R 325.1763 Deep wells; tunnels; relief and municipal wells.

Rule 263. (1) A dewatering well deeper than 40 feet below the working grade, a dewatering well used in conjunction with tunnel construction or a well used as an artesian pressure relief well and all dewatering wells located within 500 feet of a municipal well site shall be plugged during construction as prescribed in this rule. The annular space between the bore hole and the casing shall be backfilled at the time the filter material is added with a mixture of 4 pounds of bentonite to each cubic foot of fine textured inorganic soil, upward from a minimum depth of 10 feet below the working grade. The bentonite and fine textured inorganic soil shall be thoroughly mixed at the time of backfilling. (2) The following chart relates bore hole diameter to volume and quantity of bentonite needed per foot of bore hole. Boring Gallons per Cu. ft. per Pounds bentonite* diameter 1 foot of depth 1 foot of depth per 1 foot of depth 12 inches 5.9 0.8 3.2 18 inches 13.2 1.8 7.2 24 inches 23.5 3.1 12.4 30 inches 36.7 4.9 19.6 36 inches 52.9 7.1 28.4 42 inches 72.0 9.6 38.4 * Based on 4 pounds bentonite per cubic foot of excavation. (3) The plugging shall be completed after the casing and screen are removed from the dewatering well by backfilling the hole with a mixture of 4 pounds of bentonite to each cubic foot of fine textured inorganic soil for a distance of not less than 5 feet of the bore hole. The bentonite and fine textured inorganic soil shall be thoroughly mixed at the time of backfilling. A minimum of 142 pounds of bentonite shall be used in backfilling a 36- inch diameter hole and a minimum of 62 pounds of bentonite in a 24-inch diameter hole. The plugging shall be completed by adding and compacting additional inorganic parent material or fine textured soils. These soils shall be added in a manner to prevent future slumping and assure that the drill hole shall remain filled to the working grade. Approval in writing shall be obtained from the department prior to instituting any modifications in the plugging requirements of this rule.