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Site Dewatering

Seattle, WA: Soil-Cement-Bentonite Slurry Wall Westlake Mercer Cleanup Project


Geo-Con was contracted to install a soil-cement-bentonite gravity retaining/barrier wall. The gravity retaining/barrier wall was designed as part of the remediation of a site that is a former retail gasoline station in downtown Seattle, Washington. The site is a HAZWOPER site requiring work to be completed under the requirements of 29CFR1910.120.

The gravity retaining/barrier wall was installed to serve two purposes. The wall will serve as a gravity retaining wall to allow 20-foot deep excavation with a vertical face up to the wall. The removal of soil to this depth by the general contractor is the primary remediation measure on the site. The low permeability property of the wall will also serve to prevent infiltration of water on to the site during excavation, thus eliminating the need for extensive dewatering and potential migration of contamination.


The length of this gravity retaining/barrier wall is 900 linear feet with a depth of 20 feet for a total of 18,000 vertical square feet. The wall is 12-feet wide to its full depth. The center 4-foot wide section of the wall was installed an additional 5-feet deeper for a "key-way" that was designed to prevent overturning or sliding of the wall during subsequent site excavations.

Slurried bentonite was used as hydraulic shoring to keep the excavation open. The soil excavated from the wall alignment was disposed of off-site as it was excavated from within the property lines and potentially contaminated. The backfill for the gravity retaining/barrier wall was created using imported soil mixed with cement, dry bentonite, and slurried bentonite. The backfill was mixed by batches in steel rock boxes using two hydraulic excavators. This method allowed the backfill to be mixed without incorporating any of the potentially contaminated site soil. The resulting mixture created a permanent wall with compressive strength and low permeability. The design parameters called for strength (UCS) between 20 and 300 psi and a permeability of less than 1x10-6 cm/sec. Field samples of the freshly mixed material were taken once a day from the mixing operation prior to placement in the trench and remolded into cylinders for laboratory testing. Average compressive strength at 28-days was 105 psi and the permeability ranged from 2 to 7x10-7 cm/sec. All other quality control tests were done on site by the Geo-Con Project Engineer. These tests included unit weight, viscosity, pH, and filtrate on the bentonite slurry and slump and density testing on the backfill mixture.


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