2017-10-19_REVISION - M2000041 - Laserfiche WebLink

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The effective confining pressure for laboratory permeability test samples shall be based on the estimated vertical pressure at 2/3 the height of the slurry wall at the sample location. PART 3-EXECUTION 3.01 TRENCH EXCAVATION EQUIPMENT Excavation of the slurry trench cutoff wall shall be accomplished by use of earth-excavating equipment and any combination thereof such as a backhoe and/or excavator so that the required width trench can be carried to its final depth and cut continuously along the trench line. Special chopping,chiseling or other suitable equipment may be used as necessary to satisfactorily accomplish the required excavation. A heavy excavator and rock bucket with an aggressive tooth configuration,sufficient to overcome the breakout force of the bedrock is recommended. 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. All slurry for use in the trench shall be prepared using a mixer capable of mixing slurry to meet the technical specifications. Slurry hatching shall not be performed in the trench. Mixing of water and bentonite shall continue until bentonite particles are fully hydrated and the resulting slurry appears homogeneous and meets the project specifications. 3.03 BACKFILL MIXING AND PLACING EQUIPMENT Equipment for mixing and placing backfill may consist 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(i.e. Station 0+00)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 commences 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 always remain at least 2' above groundwater level and shall not be permitted to drop more than 2'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. 3.05 KEY Unless otherwise directed by the Engineer,the bottom of the slurry trench shall extend through the visible weathered bedrock and be keyed a minimum of 4 feet into the underlying suitable bedrock as indicated by soil borings or trench cuttings and as further specified on the"Stonewall Springs Water Storage Reservoir,Slurry Wall Profile Drawings". Suitable bedrock generally consists of hard to very hard,grey to dark grey shale of the Pierre Shale Formation. Suitable bedrock is sometimes referred to as bedrock or competent bedrock. The DRMS refers to suitable bedrock as"an impermeable layer at the base of the wall." For the purposes of this design,suitable bedrock is bedrock a few inches below the visibly weathered horizon that has relatively low permeability and sufficient plasticity to provide an effective water cutoff with the slurry wall. The depth to bedrock and the final depth and penetration of the trench shall be measured and checked by the Slurry Trench February 2016 Section 02168-Page 3 of 6