CHALLENGES FOR HORIZONTAL DIRECTIONAL DRILLING IN WESTERN WASHINGTON

Horizontal Directional Drilling (HDD) is a trenchless construction method widely used in the pipeline engineering and construction industry. Geologic setting and local subsurface soil, bedrock, and groundwater conditions present unique challenges to HDD projects. Variable subsurface conditions present challenges such as borehole instability, difficult drill head steering, potentially slow advancement rates, formational slurry loss and inadvertent returns, and potential ground settlement.

This case study examines a 3,200-foot HDD reroute of a 16-inch diameter petroleum products pipeline beneath the Stillaguamish River in Washington State. Considerable technical challenges were overcome to complete the HDD in 2004 after a failed attempt in 2002. A glacial geologic setting included very dense glacial till, zones of clean sand and gravel interbedded with clay, occasional wood debris and large boulders, which resulted in difficulties maintaining HDD alignment, slow advancement rates and difficulties maintaining borehole stability. A challenging HDD path was designed to ascend from river floodplain to a high terrace while maintaining a minimum depth of 100 feet below the riverbed for a total vertical change of approximately 275 feet. Surface entry and exit point locations were constrained by sensitive areas, which included steep slopes, wetlands, and private property.  

Rapid movement of groundwater and liquefied sand into the borehole during reaming operations resulted in borehole collapse near the exit point. An attempt to seal the high groundwater/sediment flux zone using a grout plug placed within the borehole had limited success. The borehole was successfully sealed and stabilized using a surface injection grouting program completed via a series of vertical borings into and above the HDD borehole. This case history clearly illustrates the need for a thorough understanding of the geologic setting and associated subsurface conditions toward the successful planning, design and construction of HDD projects. This presentation will also discuss various techniques that can be used to overcome challenging geologic conditions.