2014-12-24_REVISION - M2004051 (4)

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3.03 BACKFILL MIXING AND PLACING EQUIPMENT Equipment for mixing and placing backfill may consist of a suitable type of earthmoving or grading equipment, such as bulldozers, or blade graders, or blenders such as a pug mill, that are capable of thoroughly mixing the backfill materials into a homogeneous paste having the required gradation and properties and placing the material in the trench as hereinafter specified. 3.04 SLURRY TRENCHING Excavation shall be carried to final depth at the point where excavation is started and then the final depth of cut shall be carried along the line of the trench. Excavation shall proceed continuously from the starting point to the finishing point. Slurry shall be introduced into the trench at the same time trenching is begun and shall be maintained in the trench during excavation and until backfilled. The Contractor shall maintain the stability of the excavated trench at all times for its full depth. The level of bentonite slurry shall not be permitted to drop more than 2 -feet below the surface of the slurry trench working platform except as approved by the Engineer. The Contractor shall have personnel, equipment, and materials ready to raise the slurry level at any time. To this end, the Contractor shall have personnel on call to raise the slurry level, weekends and/or holidays included. Containment berms will be constructed as necessary to prevent excavated materials, backfill and slurry from entering the mine, rivers or ditches, pond or flowing across the project boundary. 3.05 TRENCH BOTTOM Unless otherwise directed by the Engineer, the bottom of the slurry trench will be extended or "keyed" a minimum of 5 feet below competent claystone bedrock surface. Weathered bedrock of uncertain quality overlies bedrock and shall not be counted as part of bedrock penetration. When sandstone or other bedrock types are encountered which may have a higher permeability than claystone, keyway depths of 6 feet or greater may be required. The Engineer shall be notified of such variations in bedrock. The Contractor shall provide sufficiently powered excavation equipment capable of penetrating through the bedrock to the prescribed depth. Should excavation of Key bedrock proceed to where the Key cannot be achieved or refusal, the contractor will proceed to the next "cut ". Bedrock Key "refusal" will be defined as not being able to penetrate the key material at a minimum rate of 2 -feet per half -hour per 40 -foot horizontal "cut ". The depth to the top of the bedrock will be measured after the materials are properly identified in the excavation spoils. The final depth and penetration of the trench shall be measured and checked by the Contractor and approved by the Engineer. 3.06 CLEANING TRENCH BOTTOM Upon completion of excavation, any loose material or cuttings shall be removed from the bottom of the trench with the excavation tools or other suitable means such as air lift pumps. Additionally, the excavated trench will be cleaned by the Contractor of excessive sediment that may accumulate at its bottom. This will be accomplished by dragging the Hydraulic Excavator's bucket horizontally across the bottom of the trench. The same procedure for cleaning the trench bottom will be repeated prior to backfill introduction if trench re- soundings taken immediately prior to placing backfill are found to be less than those originally recorded. If the unit weight of the slurry in the trench exceeds the specified limits, or becomes unworkable, the heavy slurry shall be removed from the trench by airlift pump, clamshell, or other methods approved by the Engineer or the excess solids shall be removed from the slurry by screening or centrifugal - type de- sander. 3.07 BACKFILL MIXING Material from borrow shall be mixed and blended in mechanical blenders or by windrowing, disk harrowing, bulldozing, blading or by other approved methods. Mixing and blending shall be performed in such a manner as to produce the required gradation of backfill. The backfill material shall be thoroughly mixed into a homogeneous mass, free from large lumps or pockets of fines, sand, or gravel. Occasional Wattenberg Resource Pond 3 Slurry Wall Section 02168 — Slurry Wall Construction Page 3