Good Things Come in Threes – Complex Soil Mixing at a Superfund Site



Good Things Come in Threes – Complex Soil Mixing at a Superfund Site

Presenter:  Paul R. Lear (Envirocon)
Authors:  Paul R. Lear and Peter Joy (Envirocon)


As part of a larger remedial effort at the St. Maries Creosote Superfund Site, installation of a perimeter wall and in-situ soil mixing of creosote-impacted soils was necessary. Envirocon used a unique combination of two in-situ and one ex-situ soil mixing techniques for the required soil mixing, marking the first time that 3 soil mixing techniques were simultaneously employed. Envirocon constructed a 735-foot long perimeter cut-off wall that extended 62-feet deep to the underlying aquiclude layer, using the modified slurry wall method. The perimeter wall was excavated using a PC 800 excavator equipped with a long stick under bentonite slurry. The excavated trench spoils were brought to the surface and mixed ex-situ in a 40 cubic yard mixing box with a Portland cement/slag grout to create a soil-crete backfill, which was then replaced into the trench through the bentonite slurry. The placed soil-crete backfill met the required performance criteria for strength (>50 psi) and permeability (<1×10-6 cm/s). For the in-situ stabilization (ISS), two soil mixing approaches were used to reduce the cost and schedule for the ISS treatment. Excsvstor mixing was conducted with a GPS-equipped, PC 400 long reach excavator in mixing cells to a depth of 30 feet. Auger mixing was performed using a Delmag RH-34 drill equipped with an 8.5-inch auger from the working surface down to the prescribed bottom depth – between 40 to 52 feet bgs. The soil was treated with 10% ground granulated blast furnace slag and 5% Portland cement. Regardless of the soil mixing technique, all ISS treated material met the required performance criteria for strength and permeability.


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