Page 155 - Water and wastewater engineering

4-10 WATER AND WASTEWATER ENGINEERING

sealed to the upper confining layer from within 1.5 m of the top of the aquifer to the ground sur-
face (U.S. PHS, 1965).
Grout made from Type I, Ia, or III cement should cure a minimum of 24 hours before drill-
ing operations resume. When a cement/bentonite mixture is used, a waiting period of 48 hours is
recommended (MSDWA, 1976). Some rules require a curing time of three to seven days.

Well Screen. A well screen is installed when the water-bearing formation is unconsolidated
material such as sand. The well screen prevents fine material from entering the well and then
causing undue wear on the pump and, potentially, filling the well.

Gravel-Packed Well. When the water-bearing formation is unconsolidated, the gravel-packed
well ( Figure 4-1 a) is used. As shown in Figure 4-4 , there are numerous configurations for the
gravel pack. The gravel wall retains the water-bearing sand while the screen keeps the gravel

Casing Casing Casing

Grout Seal Grout Seal Grout Seal Grout Seal Grout Seal Grout Seal

Grout Seal Grout Seal Grout Seal Grout Seal

Gravel
Gravel
Gravel

Underreamed—single casing Straight hole—dual casing Underreamed—dual casing
FIGURE 4-4
Some alternative schemes for packing gravel in a gravel-packed well.