FIELD DOCUMENTS AND WORK PLANS 1991

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injection wells, lysimeters and extraction well may be drilled using either method, as approved by the Engineer. The water level in each boring shall be maintained whenever drilling equipment is retracted in preparation for sampling to avoid unbalanced hydrostatic pressure that might wash in material from the sides and bottom of the boring. 2A-3. DUAL-WALL REVERSE CIRCULATION DRILLING. Dual-wall reverse la circution drilling shall include earth drilling and sampling. All borings completed using the dual-wall reverse circulation method shall be drilled vertically. Split barrel and thin-walled tube sampling shall be performed in accordance with the requirements of Section 2A-5. Dual- wall reverse circulation drilling shall be performed as required to satisfy the requirements for advancing the boring and cleaning the borehole for each sampling procedure. Borings for the vertical injection wells shall have a minimum outside diameter of 9 inches. The boring for extraction well EW-2 shall have a minimum outside diameter of 12 inches. Borings for the monitoring tubes shall have a maximum outside diameter of 5-1/2 inches or larger if approved by Engineer. The Contractor shall be prepared to use the 6-5/8 inch dual-wall size for installation of the monitoring tubes if directed to do so by the Engineer. The borings for the lysimeters shall have a minimum outside diameter of 6-5/8 inches. The drilling equipment and materials required to perform the dual-wall reverse circulation drilling shall be decontaminated. Grease and other lubricants shall not be used, unless approved by the Engineer, on the drill casing, drill rods, sampling equipment, or on tools required to perform the dual wall reverse circulation drilling. In silty formations which might be disturbed by side discharging bits, the hole shall be prepared for split barrel sampling with a bit equipped with baffles to deflect the air upward. Air used during drilling to remove the soil cuttings shall be filtered to remove organic contaminants from the air prior to the air entering the boring. A minimum of two air filtering devices shall be provided in the air supply line. Certification of the absence of organics in the air shall be provided to the Engineer upon Engineer's request. Water to aid the drilling may be added to the air used for drilling only when approved by the Engineer. The water added shall be potable and shall come from a source approved by the Engineer. Water shall not be added to the monitoring tube borings. 2A-4. HOLLOW-STEM AUGER DRILLING. Hollow-stem auger drilling shall include earth drilling and sampling. Injection wells IW-6, 7, and 8 (MARLEY SSFS ) 2A-3 (CONTRACT NO. 13) (11654.652 )