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DRAFT 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 maybe 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 BACKFILL The material for trench backfilling shall be composed of slurry, dry bentonite, and selected soils obtained from the designated borrow azea. The soil shall be friable and free from roots, organic matter, or other deleterious materials. The backfill shall be thoroughly mixed and reasonably well graded between the following gradation limits: Screen Size Percent Passing (U.S. Standard) by Dry Weight a/e° 65-100 No. 20 40-85 No. 200 25-40 The fines in the backfill soil shall have sufficient plasticity so that the material can be rolled into a 1/8-inch thread without cmmbling. The water content of the backfill material shall not exceed 20% prior to blending with bentonite and/or slurry. Material excavated from the trench is not expected to meet the above requirements for backfill. Excess sand and gravel excavated from the slurry trench will need to be "wasted" and not included in the final backfill mix. The "wasted" sand and gravel shall be replaced with approved supplemental clay soils. Supplemental clay soils may have to be stockpiled along the slurry wall alignment as directed by the Engineer. Samples of the borrow material to be used in the backfill will need to be tested well in advance of slurry wall constmction to insure an adequate supply of acceptable material is on hand. Initial laboratory results indicate an addition of dry bentonite to the soil bentonite backfill is required at a rate of 1.0 percent of the dry weight of backfill material to provide an adequate reduction in permeability. PART 3 -EXECUTION 3.01 TRENCH EXCAVATION EQUIPMENT Excavation of the slurry trench cu[-off wall shall be accomplished by use of any suitable earth-moving equipment or combination thereof such as a backhoe and/or clamshell so that the required width trench can be carried [o its final depth of cut continuously along the trench line. Special chopping, chiseling or other suitable equipment maybe used as necessary to satisfactorily accomplish the required excavation. The width of the excavating tool shall be equal to or greater than the specified minimum width of the cut-off wall. Additional equipment such as air lift pumps and slurry desanders shall be used if required to clean the Wench bottom and/or slurry in accordance with the requirements of the Specifications. 3.02 SLURRY BATCHING PLANT The slurry botching plant shall include the necessary equipment including a mixer capable of producing a colloidal suspension of bentonite in water, pumps, valves, hoses, supply lines, and all other equipment as required to adequately supply slurry to the trench. Storage ponds shall be provided to store initially mixed 7/04 Section 02168 -Page 2 of 5