Drilling in Hard Rock in Northern Virginia

In early December 2015, Mincon Group PLC, an Irish engineering company with almost 40 years of manufacturing experience in hard rock drilling equipment, and its distribution partner in Virginia, Vermeer Mid Atlantic Inc., participated in a particularly difficult pilot bore and reaming project in northern Virginia with Nichols Construction, of Leesburg Va.

The job was located in Broad Run, Va., and involved the installation of a 4-in. pipe for Verizon, underneath Highway 7. Because of the need to lower the grade in order to add on and off ramps, as well as remove an existing traffic light, the bore needed to be a minimum of 25 ft deep for the entire length of the bore. This requirement added to the length of the bore, making it quite long at 520 ft.

The geology in this area of northern Virginia is a highly abrasive hard Blue Granite and Quartz composite. The bore was to be drilled in solid rock, with the exception of a small amount of drilling going through overburden at the entry and exit points. Mincon HDD specialists Mike Nameth and Jason Miller, along with representatives of Mincon’s distribution partner, Vermeer Mid Atlantic Inc., were present onsite to assist throughout the project.

The drill rig used for the pilot bore was a Vermeer 23X30; the locating equipment used was a Digital Control Inc. F5 system.

The drill used a Mincon HDD50 Pilot Boring Hammer equipped with a 5 ¼-in. Slant Faced Bit. The Mincon HDD50 Pilot Boring Hammer was powered by an Ingersoll Rand compressor rated with 1,170 cfm at 350 psi. During the last 14 years, Mincon has developed one of the HDD industry’s most comprehensive range of Hard Rock Pilot boring Hammer Systems for trenchless drilling applications. The current Mincon range of six Pilot Boring Hammer Systems are capable of drilling pilot hole sizes from 3 ½ to 9 ¼ in. Mincon also manufactures the industry’s only patented Pull Reaming Range of Hammers. Utilizing this technology, reamed hole sizes in rock of between 7 and 16 in. and faster production rates are now possible. Mincon recently added the Mincon HDD70PR Pull Reaming Hammer to its Pull Reaming Hammer range. The tool is smaller than its predecessor the HDD80PR and is designed to ream hole sizes from 7 to 9 in. in hard rock.

The tooling entered the ground at the entry pit at a grade of -24 percent. Three drill rods were added, drilling through overburden until the tooling reached solid rock. The drilling advanced in solid rock with steering achieved at 3 to 4 percent per 10-ft drill rod until the tooling got to an inclination of 0 percent. Once this 0 percent inclination was achieved, the tooling was located at 32 ft below the surface of the highway. This depth of 32 ft was maintained for all of the drilling under the highway.

Once drilling underneath the highway was completed, the tooling was inclined at a rate of 3 to 4 percent per 10-ft drill rod. The tooling exited the ground on target at an inclination of +28 percent. The highly abrasive nature of the geology required a number of bit changes throughout the pilot bore and ended up, using three Mincon HDD50 5 ¼-in. Slant Faced Bits. A mixture of water and polymer was used at a flow rate of 2 gpm to assist with stabilization, flushing and evacuation of the drill cuttings.

The pilot bore then needed to be reamed to a hole size of 8 in. to allow for the installation of the 4-in. service pipe. Crews decided to use a new Mincon HDD70PR Pull Reaming Hammer due to the extremely hard abrasive geology encountered during the drilling of the pilot bore. A trench was excavated to a depth of 15 ft from the exit side of the bore down to solid rock. The drill rig used for the reaming of the pilot bore was a Vermeer 24X40, utilizing the same Ingersoll Rand compressor.

A Mincon HDD70PR Hammer was installed on the drill and had a standard carbide 8-in. HDD70PR Reaming Bit installed on it. Reaming commenced in hard rock with production rates of between 20 to 30 minutes per 10-ft  rod. As with the Mincon HDD50 Pilot Hammer a mixture of water and polymer was used at a flow rate of 2 gpm to assist with stabilization, flushing and evacuation of the drill cuttings. After 300 ft of drilling, it was necessary to replace the bit on the HDD70PR due to a reduction in penetration rate as the drill bit became worn out. The tooling was pushed back to the exit side of the bore, the worn bit was removed, and a new 8-in. bit was installed. The tooling was pulled back into the bore until it reached the 300-ft mark. Reaming re-commenced at this point and production rates picked up again to an average of between 20 to 30 minutes per 10-ft rod drill times. The reaming of the pilot bore was completed with this second bit.

Once reaming was completed, the Mincon HDD70PR Pull Reaming Hammer was removed from the drill string and the Mincon HDD50 Pilot Boring Hammer was reinstalled. The Mincon HDD50 Pilot Hammer chased the reamed hole back to the exit side to hook up the product pipe. At this stage, the Mincon HDD50 Pilot Boring Hammer was removed and a standard 4 ½-in. reamer was installed along with the 4-in. service pipe. The reamer, along with the service pipe, was pulled back to the drill rig at the entry location, completing what everyone agreed was a very difficult but extremely successful bore and installation.

Mincon Group would like to recognize the following groups whose professionalism and commitment to using the Mincon Range of HDD tooling solutions and methods led to this uniquely challenging jobs success: Contractor: Nichols Construction – Leesburg Va., Mincon Distribution Partner: and Vermeer Mid Atlantic Inc., Manassas Va., Jeff Jones, sales manager; Will Mielke, drill specialist; and Josh Swain, outside sales.

Tom Purcell is sales director at Mincon Group PLC.