Drilling deep under a neighborhood in Bellevue

In October 2022, Brownline USA surveyed a long and challenging HDD crossing in Bellevue, WA. As part of a sewer pipeline upgrade project, a 32” HDPE pipe was installed deep under a residential area.

The North Mercer Island/Enatai Sewer Upgrade Project has been years in the making for the Bellevue and Mercer Island areas. It involves replacing several miles of old sewage pipeline with new and more reliable pipes. A 900 m / 3000 ft stretch of pipeline was installed under a hilltop neighborhood, connecting the pipeline network to the Sweyolocken pump station.

Challenges on entry and exit

The drill trajectory started at the pump station, near a wetlands area and crossed under the ramps to highway I-90. Under the hill, the pilot bore reached a depth of 49 m / 161 ft from the surface and curved 18°. The drill punched out near the Lake Washington shoreline, very close to the piles supporting the highway bridge. Between the residential area, the lake and the highway piles, the margin at the exit side was very tight.

Accurate positional measurements and real-time logging (measurement while drilling) were required for this project, so Brownline USA brought in the Drillguide GST. In addition, the GPS track add-on was used for a secondary verification near the exit point. Despite the surrounding infrastructure, our tools do not suffer from interference. In this way, a highly accurate pilot bore is drilled.

Underground conditions

The geologic conditions for this bore were a challenge, because of transitions between hard rock and gravel layers and soft clay and sand layers. Casing was installed to protect the borehole from collapsing inward at the entry side.

The area surrounding the bore included a park and a pond near the entry side, a lake near the exit side, and an urban area above the bore. Measures were taken to avoid any mud spills into these areas. Pressure measurements logged by the GST kept the driller informed of any peaks in the annular pressure, which helps prevent inadvertent returns.

The accurately drilled pilot hole allowed the contractor to finish installing the pipe ahead of schedule.

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