TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
velop in the pressure supply line from the pump to the
another for return flow from the eductors and the
grout injection pipe, the line must occasionally be
wellpoints back to the recirculation tank and pressure
flushed to ensure that the grout being pumped into the
pump.
formation is homogeneous and has the correct viscos-
5-4. Vertical sand drains. Vertical sand drains
ity. The grout in a single-line system is flushed
can be installed by jetting a 12- to 18-inch casing into
through a blowoff valve onto the ground surface and
the soil to be drained; thoroughly flushing the casing
wasted. A recirculating system has a return line to the
with clear water; filling it with clean, properly graded
grout storage tank so that the grout is constantly be-
filter sand; and pulling the casing similar to installing
ing circulated through the supply line, with a tap off to
"sanded" wellpoints. It is preferable to place the filter
the injection pipe where desired.
sand through a tremie to prevent segregation, which
(4) Additional information on grouting is con-
may result in portions of the filter being too coarse to
tained in TM 5-818-6.
filter fine-grained soils and too fine to permit vertical
b. Slurry walls.
drainage. Sand drains should penetrate into the under-
(1) Slurry cutoff trenches can be dug with a
lying pervious aquifer to be drained by means of wells
trenching machine, backhoe, dragline, or a clam buck-
or wellpoints.
et, typically 2 to 5 feet wide, The walls of the trench
5-5. Cutoffs.
are stabilized with a thick bentonitic slurry until the
trench can be backfilled. The bentonitic slurry is best
a. Cement and chemical grout curtains.
mixed at a central plant and delivered to the trench in
(1) Cement or chemical grouts are injected
trucks or pumped from slurry ponds. The trench is car-
through pipes installed in the soil or rock. Generally,
ried to full depth by excavation through the slurry,
pervious soil or rock formations are grouted from the
with the trench being maintained full of slurry by the
top of the formation downward. When this procedure
addition of slurry as the trench is deepened and ex-
is followed, the hole for the grout pipe is first cored or
tended.
drilled down to the first depth to be grouted, the grout
(2) With the trench open over a limited length and
pipe and packer set, and the first zone grouted. After
to full depth, cleaning of the slurry is commenced in
the grout is allowed to set, the hole is redrilled and ad-
order to remove gravelly or sandy soil particles that
vanced for the second stage of grouting, and the above
have collected in the slurry, especially near the bottom
procedure repeated. This process is repeated until the
of the trench. Fair cleanup can be obtained using a
entire depth of the formation has been grouted. No
clamshell bucket; more thorough cleaning can be ob-
drilling mud should be used in drilling holes for grout
tained by airlifting the slurry to the surface for circu-
pipes because the sides of the hole will be plastered
lation through desanding units. Cleaning of the slurry
with the mud and little, if any, penetration of grout
makes it less viscous and ensures that the slurry will
will be achieved.
be displaced by the soil-bentonite backfill. After clean-
(2) Mixing tanks and pump equipment for pres-
ing the "in-trench" slurry, the trench is generally back-
sure injection of cement or chemical grouts vary de-
filled with a well-graded mix of sand-clay-gravel and
pending upon the materials being handled. Ingredients
bentonite slurry with a slump of about 4 to 6 inches.
for a grout mix are loaded into a mixing tank equipped
The backfill material and slurry may be mixed either
with an agitator and, from there, are pumped to a stor-
along and adjacent to the trench or in a central mixing
age tank also equipped with an agitator. Pumps for
plant and delivered to the trench in trucks.
grouting with cement are generally duplex, positive
(3) The backfill is introduced at the beginning of
displacement, reciprocating pumps similar to slush
the trench so as to displace the slurry toward the ad-
pumps used in oil fields. Cement grouts are highly
vancing end of the trench. In the initial stages of back-
abrasive, so the cylinder liners and valves should be of
fill, special precautions should be taken to ensure that
case-hardened steel. Chemical grouts, because of their
the backfill reaches the bottom of the trench and that
low viscosity and nonabrasive nature, can be pumped
it assumes a proper slope (generally 1V on 5H to 1V on
with any type of pump that produces a satisfactory
10H). In order to achieve this slope, the first backfill
pressure. Grout pump capacities commonly range
should be placed by clamshell or allowed to flow down
from 20 to 100 gallons per minute at pressures rang-
an inclined ramp, dug at the beginning end of the
ing from 0 to 500 pounds per square inch. The maxi-
trench. As the surface of the backfill is built up to the
mum grout pressure used should not exceed about 1
top of the trench, digging the trench resumes as shown
pound per square inch times the depth at which the
in figure 5-4. As the backfill is bulldozed into the back
grout is being injected.
of the trench, it flows down the sloped face of the al-
(3) The distribution system for grouting may be
ready placed backfill, displacing slurry as it advances.
either of two types: a single-line system or a recircu-
Proper control of the properties of the slurry and back-
lating system. Because of segregation that may de-
5-5
Previous Page
