Horizontal directional drilling (HDD) projects continue to progress into more and more challenging applications. ‘At-grade’ HDD techniques or drills where the profile is maintained at a constant grade to allow for gravity flow applications are a particularly challenging application — especially as pipe sizes get larger and alignment lengths get longer.
One recent example of HDD technology and a strong project team overcoming a unique set of technical challenges, including an ‘at-grade’ application, occurred in Seattle. A 32-in. outer diameter (OD) gravity sewer pipeline was installed using HDD methods for King County to reduce combined sewer overflows (CSO) into Elliott Bay. This large design-bid-build project included a 3,151-ft HDD that required a grade of 1.8 percent through a glacial bluff to convey flows into a 1.5 million-gal retention storage tank.
The new pipeline augments an older section of King County’s wastewater system, which was comprised of a single pipeline to carry both sewage and stormwater runoff. This combined sewer collection is routed to a wastewater treatment plant that treats the waste stream; however, during periods of heavy rainfall, the pipeline can easily surcharge, leading to a CSO into Puget Sound.
King County implemented this project to meet the requirements of the Washington State Standard and to reduce the frequency of CSOs into Puget Sound, which can pose a risk to public health and the environment. Currently King County has reduced CSOs by 90 percent countywide, and succeeded in keeping more than 2.3 billion gals of sewage out of local waterways.
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