Divers installing new control gates 150 ft below the surface of Cheesman Dam continue to blast, chop and saw-cut through granite to bring water-control systems on the 105-year-old dam up to modern standards.

The dam, in the foothills 25 miles southwest of Denver, stores 80,000 acre ft of water to help meet the needs of Denver Water’s 1.3 million customers. When it was built in 1905, the 221-ft-tall brick-and-granite dam was the tallest in the world, but its cast-iron valves are rusty and unworkable, giving engineers no reliable upstream controls to shut the water off if something happens at the dam.

The old valves are being left in place, while crews place new 3-in.-thick solid-steel gates on the auxiliary, middle and lower outlet levels of the dam, working from 60 ft to nearly 200 ft below the surface. Divers working in 12-hour shifts from a two-man diving bell blast the rock openings, clean out the holes and install rails to slide the 10-ft-long spool sleeves into the tunnels.

Once divers bolt the spools into place, they insert O-rings and grout around the openings to seal the 11-ft x 12-ft tunnels. The slide gates, prefabbed by the Rodney Hunt Co., Orange, Mass., are maneuvered into place by a barge-mounted crane. Divers with Global Diving and Salvage, Seattle, position them to complete the system.

“It’s like mining under water,” says Brian Daniels, the site construction inspector for Denver Water. “It’s really just regular heavy construction done under some very difficult conditions.”

The two-man teams work outside the elevator-sized diving bell for five hours at a time, wearing thick rubber suits heated with warm water and breathing a mixture of 90% helium and 10% oxygen “to remove nitrogen from the mix,” Daniels says. Their helmets have heavy-duty microphones that allow them to communicate with the dive supervisors, crane operators and engineers up on the barge.

Daniels calls it “saturation diving” because, once the divers return to the surface, they transfer directly from the diving bell into a pressurized living chamber so they don’t have to decompress each day. The two-man teams work 28-day shifts before decompressing and rotating out. “It’s tough work, but we take good care of them,” Daniels says.

The $18-million project, which began last winter with the construction of a new control building atop the dam, will be completed in October when all three levels of slide gates are in place. Next spring, a second phase of diving work will replace the old auxiliary valves.