NPL Construction Company turns to Ditch Witch All Terrain drill to complete difficult river crossing other HDD equipment could not.
The horizontal directional drilling (HDD) equipment available and the skill levels of experienced HDD crews make many of today’s HDD drilling installations seem “routine.”
While no two drilling jobs are the same, a high percentage of current HDD installations are initiated with a reasonable assurance that they will be successfully completed. But this is not always the case. There are difficult, challenging jobs in conditions that put the end result in question.
Utility contractor NPL Construction Company recently completed a challenging project when working to cross the Blue Earth River about 10 miles south of Mankato, Minnesota.
“The job was to install duct under the river to replace aerial electric cable,” said Mike Theis, NPL superintendent. “The project owner was Benco Electric. The overhead cable along the river was the only part of the system in the area that was not underground. Benco had been planning to take the power cable underground for several years, and we got involved after three failed attempts made by another contractor using rented equipment.”
Both surface and subsurface conditions on the job site posed challenges for operators, described Theis. The launch point was 75 feet above the river. The job then required operators to proceed 22 feet below the river bottom, cross the river and come up 188 feet to the exit point on an eroded hillside. The soils contained sand, sandstone, limestone and cobble.
“Two of the unsuccessful attempts using equipment by another contractor were made with a 36,000-pound pullback machine, and with a larger 60,000-pound pullback model,”Theis said. “Representatives of the rented equipment’s manufacturer were on site to assist with the project. On every shot, they were unable to track the bore to the exit point and had to abandon each attempt.”
NPL was asked to try what others couldn’t accomplish, and brought in a Ditch Witch® JT60 All Terrain (AT) drill unit and support equipment to get the job done with confidence.
“NPL does joint trench work with Benco,” added Theis. “We’ve done a lot of river crossings in southern Minnesota. We are known for having experience in this type of work.”
The pilot bore under the river was completed on the first attempt, but the four-inch poly duct got stuck during pullback. A second pilot hole was made without incident, and this time pullback was completed. The bore length of the finished job was 1,220 feet. “We could not have done this job without the AT machine,” said Theis.
Ditch Witch AT models are equipped with a dual-pipe, mechanical drilling system that enables them to drill and be steered effectively in rock and other hard conditions impossible for a machine with conventional equipment of comparable size. An inner rod drives a rock bit, and the outer pipe steers the downhole tool while drilling the pilot holes and providing rotary torque for the hole opener during backreaming.
The JT60 AT (60,000 pounds of pullback) utilized a 6 ½-inch bit on the pilot bore and a 8 ½-inch hole opener on the pre-ream.
A Subsite® Electronics TKQ tracking system with a locating range to depths of 110 feet, and tracker with drill-rig range to 2,000 feet was used to guide the bore. The shallow, 4 ½-foot depth of the river allowed the tracking receiver operator in waders to walk the length of the bore. The signal remained strong throughout the crossing, Theis described.
Another factor contributing to the success of the project was recycling drilling fluid. “It was imperative to avoid inadvertent returns that would release drilling fluids into the river,” Theis said. “The AT drill units use about the same amount of fluid as a conventional machine and enabled us to drill through rock and avoid using a mud motor, which was unacceptable for this job. In addition, we recycled drilling fluids, which reduced the amount of fluids on-site and brought us significant savings in total costs.”
Until recently, recycling was limited to installations made by large drilling units of 100,000 pounds pullback and above. However, the introduction of compact recycling systems enables recycling with smaller, midsize HDD equipment.
NPL used a trailer-mounted Ditch Witch MR90 recycling system designed specifically for drill units with pullback ratings from 20,000 to 60,000 pounds. It is a self-contained, drilling-fluid mixing and recycling system and is the only unit in its class with an integrated rear spoil hopper with mixer for drying additives.
“We estimate that recycling saved about 75 percent in fluid costs by reducing the amount of water needed, saving on fluid additives, and reducing hauling costs to bring in water and dispose of fluid,” Theis said.
An added benefit of the MR90 is that it can be transported when supply and spoil tanks are full.
“We completed the installation in two weeks, start to finish,” Theis said. “Benco personnel ran cable through the duct and connected it to their system at each end.”
Benco Electric is a Touchstone Energy partner and provides electricity for 18,000 co-op members in Minnesota. The association was established in 1936.
Based in Phoenix, Arizona, NPL is a national utility contractor which installs and replaces more than 10 million feet of pipe and cable each year employing a variety of construction technologies. The Blue Earth River project was done by the company’s Upper Midwest division.
The Ditch Witch product line includes HDD equipment, drill pipe, downhole tools and performance parts; fluid recyclers; trenchers; vibratory plows; vacuum excavators; mini skid-steer loaders and other utility equipment; and support accessories and equipment. Ditch Witch products are sold and serviced by the worldwide Ditch Witch dealer organization. Ditch Witch is a Charles Machine Works company.