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C. Conductor Casing. For community water supply 5. Bentonite clay mixtures shall be composed of bentonite clay and clean water thoroughly mixed before wells, the minimum thickness of steel conductor casing shall placement so that there are no balls, clods, etc. be 1/4 inch (6 millimetres) for single casing or a minimum of No. 10 U. S. Standard Gage for double casing. Steel used for 6. Used drillers' mud or cuttings or chips conductor casing shall conform to the specifications for from drilling the borehole shall not be used as sealing steel casing described in Section 12. material. D. Sealing Material. The sealing material shall The minimum time that must be allowed for consist of neat cement grout, sand-cement grout, bentonite materials containing cement to "set" before construction clay, or concrete. Cement used for sealing mixtures shall operations on the well may be resumed shall be: meet the requirements, including the latest revision thereof, of ASTU C150 "Standard Specification for Portland Cement" a. Type I cement - 72 hours types' I(common construction cement) III (high early b. Type III cement - 48 hours strength) and V (for high sulfate resistance, i.e., corrosive c. Type V cement - 6 hours waters),?,/ Water used for sealing mixtures shall be clean and of a potable quality. Materials used as additives for When necessary these times may be reduced by the use of Portland cement mixtures in the field shall meet the require- "accelerators", i.e., additives designed specifically to ments, and latest revision thereof, of ASTM C494 Standard shorten setting time. Specification for Chemical Admixtures for Concrete". 1 8. Where thermoplastic casing is used, caution . Neat cement grout shall be composed of one should be exercised to control the heat generated during the sack of Portland cement (94 pounds or 43 kilograms) to 4-1/2 curing of the cement (called "heat of hydration"). This is to 6-1/2 (depending on cement type and additives used) of special concern where casing of thinner wall thicknesses gallons (17 to 25 litres) of clean water. are to be installed. The addition of bentonite to the cement 2. Sand-cement grout shall be composed of not mixture (up to 8 percent) or circulating water inside the more than two parts by weight of sand and one part of casing will lower the temperature of the cement. Additives which accelerate the curing process also tend to increase the Portland cement to 4by to 6-1/2 (depending on cement type and additives used) gallons (17 to 25 litres) of clean water heat generated and should not be used where thermoplastic casing is installed. per sack of cement. 3. Concrete / used shall be "Class A" (6 sacks E. Thickness of Seal. The thickness of the seal of Portland cement per cubic yard or 0.76 cubic metre or shall be at least a nominal inches,f and not less than "Class B" (5 sacks per cubic yard or 0.76 cubic metre�.:�L/ three times the size of toe largest coarse aggregate used in the sealing material. Aggregates shall meet the requirements, including the latest revision thereof, of ASTM C33 "Standard Specification for F. Placement of Seal. Concrete Aggregates". 4. Special quick-setting cement, retardents to 1. Before placing the seal all loose cuttings, setting, and other additives, including hydrated lime to make drilling mud, or other obstructions shall be removed from the the mix more fluid (up to 10 percent of the volume of cement), annular space by flushing. and bentonite (up to 5 percent) to make the mix more fluid and to reduce shrinkage, may be used. -- Clay in the form of a mud-laden fluid is similar to and has the advantages of neat cement and sand-cement grout. l/ American Society for Testing and Materials. There is a disadvantage in that clay may separate from the f Corresponding API (American Petroleum Institute) cement fluid. Clay should not be used where structural strength classes are: Type I - API Class A, Type III - API Class C. o stability i the seal is required, where flowing or �f Concrete is useful in sealing large-diameter wells where moving water might break it down, es where it might dry the volume of annular seals required is likely to be out. Although there are other tyFes ti clay available, substantial. However, unless care is exercised during none have the sealing properties (particularly the ability lastanti the coarse aggregate m become separated from to expand dramatically), comparable to bentonite. There- placement, p fore, only bentonite clays are recommended. the cement. In other words, the borehole shall be nominally 4 inches f A popular concrete mix among drillers consists of 8 sacks (100 millimetres) larger in diameter than the nominal of Portland cement per cubic yard (0.76 cubic metre) and casing diameter (thus creating a 2-inch, or-50 millimetre uniform aggregate of 3/8 inch (9.5 millimetres) diameter. annular space).