Virgil Cooley has more than 25 years of experience working for the aggregates industry. He started at Welch Sand & Gravel four years ago when he joined the team to operate the 13-cubic-yard Rohr clamshell dredge. He was later promoted to plant foreman of the Ross Plant. Prior to Welch, he worked for the black-top industry and as a dragline operator for another sand and gravel company.

Rohr continues making machines bigger and bigger. It has produced clamshells with buckets as large as 20 cubic yards, and in some instances they have actually bolted machines together in order to run tandem and double production.

For Welch, this dredge was a $3.5 million investment, and each floating conveyor was $150,000, Cooley says. But it is paying off both in production output and fuel costs. The entire machine is electric, unlike its dragline ancestors, so it is more environmentally friendly. Cooley says there are 36,000 volts coming in off the street, which is reduced to 12,460 volts at a step-down transformer. And another step-down transformer onboard reduces it to 480 volts.

The clamshell takes large bites from 100 feet every 90 seconds and dumps into a hopper. Above the hopper is a 6- × 8-inch horizontal grizzly. As material sticks to the grizzly, it is raised by a hydraulic lift to dump the oversize. Often there is a barge waiting below to collect the material and take it to a waste pile on shore. Other times it just goes back to the lake.

Below the hopper is an 8- × 20-foot polyurethane dewatering screen that was built by Metso Minerals. Dried material is discharged onto a conveyor for transport to shore. The undersize falls through and into a smaller “possum belly” hopper that is actually submerged. A 6- × 48-inch Galligher pump inside moves this material to a 20-inch Krebs cyclone to remove the fines. The dried material is then blended onto the conveyor and heads to shore. Welch has the option of shutting this pump down, allowing the fines to wash back into the lake. It just depends on what gradation the customer wants, Cooley says.

The floating conveyors connect to an onshore stacker, which also is controlled by the dredge operator, to supply the feed hopper to the plant. The stacker also can be repositioned to create a stockpile. This way the dredge can continue producing material while maintenance is performed on the plant. Or if the dredge is in need of maintenance, the plant can be fed stockpiled material by wheel loaders. Occasionally, the company is lucky enough to sell bank run that also is sold from this point, Goessling says.

The hopper also marks a break in responsibility. Goessling says everything before the hopper is controlled by the dredge operator and everything thereafter is controlled by the plant operator in the control tower.

The automation system on the dredge is highly sophisticated, and the PLC monitors every activity taking place. The conveyor knows if a conveyor stalls, if a screen motor stops running, and where the trolley is on the overhead gantry at all times. All of this information is used to ensure that all parts are working in proper sequence, and if something was to fail, everything else would shut down accordingly.