Ohio contractor Capitol Tunneling Inc. has been in business for 50 years and is now setting new standards for hard rock disc cutting heads. The company has no less than three record-breaking projects, including a recently completed 440-ft long sanitary sewer line in Cold Springs, Ky. — a record for a Small Boring Unit (SBU-A) in the 24-in. diameter range.

The third generation family-owned business began in 1960 and now employs about 50 people in nine crews working in states east of the Mississippi. Capitol Tunneling’s use of Robbins SBUs dates back to 1997, when the company used one of the first models for a 345-ft long water line extension under a freeway in Louisville. “We wanted to compare disc cutter performance with standard roller cone rock cutting,” said Capitol Tunneling president and CEO Kyle Lucus, of his initial decision to try Robbins SBUs. “The technology has since proven itself on over 50 successful projects for the contractor in the last decade.”

Within the last year, Capitol Tunneling has completed 2,766 ft of bores using SBUs ranging in diameter from 24 to 72 in., including two landmark distances. Capitol is now setting its sights on more records, including two ongoing bores in Pennsylvania and Virginia. “These machines are an advantage for us, because they are a dry process, not requiring additives or slurry like other methods such as roller cone heads and microtunneling. Because of this we don’t have separation issues with spoils and can operate even in the most environmentally sensitive areas,” said Kyle Lucus.

A History of Small Boring Units


Since first using the disc cutting heads in 1997, Capitol has purchased five SBU-As and one SBU-M and has options to purchase several more. The fleet is typically used on eight to 10 jobs per year. “My father helped Robbins in the development process on the 1997 project. They have always been willing to work with us and listen to our suggestions to make the product stronger,” said Noah Lucus, project manager and estimator for Capitol.  

In 2006, the company purchased one of the first Robbins Motorized SBUs for a gravity sewer project in Ohio. The Perry Taggart Sanitary Sewer called for five crossings including two long crossings of 425 and 450 ft, which were originally specified as microtunneling. Capitol instead won the bid and successfully excavated the crossings using the 60-in. SBU-M/ABM setup, which remains a part of its fleet.

For Capitol, the setup of an ABM paired with a hard rock disc cutting head fills a specific need, particularly on long crossings in hard rock. “We have used roller cone heads, flat face rock heads and bullet bit heads. Each has its intended use, but the harder the rock, the more efficient it becomes to use a disc cutting head,” said Kyle Lucus.

General superintendent Dave McCulloch, who has worked on multiple SBU projects over the years, adds to the sentiment: “We tend to use Christmas tree heads for softer materials, but in hard rock, they wear very hard on the ABM, particularly on the gear box and transmission. Roller cone heads cut rock a little better, but there is still the problem with maintaining the ABM. With the Robbins Motorized SBU in particular, all of the rock cutting is done at the front — the ABM is simply pushing the casing and turning a small invert auger.”

Raising the Stakes in Kentucky


In 2010, Capitol completed a record-setting bore at the Ripple Creek Sanitary Sewer in Cold Springs. The 440-ft long crossing was excavated in just 24 days using a 24-in. diameter SBU-A with 4-in. hex auger and a 48-in. ABM with 60,000 lbs of thrust.

The crew ran into some difficulty when excavating in unpredictably mixed layers of shale and limestone. Mud seams ran throughout the rock and at one point the SBU-A was excavating in a full face of dirt. “We fought the head drifting for most of the bore because of the layers in the rock. We made adjustments to the steering fins to help control this and on several occasions we had to pull the casing back and forth, reaming the hole out to allow the casing and cutters to clear the spoils from the bottom and left hand side to avoid further drifting,” said Kyle Lucas. The crew continued to pull the auger string and check the grade throughout the bore.

The machine broke through on Nov. 24, 2010, 8 in. off line and exactly on grade. “I believe we could have pushed another 100 ft or so in this material. The jacking pressures were lower than expected, and our rotational pressure was next to nothing. Our foreman, Steven Barker, and his three-person crew should be congratulated for guiding the machine to its successful finish,” continued Kyle Lucus.

Despite the length, the 24-in. SBU-A required no cutter changes during the bore. “We generally don’t need to change the cutters until after the project. Typically after completion of the bore we take the cutters out, check them for wear and tear, and the quality of the seals., said McCulloch.  

The Ripple Creek project is only the most recent in the contractor’s run of record-breakers. Earlier in 2010, a 540-ft long crossing using a 36-in. SBU-A was completed in southern Ohio. Capitol’s next potential record-breakers include a 72-in. SBU-A and 84-in. ABM excavating a 250-ft long crossing in Centreville, Va., and a 48-in. SBU-M crossing in Clairton, Pa. The SBU-M is using a mixed ground cutterhead to excavate a landmark 435-ft long crossing in clay and sandstone beneath a 16th century cemetery.  

As of January 2011, Capitol was in the process of launching three more machines, with more projects planned in the near future. “At this point, it’s tough to keep up with all of our records,” said Kyle Lucus. With a 50th anniversary and scores of projects including four landmark bores, the company has much to celebrate.

Desiree Willis is a technical writer for The Robbins Co., based in Kent, Wash.

How it Works:

The Robbins Small Boring Unit (SBU-A) and Motorized Small Boring Unit (SBU-M)


The SBU-A is a type of hard rock and mixed ground cutting head mounted with disc cutters and used in conjunction with an auger boring machine (ABM). The SBU-A, available in diameters from 24 to 72 in., consists of a circular cutterhead mounted with single disc cutters capable of excavating rock from 4,000 to over 25,000 psi UCS.  For excavation, the SBU-A is welded to the lead steel casing. The ABM provides both torque and thrust to the cutting head, while a full-face auger is used for spoil removal.

In hard rock, disc cutters penetrate the rock face and create a “crush zone” through which fractures propagate. Material is then chipped from the face into openings in the cutterhead called muck buckets, which transfer the material to the auger. In mixed geology operation of the machine is the same, though the cutterhead may be fitted with a combination of tungsten carbide insert cutters, disc cutters, and drag bits to excavate the ground.

The Robbins SBU-M operates similarly to the SBU-A. It is designed as a manned entry, hard rock boring machine for use on both long utility installations (300 ft and over, depending on geology) and crossings with specific line and grade requirements.

At machine launch, the SBU-M is bolted to the lead casing and a small invert auger inside the casing
provides spoil removal. An in-shield motor and drive train provides torque to the cutterhead, while forward thrust is provided by an ABM or pipe jacking unit through the casing.

The machine can be steered throughout the drive from an operator’s console inside the rear shield. Control of line and grade is made possible through articulation cylinders and adjustable stabilizer pads, while a laser targeting system provides accurate monitoring of the machine’s position.

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