2017-10-19_REVISION - M2000041 - Laserfiche WebLink

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Specialist and reported to the Quality Control Representative immediately following excavation at a minimum frequency of once per every 10 lineal feet of trench. 3.06 EXCAVATION REFUSAL Bedrock key refusal is defined as excavation using a PC800(or equivalent excavator)with a minimum rate of 1'of depth per hour for a 30'to 40' long"set"or"cut"at an excavation depth of 30 feet or less. Deeper excavation will require more time due to longer cycle times and required breakout force at the point of digging. We expect very hard Pierre Shale Formation bedrock will be encountered during construction. It will be necessary to continue to excavate difficult bedrock to achieve the minimum 4 foot deep key. When these refusal conditions are encountered,the Contractor and Engineer shall document the length of trench affected and the time expended in achieving excavation of the key. Bedrock excavation refusal shall be determined by mutual agreement of the Contractor,Engineer,and the Owner's Quality Assurance Representative. When bedrock refusal is determined,the area of refusal shall be scraped clean using the Komatsu PC 800 and soil- bentonite backfill shall be placed over the bedrock within four hours of scraping and prior to the end of the current shift. 3.07 CLEANING TRENCH BOTTOM Upon completion of each excavation set(typically 30'to 40'long),any loose material or cuttings shall be removed from the bottom of the trench with the excavation tools or other means such as air lift pumps. The Slurry Trench Specialist and Quality Control Representative shall verify that the trench bottom has been cleaned of all granular material prior to allowing backfill placement. The depth to bedrock and total trench depth shall be measured and recorded every 10' as the trench excavation advances. Backfill slope profile soundings and trench backfill bottom soundings will be measured at the start and the end of each shift,and in events where trench backfill operations are suspended for more than 2 hours. If the soundings indicate a difference of more than 6"to those previously recorded,trench cleaning will be done prior to additional backfill placement. 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. 3.08 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 backfill gradation. The backfill material shall be thoroughly mixed into a homogeneous mass,free from large lumps or pockets of fines or sands. Occasional lumps of up to 6"in their largest dimension will be permitted. Just prior to placing,the backfill material shall have a slump between 2" and 6". To this end,the material shall be sluiced with slurry during blending operations. Sluicing with water will not be permitted. 3.09 BACKFILL PLACEMENT The backfill shall be placed continuously from the beginning of the trench,in the direction of the excavation, to the end of the trench. Backfill placement shall be such that the toe of the slope of the backfill in the trench shall not be greater than 50%of the reach of the excavator's effective"set"and"cut"length given the depth to the bottom of the key trench,as measured from the back of the active cut,or as required to permit inspection and measurement. Placing operations shall proceed in such a fashion that the surface of the backfill below the slurry wall follow a reasonably smooth grade and shall not have hollows,which may trap pockets of slurry during subsequent backfilling. Free dropping of backfill material through the slurry will not be permitted. Initial backfill shall be placed by excavating a"lead-in trench"within the slurry wall alignment. The"lead-in trench"will be excavated to a maximum slope of 1:1. Initial backfill will be placed in such a manner that the backfill enters the trench sliding down the lead-in slope until the backfill rises above the surface of the slurry trench. Additional backfill may then be placed in such a manner that the backfill enters the trench by sliding down the forward face of the previously placed backfill. To accomplish this,sufficient backfill shall be piled on the edge of the existing backfill to cause a slump and sliding action on the face of the in-place backfill. The backfill shall not be dropped or deposited in any manner that will cause segregation of the courser fraction of the backfill from the finer fraction of the backfill. Section 02168-Page 4 of 6 February 2016