Rock-Water-Sewers

October 2012, Vol. 67 No. 10

The Pottstown sewer relocation was composed of four separate bores of 300, 100, 158 and 185 linear feet respectively. The licensed professional engineer on Rice’s staff staked-out the location for each bore, and launch pits were excavated with consideration for the strictly prescribed alignments. Launch pits were over-excavated to accommodate crushed rock that was placed in the bottom of each pit to serve as a means for dewatering a continuous flow of groundwater that seeped into the pits as a result of the proximity to the nearby river. The rock foundation also doubled as a stable base for Rice’s boring machines. Two-inch submersible electric pumps -- operating 24 hours a day -- were positioned in well points within each pit to remove the continual flow of groundwater; volumes fluctuated daily depending on what the river was doing. As Irene blew through, however, the water level, as expected, reached unprecedented levels.

Necessary adjustments
Results of soil samples extracted from the 20-plus-foot depths required to comply with the specified cover tolerances of the project plan indicated a clean alluvial silt and alluvial grout with minimal blow counts. The plan was to use a guided pilot system approach; but after excavating the first launch pit, and encountering cobble up to eight inches in diameter, followed by fairly hefty boulders, Rice was concerned.

The amount of rock and cobble encountered on the first 300-foot bore prevented Rice’s crew from pushing the guided pilot beyond the 180-foot mark; forcing them to abandon the guided pilot method. After performing several adaptations and employing a combination of techniques, they were finally able to complete the first bore but the process added five weeks to the amount of time Rice had originally estimated.

“That was the point when I started talking to the folks at McLaughlin about the ON Target system,” says Rice. “I knew I couldn’t depend on the guided pilot system for the remaining crossings. I knew something different would need to be used if we were to be successful completing the remaining bores.”

Rice’s auger boring machine was outfitted with a 24-inch ON Target steering head.

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