before installation. Filter pack material shall be transported to the well
site in a manner which prevents contamination by other soils, oils, grease,
and other chemicals. Temporary drill casing, if installed, or hollow stem
auger, shall be removed simultaneously with the above operation. Filter
pack material shall be placed in no greater than 1 m 3 foot lifts prior to
retraction of the temporary casing/auger. A minimum of 150 mm 6 inches of
filter pack shall remain in the temporary casing/auger at all times during
filter pack installation. Frequent measurements shall be made inside the
annulus during retraction to ensure that the filter pack is properly placed.
3.3.5
Bentonite Seal
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NOTE: Sufficient time should be allowed for the
bentonite seal to hydrate and form a low permeable
seal before grout is placed in the annular space
above the bentonite seal. By not allowing enough
time, grout material could infiltrate into the seal
and possibly into the filter pack. It is
recommended waiting a minimum of 3 to 4 hours for
hydration of bentonite pellets, or tablets. If
bentonite chips are used, the minimum hydration time
could be twice as long. Normally bentonite chips
should only be used if it is necessary to install a
seal in a deep water column. Because of their high
moisture content and slow swelling tendencies, chips
can be dropped through a water column more readily
than a material with low moisture content, such as
pellets or tablets. It is recommended that the
bentonite seal be placed in lifts, with each lift
allowed to hydrate for a minimum period of time.
Slurry seals may be used when the seal location is
too far below water to allow for pellet or other
containerized-bentonite placement, or within a
narrow well-borehole annulus. For more guidance
consult EM 1110-1-4000.
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After the inner casing and [well screen] [and filter pack] have been
installed,[and after predevelopment of the well,] the annular space between
the inner and outer casings shall be sealed by use of a bentonite seal. A
minimum 1 m 3 foot thick hydrated bentonite seal shall be placed on top of
the filter pack in a manner which prevents bridging of the bentonite in the
annulus. The bottom of the bentonite seal shall be a minimum of 2 m 5 feet
above the top of the well screen. The depth to the top of the bentonite
seal shall be directly measured, and recorded immediately after placement,
without allowance for swelling. If the bentonite seal is located above any
borehole fluid levels, a [300] [_____] mm [1] [_____] foot layer of fine
sand shall be placed at the top of the bentonite seal.
3.3.6
Grout Placement
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NOTE: Before installing an annular seal, the state
regulatory agency should be consulted. The state, or
local municipality, where the well is being
installed, may have specific requirements for
sanitary, and/or wellhead protection. There is a
provision for placing a high-solids bentonite grout
SECTION 02521
Page 26
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