Keyhole Basics: Coring 101

By E. Marshall Pollock | February 2010 Vol. 65 No. 2
Truck-mounted coring unit with stabilizers

There are many small, portable coring stands on the market that can effectively cut a small core of between four and 6-inches in diameter. These are usually driven by a small, top-mounted gasoline engine, and are either controlled by a hand-held, horizontal “T-bar” or anchored to the pavement with lag bolts or a vacuum mechanism. Either do a reasonable job with small holes. But as the diameter of the hole reaches the 12 to 24-inch range (a typical keyhole is 18-inches in diameter) or the depth of the pavement increases beyond four to 6-inches, the choice becomes more limited. Only a truck-mounted, skid-steer mounted or trailer-mounted coring unit has the power and stability needed for such a job.

As with every piece of construction equipment, the size, design and performance capabilities of the coring unit, as well as its price, will depend on workplace conditions and the volume of work to be performed.

Proper setup, coring procedures
The two most important factors to be considered in the selection of truck or trailer mounted coring equipment are the stability of the coring platform itself and the ability to properly adjust the angle of the coring drill so that the core is cut perpendicular and true, or plumb with the horizon and not the road surface. This latter measure is important to neutralize the effect of gravity on both the coring and reinstatement processes.

Because most roads are crowned, or angled to allow surface water to run-off, if you cut the core perpendicular to the road surface, one side would be lower and affected by gravity more than the other and the core itself would not have “true” or vertical sides. Not only is the vertical nature of both the core and the hole important during reinstatement to prevent the bonding compound from pooling or migrating to the low side, but with deeper cores there is the added danger that the pull of gravity will cause the core to bind up inside the core drum, or, even worse, cause the core drum itself to become stuck as it is driven deeper into the pavement, causing damage to the coring unit.

As for stability, because springs are integral to most wheeled vehicles, it is important that the suspension of the truck or trailer be isolated during the coring process. Otherwise, much of the downward pressure or vertical force required to initiate the coring action will be lost or absorbed by the suspension and result in erratic cutting action.