Robbins Small Boring Unit Earns a Place in the Record Books

On December 7, 2009 a new milestone was reached for ABMs using disc cutterheads. Contractor Gonzales Boring & Tunneling bored a landmark crossing length of 600 ft (183 m) using a 42 inch (1.0 m) 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, Oregon, USA 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 ft (90 m) in length, though there have been several crossings of approximately 500 ft (150 m) in length for larger diameter cutterheads.

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.

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.

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.

After completing its first 230 ft (70 m) crossing in clay and basalt, the SBU-A was launched for its second bore on October 28, 2009. The disc cutterhead was used with a 72 inch (1.8 m) ABM and 42 inch (1.0 m) steel casing. Rock conditions on the second crossing consisted of basalt at various rock strengths (7,000 to 12,000 psi / 48 to 82 MPa UCS). Crews monitored line and grade, and were able to maintain advance at about 40 ft (12 m) per 10-hour shift.

A contractor-designed steering system guided the SBU-A to within one hundredth of an inch design grade after 600 ft (183 m) of excavation. Despite the mixed ground conditions, no disc cutter required changing after 830 ft (250 m) of boring. A third 320 ft (98 m) crossing will be excavated in early 2010.

The three crossings were initially designed as a pilot tube microtunneling project using vitrified clay pipe. "The owner has saved over a million dollars on the trenchless section alone over their original cost estimates for pilot tube microtunneling. Because the owner listened to the construction community, they saved both time and money, and kept the dollars local," said Gonzales.

The crossings form part of the Locust Street Sanitary Improvements Project, No. 6335. Approximately 1.1 mi (1.8 km) of gravity sewer are being installed by general contractor Northwest Earthmovers, Inc. for project owner Clean Water Services. The 18 inch (450 mm) diameter PVC carrier pipe will increase capacity in the area and stop overflows currently plaguing the system.