Going Underground

Underground service repair beneath city streets is one of the most disruptive operations a public works department has to deal with. Not only does it create traffic problems, but the repair itself may result in poor pavement performance, becoming a source of ongoing maintenance or premature deterioration.

A less intrusive repair technique employs a rotary core cutter and pavement reinstatement using a bonding compound to replace the core as a permanent repair. This process, called keyhole coring and reinstatement, is currently used by more than 25 leading gas distribution companies in North America. The process has applications in the rehabilitation and replacement of water and wastewater infrastructure.

Keyhole coring and reinstatement allows access to underground pipe or other buried infrastructure from the road surface without having to resort to more costly, disruptive, and dangerous excavation methods. It facilitates repairs to underground utilities and lets horizontal directional drillers and other excavators view and confirm the exact location of potential buried conflicts before digging.

The technology involves a rotary cutting unit or core drill that can safely and accurately core an 18-inch or larger diameter hole through asphalt, asphalt-concrete, or reinforced concrete road systems and sidewalks. Crews can then vacuum excavate and view subsurface activity or make repairs from the road surface using longhandled tools.

After the repair or underground work has been completed, the excavation is backfilled to the level of the base of the pavement. The core that was originally cut from the pavement is bonded back into the roadway. This process results in excellent pavement performance and saves hundreds of dollars per hole in repaving costs.

Selection of the Coring Unit

If you are going to throw the core away, any core drill will do. But precision and accuracy are essential if you plan to reinstate the pavement core. Because most roads are crowned to allow surface water to run off, they don't present a level or horizontal surface. In order to get a perfectly cylindrical shape and prevent the core from binding in the drum, pavement cores should be cut perpendicular to the horizon rather than to the road surface. To do this, coring units use hydraulic stabilizers to adjust the angle of the coring mast so that the cutting edge of the core drum is level with the horizon. These stabilizers also help to take the 'spring action' out of the vehicle to allow its full weight to be focused onto the pavement to ensure an accurate and precise core and a uniform kerf.

Cutting the core precisely at right angles to the horizon is also important for the reinstatement process. With a slanted core, the effect of gravity can cause the bonding compound to settle on the lower side of the hole and result in a weakened and irregular joint that might not be waterproof or that could shift or fail.

For roadwork, except for very small cores less than 4 inches in diameter, a vehicle-mounted coring unit (truck, skidsteer, or trailer) is heavy enough to provide the weight and stability needed for the coring job and to safeguard the operator from potential mishaps. Handheld or portable coring drills can injure operators if the drum binds in the hole and the drive unit (and sometimes the operator), rather than the drum, begins to rotate at 200 rpm.