CEMP-E
TI 814-01
3 August 1998
(2) Thickness. The thickness of the filter pack will range from a minimum of 80 mm (3
in) to approximately 200 mm (8 in). A filter envelope thicker than about 200 mm (8 in) will not
greatly improve yield and can adversely affect removal of fines, at the aquifer-filter pack
interface, during well development.
(3) Pack length. Filter pack should extend full length of the screen but not above the
top of the aquifer. A tremie pipe may be used to evenly distribute the filter material around the
screen and also to prevent bridging of the sand grains. A filter -pack well has been shown
schematically in figure 5-5.
(4) Disinfection. It is important that the filter used for packing be clean and that it also
be disinfected by immersion in strong chlorine solution (200 mg/L or greater available chlorine
concentration, prepared by dissolving fresh chlorinated lime or other chlorine compound in
water) just prior to placement. Dirty filter must be thoroughly washed with clean water prior to
disinfection and then handled in a manner that will maintain it in as clean a state as possible.
f. Grouting and Sealing. The well should be constructed to prevent water that is polluted
or of otherwise unsuitable quality from entering the well. Grout should extend from the surface
to the top of the bentonite seal overlying filter pack of the well. Grouting and sealing of wells
are necessary to protect the water supply from pollution, to seal out water of unsatisfactory
chemical quality, to protect the casing from exterior corrosion and to stabilize soil, sand or rock
formations which tend to cave. When a well is constructed there is normally produced an
annular space between the drill hole and the casing, which, unless sealed by grouting,
provides a potential pollution channel. A bentonite seal with a minimum thickness of two feet
should be placed directly above the filter pack to prevent vertical infiltration of contaminants
through filter material into the well. The wellhead must be grouted and sealed at the surface to
prevent contaminants from migrating along the casing into the aquifer.
(1) Prevention of contamination from surface. The well casing and the grout seal
should extend from the surface to the depth necessary to prevent surface contamination via
channels through soil and rock strata. The depth required is dependent on the character of
the formations involved and the proximity of sources of pollution, such as sink holes and
sewage disposal systems. The grout around the casing should extend from the top of the
bentonite seal to the surface of the well. Local regulations may govern the composition and
placement of the grout. Materials for sealing and grouting should be durable and readily
placed. Normally, Portland cement grout will meet these requirements. Grout is customarily
specified as a neat cement mixture having a water-cement ration of not over 23 L (6 gal) per
43 kg (94-pound) sack of cement. Small amounts of bentonite clay may be used to improve
fluidity and reduce shrinkage. Grout can be placed by various methods, but to ensure a
satisfactory seal, it is essential that grouting be:
- done as one continuous operation
- completely placed before the initial set occurs
- introduced at the bottom of the space to be grouted
Establishment of good circulation of water through the annular space to be grouted is a highly
desirable initial step toward a good grouting job. This assures that the space is open and
provides for the removal of foreign material.
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