Robbins Small Boring Unit Earns Place In Record Books

February 2010 Vol. 65 No. 2

On Dec. 7, 2009, a new milestone was reached for auger boring machines (ABMs) using disc cutterheads. Contractor Gonzales Boring & Tunneling bored a landmark crossing length of 600 feet using a 42-inch diameter small boring unit (SBU-A).

The feat is a record for any diameter of SBU-A, making it the longest distance ever excavated with the boring attachment. The Tigard, OR, project consists of three gravity sewer crossings in rock and mixed ground below houses, neighborhood streets, small creeks and a service facility.

“A combination of preparation, qualified crew, accurate machine design and Robbins’ unmatched support services made this a successful crossing,” said Jim Gonzales, president of Gonzales Boring & Tunneling. More typical jobs for the SBU-A are below 300-feet in length, though there have been several crossings of approximately 500 feet for larger diameter cutterheads.

The Robbins SBU-A is a type of trenchless boring attachment for use with standard 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, as with the Gonzales boring project, cutterheads can be fitted with a variety of tungsten carbide bits and single or multi-row disc cutters. The cutterhead used on the record-breaking SBU-A features single 6.5 inch diameter disc cutters and larger muck bucket openings to better handle conditions consisting of solid basalt, interspersed with clay and dirt sections containing small boulders.

During machine launch, the SBU-A is welded to the lead steel casing. Throughout the bore, the ABM provides both torque and forward thrust to the cutterhead. Openings in the cutterhead called muck buckets collect spoil from the face, where they are transferred to a full-face auger for removal.

After completing its first 230-foot crossing in clay and basalt, the SBU-A was launched for its second bore on Oct. 28. The disc cutterhead was used with a 72-inch ABM and 42-inch steel casing. Rock conditions on the second crossing consisted of basalt at various rock strengths (7,000 to 12,000 psi). Crews monitored line and grade, and were able to advance at about 40 feet per 10-hour shift.

A contractor-designed steering system guided the SBU-A to within one hundredth of an inch design grade after 600-feet of excavation. Despite the mixed ground conditions, no disc cutter required changing after 830-feet of boring. A third, 320-foot crossing will be excavated in early 2010.