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n 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 BACKFILL The material for trench backfilling shall be composed of slurry, dry bentonite, and selected soils obtained from the designated borrow area. The soil shall be friable and free front 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 3/e" 65-100 No. 20 4o-ss No. 200 15-40 The fines in the backfill soil shall have sufficient plasticity so that the material can be rolled into a I/8-inch thread without crumbling. 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 [he 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 construction to insure an adequate supply of acceptable material is an hand. Initial laboratory results indicate an addition of dry bentonite at a rate of I.5 percent of the dry weight of backfill material will provide an adequate reduction in permeability. PART3-EXECUTION 3.01 TRENCH EXCAVATION EQUIPMENT Excavation of the slurry trench cut-off wall shall be accomplished by use of any suitable eanh-moving equipment ar combination thereof such as a backhoe and/or clamshell so that the reyuired width trench can be carried to its final depth of cut continuously along the trench line. Special chopping, chiseling or other suitable equipment may be used as necessary to satisfactorily accomplish the reyuired 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 trench bottom and/or slurzy in accordance with [he requirements of the Specifications. 01 /99 Section 02168 -Page 2 of 5