against a column of water or mercury. F o r accurate low pressures, low-
pressure gages should be used. The dial of any gage in use should be care-
fully inspected. Many gages require a pressure equal to that measured by
one increment on the dial to initiate movement of the indicator needle. In
such a case, the first mark on the dial of a gage showing increments of 5 psi
may actually indicate a pressure of 10 psi. This could be critical for near-
surface grouting where low pressures have to be carefully controlled. For
very low pressures and sensitive conditions, a standpipe is sometimes used
to prevent excessive pressures from being applied. The standpipe extends
only high enough above the top of the hole to obtain the desired pressure by
the weight of the grout column in the pipe. The grout line is inserted into but
not connected to the standpipe. Thus, grout will overflow if it is supplied
faster than the hole can accommodate it. An adjustment in the height of the
standpipe is required for each mix used if the same pressure is maintained.
(2) There is no way to precisely determine the maximum safe grouting
pressure for a particular zone of grouting. A rule of thumb states that 1 lb
of pressure per square inch can be used for each 1 ft of rock and each 2 ft
of soil vertically above the point of grout injection. ( S i m i l a r coverage is
needed in directions other than vertical. ) The rule of thumb can be modified
with caution as indicated in figure 3. The weight of the column of grout in
Figure
3.
Rough guide for grouting pressures
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