On 7 December 2009, a new milestone was reached for ABMs using disc cutterheads. Contractor Gonzales Boring & Tunneling bored a landmark crossing length of 183 metres using a 42 inch (one metre) 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 project in Tigard, Oregon, US, consists of three gravity sewer crossings in rock and mixed ground below houses, neighbourhood 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 90 metres in length, though there have been several crossings of approximately 150 metres in length for larger diameter cutterheads.

The Robbins SBU-A is a type of trenchless boring attachment for use with standard Auger Boring Machines (ABMs). The SBU-A, available in diameters from 24 to 72 inches (600 mm to 1.8 metres), 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, 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 70 metres crossing in clay and basalt, the SBU-A was launched for its second bore on 28 October 2009. The disc cutterhead was used with a 72 inch (1.8 metre) ABM and 42 inch (one metre) 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 twelve metres per ten hour shift.

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

The three crossings were initially designed as a pilot tube microtunnelling 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 microtunnelling. Because the owner listened to the construction community, they saved both time and money, and kept the dollars local,” said Mr Gonzales.

The crossings form part of the Locust Street Sanitary Improvements Project No. 6335. Approximately 1.8 kilometres 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.