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I Materials needed to increase the fines content of the backfill mixture shall contain at least <br />65 percent fines (minus 200 sieve) by weight, a Plasticity Index of or greater than 10 and <br />have a moisture content less than 15 percent, measured prior to loading in the truck at the <br />borrow pit, unless approved by the Owner/Engineer on a case-by-case basis. <br />F. Backfill <br />1. Materials for the backfill mix shall consist of slurry, dry bentonite, if necessary excavated <br />trench materials, and approved off-site soil. <br />2. Backfill shall be free of roots, organic soil, lumps, trash and debris. <br />3. Frozen materials shall not be used. <br />4. The backfill mixture shall have the following gradation limits: <br />% U.S. STD Passing Sieve No. <br />100% 3" <br />70-100% #4 <br />>30 % #200 <br />5. However, occasional lumps of up to 5 inches in their largest dimensions may be permitted <br />provided that the lumps are not close together and, in the Owner/Engineer's opinion, will <br />not affect the performance of the slurry wall. <br />6. If material from off site is needed to increase the fines content of the backfill mixture, the <br />fines shall not exceed 45 percent, unless approved by the Owner/Engineer on a case-by- <br />case basis. <br />7. The backfill mixture shall have a consistency that has the appearance of a wet concrete <br />and a slump of 3 to 5 inches, just prior to placing. <br />8. The density of the backfill shall be a minimum of 15 pounds per cubic foot more than the <br />slurry at any depth in the trench. <br />9. The five test running average hydraulic conductivity of the backfill shall be 5 x 10-7 <br />cm/sec, or less. The maximum permissible value shall be 1 x 10-6 cm/sec as measured <br />by the modified filter press test. <br />10. The Contractor shall vary the bentonite content and mixing operations periodically, based <br />on varying in-situ soil properties, to achieve the specified hydraulic conductivity value. <br />2.02 EQUIPMENT <br />A. Trench Excavation. Equipment for excavating the slurry trenches shall consist of a cable clam shell <br />and/or hydraulic excavator. The equipment shall be capable of excavating the minimum required <br />trench width of 2.5 feet in a single pass of the excavating tool. The equipment shall be capable of <br />excavating at least 5 feet deeper than the maximum depth shown on the drawings. If used, <br />dragline buckets shall be free of protrusions. <br />B. Slurry Mixing. The Contractor's slurry plant shall include a suitable mixer capable of producing a <br />colloidal suspension of bentonite in water, plus necessary pumps, sumps, valves, lines, hoses and <br />storage tanks or ponds to provide a continuous supply of slurry. <br />C. Cleaning Slurry. The method of cleaning used for removal of suspended solids and bottom <br />sediments from the slurry in the trench shall be capable of maintaining the slurry density within the <br />specified limits. Equipment shall include clamshell, air lift pumps, mud shakers, settlement ponds, <br />pipes hoses, and fittings, or other equipment approved by Owner/Engineer. <br />D. Mixing and Placing Backfill. Equipment for mixing and placing the backfill shall be any type of <br />earthmoving or grading equipment, such as bulldozers, disk harrows, and blade graders, or <br />blenders and pug mills that are capable of thoroughly mixing the backfill material (soil and