Case Study: Building the Basin - Feature Articles

Controlling Floodwaters

The U.S. Army Corps of Engineers has been instrumental in determining an effective way to collect the massive amounts of water that stream from the mountains and threaten to flood Las Vegas streets and neighborhoods. But it didn’t come without its challenges. As designers and planners reviewed several methods and solutions to prevent flooding in the valley, they ran into several obstacles.

Due to the development of cities east of the Vegas strip, existing channels couldn’t be enlarged because there was no way to collect and confine water on the alluvial fan. Further, water embankments could no longer be relied on as protective shields.

Ultimately, the Corps designed a retention system that would manage alluvial fan flows entering the Tropicana and Flamingo washes and release those flows through the city at steady rates. However, the Flamingo Basin in Southwest Las Vegas built in 2000 wasn’t enough to prevent water from spreading into communities. So, a second basin, known as the F4 Flood Basin, was slated to control water and pull channels from the west side of Las Vegas to Lake Mead on the city’s east side. American Asphalt and Grading, which specializes in grading, paving, crushing and water control, was contracted to assist with the building of the additional basin.

The Caliche Challenge

To create a stress relief through the site’s hard rock, American Asphalt and Grading dug in front of its tractor with a reconfigured Vermeer T1055 track trencher.

Since natural waterflow continues to stream from the mountains, the F4 Flood Basin was created at the foot of the mountains. As construction began in 2005, onsite conditions proved difficult. When contractors tackled the soil conditions, they ran into caliche, a hardened conglomeration of gravel, rock and soil. This rock type is found in layers of varying thickness throughout the southern part of the Las Vegas Valley and is very difficult to navigate through. While blasting could handle the site’s ground conditions, it wasn’t a recommended technique because of its potential to fracture the ground material along a seam, which would result in leakages in the embankments.

Dave Moyer, general superintendent of public works for American Asphalt and Grading, says that to maneuver through the caliche, he needed more productive equipment than the hydraulic ram and excavator in his fleet at the time. Acting on the advice of his Vermeer dealer, Moyer rented a Vermeer T1255 tractor with a Terrain Leveler surface excavating attachment, which was capable of cutting the existing rock.

The Terrain Leveler attachment was created when Vermeer took a rock trencher, removed the trencher attachment, and added the patented tilt-head attachment with top-down cutting action. This offered greater efficiency and deeper tooth penetration. The Vermeer T1255 Terrain Leveler attachment is a 600-horsepower unit that can dig up to 24 inches deep and 144 inches wide. The top-down technique, which is powered by a large direct-drive, hydrostatically controlled cutting drum, provides greater speed control and results in proper material sizing with more usable fines.

With TS19C and TS31C Kennametal cutters on the digger chain drums, the Terrain Leveler was able to produce material less than 6 inches in size, which resulted in less material hauling for the crew. Moyer says the cutter setup on the digger chain drums proved successful since the cutters were never lost--just worn.

“This was not the first time that we had used a Terrain Leveler machine. However, it was the first time that we used one for this particular job,” Moyer says. “We started out renting the machine but decided that, with the amount of work that was ahead of us, it was a worthwhile investment to purchase it. We ran the Terrain Leveler machine approximately 80 percent of the time over a five-day workweek until the project was completed.”

Moyer and his crew used the Vermeer unit to deepen a 3.2-mile-long channel, which averaged 15 feet deep and 45 feet wide in the low-lying areas. The machine averaged up to 2,500 cubic meters on a typical nine-hour workday. Once material was dug from the basin and channel, the crew used Caterpillar 623 paddle wheel scrapers to pull material out and dump it in a stockpile. After a significant amount of material was stockpiled, it was taken to an underground water channel to backfill concrete channel walls and build maintenance roads.

In addition to the Vermeer T1255 Terrain Leveler, Moyer and his crews also used a Vermeer T1055 track trencher. But it was no ordinary T1055. Moyer says his Vermeer dealer reconfigured the unit, turning around the digger chain on the trencher to allow the machine to cut from the top down. This reduced the possibility of fracturing rock and creating leakages in the basin. The reconfigured trencher was also utilized to create a stress relief through extreme hard rock by digging in front of the terrain leveler.

“The changes made on the track trencher to reverse the chain were a great additive in the extreme hard rock conditions that we were facing,” Moyer says. “Even in conditions such as caliche, we were able to reach a 12-foot-wide and 24-inch-deep maximum cut with the Terrain Leveler unit and removed nearly 380,000 cubic meters of material from the basin site. For the most part, the ground averaged between 4,500 and 7,000 psi, and in some areas was as high as 22,000 psi.”

With American Asphalt and Grading’s help, the F4 Flood Basin was completed in April 2007 and runs across four miles to meet up with the Flamingo Basin. Together, the two containers retain and filter water into Lake Mead and prevent water from flowing into Las Vegas city streets.

Moyer believes that with the addition of the F4 Flood Basin, flooding in the Las Vegas Valley will diminish considerably. “We worked hard to protect homeowners in the low-lying areas by controlling floodwaters,” he says. “We are confident that we accomplished just that.”