The Keyhole Drilling method uses the trenchless techniques of coring, potholing and horizontal directional drilling (HDD) to install pipes underground. A core is cut out of the road, a pothole is excavated, and the drilling machine is inserted into the pothole. A horizontal pilot bore is drilled, then the pipe is pulled in.
Typically, keyhole drilling is used for connections from service mains to properties across roads for distances of up to about 50 m. The installed pipe is a sleeve that can be up to 90 mm diameter into which the product pipe or cable is inserted. The product pipe can be cables, gas or water pipes.
Usually, if the location is in a road, a coring machine is used to cut a core disc out of the road surface (about 600 mm diameter) and kept for the road reinstatement. The pothole is excavated using vacuum equipment down (typically 0.6 m to 1.5 m) to the required level next to the service main. The pothole must be excavated accurately to accommodate the keyhole drilling machine.
The keyhole drilling equipment is prepared with the drill head and tracking sonde, and is inserted into the pothole and accurately set up with hydraulically operated stabilising braces. By necessity the drilling machine protrudes above ground level. Power must be supplied to the drilling machine. The destination pit or sometimes basement, is prepared to receive the pilot bore.
Drill rods (typically 250 mm long) are loaded into the drill rod feed hopper. The pilot bore is commenced and tracked using an HDD type walk over tracking system. Steering corrections are made from the drilling machine control console.
Once the pilot bore is completed, the drill head is replaced with a back reamer and the product pipe attached. The drilling equipment is reversed to pull back the reamer and product pipe, recovering and storing the drill rods as it proceeds. Additional incremental back reaming may be necessary in advance of the final pull back to ensure the bore is large enough for the product pipe to be installed. Once the pipe sleeve is installed, the drilling machine is removed, the service pipe is inserted through the sleeve and connected to the main using specialised long handled tools and keyhole techniques. The work is completed by backfilling the excavated hole and reinstating the surface by grouting the core disc back in place.
Cutting the core out of the road surface
Removing the core
Vacuum excavation of the pothole
Keyhole drilling machine installed, set up and drilling the pilot bore
Making the connection with long-handled tools
Replacing the core and reinstating the road
Typical keyhole drilling installation
The ISTT gratefully acknowledges TRACT-TECHNIK, who provided the illustrations.