Underground Report

Extending broadband services to more customers is a priority for telecommunications and cable television companies who are locked in a fierce battle to consolidate and expand their shares of the high-speed Internet market and, with it, other bundled services; fiber optic technology is a key element in most growth plans—more bandwidth and higher speeds appeal to residential customers, and large corporate enterprise customers and non-profit organizations demand them.

To achieve maximum speed and capacity, fiber connections must be brought directly to homes and businesses. FTTP (fiber to the premises) and FTTX (fiber to wherever it’s needed) are hot acronyms in today’s telecommunications market.

Even so, the United States lags far behind other countries in deploying fiber. At the beginning of 2007, only Verizon has made significant progress bringing fiber to residential and small-business customers.

Some large organizations are not willing to wait for conventional sources to bring the broadband solutions they need, while enterprise customers, municipalities, educational institutions and governmental agencies are building private fiber networks to connect to existing ones. This way, they can distribute high-speed datacom services to their organization’s buildings.

For example, the DeKalb County School System in Georgia recently put in service—a new 200-plus mile, all-fiber, all-underground network, linking the district’s more than 125 buildings.

Depending on site conditions and other factors, much of the outside fiber is going underground, and trenchless construction methods are used to limit surface disturbance and disruption of regular activities. For example, pneumatic piercing tools and horizontal directional drilling (HDD) equipment trenchlessly installed conduit in which fiber was placed for the DeKalb network.

Such projects may be especially attractive to qualified electrical contractors who can install structured wiring in buildings and also install outside underground and aerial cable.

Commonwealth Communications, a division of Commonwealth Electric of the Midwest (CECM), has much experience on such projects, said Todd Havlat, RCDD, Lincoln (Neb.) Division communications manager.

Recent examples include work for the City of Lincoln burying fiber optic cable that links various city buildings, including fire stations, to the city’s communications system. CECM workers also installed underground innerduct among all buildings on the University of Nebraska’s Agricultural Research Development Center.

“For the city,” Havlat said, “we installed 25,000 feet of 4-inch HDPE duct by horizontal directional drilling, which permits installation of duct in developed areas with minimal surface disturbance and less interruption of regular activities.”

Cable routes generally were in green spaces of city street easements. Directional drilling was subcontracted and accomplished with a compact, 11,000-pound pullback drill unit. Crews also installed innerduct in existing underground raceways, which already contained fiber.

At the agricultural center, 15 miles of 3¾-inch HDPE ducts were installed using a 6,000-pound pullback drilling unit, Havlat said. In some areas, vibratory plowing also buried duct.

The underground construction methods used vary with job specifications and site conditions.

Horizontal directional drilling is perfectly suited to FTTP construction in areas where excavation is inconvenient, impractical or impossible. And while HDD is widely used for FTTP work, a surprising amount of the underground construction continues to be completed the old-fashioned way—with trenchers, vibratory plows, loader-backhoes, compact excavators and skid-steer loaders with attachments. Although attracting little attention, the relatively simple and inexpensive technique of making trenchless installations with pneumatic piercing tools is being used on many FTTP projects.

Horizontal directional drilling

Directional drilling permits the underground installation of telecommunications duct with a limited amount of excavation. It permits network segments to be installed in areas with surface improvements and in already-crowded easements with little disturbance to landscaping and paved surfaces. Reducing the high restoration costs required after open-cut construction usually more than offsets the higher per-foot cost of HDD.

These benefits are perfectly suited to conditions found on many telecommunications projects and especially FTTP installations, which often are in highly developed areas where excavation would be unacceptably obtrusive.

To install a segment of communications duct, a HDD machine makes a carefully guided pilot bore. When the pilot bore reaches its exit point—either in a receiving pit or by being guided up to the surface—the drill head is removed. Then, duct to be installed is attached to the string of drill pipe and pulled back through the hole. If necessary, the pilot hole is enlarged by backreaming to accommodate large-diameter or bundles of multiple ducts.

The pounds of pullback force it can generate specify the size of a drill rig. For FTTP work, small 5,000-pound pullback units are widely used for services, usually 100 feet or less. Longer runs are made by more powerful machines with pullback up to about 40,000 pounds. Although larger, the footprints of these models still require relatively little space, allowing construction in street easements without rerouting traffic.

