Three years ago, Honduras' Francisco Morazán hydroelectric project, the country's main source of electricity, was on the verge of collapsing into a $775 million pile of rubble. "Water was leaking everywhere," recalls the Inter-American Development Bank's William Large, who was sent to Honduras on an emergency mission to see how the IDB could help save the project it had originally financed. "Without a doubt, it looked like the power house would flood."
Some of the world's leading specialists had spent two years trying to contain the leaks without success. No one imagined engineering history was about to be made with 8,650 plastic and wooden balls, 25,000 feed sacks and some very unconventional thinking.
The dam, completed in 1985, was the largest civil engineering project ever carried out in Honduras. It took nearly two years to pour the concrete to span the narrow box canyon that gives the dam its popular name, "El Cajón." Its 226-meter-high double curvature arch dam is the sixth tallest in the world.
With a capacity of 300,000 kw, El Cajón was to provide 70 percent of the country's electricity and produce a surplus that could be sold to neighboring Nicaragua.
But problems became evident as soon as the reservoir filled up in 1986. Engineers were alarmed to discover that the tremendous weight of the water caused tiny cracks to form in the cement grout curtain behind the dam. Water had begun seeping through and eroding pockets of clay in the limestone under the dam.
Hydraulic engineers always expect some leaking as a result of the pressures created when huge amounts of water are impounded. A large sump was excavated at one end of El Cajón's powerhouse to gather leaking water and pump it outside. But by 1993, water was pouring in at the rate of 1,600 liters per second. Even more alarming, the water was discolored with clay, indicating that the fissures and karsts (cavities in the limestone bedrock) were getting larger. If the leaks were not plugged, the power house would eventually flood, and even the dam itself would be threatened as the limestone under it became more porous.
To the rescue
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The World Bank loaned Honduras $12 million to finance a rescue operation. Conrodio, a European consortium and a world leader in grout injection, was contracted to handle the daunting job.
Special drilling and injection machinery had to be designed to withstand the tremendous pressure produced by the high head of water behind the dam. Hundreds of holes, up to 250 meters deep, were drilled from the dam base into the limestone and many tons of different combinations of cement, sand and gravel grouting were injected. But most of it just ran through and ended up in the powerhouse sump. Nothing was working, and time and money were running out.
Then, on April 24, 1994, a power surge knocked out electricity throughout Honduras. The sump pump in the powerhouse stopped, the water started rising, and the emergency generators wouldn't start. "It was tense," recalls Jorge Flores, El Cajón's chief geologist. "The water nearly reached floor level." The powerhouse was on the verge of being flooded when the turbines finally started up, producing power to drive the pump.
Had El Cajón's powerhouse turbines flooded, the hydroelectric plant would have become an enormously expensive white elephant, literally leaving the country in the dark, crippling business and the economy. Honduras was already rationing electricity, because a two-year drought combined with the leaks had lowered El Cajón's reservoir and cut its generating capacity in half.
But while the immediate crisis was over, the leakage continued to worsen. "It was demoralizing to work so hard without seeing any results," recalls Flores.
Toward the end of 1994, the World Bank money was nearly gone. The IDB quickly approved a $37 million emergency loan that included up to $5.14 million for the El Cajón problem.
Geologists and engineers decided they needed to inject larger objects (about 5 to 7 cm.) that would stick inside the fissures and karsts and prevent the grouting from passing through before hardening. But there was no well sorted, round gravel of this size anywhere nearby. Flores then came up with a bizarre idea: plastic balls sold for toys were just the right size and could be filled with concrete. So his staff went to every store and market in the region and bought balls by the hundreds.
"We told the fish at El Cajón needed soccerballs," Flores says.
It was a crazy idea, but it began to work. The engineers also needed something that would float to plug holes at the top of the karsts. They tried filling the plastic balls with dried corn kernels, but everything ran through to the sump. Next they tried balls made of wood. That worked. Altogether, the engineers injected 8,650 plastic and wooden balls into the hundreds of karstic flow holes they had drilled into. But while the balls remained in place, they didn't hold back enough of the cement grout material injected to plug the cavities.
Flores came up with another bizarre idea: rolled up polypropylene feed sacks injected into the drill holes might spread out like mesh inside the cavities, and in this way prevent the cement grout from passing through. Finding sacks in an agricultural country was easy, and during the next six months, 25,000 feed sacks were injected into the limestone cavities under the base of El Cajón, followed by the grout mixture. By April of 1995, leakage had been reduced from 1,600 to less than 100 liters per second. Hydrostatic pressure under the dam was cut by 60 percent.
Today, El Cajón's reservoir has nearly refilled to capacity, and the powerhouse is again generating at full capacity. The grouting so far has withstood the tremendous weight of the filled reservoir. This spring, the story of how the $775 million hydroelectric project and the economic life of a country was saved by plastic and wooden balls and 25,000 polypropylene feed sacks was presented as a scientific paper at the International Congress of Large Dams by Jorge Flores and engineers from Conrodio.
"What we learned at El Cajón," says Rolando Yon-Siu, the IDB specialist who supervised the rescue work, "may someday save a dam somewhere else in the world."