Crews finish Magnolia pipeline drilling work

King County’s contractor has brought the Horizontal Directional Drilling (HDD) effort to a close. On Friday, Dec. 12, Walsh Construction and Mears Group Inc. began pipeline pullback, a step that involves pulling 3,000 feet of fused pipeline through a prepared borehole from 32nd Avenue West to the exit point on 23rd Avenue West. This pipeline will connect to a new underground storage tank in the Smith Cove area that will help reduce discharges of stormwater and sewage to Puget Sound.

Because the Magnolia project is building a gravity sewer pipeline, the orientation of the drilling alignment is downhill. The downhill orientation — with an exit point 55 feet lower than the entrance — requires large amounts of grout to keep the borehole open during the drilling process, and controls at the bottom to keep pressure on the borehole. Before the pullback, pressure was relieved by draining the borehole into tanks on 23rd Avenue West.

For the last few months, Mears’ crews carried out several steps to get to the pullback. They first established a pilot bore along a pre-determined alignment. In three reaming passes, they widened the borehole to 26, 36 and, finally, 46 inches in diameter. They then installed a casing to protect the entry at 32nd Avenue West during pullback and smoothed the borehole to help the pipeline travel through.

Over the weekend, they used a drill rig at 23rd Avenue West to begin pulling the pipeline through the borehole. Five pipeline strings about 600 feet long (and about 34 inches in diameter) were fused during pullback to form one continuous pipeline.