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poured in one continuous operation. <br /> 3. Sealing material shall be placed in the interval or intervals to be sealed by <br /> methods that prevent free fall, dilution, and/or separation of aggregate from <br /> cementing materials. <br /> 4. Where the head (pressure)producing flow is great, special care and <br /> methods must be used to restrict the flow while placing the sealing <br /> material. In such cases, the casing must be perforated opposite the area to <br /> be sealed and the sealing material forced out under pressure into the <br /> surrounding formation. <br /> 5. In destroying gravel-packed wells, the casing shall be perforated or <br /> otherwise punctured opposite the area to be sealed. The sealing material <br /> shall then be placed within the casing, completely filling the portion <br /> adjacent to the area to be sealed and then forced out under pressure into the <br /> gravel envelope. <br /> 6. When pressure is applied to force sealing material into the annular space, <br /> the pressure shall be maintained for a length of time sufficient for the <br /> cementing mixture to set. <br /> 7. To assure that the well is filled and there has been no jamming or <br /> "bridging" of the material, verification shall be made that the volume of <br /> material placed in the well installation at least equals the volume of the <br /> empty hole. <br /> D. Materials. Requirements for sealing and fill materials are as follows: <br /> 1. Impervious Sealing Materials. No material is completely impervious. <br /> However, sealing materials shall have such low permeability that the <br /> volume of water passing through them is of small consequence. <br /> Suitable impervious materials include neat cement, sand-cement grout, <br /> concrete, and bentonite clay, all of which are described in Section 9, <br /> Subsection D, "Sealing Material' of these standards; and well-proportioned <br /> mixes of silts, sands, and clays (or cement), and native soils that have a <br /> coefficient of permeability of less than 10 feet per year.Note 24 Used drilling <br /> muds are not acceptable. <br /> 2. Filler Material. Many materials are suitable for use as a filler in destroying <br /> wells. These include clay, silt, sand, gravel, crushed stone, native soils, <br /> mixtures of the aforementioned types, and those described in the preceding <br /> paragraph. Material containing organic matter shall not be used. <br /> E. Additional Requirements for Wells in Urban Areas. <br /> In incorporated areas or unincorporated areas developed for multiple habitation, <br />