YACYRETA HYDROELECTRIC POWER PLANT

 

WORKS FOR THE PROTECTION OF THE AGUAPEY STREAM

 

Introduction

 

Aguapey Stream Basin Protection is one of the main works for the finishing of Yacyreta Reservoir Project. This Project is fundamental since its completion will impede the flooding of vast territories of the Republic of Paraguay thought the elevation of the reservoir storage level to its definitive height of 83 m above sea level.

Main works consist of 64.5 km of land dams which begin in the left margin of the river in Rincon de Santa Maria, cross over the main branch where they reach a maximum height of 43 m. After covering part of Yacyreta Island, cross over Aña Cua branch, then continue through the right margin 25 km, finishing in Paraguay’s San Cosme and Damian cities.

Nearby Damian city is Aguapey Stream, tributary of Parana River, which discharges an average flow of 25 m3 / s over the reservoir at a level of approximately 78 meters.

The Elevation of the Reservoir to its definite storage level of 83 masl would flood Aguapey Stream Valley, generating a sub reservoir of over 450 km2 of which 360 km2 correspond to mainland and wetland, and the remaining 200 km2 corresponding to productive land.

To prevent this situation, Protection Works of Aguapey Stream have been developed consisting of a land dam in the stream mouth to avoid flooding when the reservoir in filled with more than the current storage level and, a drainage channel of 12.5 km long downstream which connects with the collection channel that exists in the dam’s bottom until it reaches the Aña Cua branch.

 

Aguapey II – Aguapey Stream Closure Dam

 

The second stage of the work consisted in the construction of Aguapey Dam with an approximate length of 4.3 km. reaching to a crest elevation over the asphalt pavement of 86.45 masl, the construction of an irrigation channel in the Paraguayan margin and the Closure of the Temporary Deviation and the Opening of Aguapey Deviation Channel simultaneously.

The original Dam Project included in the bidding documents consisted on a body of soil and a blanket towards the reservoir. The Dam contained two lower clay cofferdams in the stream channel bed that along with the blanket were works performed over 14 years ago. All the leaking control in the foundation relied on the blanket and wells of relief.

The Work consisted on the drainage of the sector between both cofferdams and the continuation of the Dam building works until it reaches a final profile of 86.45 masl.

Before the construction started, work direction had begun, by conducting a geological-geotechnical auscultation of the stream channel, the cofferdams and the blanket, in order to reassure that the emptying of the precinct was done in a safety manner.

This study was meant to contemplate the current state of the work that was going to be part of the final project, and the fact that the reservoir level was higher than the level expected when the dam was planned.

As a result, the conclusion was that the bottom of the stream presented significant soil heterogeneity with important permeable sand layers and that the blanket had low waterproofing capacity.

After calculations of thin matter in which filtrations and slopes stability were analyzed, the conclusion was that the central precinct couldn’t be emptied without risking   siphoning effect and cofferdams failure. Even though it was possible to empty the precinct up to a level of 76.50 masl, the work between both cofferdams should be done under a strap of approximately 4 m of water.

Since the blanket provided a poor water proving capability, flow lines were to be prolonged underneath the dam so as to decrease the gradients to acceptable levels.

During design adjustment, 3 constructive alternatives were analyzed to enlarge the flow lines underneath the cofferdams precinct:

  • A bentonite screen constructed underneath the cofferdam in Yacyreta side that goes up to the waterproof mantles
  • A waterproofing of the bottom of the central precinct between the cofferdams through the placement of a concrete layer poured under water
  • The waterproofing of the mentioned precinct through the collocation of a waterproof membrane under water. 

Comparative Studies were done taking into account constructive complexities, costs, work safety and construction time since there were contractual key dates related to the expected time for the elevation on the storage level of Yacyreta Reservoir.

From those studies, the last alternative resulted as the most convenient being the new dam project as indicated in the diagram below:

With the alternative defined, the constructive challenge was the correct placement of the waterproof membrane under a strap of approximately 4 m of water.

Giving the critical conditions of the Project, after having the bottom waterproofed and before the drainage of the precinct, this one had to be filled up with sufficient weight so as to prevent a siphoning effect and stability mentioned before. Filling the area with sand coming from the Parana River was the most viable solution.

The selected membrane was made up of Polyvinyl chloride of 1.2 mm. thick and a resistance of 15 Mpa according to ASTM 882.

The membranes were rolled in rolls of about 1.80 m. The panels were welded by heat with controlled temperature to achieve complete fusion of juxtaposed panels, forming a single piece of the size of the precinct to be fulfilled.

The joints were tested in perpendicular tension to the seam making sure that they possessed greater strength than the membrane itself.

Before the placement of the waterproof membrane, another geotextile membrane was spread on the bottom to protect the former of any tearing element that may be deposited in the bed of the stream.

Alter being welled, the membrane was rolled and placed over a pontoon withheld and mobilized by a hoisting engine for a controlled placement of it over the bottom of the stream.

Before the filling of the precinct it was necessary to counterbalance the membrane to avoid dislocation or movements.

For this reason geocells of 7.5 cm high, 1.1 mm thick of geotextile were employed as well as concrete slipped in a working platform on a maximum area of 1500 cm2 to transform them into counterweight articulated sheets. Afterwards, they were lifted with a rocker arm and collocated over the membrane with a crane.

The positioning and the correct placement of the membrane as well as the geocells were inspected underwater by divers from the Consulting Consortium.

Once the membrane had been counterweighted, the hydraulic filling of sand took place up to the level of the new project.

Alter the filling was done, the precinct was drained to the expected levels continuing in this way with the work in the traditional way.

The new design is stable even though if the membrane deteriorates or even if it disappears during the work. Something that is highly improbable.

The Dam Project is completed with the construction of an irrigation channel on the Paraguayan side with a volume of 600m3 of reinforced concrete.

Special Tasks had to be made for the coordination of the closure of the stream discharge over the reservoir and the simultaneous opening of Aguapey Channel.

Those tasks were performed under strict supervision and having developed an Operative Plan and a Plan for Contingencies on the handling of the plug cofferdams and the Channel gates that provide all the possible contingencies and other exceptional events that could appear while the Aguapey sub reservoir drainage was done.

 

The main data of the project is the following:

 

The Project Adjustment, Inspection, Contract Administration and Work Direction were in charge of the Joint Venture COINTEC-INCONPAR-GCM-ELEPAR-GEIPEX-GCA UTE under the denomination ENERYA Consortium.