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New Boring Record Set In Louisville
After 35 working days of excavation, Ohio contractor Turn Key Tunneling Inc. had reason to celebrate. Their trenchless crossing beneath Interstate Highway 265 in Louisville, KY, had just been completed at an unprecedented bore length of 352 feet.
The machine, a Robbins Small Boring Unit (SBU A), emerged with a record for trenchless boring in its size class of 54-inch diameter bores, improving upon the previous record of 350-feet set by C.B. Services of Dallas, TX, 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. Typical jobs for the SBU A are below 300-feet in length; smaller diameter SBU As have successfully excavated crossings up to 500 feet.
The Robbins 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, 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. 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 Jan. 22 in conjunction with a 60 1200 ABM. The rock, consisting of limestone up to 20,000 psi, and the 2.91 percent grade presented early challenges. 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 feet 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 inch off of line and grade and well within its contractual three-inch tolerance requirement.
“Conventional tunneling for a 54-inch tunnel with a drill and blast technique would produce about a three to four feet per shift rate of production,” Lewis explained. “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 five times faster than hand mining.”