from one compartment to another, it will quickly enlarge its channel in the
soft filling, causing a pressure drop and an injection rate increase.
6. GROUTING IN FLOWING WATER. A channel carrying flowing water en-
countered during grouting operations should be closed by excavation and
backfill if feasible. If at depth, this can sometimes be accomplished through
l a r g e - d i a m e t e r drill holes. Portland- cement grout is not usually effective
at sealing voids filled with rapidly moving water. If the movement of water
is caused by dewatering or other pumping activities, it is likely that such
operations will have to be suspended before grouting can be successful. In
the case of fairly large underground channels filled with naturally flowing
water discovered by drilling for grouting, the flow must be essentially stop-
ped before attempting to fill the channels with cement grout. As much infor-
mation as possible on the size and shape of the channel should be obtained.
The borehole television camera can be used to augment information from
drilling and sounding. It may be possible to construct a barrier across the
channel by drilling and backfilling cased 6 -in. or larger holes, one at a time,
spaced so that intermediate holes will overlap the primary holes. Since the
casings would be left in place, they should be something other than steel so
that they can be readily cut with a drill. A barrier may be created by intro-
ducing an inflatable bag attached to an injection pipe into the water-filled
channel through a 6-in. hole. Grout or mortar is then pumped into the bag
through the pipe to inflate it against the walls of the cavity. After the flow of
water is stopped, grout can be pumped into the channel upstream from the
barrier until it is filled sufficiently to satisfy the requirements of the
CLEANING GROUT HOLES. The injection of grout is only one of the
subsurface operations that can result in lifting and damage to rock. Lifting
can be done by water during drilling operations, and it can be done by com-
pressed air used to blow sludge and cuttings out of grout holes. The best
way to clean grout holes after drilling is completed is by washing them
through the drill rods with the drill raised just above the bottom of the hole.
Washing should be continued until the water returning to the surface is clear.
If the hole has been stopped because of water loss, the washing should be
continued for approximately 5 min unless it is certain that the drill water
was lost into a large cavity. No washing is needed in that event. If air is
used to clean the hole, the injection pipe should not have a diameter greater
than half the diameter of the hole. If the annular space around the pipe is
small, it may become clogged with debris ejected by the compressed air
causing full compressor pressure to be exerted against the walls of the hole.
Lifting is almost certain to take place if the compressor pressure greatly
exceeds the load of overlying rock and soil. This risk also exists for per-
cussion drilling when air is used to remove the cuttings.
SAFETY. Some of the dangers that attend pressure grouting with cement
grout are briefly described here. An important thing to keep in mind is the