SUBMARINE FLOW SLIDE IN PUGET SOUND

In December 1985, construction began on the offshore portion of the Renton Effluent Transfer System. Under the auspices of the Municipality of Metropolitan Seattle (Metro), treated effluent is pumped to Duwamish Head where the flow is separated into two 1.63-m-diameter steel pipes that extend about 3,050 m offshore into Puget Sound. A 155-m long diffuser section at the end of each pipe rests in about 185 m of water. A submarine slide occurred in Puget Sound during the dredging operation for the construction of the offshore portion of thc pipeline. The slide occurred during low tide and involved 400,000 m3 of medium-dense sands to a thickness of up to 12 m. The failure and an evaluation of the contributing causes are described in this paper. The failure is attributed to a collapse mechanism in the soil causing liquefaction as a result of low tidal conditions combined with details of the dredging operations.