Section 812.16. Gravel-pack well construction.

 

(1)   Conditions. Gravel-pack well construction may be used under any of the following conditions:

(a) In an unconsolidated formation well when the natural formation comprising the aquifer is very fine grained or when the aquifer formation is highly variable in size.

(b) In loosely-cemented sandstone aquifers that require screening to prevent sand pumping conditions.

(c) In badly fractured or decomposed crystalline bedrock formations.

(2)   Material. The gravel or coarse sand to be used for the gravel-pack shall be uniformly graded, well rounded, washed and sterilized silica-based rock gravel or coarse sand, shall be free from other material and shall comply with the requirements of AWWA A100-84, section 6. The size of the gravel-pack should be based on a sieve analysis of the aquifer material. The 70% retained size of the gravel-pack shall be 4 to 6 times greater than the 70% retained size of the aquifer sample. The uniformity coefficient of the gravel-pack may not exceed 2.5.

(3)   Screens. Screens to be placed in gravel-pack wells shall have a set of centering guides on the top and bottom of the screen to ensure an even gravel-pack. The screen slot openings shall be sized to retain 90% of the gravel-pack.

(a) Screen type. Screens installed within gravel-packs for nonpotable high capacity wells and for low capacity wells, including school and wastewater treatment plant wells, shall be continuous-slot. Screens for potable high capacity wells shall be continuous-slot, V-shaped wire wrap on a rod base. As an alternative, louvered, shutter-type screens may be used with gravel-packs for any type of well provided they are designed and installed in a manner such that water from the well is practicably free of sand. Screen selection for dewatering wells is optional.

(b) Screen material. For potable wells, the screen material shall be stainless steel, everdur, monel or brass. Brass screens may not have a lead content of more than 8%. Low carbon steel or low carbon steel-galvanized screens may also be used for nonpotable wells, but burlap or other organic material may not be used as a filter wrap surrounding screens.

(c) Method of screen attachment. The screen may either be permanently welded or threaded onto the bottom of the well casing pipe or may be installed with the telescope method.

(4)   Placement. The gravel-pack shall be placed through an adequately sized metal conductor pipe placed in the annular space. The gravel-pack shall surround the entire screen and shall extend at least 2 feet above the top of the screen. At least a 2-foot sand seal shall be placed upon the top of the gravel-pack to prevent the required annular space sealing material from entering the gravel-pack. The gravel-pack may not extend closer to the ground surface than the minimum depth for the upper enlarged drillhole specified in Tables I to IV.

(5)   Thickness. To allow for adequate well development the department recommends that the gravel-pack not exceed 4 inches in thickness, on one side. The gravel-pack may not be less than 2 inches in thickness, on one side. The diameter of the upper enlarged drillhole shall be sized to accommodate the thickness of the gravel-pack.

History: Cr. Register, January, 1991, No. 421 , eff. 2-1-91; am. (3) (a), Register, September, 1994, No. 465 , eff. 10-1-94.