Robbins Small Boring Unit (SBU-A) Breaks World Record in Kentucky

May 19, 2009

Contractor Pushes Limits on Hard Rock Auger Bore: After 35 working days of excavation, Ohio contractor Turn-Key Tunneling, Inc. had reason to celebrate.  Their trenchless crossing beneath Louisville, Kentucky, USA's Interstate 265 had just been completed at an unprecedented bore length of 352 ft (107 m).  The machine, a Robbins Small Boring Unit (SBU-A), emerged with a record for trenchless boring in its size class of 54 inch(1.4 m) diameter bores, improving upon the previous record of 350 ft (106.7 m) set by C.B. Services of Dallas, Texas in 2000.

"We are confident, even after testing the limits of the 54 inch SBU, that we could bore longer lengths given the right rock and project specifications," said Deborah Tingler, Vice President for Turn-Key Tunneling, Inc.  Typical jobs for the SBU-A are below 300 ft (90 m) in length, as smaller diameter SBU-As have successfully excavated crossings up to 500 ft (150 m).

The Robbins Small Boring Unit (SBU-A) is a type of trenchless boring attachment for use with standard Auger Boring Machines (ABMs).  The SBU-A, in diameters from 24 to 72 inches (600 mm to 1.8 m), consists of a circular cutterhead mounted with disc cutters.  The disc cutters are capable of excavating rock from 4,000 to over 25,000 psi (25 to over 175 MPa) UCS.  In mixed ground, the cutterheads are fitted with a variety of tungsten carbide bits and single or multi-row disc cutters.

In the launch pit, the machine is welded to the lead steel casing, while the ABM provides both torque and forward thrust to the cutterhead.  Openings in the cutterhead called bore scrapers collect spoil from the face, where they are transferred to a full-face auger for removal.

Turn-key Tunneling launched the SBU-A on January 22, 2009 in conjunction with a 60-1200 ABM.  The rock, consisting of limestone up to 20,000 psi (138 MPa) UCS, and the 2.91% grade presented challenges early on.  The cutterhead drifted while boring through several dirt seams as well as a mud-filled cavern, which momentarily slowed progress. "We continued to make progressively smaller adjustments and the SBU responded very well.   The last 200 ft we bored were very true and required little fine-tuning," said Roger Lewis, Project Superintendent for Turn-Key Tunneling.  The machine broke through on March 11, just 0.17 in (4.3 mm) off of line and grade and well within its contractual ± 3 in (8 cm) requirements.

The method proved highly successful compared to other trenchless techniques. "Conventional tunneling for a 54" tunnel with a drill and blast technique would produce about a three to four feet per shift rate of production.  On this crossing, blasting was not permitted so hand mining would have been no more than a foot or two per shift.  Given these project restrictions, the SBU proved to be at least 5 times faster than hand mining," said Lewis.

The crossing is part of the 5.4 mi (8.7 km) long Gene Snyder Transmission Main, built by general contractor MAC Construction of Indiana.  The USD $6.4 million project, for the Louisville Water Company, will connect an existing community line with expansion on the other side of the highway.  The project is scheduled for completion by early 2010.

The Robbins Company
Brianna Home
29100 Hall Street
Solon, OH 44139, USA
PH.: +1 253.872.4484

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The Robbins Company

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