Trenchless Engineering Profile: Force Main Navigates Under Challenging Sites

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RJN Group Inc ProjectDesigning and constructing an 18-in. force main under a creek with certified levees and through a designated floodplain, wetlands traversing along the creek, a pond, and a heavily wood area with limited accessibility was only feasible using horizontal directional drilling (HDD). Engineers at RJN Group Inc. designed the creek crossing with the 18-in. force main installed in casing at a minimum depth of 5 ft below the creek bed.

While only one drill/receiving pit location was needed within the floodway and on the edge of the wetland, preparing designs for construction through the wetlands presented the greatest challenges. A single, 700-ft, directional drill was required through soil with minimal bearing strength, and the groundwater table and potential sand seam varied between 6 and 8 ft (the preferred depth of the drill).

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The insertion pit site also had limited layout space due to vegetation and property boundaries. This made selection of drilling pipe material critical. PVC was the selected as it is much lighter and has a smaller profile that HDPE. Fusible PVC required at least 800 ft of layout space which was not available.

Restrained joint PVC pressure pipe was the best option, but at the time of design was only available in 12- and 24-in. diameters. RJN worked with the manufacturer to determine if there were plans to produce an 18-inch pipe. While the manufacturer did not have production dates for the 18-in. pipe, design and permitting were completed assuming the 18-in. pipe.

As the project was bid, the manufacturer announced that the 18-in., restrained joint PVC pipe was available.  The force main was constructed successfully with minimal issues.

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