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SBUs, Auger Boring Equipment Play Critical Role In Pipeline Expansion
The Phoenix Expansion Project traverses rough geography that parallels the existing El Paso Natural Gas line and minimizes environmental impact. The path cuts through two national forests – the Kaibab National Forest (nine miles of pipeline) and the Prescott National Forest (24 miles of pipeline). Much of the protected area did not require additional clearing since it was in close proximity to the already cleared path of the El Paso line; however, a unique crossing was required at the protected Verde River in the Prescott National Forest. Crews used the flume and dam method, constructing a temporary dam across the river to install pipe.
The pipeline project was awarded to two main contractors – Price Gregory Services and Rockford Corporation. Subsequently they have subcontracted some of the work and created hundreds of jobs for contractors since construction began in early 2008. Approximately 114 miles of the pipeline were contracted to Price Gregory Services, who installed both the main pipeline and 40 crossings under roads and canals. Work is ongoing on another 150 mile section of the line contracted to Rockford Corporation of Hillsboro, OR. Rockford then subcontracted T&D Services for a total of 19 crossings, four of which used Small Boring Units.
Choosing effective technology
Given the wide ranging geology, contractor T&D Services had a number of methods to choose from. The contractor elected to use a hard rock cutting head with disc cutters on four crossings ranging from 200 to 310 feet. The crossings passed through a mix of cemented cobbles ranging from six inches to 3 feet in diameter. After an initial bore wore out the cutter heads after only 30 feet, T&D switched to the Robbins equipment, said Rex Vandyke, superintendent for T&D Services.
The company rented a 48 inch diameter SBU A. The circular cutterhead was first welded to an 80-foot length of steel casing. A custom made, 72-inch auger boring machine was then used on the bores to provide both torque and forward thrust to the cutterhead, while allowing for spoils removal through a full face auger. “We’ve averaged about 10 feet per hour,” said Vandyke.
T&D first became interested in small boring units after completing a 54-inch diameter, 250 foot long crossing in Mexico in 2007. “The ground was very bad, mostly solid volcanic rock. We used a roller cone cutting head, which was a disaster – the crossing took about two and a half months,” said Tom Vandyke. After their initial experience, T&D began familiarizing themselves with small boring units.
Excavating the crossings