In turn, the high pressure trailer pump pumped the mix through 20 feet of Putzmeister delivery line to the placing boom mounted on a 20-foot-tall freestanding tower. No counterweight was required for the Putzmeister placing boom, so it offered a lighter weight on the equipment barge floating atop the water. The placing boom, capable of extending 108 feet, 7 inches horizontally, maneuvered its four-section boom into a 10-foot-diameter tremie pipe that delivered the concrete 390 feet to the lake’s bottom.
“The placing boom was powered by a 3-cylinder diesel power pack instead of the more commonly used electric power pack, which would have required a great deal of power from a generator,” says Gribble.
No divers were in the water guiding the work. Instead, the crew at the surface used sonar, GPS sensors and a single remote-controlled submarine to monitor what was happening below the water’s surface.
“At times, concrete output peaked over 100 cubic yards an hour, which is a high volume for this type of application, especially with the distance and steps involved to consistently move the concrete from the shore to the bottom of the lake,” adds Gribble.
The pour was proceeding as planned until the sixth day when strong winds gusted up to 60 miles per hour on the lake, sending waves crashing over the platform and causing treacherous conditions for the barges ferrying ready mix trucks. The job shut down for nine hours.
Since the concrete mix had a 30-hour set time the strong winds didn't affect the quality of the finished product. The set time was initially specified to provide ample time to inch the concrete around the intake structure and add new layers before hardening; however, the 30-hour set time also accommodated the unexpected wrath of nature.
Eventually, the intake structure will connect to a 20-foot-diameter, 15,000-foot-long concrete panel reinforced tunnel under the lake bed. This will occur when a tunnel-boring machine on a 1,571-day mission reaches the intake structure, after digging three miles through solid rock beneath Lake Mead. The entire raw water intake project is presently slated for completion in early 2014.