Underground Construction

Utility providers routinely bury electrical and communications cable and the conduit in which cable is placed for protection. And for years, trenching and vibratory plowing were the primary methods contractors used for underground construction.

Both remain in wide use today, but excavating is becoming increasingly difficult--and sometimes impossible--on projects with easements already filled with utilities, in busy urban areas where paving covers most surface areas, and in established residential neighborhoods where aging cable must be replaced or distribution systems upgraded to meet increasing demands.

Two types of specialized equipment facilitate construction in such situations:

¥ Trenchless construction procedures--horizontal directional drilling (HDD) and compaction boring--provide cost-efficient alternatives to trenching. Savings in restoration costs alone often justify their use.

¥ Versatile vacuum excavators, with "soft" excavation capabilities, are changing how several underground procedures are done.

Horizontal directional drilling

The best-known trenchless construction procedure is horizontal directional drilling. Directional drills are used to install distribution and feeder lines, and individual services.

Without making large excavations or blocking traffic, compact directional drills install conduit below sidewalks and streets; in easements too crowded with other utilities to permit excavation; under lawns, parks and landscaped areas; beneath streams and rivers; and in environmentally sensitive areas where excavation is prohibited.

Rapid improvements in HDD technology make today's equipment more productive and dependable, and easier to use than models available only a few years ago. Self-contained drill units pack more power in smaller sizes and have convenient features that speed setup and operation. The latest electronic guidance systems are more accurate and versatile, and downhole tools permit medium-size models to drill through many types of rocky conditions.

The first step in an HDD installation is to make a carefully guided pilot bore. When the pilot hole is complete, conduit or cable to be installed is attached to the string of drill pipe and pulled back through the hole. When necessary, pilot holes are enlarged to accommodate large pipe by backreaming. Depending on the power of equipment and soil conditions, HDD equipment used for utility work can make installations up to 1,000 feet of pipe up to 30 inches in diameter.

Compaction boring

Pneumatic piercing tools provide an economical way to make short bores under sidewalks, driveways and streets.

The tool is a rugged steel cylinder containing a striker mechanism. A hose extending from the rear connects to a portable air compressor that supplies power. Compressor requirements vary with tool size.

To make an installation, the tool is placed in a short trench or small pit and connected to a compressor. When the air supply is activated, the tool literally pounds itself through the ground, compacting surrounding soil. Material to be installed is either pulled behind the tool, or pulled back through the completed bore hole.

Piercing tools are inexpensive to purchase and easy to transport, set up and operate. With only one moving part, pneumatic boring tools require little maintenance.

Until recently, installation lengths were limited because tools could not be steered and frequently strayed off course. Models that can be tracked with surface locators first addressed this issue, and the introduction of a steerable tool has extended the lengths to which piercing tool installations can be made.

Vacuum excavation

Mounted on trailers or trucks, excavators are both powerful, all-purpose vacuums and efficient "soft" excavation tools. They are regularly used to clean up drilling fluid on HDD sites and for digging "potholes" to expose existing utilities before construction.

Using high-pressure air or water, a vacuum excavator can dig a 1-foot-square, 5-foot-deep pothole in 20 minutes or less, depending on soil conditions and machine size. While the hole is being dug, the machine's vacuum sucks up soil displaced by excavation. Soft excavation eliminates the inherent risk of damaging buried utilities with mechanical equipment and tools.

The versatility of vacuum excavators extends their value far beyond supporting HDD operations, and innovative contractors are using them to dig holes for utility and light poles. They can trench in easements where there are too many buried utilities to safely use conventional equipment and can dig small excavations in areas that cannot be accessed by conventional equipment.

As vacuums, the machines clean out manholes, vaults, conduit and pipe, and perform a variety of general clean-up tasks. Some contractors also find the vacuum function effective for pulling cable through conduit; cable is attached to a pig, which is inserted in the conduit. The vacuum excavator, positioned at the opposite end, sucks the pig and cable through the conduit. EC

GRIFFIN, an experienced construction and tools writer from Oklahoma City, Okla., can be reached at 405.748.5256 or upfront@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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