Originally a pile driving company, Hub Foundation Company of Harvard, Mass., first added wide-diameter bore drilling to its operations 23 years ago.
Most of the region’s ground conditions generally call for augers. Greg Maxwell, grandson of company founder Frank Maxwell, said Hub has always persevered through each hard rock socket, but they have continually sought easier, faster ways to drill out hard rock. The continuous tweaking of core barrels and rock augers to get as much production as possible had seemed to run its course and a new technology was needed. Maxwell said they had been studying the cluster drill offerings from a few manufacturers in particular for about five years, but the occasional hard rock job did not justify the significant investment required for a good cluster drill.
Forty-five years of careful acquisition planning has taught the company, which Maxwell said is one of the few family-owned, wholly U.S.-based foundation specialists in the Northeast, to wait for the right opportunity to expand their capital.
That opportunity came with a contract under general contractor Manafort Brothers on a Providence, R.I., bridge project. The job called for 23 drilled shaft pilings 42 inches in diameter bored 7 feet into the basalt-like rock conditions beneath the banks of the Pawtuxet River.
“We chose Atlas Copco because of its integration of the hammers with the barrel. The engineering just seemed well conceived,” Maxwell said.
Tipping the Scales
Maxwell said the decision took into account a lot of factors, not just penetration rate—though it was a big plus. Hub needed to consider ease of setup, air requirements and fuel consumption, as well as personnel training and equipment maintenance. Running two 1,600 cfm air compressors, for instance, would require about 50 gph of fuel. When the cluster drill could be declared a good value after considering total costs, Hub made its purchase.
Maxwell said, “It was important to know I didn’t have to hire specially trained operators to run it, but I still want to have a dedicated team who knows how to care for it.”
Ken Kasavage, Northeast sales manager for Atlas Copco Geotechnical Drilling and Exploration tools, said, “While it’s drilling, water and mud are not a problem, but you wouldn’t want to let it sit idle overnight at the bottom of a hole where silt or mud can back up into the hammer components. A trained dedicated crew will understand this.”
High Penetration Rate
In these conditions, Maxwell estimated the cluster drill was outperforming the auger by about 4,000 percent.
Before sending the cluster drill in, the crew augers until they are at least a foot into solid rock, which project planners had described to Maxwell as metamorphosed schist and sandstone. He observed it to be quite hard. Drilling with these techniques in the same formation allowed Maxwell to compare them.
“We get about an inch an hour with the auger—and that’s being optimistic. With the cluster drill, we’ve gotten over 3 feet an hour.” He said they got less where the rock was heavily fractured. He believed there may have been a lot of overblast in a prior construction project where a water main was installed alongside the new bridge construction. Such areas of fractured rock decreased the efficiency of the cluster drill’s impact, reducing penetration rates.
No additional down force is required, as the drill and drill string weight are sufficient. Rotation is “as slow as humanly possible,” Mike Gushyn said, noting that the drill tended to rotate itself as it worked. Gushyn has been training on the drill under operator Shawn Skinner, who has drilled with Hub for 15 years.
Maxwell said they had intended as usual to complete the holes one at a time. The crew would begin a hole with 63-inch starter casing. They worked a 48-inch core barrel inside of that down to the metamorphosed schist, scoring a kerf into the rock before withdrawing it to send down a 48-inch casing with sacrificial carbide teeth.
They turned this casing into the rock at least a foot to form a good seal.
Then the plan was to have the 42-inch cluster drill hammer out the 7-foot socket. But the cluster drill was so fast that Hub found it was more efficient to set aside the cluster drill until all the casings were set, and then form the rock sockets for all of them with the cluster drill in a single sweep.
Asked if Hub had not gone with cluster drilling, what technique they would have chosen instead, Maxwell quipped, “Exactly. No other technique in these conditions compares to the efficiency of pneumatic percussion.”
Atlas Copco cluster drills use self-indexing chucks that allow bits to rotate when the cluster drill is off bottom and rotated. This is done to rotate the bits so carbide wear is even between gage and face carbides.
“If the bits didn’t do this,” said Kasavage, “the gage row of carbide inserts on the outer bits would wear faster than the others. It would wear out the bits and you’d not only be changing bits too often, you’d also be losing hole diameter.”
And the time required to change out bits, Kasavage said, can be cut by Atlas Copco’s patented quick-change bit system to less than 55 minutes for the six bits. The system uses special chucks with one set of retaining shoulders that secure bits in place. The chucks have two safety shoulders to reduce the possibility of losing a bit cycling over a void. The chucks permit bit changes without requiring access to the hammer.
Large Diameter Drilling Specialist Al Watry talked to the Hub crew at their headquarters and spent about three days on the drill site to ensure everything was working well.
Watry said, “Once cluster drills are built for customers, they get education on not only their operation but also maintenance. The operator needs to know what he’s looking for while it’s working and how to get the most out of the product—there’s compressors, lubricators and hammers that all function together.”
Watry said Hub’s staff brought great skills to the job and will see a lot of success with the cluster drill.
In general, he said customers who rent or own a cluster drill are impressed. “They might initially think about capital cost, but they change their mindset when they see the drilling speed and not wasting time waiting for an auger or having extra guys on standby for welding. They are really accomplishing something.”
Now that they have the cluster drill, it isn’t just big projects like the Warwick bridge replacement where it will pay off. When a call comes from another company to help out with a surprise hard rock hole, Hub Foundation might just have the perfect tool for the job, with 4,000 percent less time for aggravation.