2008-05-27_REVISION - M1997058 - Laserfiche WebLink

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2.05 BACKFILL The material for trench backfilling shall be composed of slurry, dry bentonite, and selected soils obtained from the trench and designated borrow areas. 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 2" 80-100 3/8" 65-100 No. 20 40-85 No. 200 > 25 The fines in the backfill soil shall have sufficient plasticity so that the material can be rolled into a 1/8" thread without crumbling. The water content of the backfill material shall not exceed 20% prior to blending with bentonite and/or slurry. The percent passing the No. 200 sieve will be, in general, maintained at greater than 30%. When testing reveals a value less than 30%, corrective action will be taken to prevent a fines content value below the specified 25 percent minimum. 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 construction to prove an adequate supply of acceptable material is on hand. Laboratory results indicate an addition of dry bentonite at a rate of 0.5% of the dry weight of backfill material will provide an adequate reduction in permeability. PART 3 - EXECUTION 3.01 TRENCH EXCAVATION EQUIPMENT Excavation of the slurry trench cutoff 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 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 required excavation. The width of the excavating tool shall be equal to or greater than the specified minimum width of the cutoff wall. Additional equipment such as air lift pumps and slurry desanders shall be used if required to clean the trench bottom and/or slurry in accordance with the requirements of the Specifications. 3.02 SLURRY BATCHING PLANT The slurry batching 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 slurry to allow hydration and to serve as a reserve in cases where substantial slurry loss from the trench through underlying pervious zones or other reasons may occur. The slurry shall be agitated or recirculated in the storage ponds as required to maintain a homogeneous mix. All slurry for use in the trench shall be prepared using a suitable mixer. No slurry is to be made in the trench. Mixing of water and bentonite shall continue until bentonite particles are fully hydrated and the resulting slurry appears homogeneous. 4/08 Section 02168 - Page 2 of 5