Much has been written about Small Boring Units (SBUs) — rock cutting attachments that work with Auger Boring Machines (ABMs) or Pipe Jacking Units. The first machine to be shipped out of the Robbins factory was in 1996 for a project on the New Jersey Turnpike. However, there was a project prior to that, a prototype that paved the way for the design and development of the Small Boring Unit.
The story of the first SBU is one of necessity — New York-based contractor Bortech Co., Inc. was excavating a 140-ft long crossing in hard diabase rock tested at 16,000 to 20,000 psi UCS, a strength far above what typical ABM roller cone cutting heads could do. The SBU that was developed would not only go on to successfully bore that crossing, but would also be refurbished in 2017 for yet another project with a different contractor after boring multiple crossings in its long history.
The First SBU
When Bortech took on the original crossing in Leesburg, Virginia, the contractor knew the rock would require a new type of boring attachment for its ABM. The crossing, requiring 42-in. steel casing to be placed under Goose Creek at the mouth of the Potomac River for a new water main, would need something more than a roller cone cutting head.
The contractor first tried a prototype cutting head with limited success in the hard rock, and then called The Robbins Co. “I contacted Robbins and spoke to David Long (now retired), and we spoke in length about the Robbins design. As an engineering graduate, I felt comfortable with the design they were developing,” said Bortech president, CEO and founder Robert Titanic.
Also on that team at Robbins was current project engineer John Kocab: “The first thing we made was the cutter itself. We wanted to develop a small cutter (6.5 in. diameter). We also started conversations with a few different companies to see if we could make a cutterhead to mount on their equipment. We knew that we needed to mount the head on a bearing rather than on the end of the auger. Just hanging the head on the end of the auger string wouldn’t work.”
Kocab’s prototype 6.5-in. diameter disc cutters turned out to be a durable design that is still used on the smaller size SBUs today. “All we knew at the time were larger diameter cutters for TBMs, so it was based on that,” he said. “The main thing that was different was to combine bearing cup surfaces, hub and disc all into one piece, which was due to lack of space.”
In larger cutters the bearing cups, hub and disc are separate pieces. The design was tested on the jobsite, to great success. In the years that followed, Kocab would design 9.5- and 11.5-in. cutters for larger size SBUs, but only the 6.5-in. cutters would require the unique all-in-one setup due to their small size.
At the jobsite, the Bortech crew prepared the launch pit with a concrete starter block 7 ft wide x 7 ft high x 3 ft thick. The starter block was formed and poured on the face of the rock at the point of entry. Titanic explained that they chose to install a starter block because the rock face was not perpendicular to the cutting head. The crew welded the SBU to the lead steel casing and launched it mounted with one set of 6.5-in. single disc cutters in the gage, or perimeter, area of the cutting head, with double-row carbide button disc cutters in the center. The crew used an ABM as the power source to turn the cutterhead and provide thrust to the SBU.
“We applied 125,000 lbs of thrust, 25,000 ft-lbs of torque and 25 rpm to get the optimum cutting penetration through this rock formation,” said Titanic.
The newly designed machine worked — it bored through the rock in nine days, with eight hours of drilling each day. Only one set of disc cutters was needed for the hard rock crossing. Titanic recalled that the bore “went quite well, and everything stayed on line and grade.”
Bringing the Idea to Market
Since the initial bore, Bortech has excavated approximately 1,800 ft of crossings using Robbins SBUs. The company, founded in 1988, has also expanded to construction of guided bores for line and grade applications, microtunneling, HDD, and even concentric hammering using a company-built machine for unconsolidated rock formations. Throughout it all, the contractor has found auger-driven SBU crossings to be a solid bet to take: “It’s a straightforward and easy-to-implement technology. We’re still using SBUs to this day,” said Titanic.
SBUs gained popularity as word of their usefulness spread. “We were introducing a different kind of technology to the trenchless industry. Any time contractors used roller cone cutters in hard rock, there was a problem. We found that with disc cutters you could cut rock much better. The more rigidly you can hold everything in place the better it would cut — this was a different philosophy than what contractors were used to at that time. All of a sudden, contractors found they were able to cut things they couldn’t cut before,” said Kocab.
“It took a while for the technology to become commonplace in the industry. Many methods seem to have an application in some geology; some do cross over to the mainstream but many don’t. Good solid rock, though, was made for the SBU,” said Titanic.
One of the reasons Titanic cites for the method’s popularity is its usability in urban settings. “The SBU doesn’t have as much vibration as other methods. It is a good method around existing utilities because it doesn’t work off of percussion or force, just pressure,” he said. “It has a slow grinding effect looked upon favorably for urban projects. Since I work a lot in city environments, I am more aware of that.”
Some modifications were of course made after that initial project, and the line of Small Boring Units expanded to encompass diameters anywhere from 24 to 72 in.. “We refined the stabilizers — the first stabilizers were not as sturdy. We made them heavier duty along with everything else. A lot more steel goes into modern SBUs compared to the original. After that initial unit, I would say we increased the strength of the stabilizers by at least 50 percent,” said Kocab.
20 Years Later
As of 2017, more than 400 small boring units have been built and have operated both nationally and internationally — more than 90 percent of these are still in use. The original 42-in. SBU (now known as the SBU-A for SBU Auger) used by Bortech was rebuilt in the Robbins Solon, Ohio, shop for use by a different U.S. contractor. SBU-As have tackled rock as hard as 38,000 psi UCS, they have bored crossings more than 600 ft long, and the most-used SBU-A, a 30-in. diameter model, bored at least 11 crossings, totaling 2,800 ft. The legacy is a long one, as the technology has proven itself again and again.
For Titanic, the simplicity of the design is key to its longevity. “We like that the setup of the cutting head requires one weld. It does not have a steep learning curve. It’s an easy-to-use technology, and we’ll continue to use it,” he said.
This article was written by Desiree Willis, public relations manager at The Robbins Co.