Irrigation Notebook: April 1998, In The Trenches

Using the proper trenching equipment to install an irrigation system can make or break the successful completion of the job and impact a contractor’s profitability.

Mike Hubbard, rubber tire trenching specialist, Vermeer Mfg., Pella, Iowa, outlined the considerations to take into account to select the correct size trencher for irrigation installations: "Depth of service, width of the service line, project deadlines, size of the project and ground conditions are the five main variables for picking the right rubber tire trencher," he said.

For example, say you need a trench for an an 8-inch water line feeding off a main supply source with a bed of pea gravel backfill. This will put you in the 50-horsepower trencher class, especially if the service will be laid 4 to 8 feet deep, Hubbard noted.

"You can get a smaller horsepower unit to cut a fairly narrow trench - say 4 to 6 inches at shallow depths - but at deeper depths, only a larger horsepower trencher will get you the desired productivity," he added. "Then you have to inspect and respect the ground conditions." If ground conditions are good, then going with a smaller horsepower unit is a viable option. However, tough ground conditions are better handled with a more powerful, heavier and more durable unit.

One project that illustrates Hubbard’s points was the Raleigh-Durham Airport, where 77,000 feet of irrigation piping was installed in the rocky ground surrounding it. Davis Landscaping Inc., Harrisburg, Pa., handled the installation.

The entire irrigation system is run by two different water supplies, one from the potable water and the other from a 400-gallon-a-minute pump station that pumps water out of a lake on the site.

The pipeline diameters that Davis Landscaping installed ranged from 12-inch mains at the source of the water supplies down to ¾-inch diameter lines for the laterals.

Davis Landscaping used its 40-horsepower mechanical trencher. "The specifications called for the 8-inch line to be surrounded by 2 inches of crushed gravel backfill, so we needed to make a 12-inch wide trench up to 4 feet deep for these main lines," explained Scott Maherg, irrigation manager. "As the lines progressively decreased in size, we strategically used other rubber tire trenchers in our fleet. The riding trenchers were used to put in the 2½- to 3½-inch diameter PVC lines, and the lawn plow was used to plow for the laterals," he noted.

"The benefits of using the 40- to 50- horsepower trenchers in these conditions became evident to us," said Maherg. "If we were trenching and ran into a boulder, we could turn the machines around and use the backhoe bucket to remove the rock and keep our productivity up. But the rock trenching took its toll, as we expected. We wore out chains and buckets of shark teeth on the trenching."

The innovative use of the lawn plow in these conditions was perhaps the most surprising construction development for Maherg’s crew. When it came to laying in the ¾-inch PVC laterals, rather than cut a trench 18 inches deep the crew used the vibratory plow to pull the pipe.

"This vibratory plow was capable of pulling in glued 20-foot lengths of PVC up to 120 feet," recalled Maherg. "This greatly improved our productivity for the thousands of feet of lateral we had to install.

"Since the specifications called for PVC pipe rather than flexible polyethylene, we had to use the best method and the right machine to get the small diameter PVC in," he explained. "What allowed us to pull PVC with the lawn plow was the fact that we had many 120-foot straight runs. And with the runs straight, pulling in glued sticks of PVC was no different than pulling in poly pipe. We lessened ground friction and reduced the chance of the PVC pipe scoring on the trench wall by placing an 1½-inch diameter bullet on the plow blade that gave the pipe added clearance as it was pulled in."