USSD Rio Descoberto Dam - Sabino Freitas Correa-ENGL -

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internal drainage holes near the downstream face. Right after the conclusion of those works dam appearance was very good, without leakages at the downstream face and only some weak moisture spots could be observed at the upstream inspection gallery’s walls. The inspection gallery was clear and the foundation drains of the dam were operating well. Unfortunately, only three years after the conclusion of those works, the dam structure was worse than it was before. 3 rd Intervention In 1993 the Descoberto Dam was confronted with several serious problems that continued to get worse. More than just the leaching horizontal planes at the downstream “E”, “I”, “G” and “O” block’s face with significant flow, the fissure problem had also been increased. At the leaching blocks, new percolating horizontal planes were observed. The inspection tunnel was flooded and full of carted material from the rock bottom drains. During the course of this intervention CAESB decided to adopt a new approach in order to reach the cause of those chronic problems. At this time was constituted a Consultant Committee, formed by the Professor Victor de Mello, Eng. Francisco Andriolo and Eng. Walton Pacelli. The committee ordered the collection of samples of inner inspection gallery drains sediment, dam’s concrete, reservoir and drain water analyses. Those analyses indicated the presence of pyrite on the concrete aggregate and foundations. The combination of the presence of that mineral with the action of acidic (pure) water, was contributing for the degeneration of the concrete structure of the Descoberto Dam. The main pathologies observed were pyrite reactions. The best solution was to implant an upstream waterproof barrier, avoiding the contact between the reservoir water and the body of the dam. The injection works were suspended and a 4.2 meter long pilot drive of the diaphragm secant piles wall was built for testing and experience purposes. Situation Analysis OWNER CONDITIONS After those three interventions without successful results, now with a deeper research of the problem developed by the committee, CAESB decided to fully rehabilitate the Descoberto Dam. Starting from the conclusions of the committee, observing the increase of the structural problems, CAESB started to search alternatives for the dam rehabilitation in order to bid the best options.
Picture 1.a (left) and 1.b (right): A. Downstream face of “O” block damaged by aggregate reaction (Sept. 1999); B. Alkali-Aggregate reaction at the upstream _face of the insp. gallery (Sept. 1999) Basic Requirements Five basic requests were imposed by the owner at his search for rehabilitation techniques: 1) The rehabilitation method must prevent the water access to the dam concrete body and foundations; 2) The water supply must be fully maintained during the rehabilitation works; 3) The water quality must be fully maintained during the rehabilitation works; 4) The method efficiency must be able to be tested by sections during the performance of the rehabilitation works; 5) Grouting based impermeabilization solutions wouldn’t be accepted. PROBABLE METHODS A – Alternative 1: Diaphragm Wall across the body and concrete and foundations contact B - Alternative 2 : Geocomposites applied by caisson C - Alternative 3 : Geocomposites applied with an upper cofferdam D - Alternative 4 : Geocomposites appllied combined with an underwater concrete mat Figure 3, illustration of rehabilitation methods previously choice alternatives: A. Secant pile diaphragm wall, performed from the crest, drilling sequences near of the upstream face reaching until 5m into the rock foundations; B. Geocomposite barrier applied at the upstream face of the
Page 1 and 2: RIO DESCOBERTO DAM - "IN THE WET" R
Page 3: Opening Remarks The consequences of
Page 7 and 8: First Evaluations Before the beginn
Page 9 and 10: Diaphragm Continuity The continuity

USSD Rio Descoberto Dam - Sabino Freitas Correa-ENGL -

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