rate of injection can be made, if desired, by using the stick gage to measure
the amount of grout pumped during any given time interval. The inspector
may want to check the rate just before changing mixes to be sure that the
last computed rate is continuing. Such observations may be recorded or not
as the inspector wishes. According to the log the hole accepted 4:1 grout
readily, so it is understandable that a thicker mix should be tried. The
thicker 3:1 grout increased the rate of cement injection, although not as
much as shown for the first batch. The first batch of a thicker new mix
dumped into the remnants of the old mix is diluted. In this case the 10.5 cu ft
of the 3:1 mix was diluted by about 6.0 cu ft of 4:1 mix remaining in the tank
and circulating through the pump system. This dilution would give a mix of
about 3.3:1 and a cement-injection rate for the first batch of 12.7 cfm rather
than 13.8 as shown. The only time it is necessary to make this computation
is at the completion of grouting if the new mix has not been used long enough
to have its " a s -mixed" proportions in the sump tank.
4.
A delay of 2 min (1828-1830 hr) occurred as the result of a broken water-
line. The water for the batch of grout discharged at 1816 hr was in the
mixer when the line was broken, otherwise the delay would have been 12 min
longer, since the line was broken 6 min before the batch was needed. It is
good practice to charge the mixer with the water for the next batch imme-
diately after discharging. This helps to keep the mixer clean and provides
a small supply of water for emergency use.
5.
After a few batches of 3:1 grout, it appeared that the hole would accept
a thicker grout and the inspector changed the mix to 2:1 grout. The change
from 3:1 to 2:1 grout causes a much greater increase in the rate of cement
injection than changing from 4:1 to 3:1 grout. The inspector should carefully
observe the effect of a change to a thick or moderately thick grout on the
injection rate. In the case of the sample log there was some evidence of a
reduction in rate `after four batches. In reality it was not until the third
batch of 2:1 grout that the hole was actually receiving 2:1 grout because of
the diluting effect of the 3:1 grout left in the system when the 2:1 grout was
introduced.
The first evidence of a slowing rate of injection appeared in the
fifth batch of 2:1 grout, which was dumped into the sump tank at 1928 hr. De-
spite the increase in pressure, the injection rate was the same as for the
preceding batch with less pressure. It is probable that the inspector was
aware of this slowing and raised the pressure as a consequence of it. The
rate continued to decrease even with the pressure at the maximum allowable
(26 psi on the gage and an additional 42 psi by weight of the grout column).
The average rate of injection for the last batch of 2:1 grout mixed (2005 hr)
was 0.5 cfm. It is probable that the rate at the end of the 24-min period of
injection for this batch was about 0.3 cfm, although this was not recorded.
Thus, there was reason to think that one more batch of 2:1 grout would fin-
ish the hole. Therefore, the mix was thinned toward the end of prolonging
the grouting period and injecting additional cement. The gage pressure was
raised to compensate for the reduction of pressure from the lighter weight
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