HORIZONTAL DIRECTIONAL DRILLING
Bennett Trenchless Engineers has extensive experience designing horizontal directional drilled projects. We have designed projects for many different sectors, including water, sewer, fiber optic, gas and electricity. We have worked on projects with pipe diameters from 4” to 36” OD, and with lengths over 6,800 feet.
Below are descriptions of a few of the projects that Bennett Trenchless Engineers has designed.
Netarts Force Main Replacement Project – Netarts, Oregon
As a subconsultant to Westech, Bennett Trenchless Engineers provided design assistance and construction inspection services for the Netarts Force Main Replacement Project. The Netarts/Oceanside Sanitary District on the Pacific coast near Tillamook, OR had only one sewer force main pipeline that ran along the beach and up a 120-foot high bluff. The force main was susceptible to landslide movements. The District elected to replace the sewer, using HDD to install 1386-feet of 11.1-inch inner diameter DR 9 HDPE sewer away from the beach and out of the landslide area.
Four borings completed along the alignment encountered clay, silt, sand, gravel, cobbles, wood, and embankment fill, providing serious challenges. A starter casing was used to bridge and support the near-surface gravels and cobbles along the beach. However, gravels and cobbles were also encountered within the bluff, and random roadway fill was anticipated near the exit. Therefore, an alternative exit was developed if the bore could not be completed through the fill.
Design issues included avoiding soils containing gravels and cobbles as much as possible, minimizing disruption while working on the beach during the tourist season, 120-foot elevation changes between entry and exit, random fill materials at the upper end of the alignment, controlling drilling fluid pressures to prevent hydrofracture, steering control in gravels and cobbles, and pipe layout along the highway. The project was successfully completed July, 2006.
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Click on to view Bore Profile
 South River Pump Station: Power Supply Project – Sacramento, California
Bennett Trenchless Engineers provided trenchless design assistance for a power supply crossing of the Sacramento River. The crossing provided electrical power from an existing substation in the Pocket neighborhood of Sacramento to the new South River Pump Station on the Lower Northwest Interceptor Project west of the Sacramento River. The crossing consisted of a 1,475-foot long, 24-inch diameter steel casing pipe containing four 6-inch PVC conduits for the electrical conductors and one 4-inch PVC conduit containing fiber optics for a SCADA system. The casing passed beneath flood control levees on both sides of the river.
The design of the crossing required special sequencing to minimize disruption to the residents in the Pocket neighborhood. The contractor was required to both drill the bore and fabricate the pipe on the west side of the river. Only after the bore was prepared was the contractor allowed to mobilize the drill rig to the neighborhood on the east side of the river to pull the casing and conduits into place. The contractor was also required to contact grout the ends of the bore to prevent seepage of floodwaters along the bore annulus beneath the levees. A relief well was installed near the landside toe of the levee in the Pocket neighborhood to protect against hydrofracture near the levee. The project was completed succesfully in October, 2005.
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 Lower Northwest Interceptor Project: Barge Canal Crossing – Sacramento, California
As a subconsultant to CH2M Hill, Bennett Trenchless Engineers provided trenchless design assistance for the Barge Canal HDD Crossing, which comprised one design element of the larger Lower Northwest Interceptor Project containing seven microtunneled crossings and a twin HDD crossing. The HDD crossing consisted of twin 900-foot HDD bores beneath a shipping canal, several railroad spurs, and a flood control levee. The twin installations of 20-inch DR 11 HDPE pipe were separated horizontally by 20 feet. The alignments of these force mains included both vertical and horizontal curves to better position the exit location for the tie-in work.
Bennett Trenchless Engineers performed full design services for the crossings including a constructability analysis to determine the preferred trenchless construction method, basis of design report, and alignment for the pipelines. The design of the twin bores required minimizing the potential for hydrofracture into the Barge Canal, as well as methods to reduce the risk of settlement damage to the rail lines, levee, and canal bottom. Challenges included a complex geotechnical conditions and the presence of highly permeable alluvial conditions at the entrance and exit. Finally, the design called for contact grouting of the annular space along the initial and final 150 feet of the alignment to prevent preferential seepage paths for floodwaters beneath the levee.
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