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3.03 BACKFILL MIXING AND PLACING EQUIPMENT <br />Equipment for mixing and placing backfill may consist of a suitable type of earthmoving or grading <br />equipment, such as bulldozers, or blade graders, or blenders such as a pug mill, that are capable of thoroughly <br />mixing the backfill materials into a homogeneous paste having the required gradation and properties and <br />placing the material in the trench as hereinafter specified. <br />3.04 SLURRY TRENCHING <br />Excavation shall be carried to final depth at the point where excavation is started and then the final depth of cut <br />shall be carried along the line of the trench. Excavation shall proceed continuously from the starting point to <br />the finishing point. Slurry shall be introduced into the trench at the same time trenching is begun and shall be <br />maintained in the trench during excavation and until backfilled. The Contractor shall maintain the stability of <br />the excavated trench at all times for its full depth. The level of bentonite slurry shall not be permitted to drop <br />more than 2' below the surface of the slurry trench working platform except as approved by the Engineer. The <br />Contractor shall have personnel, equipment, and materials ready to raise the slurry level at any time. To this <br />end, the Contractor shall have personnel on call to raise the slurry level, weekends and/or holidays included. <br />3.05 TRENCH BOTTOM <br />Unless otherwise directed by the Engineer, the bottom of the slurry trench will be extended a minimum of 4' <br />below the surface of competent bedrock. Weathered rock of uncertain quality overlies bedrock. Key trench <br />depth will be measured from the surface of competent bedrock as determined by the quality control testing <br />technician and the Engineer. The Contractor shall provide sufficiently powered excavation equipment capable <br />of penetrating through the bedrock to the prescribed depth. The final depth and penetration of the trench shall <br />be measured and checked by the Contractor and approved by the Engineer immediately following excavation. <br />3.06 CLEANING TRENCH BOTTOM <br />Upon completion of excavation, any loose material or cuttings shall be removed from the bottom of the trench <br />with the excavation tools or other suitable means such as air lift pumps. If the unit weight of the slurry in the <br />trench exceeds the specified limits, or becomes unworkable, the heavy slurry shall be removed from the trench <br />by airlift pump, clamshell, or other methods approved by the Engineer or the excess solids shall be removed <br />from the slurry by screening or centrifugal-type desander. <br />3.07 BACKFILL MIXING <br />Material from borrow shall be mixed and blended in mechanical blenders or by windrowing, disk harrowing, <br />bulldozing, blading or by other approved methods. Mixing and blending shall be performed in such a manner <br />as to produce the required gradation of backfill. The backfill material shall be thoroughly mixed into a <br />homogeneous mass, free from large lumps or pockets of fines, sand, or gravel. Occasional lumps of up to 6" in <br />their largest dimension will be permitted. Just prior to placing, the backfill material shall have a slump of 2" to <br />6". To this end, the material shall be sluiced with slurry during blending operations. Sluicing with water will <br />not be permitted. Containment berms will be constructed where necessary to prevent slurry from entering <br />rivers or ditches or flowing across the project boundary. <br />3.08 BACKFILL PLACEMENT <br />The backfill shall be placed continuously from the beginning of the trench, in the direction of the excavation, <br />to the end of the trench. The toe of the slope of the trench excavation shall precede the toe of the backfill slope <br />so that the toe of the backfill shall not be less than 50' following the toe of the excavation, or as required to <br />permit proper cleaning of the trench bottom and to permit inspection and measurement. Placing operations <br />shall proceed in such a fashion that the surface of the backfill below the slurry shall follow a reasonably <br />smooth grade and shall not have hollows which may trap pockets of slurry during subsequent backfilling. Free <br />dropping of backfill material through the slurry will not be permitted. Initial backfill shall be placed by <br />lowering it to the bottom of the trenches with clamshell bucket until the surface of the backfill rises above the <br />surface of the slurry trench at the end of the trench. Additional backfill may then be placed in such a manner <br />that the backfill enters the trench by sliding down the forward face of the previously placed backfill. To <br />4/08 Section 02168 - Page 3 of 5