2010-02-11_REVISION - M2000158 (6)

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The Contractor shall, if directed by the Owner/Engineer, employ one or more of the following techniques to satisfy the hydraulic conductivity criterion: A. Use a higher percentage of clay and silt materials in the backfill by separating and spoiling the excavated sandy layers. B. Add dry bentonite, up to 3.5 percent by dry weight, to the backfill. Bentonite shall be spread over 6 to 8 inch thick layers of soil backfill material and mixed thoroughly. C. Add more bentonite to the slurry mixture. D. Use material from an approved off-site area with a high percentage of clay and/or silt in the backfill by importing material. 3. Stockpiled material from excavation and off-site materials for slurry wall backfill shall be thoroughly mixed and blended into a homogeneous mass, free from large lumps or pockets of fines, sand, or gravel, by windrowing, disk harrowing, bulldozing, blading, or by other methods such as a pug mill, as approved by the Owner/Engineer. 4. Slurry may be sluiced into the backfill mixture during blending operations to obtain the required consistency and slump. Sluicing with water will not be permitted. 5. Excess slurry that may drain away from the mixing operations shall be allowed to drain back into the trench or shall be collected if mixing is done at a remote location. In no case should excess slurry flow outside of the construction limits. 6. The Contractor may be required to add dry bentonite to the backfill material by broadcasting, pug mill or other methods approved by the Owner/Engineer to achieve the desired hydraulic conductivity value. 7. If the backfill material is mixed adjacent to the trench, the Contractor shall construct a small dike (0.5-1 foot high) parallel to the slurry trench, if required by the Owner/Engineer, in order to keep the backfill from flowing uncontrolled into the trenches as a result of wave action created by the dozer mixing the backfill. Intermittent openings in the dike will be allowed so that excess slurry may flow back into the trenches. 8. The Contractor shall mix backfill at a remote mixing area if conditions are not acceptable to the Owner/Engineer along side of the trench. 9. No mixing of backfill material shall be performed when the air temperature is continuous below 25 degrees Fahrenheit unless otherwise approved by the Owner/Engineer. 10. Mix only the quantity of backfill that is needed to construct the wall. B. Placing 1. The backfill mixture shall be in compliance with the backfill requirements, immediately prior to introduction into the trench. 2. The backfill shall be placed so that no pockets of slurry are present in the completed slurry wall. The backfill shall not be deposited in any manner that will cause segregation. Dropping of backfill material into the slurry will not be permitted. 3. The Contractor shall backfill continuously in the direction of the excavation from the beginning of the trench to the end of the trench, unless approved by the Owner/Engineer. 4. If a lead-in trench is not excavated, the initial backfill shall be placed by lowering the material to the bottom of the trench by means of a clamshell bucket or other approved equipment until the backfill emerges from below the slurry surface and achieves its natural angle of repose from the bottom of the trench to the surface. 5. Placing operations shall proceed in such fashion that the top of the backfill below the surface of the slurry shall follow a reasonably uniform grade and shall not have hollows that may trap pockets of slurry during subsequent backfilling. The bulldozer operator shall pile sufficient backfill on the edge of the existing backfill to cause the backfill to enter the trench by sliding down the forward face of the previously placed backfill. 6. The toe of the backfill slope shall not be less than 30 feet following the toe of the excavation to permit proper cleaning.