TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
CHAPTER 5
INSTALLATION OF DEWATERING AND GROUNDWATER
CONTROL SYSTEMS
5-1. General. The successful performance of any
mied in. The tremie pipe should be 4 to 5 inches in dia-
dewatering system requires that it be properly in-
meter, be perforated with slots /16 to /32 inch wide and
stalled. Principal installation features of various types
about 6 inches long, and have flush screw joints. The
of dewatering or groundwater control systems are pre-
slots will allow the filter material to become saturated,
sented in the following paragraphs.
thereby breaking the surface tension and "bulking" of
the filter in the tremie. One or two slots per linear foot
5-2. Deep-well systems.
of tremie is generally sufficient. After the tremie pipe
a. Deep wells may be installed by the reverse-rotary
has been lowered to the bottom of the hole, it should be
drilling method, by driving and jetting a casing into
filled with filter material, and then slowly raised,
the ground and cleaning it with a bailer or jet, or with
keeping it full of filter material at all times, until the
a bucket auger.
filter material is 5 to 10 feet above the top of the
screen, The filter material initially poured in the tre-
b. In the reverse-rotary method, the hole for the
mie should be wasted in the bottom of the hole. The
well is made by rotary drilling, using a bit of a size re-
level of drilling fluid or water in a reverse-rotary
quired by the screen diameter and thickness of filter.
drilled hole must be maintained at least 7 feet above
Soil from the drilling is removed from the hole by the
the natural groundwater level until all the filter ma-
flow of water circulating from the ground surface
terial is placed. If a casing is used, it should be pulled
down the hole and back up the (hollow) drill stem from
as the filter material is placed, keeping the bottom of
the bit. The drill water is circulated by a centrifugal or
the casing 2 to 10 feet below the top of the filter ma-
jet-eductor pump that pumps the flow from the drill
terial as the filter is placed. A properly designed, uni-
stem into a sump pit. As the hole is advanced, the soil
particles settle out in the sump pit, and the muddy
out tremieing if it is poured in around the screen in a
water flows back into the drill hole through a ditch cut
heavy continuous stream to minimize segregation.
from the sump to the hole. The sides of the drill hole
are stabilized by seepage forces acting against a thin
d. After the filter is placed, the well should be devel-
film of fine-grained soil that forms on the wall of the
oped to obtain the maximum yield and efficiency of
hole. A sufficient seepage force to stabilize the hole is
the well. The purpose of the development is to remove
produced by maintaining the water level in the hole at
any film of silt from the walls of the drilled hole and to
least 7 feet above the natural water table. No bento-
develop the filter immediately adjacent to the screen
to permit an easy flow of water into the well. Develop-
the filter and aquifer adjacent to the well. If the hole is
ment of a well should be accomplished as soon after
drilled in clean sands, some silt soil may need to be
the hole has been drilled as practicable. Delay in doing
added to the drilling water to attain the desired degree
this may prevent a well being developed to the effi-
of muddiness (approximately 3000 parts per million).
ciency assumed in design. A well may be developed by
(Organic drilling material, e.g., Johnson's Revert or
surge pumping or surging it with a loosely fitting
equivalent, may also be added to the drilling water to
surge block that is raised and lowered through the well
reduce water loss.) The sump pit should be large
screen at a speed of about 2 feet per second. The surge
enough to allow the sand to settle out but small enough
block should be slightly flexible and have a diameter 1
so that the silt is kept in suspension.
to 2 inches smaller than the inside diameter of the well
c. Holes for deep wells should be vertical so that the
screen. The amount of material deposited in the bot-
screen and riser may be installed straight and plumb;
tom of the well should be determined after each cycle
appropriate guides should be used to center and keep
(about 15 trips per cycle). Surging should continue un-
the screen plumb and straight in the hole. The hole
til the accumulation of material pulled through the
should be some deeper than the well screen and riser.
well screen in any one cycle becomes less than about
(The additional depth of the hole is to provide space
0.2 foot deep. The well screen should be bailed clean if
for wasting filter material first put in the tremie pipe
the accumulation of material in the bottom of the
if used.) After the screen is in place. the filter is tre-
screen becomes more than 1 to 2 feet at any time dur-
5-1
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