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Locust Street Sanitary Improvements Project No. 6335

  • Small Boring Unit (SBU-A)
  • 1.0 m (42.0 in)
  • Gravity Sewer
  • 70 to 183 m (230 to 600 ft)
  • Clean Water Services
  • Gonzales Boring & Tunneling
  • Tigard, Oregon, USA

Project Overview

SBU-A for Locust Street Sewer ProjectIn Tigard, Oregon, upwards of 1.8 km (1.1 mi) of gravity sewer were installed by general contractor Northwest Earthmovers Inc. for project owner Clean Water Services. Gonzales Boring & Tunneling was subcontracted to complete three crossings, which formed part of the Locust Street Sanitary Improvements Project No. 6335. The three crossings measured 70 m (230 ft), 183 m (600 ft), and 98 m (320 ft) in length. The 450 mm (18 in) diameter PVC carrier pipe was laid to increase wastewater capacity in the area and stop overflows currently plaguing the system.

Gonzales Boring & Tunneling purchased a Robbins 1.0 m (42 in) SBU-A to complete the three crossings located below houses, neighborhood streets, a small creek, and a service facility.


Rock conditions on the first crossing consisted of clay and basalt, while the second crossing was composed of basalt at various rock strengths ranging from 48 to 82 MPa (7,000 to 12,000 psi). Sections of the crossings were also interspersed with dirt containing small boulders.

SBU-A Features

Robbins SBU cutterheads can be equipped with a variety of tungsten carbide bits and single or multi-row disc cutters depending on the ground conditions. The SBU cutterhead for the Gonzales Boring crossings was fitted with 165 mm (6.5 in) single disc cutters and larger muck bucket openings to handle the mixed ground conditions.

During the launch of the machine, the SBU-A was welded to the lead steel casing. The cutterhead was propelled forward via torque and thrust from the Auger Boring Machine (ABM). Spoil was removed through openings in the cutterhead called muck buckets and discharged using a full-face auger.

Tunnel Excavation

SBU-A for Locust Street Sanitary ProjectWorkers monitored line and grade, and were able to maintain advance rates of 12 m (40 ft) per 10-hour shift. Using a contractor-designed steering system, the SBU-A holed through within one hundredth of an inch design grade after 183 m (600 ft) of excavation. In addition to having the record for the longest single crossing length, 183 m (600 ft), the machine required no cutter changes after 250 m (830 ft) of boring. A third crossing of 98 m (320 ft) was also excavated in early 2010.