables. In addition, accumulated variations in time are identified by isolating the influence of external components. Non-elastic or drifts in dam behavior can be identified thanks to this type of complex analysis on statistical residues; rather than actual measurements

Statistical analysis of a piezometer with AUSMODEL: Accumulated variation of residuals over a certain sensor since 1980.

ADDITIONAL STUDIES

Due to the high quality and quantity of the available monitoring data, it’s been possible to examine the dam behavior more thoroughly. A transitory deterministic model was developed and calibrated with monitoring data, allowing to make forecast on pendulum deformations, temperatures and stress-strain scenarios in the extensometers, depending on the temporal evolution of thermal and hydraulic loads. In this way, areas of maximum stresses were determined and different risk scenarios identified.

Principal stresses in downstream face of the arch dam

Moreover, a Neuronal Network Model with monitoring data was created in order to make more precise forecasts on dam behavior and provide an alternative approach to the above mentioned deterministic and statistical models. For the prediction of extraordinary situations, output data from the deterministic model was used to provide the neuronal network with input data.

Forecast of seepage in V-notch weir carried out by means of Neuronal Network Models

CONCLUSION

The presented showcase is a good example of an efficient and thorough dam safety management throughout the life of a remarkable infrastructure. Surveillance and monitoring have played a key role in this process.

Relying on a good design of the monitoring system, and following a careful installation of monitoring sensors during its construction from 1974 to 1976, a strict and constant gathering of monitoring data, periodical dam behavior evaluations and regular maintenance of all components have proven to be of capital importance.

This dam safety management has allowed the dam owner to understand, control and forecast the behavior of the dam, identifying potential risks, prioritizing investments and always taking the necessary corrective measures.