batch of the grout and the volume of the batch must be known. The most cer-
tain way to determine the volume of a batch containing several ingredients is
by gaging the sump tank after a batch has been discharged.
4.
C H E C K I N G LOW-PRESSURE GAGES. Low-pressure gages should be
checked before each use when that use is to register pressures of less than
5 psi. It is necessary that gages used for the first stage of grouting in stage
grouting and the topmost stop in stop grouting be sensitive to pressures of
2 or 3 psi. The needle of a gage whose dial is marked to show pressures
less than 5 psi can be moved from the peg by lung pressure. This is a quick
check of gage sensitivity. Precise tests can be made as follows: A trans-
parent plastic tube several feet long that can be attached to a gage and filled
with water provides a means of an accurate check of low pressures. E a c h
vertical foot of water in the tube above the level of the gage exerts a pres-
sure of 0.43 psi. Thus, if the tube is held so that the water level is 5 ft
above the gage, the gage should read slightly more than 2 psi. A U-tube of
mercury can also be used to check low-pressure gages. One end of the open
U-tube is connected to the gage by a tube containing a valve for the injection.
of air. Air pumped into the connecting tube causes the mercury to stand at
different levels in each arm of the U-tube. Each 2 in. of differential between
t h e mercury-column levels represents a pressure of approximately 1 psi. A
differential of 10.17 in. of mercury should register on the gage as 5 psi.
5.
LIFTING CLUES. When grout is injected at pressures greater than the
rock can withstand, the rock is lifted or heaved. Surface evidences of lifting
other than grout leaks are sometimes readily discernible, and where struc-
tures are involved damage may be substantial. When structures are present
grouting should be accomplished without any lifting, and if lifting takes place
grouting should be stopped immediately. It is important, therefore, to rec-
ognize signs or clues that lifting may be occurring. The inspector should
watch for changes in the behavior of the hole each time pressure is raised.
After the initial rupture of the rock, it may be noted that the pump labors
less, the gage pressure may drop a few pounds, and the injection rate may
increase.
All these signs may occur simultaneously. If lifting takes place
at a depth of several feet and is caused by cleaving of bedding planes, the
hole may have a relatively high back pressure. This is a result of the rock
pushing back on the grout. It can be checked by closing the valve between the
grout line and the hole. The gage will then reflect the pressure of the grout
in the hole. If it is nearly the same as the injection pressure and does not
fall at a readily visible rate, it should suggest the possibility of lifting. If it
falls rapidly it is pump pressure that is dissipating. Unfortunately, these
signs and clues are not infallible. Some of the clues can be produced by
grouting at least one other subsurface condition. During the grouting of so-
lution channels or cavities compartmented by muck, a hole may show most of
the indications of lifting without having any lifting involved. If grout breaks
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