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C. The slurry to be mixed with backfill material shall be either slurry taken from the trench or slurry meeting the requirements of slurry introduced into the trench. 2.02 BENTONITE Bentonite used in preparing slurry shall be pulverized(powder)premium grade sodium cation montomorillonite and shall meet the most current API Standard 13A,"API Specification for Oil-Well Drilling- Fluid Materials." 2.03 WATER Fresh water,free of excessive amounts of deleterious substances that adversely affect the properties of the slurry shall be used to manufacture bentonite slurry. It is the responsibility of the Contractor that the slurry resulting from the water used for mixing shall always meet the standards of this specification. 2.04 ADDITIVES Admixtures of the type used in the control of oil-field drilling muds such as softening agents,dispersants, retarders or plugging or bridging agents may be added to the water or the slurry to permit efficient use of bentonite and proper workability of the slurry. However,no additives shall be used except as approved by the Engineer. 2.05 SUPPLEMENTAL FINES Supplemental fine-grained soils shall be necessary to blend with the material excavated from the trench to meet the backfill gradation. Supplemental fines shall be soil free from roots,organic matter,or other deleterious materials. The supplemental fines shall have a percent passing the No.200 sieve of at least 50 percent and shall have sufficient plasticity so that the material can be rolled into a 1/8"thread without crumbling. Supplemental fines shall be staged in sufficient quantities along the slurry wall alignment. The supplemental fines plus the added dry bentonite and bentonite slurry shall be proportioned to provide the final slurry wall backfill mixture with a percent passing the No.200 sieve of 30%or greater by dry weight. 2.06 BACKFILL MIXTURE The material for trench backfilling shall be composed of slurry and soils obtained from the trench excavation. Supplemental fine-grained soils and dry bentonite will be blended into the backfill as outlined on the construction drawings. The backfill mixture shall be free from roots,organic matter,or other deleterious materials. The backfill shall be thoroughly mixed. The backfill mixture shall have sufficient plasticity so that the material can be rolled into a 1/8"thread without crumbling. The percent passing the No.200 sieve shall be maintained at greater than 30%. The backfill shall be reasonably graded between the following gradation limits: Screen Size Percent Passing (U.S.Standard) by Dry Weight 3/8" 65-100 No.20 40-85 No.200 >30 The material excavated from the trench is not expected to meet the fill requirements. Excess sand and gravel excavated from the slurry trench will need to be"wasted"and not included in the final backfill mixture. The "wasted"sand and gravel shall be replaced with approved supplemental clay soils. Supplemental clay soils shall be placed along the slurry wall alignment in sufficient quantities to meet the minimum percent passing the No.200 sieve. Samples of the borrow material to be used in the backfill will be tested in advance of slurry wall construction. Section 02168-Page 2 of 6 February 2016