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ENVIRONMENTAL CONSTRUCTION AND REMEDIATION Deep Soil Mixed Cut-Off Wall A large manufacturing firm required an underground vertical barrier (cut-off) wall to isolate two areas of PCB-contaminated soils and prevent contaminated groundwater from leaching into a nearby river. The owner chose Geo-Con's Deep Soil Mixing (DSM) method over other conventional methods for the following reasons: (1) placement of the cut-off wall approximately four feet from the river's edge, representing very soft soil conditions along with the hydraulic forces of the river, causing concern that a conventional slurry cut-off wall trench might fail during construction, and (2) since the cut-off wall would be installed in contaminated soil, DSM was an overwhelming choice because it does not require any soil to be excavated and disposed of during construction.
Geo-Con's specially designed four-auger DSM rig was utilized for this project. The uniquely designed overlapping hollow shaft augers, complemented with staggered angled mixing paddles, allowed for 25 cubic ft of wall to be installed for every vertical ft drilled. As the augers advanced vertically, a cement-bentonite grout was injected into the soil and blended. At the same time, Geo-Con's continuous mix grout plant supplied the large volumes of grout required by the four-shaft rig.
Technical requirements for the cut-off wall specified a permeability of 10-6 cm/sec and structural properties similar to a retaining wall. To meet these requirements, Geo-Con evaluated more than 15 soil mix designs. The soil mixes were tested for compatibility with the leachate, permeability and strength before a cement-bentonite mix (grout) was selected.
Geo-Con operated on a two-shift basis to install the 250,000 sf of cut-off wall required. The cut-off wall was keyed two ft into a dense glacial till layer, which in some areas was almost 70 ft below the existing ground surface. The client requested that approximately 2,600 lf of the cut-off wall be structurally fortified by the addition of structural steel beams to allow for future excavation of contaminated soils inside (away from the river) the wall area. Geo-Con added the steel beams and provided the client with a cantilevered retaining wall capable of holding back the load applied. Comprehensive health and safety training was given to all employees prior to the start of the project. Trained Geo-Con personnel monitored the site and the health and safety program through the project duration.
DSM is an appropriate form of vertical barrier construction in situations where traditional vertical barrier techniques cannot meet the specified requirements due to soil conditions or contaminated soil conditions present. This project represents the first application of DSM to contain hazardous wastes in the United States.
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