Most pilot bores are surface-launched, requiring little or no excavation. Pit-launch models usually are used when a job site is too small to allow surface launches. The compact drill frame is lowered into a pit and initiates the pilot bore at the required depth. A separate power source is positioned on the surface near the pit.

Pneumatic piercing tools

Piercing tools, also called moles, have been in use for years, providing an economical method for making short compaction bores under sidewalks, driveways and other surfaces. The tools—steel cylinders with a hammer mechanism inside—are available in a variety of diameters with those in the 3 to 6 inch range most often used for telecommunications and electrical work. The air compressor must be correctly matched to the tool it powers.

To make an installation, a tool is placed in a starting pit or trench, is connected to an air compressor and literally pounds itself through the ground.

Pipe or cable to be installed is either pulled behind the tool, pulled back through the completed hole, or simply shoved or pulled through the hole after the tool is removed. Installation lengths are limited because unsteerable tools easily stray off course, and their paths can be deflected by striking a rock or other buried object. Finding a lost tool can be frustrating and time consuming. Some models can be equipped with a small radio transmitter that allows a crew member to monitor its location with an electronic tracker. More recently, a guided version using a method similar to that used for directional drilling became available, but most underground specialists prefer HDD equipment for all but short bores.

Vibratory plowing

Before the advent of directional drilling, vibratory plowing often was touted as a trenchless way to bury pipe and cable. Although it does not involve digging a trench, plowing is not considered a trenchless technology because the technique involves pulling a blade through the ground, which will strike and damage any existing utilities in its path.

The machine takes its name from the vibrator component mounted on the rear of the plow unit. A blade attaches to the back of the vibrator unit, and the force of the vibrating action facilitates the blade’s cutting action as it moves through the earth.

An installation is made by lowering the blade into the ground, activating the vibrator. By engaging the ground drive, the vehicle moves forward as the shaker continues to operate. Cable or small-diameter polyethylene pipe feeds through a chute at the back of the blade. As the unit moves forward, cable goes into the top of the chute and out of the opening at the blade’s base and into the ground. Larger-diameter pipe requires a pulling blade to which pipe is attached with a pulling grip and the conduit or pipe is pulled into the ground as the unit moves forward.

Whether material is fed or pulled into the ground, turf damage is significantly less than with open-cut methods, and surface restoration is much faster than filling in open trench.


  • Trenchers: When surface damage is not an issue and there are no existing buried utilities at the job site, digging trench, putting duct or cable in it and returning spoil to the excavation is a fast, economical method of underground construction. Trenchers dig continuous, narrow trenches, depositing spoil on either one or both sides for fast restoration. For FTTP work, trenchers likely will be compact, walk-along models or small riding units. Most travel on wheels, although some track models are available. Typically, walk-along models, starting at about 10 horsepower, can do nothing but trench. Riders—starting at about 20 horsepower—accept a variety of attachments. Most popular are small utility backhoes and “combo” attachments with an offset trenching component and vibratory plow. Combos are popular for telecommunications and electrical work because a small starting trench can be dug, then the plow unit dropped into position and the installation can begin at the specified depth.
  • Loader-backhoes: Although much slower than a trencher, tractor-mounted loader-backhoes are widely used to dig trench. Even though the width of the bucket is much larger than needed for a telecom installation, a backhoe on-site is employed rather than bringing a trencher to the job.
  • Compact Excavators: In the past few years, compact excavators have done a growing amount of utility work. Unlike a full-sized excavator, compact models can dig offset trench adjacent to streets, fences, buildings and other obstacles. With a variety of attachments, they also can do numerous other tasks. Track-mounted compact excavators can work on a variety of terrain and in wet conditions where wheeled equipment is difficult to use.
  • Skid-steer loaders: Today’s skid-steer loader models are multipurpose tool carriers with an array of attachments that include several perfectly suited for FTTP and utility projects, including trencher, backhoe, vibratory plow, pole-digger auger and tampers. Wheel-mounted skid-steers turn on a dime and can move quickly from place to place on the job site, also performing many other jobs. Track-mounted mini skid-steer loaders can work in confined spaces and in wet areas where wheeled equipment cannot operate.                EC

GRIFFIN, a construction and tools writer from Oklahoma City, can be reached at up-front@cox.net.

About the Author

Jeff Griffin

Freelance Writer
Jeff Griffin, Oklahoma City, is a construction journalist specializing in the electrical, telecommunications and underground utility construction industries. Contact him at up-front@cox.net .

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