Rock Trenching The Right Way

By Jeff Griffin, Senior Editor | May 2014, Vol. 69 No. 5

“In some situations,” said Morris, “customers ask us to jump on their equipment and backfill, and we’re fine with that. We do carry a few contracts directly with owners. This year we have the work that could justify adding a couple of more operators, but we don’t want to get overextended. We are taking care of customers with what we have.”

Most of Custom Trench’s excavation is done with trenchers – they can produce substantially more trench per hour than is possible with an excavator.

“We have occasionally used excavators in two types of conditions,” said Morris. “If we hit wet clay, a trencher’s brakes come on and production can drop over 50 percent. It takes wet clay a while to dry enough to stop sticking to digger chains. That is a good time to bring in excavators with buckets.

“The other condition is if the rock becomes so abrasive that we can’t achieve production to cover the cost of the trencher teeth being used. We bring in our 10,000-pound hammer mounted on an excavator to fracture the rock. Then we bring the trencher back in to finish the job.”

On a recent project for the expansion of a residential subdivision, Custom Trench crews dug nearly 18,000 feet of trench for the water, sanitary sewers, storm drain, electrical, and gas utilities. Pipe and conduit installed was reinforced concrete in diameters from two inches for electrical duct to 60 inches for storm drains. Trench depths ranged from six to 18 feet, widths from 12-inches to 13-feet. Soil near the surface was clay with some small rock, said Morris.

“When we got 12 to 18-inches deep,” he continued, “it became abrasive and hard with limestone and layers of flint rock. This made it important to find the right teeth to make the job cost effective and to have enough equipment to stay on schedule.”

Five Trencor trenchers were employed on this project: three model 1260HD units powered by 425-horsepower diesel engines, one 1460HD with a 630-horsepower diesel engine and one 765HD, powered by a 250-horsepower diesel engine. All are track machines with mechanical digging chain drive and conveyor spoil removal systems. Trench width and depths of each depends on set up of the boom, chain and teeth.

As on most jobs, a welder was on the site throughout the project.

“These machines are put in extreme conditions, and eventually tooth holders get ripped off and have to be welded back on, or frames get stress fractures and need repair,” he explained. “Welders also are sometimes needed for fabrication, “In addition, our field mechanics have welding experience.”

Unexpected