A great deal was riding on one shot at a record pilot tube bore under a taxiway at Mobile Regional Airport in Alabama recently. The ideal combination of geology and operator skill allowed for pinpoint accuracy on a 595-ft pilot tube guided auger bore drive for a long-time boring contractor.The project is believed to be the longest installation by the pilot tube/guided boring method ever completed, surpassing the previous mark of 580 ft.
Alabama Power needed 595-ft of 16-in. OD steel casing installed to house electrical conduits under the taxiway, although it was first suspected that the length was closer to 550-ft. Once the steel casing was installed, general contractor Pike Electric was to insert two 6-in. and one 4-in. HDPE pipes inside to house power lines. D&G Boring Inc. of Smyrna, Ga., has been serving the southeast since 1975. D&G operators were familiar with the method of guiding an auger bore rig with a GBM system, having used one on a project in Baton Rouge three years prior. When the Mobile Airbus Project became available for bid, D&G owners knew that the GBM system would be ideal.
In early February, D&G received its shipment of a leased GBM 240A jacking frame, P100Q power unit, a generation four GBM guidance system, 20 racks of dual-walled pilot tubes, and a 16-in. weld-on reaming head to upsize to the final casing diameter.
Crews were mobilized and began excavating and shoring the 12-ft by 36-ft launch pit in early February. An Akkerman field technician was dispatched to the job site shortly thereafter, and the GBM jacking frame was placed on the auger bore rig rails in front of their Barbco 48/60 unit. The guidance system was carefully calibrated for the 4-in. line and grade tolerance. It was suspected and later confirmed that there would be 8-ft of cover above the alignment.
Pilot tube installation began on Feb. 18. Day 1 consisted of training for five crew members, who were able to get 210-ft across before the shift’s end. On Day 2, crews maintained a clear sight path of the target and quickly found their groove to install all but 10-ft of the full drive. Day 3 began with crews excavating and shoring a 12-ft by 12-ft reception trench box. While the last few lengths of pilot tubes were launched, some of the crew was trained on pilot tube removal as the lead pilot tube with steering head emerged into the reception pit. D&G completed the drive in the next few weeks by way of standard auger bore operation and reported the final line and grade was well within the tolerance. They wrapped up and pulled out in early March.
The project’s geological conditions represented the perfect storm of variables to achieve this distance. Average industry lengths have evolved as new technology has emerged since the advent of early pilot tube guided auger boring. Today’s average length for this application is right around 400 ft in most displaceable soil. When asked what factors gave them the confidence to bid on a project of this length, foreman Monk Thompson cited the soft and smooth clay soil in addition to the company’s level of confidence in the system based on past experience. When advised of the current industry average, Thompson jokingly stated, “What’s a couple hundred more feet?”
Regarding his satisfaction with the project, Thompson said that the outcome “was above and beyond our expectations to emerge dead perfect on line and grade.” D&G Boring Inc. will begin another pilot tube guided auger boring project in the near future, relocating an emergency sewer line in Valdosta, Ga.