Large Pilot Tube Guided Bore in Downtown Jacksonville—A Solution for Challenging Ground Conditions

This paper describes the design and construction of a 155-ft long bored crossing of the Arlington Expressway in downtown Jacksonville, Florida. The crossing is for JEA’s Main Street Water Treatment Plant water transmission main project. This project assists JEA in meeting future water supply needs and increases system reliability. The crossing is part of a 36-in. water transmission main that will increase the amount of water that JEA can transfer between their North Water Grid and South Water Grid. The transfer of water between their major grids is a key component of JEA’s long term water supply plan. The Arlington Expressway crossing was constructed using pilot tube guided boring (PTGB) to install a 54-in. diameter steel casing pipe under the expressway. PTGB is a technique for installing casing accurately to design line and grade by first advancing a steerable steel pilot tube, followed by one or more boring passes centered on the tube to upsize the bore and install the casing. The Arlington Expressway crossing design required the boring contractor to use PTGB in conjunction with dewatering to overcome challenging ground conditions consisting of sand deposits below the water table. The designers selected the bore horizon to provide a minimum soil cover thickness of approximately 1.5 times the casing diameter, and to avoid an underlying layer of very loose sand, which presented a risk of difficulties maintaining soil stability at the face of the bore, and with steering. The designers specified PTGB considering the importance of preventing grade deviations and concerns for maintaining the required grade tolerance with conventional auger boring. Dewatering was required to stabilize the launch and exit pits, and to draw down the water table under the expressway to promote face stability. The casing was installed within 1.5 in. of design, and settlement along the alignment was negligible. The 36-in. carrier pipe was installed in the casing, and the annulus was backfilled with cellular concrete to complete construction of the crossing.