Mexican powerhouse

08 May 2008

The outfalls of the dam's two spillways

The outfalls of the dam's two spillways

The El Cajón Dam is located approximately 200 km Northwest of Guadalajara in the Mexican municipalities of Yesca and Santa Maria del Oro. The development is part of the larger Santiago hydroelectric system that consistsof 27 schemes in all. When complete, the El Cajón dam will have an installed generating capacity of 750 MW ” about 2% of Mexico's total electricity consumption.

The contract to construct the dam was put out to tender by Mexico's Comisión Federal de Electricidad (CFE) ” the national electricity authority ” in September 2002. It was won in March 2003 by the Constructora Internacional de Infraestructura S.A. de C.V. (CIISA) consortium, a group of four companies led by Ingenieros Civiles Asociados (ICA), Mexico's largest construction company. ICA has a 65% interest in the consortium, split between two separate subsidiary companies. Energomachexport, the Russian turbine supplier has a 20% stake, and Peninsular Compania de Constructora owns the remaining 15%

Preparatory work got underway in April 2003, with work on the main structures beginning a year later, following the successful financing of the project by the consortium. The group secured US$ 682 million on the money markets by means of a syndicated bank loan and bond debt to cover construction costs on the US$ 748 million contract.

These arrangements were the largest structured transaction for this type of project in Mexico's history, and the scheme was given investment-grade ratings by both Moody's and Standard &Poor's. Backing for the finance was in the form of the consortium's contractual payment rights, along with a structured set of guarantees from CIISA's shareholders.

Massive task

As the financial costs indicate, the El Cajón Dam is a massive construction undertaking. The dam across the Rio Grande de Santiago River will be a Concrete Face Rockfill Dam, (CFRD), measuring 186 m high and 550 m along the crest, with a volume of 10.2 million m3.

Underground, the hydroelectric installation will entail the construction of more than 3.2 km of tunnels comprising one 706 m and one 786 m diversion tunnel, one 310 m relief tunnel, some 750 m for pioneer tunnels for the generation works, and two 120 m sloped tunnels with their respective 220 m horizontal spans to conduct water to the generation house. These will take the form of two massive underground caverns, measuring 22 m wide and almost 100 m long, the second being 15 m wide and 65 m long. Each will house a 375 MW turbine.

Total rock excavation on the project will come to some 11.5 million m3, sothe successful execution of the project required careful equipment selection. Constructora el Cajón, S.A. de C.V. (CECSA), the company incorporated by the CIISA consortium to act as general site manager contacted Sandvik Mining and Construction de Mexico (formerly known as Tamrock de Mexico S.A. de C.V.), a long-time supplier in the region with a strong working relationship with ICA.

Working with Ameco Services, the contracting company responsible for supplying the drill units for the El Cajón project, Sandvik Mining and Construction de Mexico, worked out a rental program for 12 Tamrock Ranger 500-2 surface rigs to add to the three Tamrock CHA 550 drills already owned by Ameco Services.

A key requirement was that the machines had to be able to drill both horizontally andvertically, something the Ranger 500-2 can do by means of a hydraulic piston change. Thecompany also needed equipment with low pollution, different drill functions and enough capacity to drill to a depth of 15 m.

All the Tamrock drill units are in action 20 to 22 hours per day, 300 days a year. Onthe quarry bench, the Rangers drill 3 “ (76 mm) diameter blast holes at typical depths of 7 to 10 m in the hard igneous and sedimentary rock. In order to meet deadlines, 25000 m3 of rock must be blasted and removed each day for excavation of the spillway.

In addition to quarrying ” concrete for the dam will require 600000 m3 of aggregate ” the Rangers are involved in various aspects of surface rock excavation. The spillway construction entails some 5 million m3 of excavation, as does the construction of a bank. Construction of the dam itself entails 10 million m3 of rock removal.

For the underground work there are three modified Tamrock jumbo Paramatic and two Maximatic drills working on the tunnels. The Tamrock jumbo Paramatic drills, with three drill booms each, are being used to drill the tunnelroof at the rate of 50 drill metres per hour per drill arm, or 150 m per hour in total. When complete, each diversion tunnel will be 14 m high and 14 m wide. One of them will be 706 m long, while the other will be 786 m. Thetotal rock excavation will come to 136000 m3, and 29500 m3 of concrete will have been used to line the tunnel walls.

The generation house vault was completed in December 2004 so the building of the equipment that will house the two generators, each with a capacity of 61000 m3 of water, could begin. Modifications were made to the jumbo drills' compressor units to allow drilling on an incline to excavate the penstock area, or the water entryway to the generation house. Each of the two inclines is on a 60º angle, 120 m long and 9.5 m wide.

On target

The El Cajón Project is on schedule to meet its aggressive completion date of February 2007, when the first generation plant will go into operation. The second turbine will be completed two months later and the dam will be turned over to CFE in August 2007.iC

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