RCAA ON-LINE LIBRARY
Embankment
Issues in Brief: Sea-Tac Airport was built on a narrow plateau, which runs north & south. To add the third runway, the Port plans to build an artificial extension of that plateau, toward the west. The runway itself is planned for a length of 8500 feet & width of 150 feet, with several hundred feet of mandatory runway safety areas north and south, bringing the length of the embankment to well over 9000 feet. The runway would slope, north to south, & would run north & south, parallel with the existing two major runways, with its north end on a line with the ends of the other two runways.
The midline of the third runway would be 2500 feet west of the midline of the longer of the existing runways (the more easterly). The midline would be fall on 12 th Avenue So., as it is shown on most area maps.
Fill would be required to a depth as great as 160 feet in some places. The total amount of fill required for the embankment is 19.84 million cubic yards, to be compacted down to 17.25 million cubic yards. About one-quarter of this amount has been brought to the site in four years of work. Port staff have consistently under-represented the amount of required fill, even to the Commission, apparently trying to make the project look less daunting, or to make it seem that more work has been completed than is the case. It would be the largest earth–moving project in this state since the Grand Coulee Dam was built.
The embankment would have a slope of 2:1 (greater in some areas), & would be held in place, not by concrete walls, but by the “Mechanically Stabilized Earth” method – fill would be compacted in layers, with horizontal reinforcements (steel matting, polymerized sheets) between the layers (analogous to “stickers” in a load of lumber). The only concrete would be a decorative facing. Although the Port's experts claim this will be perfectly safe, no “MSE” wall of comparable height & length has ever been identified. Several much-smaller MSE walls failed during the Nisqually 'quake, including one on a State highway in Thurston, and a very small one, partly damaged, at a Port of Seattle pier in Seattle.
There is no similar runway project built or planned anywhere. Runways are usually built on reasonably level ground, requiring only minor amounts of fill, little leveling, and not much compacting - & certainly no MSE walls.Documents:
Geosyntec Consultants, February 2001. Comments to the WA State Department of Ecology on the critiquing the engineering of the great wall (as of 2001, there are four mechnically stabilized walls.) MSE walls: "significant deficiencies in the field and laboratory investigation, and in the analysis." [Acrobat version, 452KB, 22 pages]
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