Casing, Screens, and Annular Space Sealing and Grouting
This domain tests the barrier between contaminated surface water and the aquifer. Casing holds the hole open; grout seals the annulus; the screen admits water while holding back formation material. The exam rewards precise answers about materials, depth, and placement method.
Casing and wellhead
Casing must be new, structurally sound material suitable for the use, with adequate wall thickness and joints that do not leak. It must extend above finished grade, commonly at least 12 inches, and be fitted with a sanitary seal or approved well cap. The casing must be seated into competent material or to the required depth so that surface water cannot short-circuit down the outside.
Grouting
The annular space is sealed with neat cement or bentonite grout placed by tremie or positive displacement, from the bottom up, without voids. A minimum 1.5-inch radial annulus is the usual benchmark. Grout must be continuous to the surface and must not be mixed so thin that it settles away from the casing. Drill cuttings and gravel are not grout.
Screens
Screen slot size is selected from the formation gradation. A filter pack may be needed in fine sand. The screen must be placed in the water-bearing zone without allowing collapse of the formation around it.
Worked example
A 6-inch casing is set in an 8-inch hole. The annulus is only 1 inch, below the 1.5-inch minimum. Using a 9-inch borehole gives the required 1.5-inch radial space, provided the hole is stable and the grout is tremied from bottom to surface.
Traps
Pouring grout from the top; using cuttings as fill; leaving a shallow casing seat; and choosing screen slots without examining the formation sample.