The Tahoe Forest Hospital is an expanding healthcare center located in Truckee, California. In autumn 2010, they were in the process of designing a new cancer wing when it was determined that additional utility and mechanical lines were required before the new building could be built. To house the bundled utilities, three crossings needed to be bored directly below the active main hospital.
General contractor AM-X Construction & Excavation, Inc. subcontracted the three parallel 21 m (70 ft) sections to Silver State Boring Inc. Due to variable ground conditions and hospital noise constraints, the contractor opted for a Robbins SBU-A with a mixed ground head. This was the first time Silver State had used this type of cutterhead, but they were confident that it was the right machine for the job.
The hospital is set on ground containing large granite boulders, and Silver State was worried that one of these rocks would be hit during boring. This concern was actualized 7.6 m (25 ft) into the first bore when the SBU-A caught the edge of a boulder about 3.7 m (12 ft) in diameter. During the second bore, the machine drilled straight through the same boulder. Additional boulders of 175 MPa (25,000 psi) UCS were encountered during the third bore, and the SBU-A successfully powered through them as well.
The Robbins SBU-A is a circular cutting head mounted with a combination of tungsten carbide bits, single disc cutters, and multi-row disc cutters. The SBU-A is available in diameters from 600 mm to 1.8 m (24 to 72 in), and can excavate a wide variety of hard rock and mixed ground conditions including dry soils, clay, cobbles and boulders from 25 to 175 MPa (4,000 to 25,000 psi) UCS.
During boring, the SBU-A is welded to an Auger Boring Machine (ABM), which provides both torque and thrust to the cutting head. Drag bits scrape soil from the machine face, while disc cutters excavate sections of rock. Muck scrapers scoop the excavated rock into large openings in the cutterhead, allowing for a smooth flow of muck from the face to the auger string.
The SBU-A bored 2 m (8 ft) below the hospital’s foundation for the three jack-and-bore crossings. Straight bores were needed due to tight exit space, and each one began with a 609 mm (24 in) thick concrete starting block to keep the machine properly aligned. In addition, the older hospital has many buried utilities that needed to be cut and bypassed during site excavation. To make sure that everything would run smoothly, Robbins field service technicians visited the site prior to launch to assist with setup and crew training.
The first of the three bores began in October 2010, and by November 2010 all three bores were successfully completed. Advance rates for the duration of the project were approximately 3 m (10 ft) per day.
Due to the low torque and impact of the SBU-A, little stress was put on the ABM, no cutter changes were needed between bores, and the hospital was unaffected by noise or vibration during boring. Silver State was very pleased with the results, and used the technology again in 2011 for 609 mm (24 in) diameter bore in South Lake Tahoe, Nevada.