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EXH AR025785: Ecology Memo — Retaining Wall/Slope Alternatives, Miller Creek, Sta 174+00–186+00

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Sea-Tac Airport
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A 1999 memo from the Washington State Department of Ecology evaluates construction techniques for a retaining wall near Miller Creek at Sea-Tac International Airport's Third Runway project. The memo concludes that specialized methods such as tied-back walls, bored piles, soil nailing, or ground freezing can address difficult underground soil conditions while keeping construction within the approved wetlands disturbance footprint, though at significant cost. The author also recommends installing monitoring devices in the wall and backfill to confirm the structure performs as designed throughout its service life.

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Memorandum dated August 9, 1999 from Jerald LaVassar, M.S., P.E. (Washington State Department of Ecology) to Tom Luster reviewing the retaining wall/slope alternatives report for the Third Runway at Sea-Tac Airport, Embankment Station 174+00 to 186+00. Addresses whether construction techniques exist to handle adverse foundation conditions — including very dense glacially overridden soils at depths of 10 to 30 feet — while restricting wetland disturbance to the proposed footprint. Concludes that tied-back walls, bored piles, soil nailing, and ground freezing are viable specialty-contractor techniques. Evaluates the mechanically stabilized earth (MSE) wall proposal and references Shannon & Wilson, Inc. peer review comments. AR 025785–025786.

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