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12 2.3.6 GRAVEL ENVELOPE AND INTERMEIDIATE SEALS A. Scope -- This item shall consist of providing and installing gravel or coarse grained sand opposite the screen intervals and intermediate bentonite or sand/cement seals between the screen intervals,as specified by the Engineer, in the annulus between the casing and screens and the well bore of each well. Intermediate bentonite seals may be used in saturated zones. Sand/cement grout shall be used for intermediate seals in unsaturated zones. B. Methods 1. Prior to placement of the gravel pack and intermediate seals in the well,the drilling fluid shall be thinned with clean water. Thinning shall be accomplished by reducing the viscosity of the drilling fluid in the sump to a maximum marsh funnel viscosity of 30 seconds and a maximum weight of 8.9 pounds per gallon by the addition of clean water to the sump. The Contractor shall avoid the direct injection of water into the well bore through the tremie pipe in order to prevent unbalancing the fluid consistency in the borehole. 2. Gravel packing and sealing material shall be pumped or gravity fed through the tremie pipe. The gravel pumping system shall consist of a hopper, which will allow for the calculation of the amount of gravel packing material entering the borehole. The Contractor shall provide the Engineer with a schematic drawing of the system of gravel placement he intends to employ prior to the installation of casing. 3. The tremie pipe shall be removed in approximately twenty-foot intervals when the gravel in the borehole reaches the tremie pipe. 4. The quantities of gravel placed in the annulus of each well shall not be less than the computed volume of the annulus. A quantity less than the computed value will be judged as an indication of voids, and corrective measures shall be taken by the Contractor. 5. If the volume of gravel installed in the annulus is less than the theoretical volume, the well may be rejected by the Engineer. 6. Gravel packing and bentonite seal placement shall continue uninterrupted until the gravel pack reaches the depth of the surface seal. 2.3.7 ANNULAR SEAL A. Scope --. This item shall consist of providing and installing a sand/cement grout annular seal in the annulus between the casing(s) and the bore of each well. B. Methods-Installation of the annular seal shall conform to State Water Well Standards and the requirements of the well permitting agency. 1) The Contractor shall proceed with sealing operations after the Engineer verifies the depth of the top of the gravel in the well annulus. 2) The tremie pipe shall be installed no more than 5 feet above the placed gravel envelope before beginning seal placement. The bottom of the tremie pipe shall remain