TM
5-818-6/AFM
88-32
not drop to indicate a cavity at the point of water loss or gain, it is advisable
to advance the hole 1 or 2 ft beyond that point to be sure that the hole is well
into the permeable zone before grouting. Many cases of a second water loss
within a foot of the first have been recorded. In these cases a cycle of drill-
ing and grouting could have been avoided with the extra drilling. Sometimes
specifications are written to provide for grouting if approximately half of the
drill water is lost abruptly or if cumulative losses aggregate about half of
the water being pumped into the hole. Judgment should be exercised in de-
ciding that apparent water loss or gain is real. If the water source for the
drill also supplies other operations, pressure fluctuations may cause volume
changes in the drill water that are easily mistaken for losses or gains. L o s s
of return water caused by blocked bit or a collar of cuttings around the drill
pipe may be construed as complete loss of drill water. In porous rock the
water loss may increase gradually as the hole is deepened. If the pores are
too small to accept the grout, nothing is accomplished by suspending drill-
ing operations to grout.
--.
i
Pressure
Testing
and
Pressure
Washing.
(1) Pressure testing as used in drilling and grouting operations is the
measured injection of water into a grout hole prior to grouting. Pressure
washing is the term applied to washing cuttings and other filling out of open-
ings in the rock intersected by the hole. Both operations are done through a
packer set in the hole or through a pipe grouted in the top of the hole. In a
stage-grouting operation (para 11a), pressure testing is used primarily to
determine whether grouting is needed. If the hole does not take water at a
given pressure, it will not take a grout containing solids at that same pres-
s u r e . Pressure testing will also disclose the likelihood of and/or the poten-
tial locations of surface grout leaks and the depth at which a packer must be
set to avoid them. In stop grouting (para 11c), normal pressure-testing
techniques can be used to determine whether grouting is required in the low-
est zone; but in the higher zones, this can be' done only if the lower zone or
zones are tight at the pressure desired for the upper zone. The use of pres-
sure testing with water in a stop-grouting operation to ascertain whether one
or more stops can be eliminated costs as much as checking the hole with
grout. Thus, if the lower zones are not tight, pressure tests in the upper
zones need only be used to find locations for seating the packer in fractured
rock or to check for potential surface grout leakage. In stage grouting it is
good practice to always grout the first stage unless the water take in pres-
sure testing is zero. The filling of small openings with low-pressure grout
precludes high-pressure grout entering upper rock and heaving it while
grouting lower zones. The maximum pressure for pressure testing should
never exceed the maximum grouting pressure proposed for the same zone.
Generally, it should be lower than the grouting pressure to ensure that the
rock is not damaged. Careful control of pressure tests in stage grouting is
especially important in this respect. If a hole is tight the pressure test can
be completed in 5 to 10 min after the hole is full of water. If the hole takes
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