2010-02-11_REVISION - M2000158 (6)

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PART 3 - EXECUTION 3.01 TEMPORARY WORKING SURFACE PREPARATION A. General. The Contractor shall prepare the temporary working surface in accordance with Section 02200 Earthwork, and Section 02290 Clay Embankment & Slurry Construction Pad, of the Specifications. The Work shall include stripping topsoil and organic materials from the area of the temporary working surface, clearing trees, excavating soil, salvaging and stockpiling of selected materials, disposing of surplus and/or unsuitable material, furnishing and placing embankment or fill materials, trimming the earth grade and maintaining the Work as required. B. Grade. The longitudinal grade of the working surface shall not exceed 1 percent, unless approved by the Owner/Engineer. The level of the temporary working surface shall be at least 3 feet above the measured groundwater level. The temporary working surface shall be graded to ensure excavation equipment produces a vertical trench. The Contractor shall provide and maintain gravity drainage control along the temporary working surface. C. Maintenance. The Contractor shall maintain the working surface free of excessive amounts of debris and slurry. D. Remote Mixing Area. The Contractor shall prepare and maintain, as needed, a temporary working surface at a remote location from the slurry wall alignment in which the soil bentonite backfill will be proportioned and mixed. The areas available for remote mixing are identified on the Drawings. The Contractor may request to the Owner/Engineer for additional areas. The request for additional areas may or may not be approved by the Owner/Engineer. 3.02 SLURRY PREPARATION AND PLACEMENT A. Slurry Preparation. The Contractor shall prepare slurry using a flash or paddle mixer to provide stable colloidal suspension of bentonite in water. Freshly mixed slurry shall be allowed to hydrate initially in storage ponds or tanks. The slurry shall be occasionally agitated or recirculated in the storage ponds until bentonite is fully hydrated, based on stabilized Marsh Funnel viscosity readings, and until the slurry appears homogeneous. No slurry is to be made in the trench. The slurry shall meet the requirements set forth in Paragraph 2.01 C, Initial Slurry Mixture, before the slurry is introduced into the trench. B. Slurry Placement. Slurry shall be introduced into the trench at the time excavation begins. The level of the slurry in the open trench shall be maintained at all times at least 3 feet above the groundwater level and no more than 2 foot below the temporary working surface, unless approved by the Owner/Engineer. Dilution of the slurry by surface water shall be minimized. The slurry shall be maintained at all times in a condition which meets the requirements set forth in Paragraph 2.01 D, Slurry -Mixture in Trench. The Contractor shall maintain the level of the slurry in the trench at all times, including weekends, nights and holidays. C. Slurry Cleaning. The Contractor shall remove, from the trench, any slurry which contains excessive suspended solids as indicated by a slurry unit weight exceeding 85 pounds per cubic foot or within 15 pounds per cubic foot of the unit weight of the backfill mixture. The excessive suspended solids may be separated and removed from the slurry, and the "desanded" slurry may be re-introduced into the trench; or the slurry with the excessive suspended solids may be used for the preparation of the backfill mixture. If the density of the slurry becomes greater than 85 pounds per cubic foot, the slurry shall be removed by methods approved by the Owner/Engineer and the excess solids shall be removed by screening or by a centrifugal-type desander.