the hole may necessitate further modification of the gage pressure. F i g u r e 4
shows the pressure exerted by a column of grout 1 ft high for various grout
Figure
4.
Pressure
of
neat
cement
grout
m i x e s . If an installation 100 ft below the surface is to be grouted from the
surface, a pressure of 73 psi for 1:1 grout should be added to the gage pres-
sure at the collar of the hole to obtain the effective grouting pressure at the
level of the installation. In any grouting in which the grout may come in con-
tact with a structure partially or entirely underground, the strength of the
structure should be considered. This, rather than the rock or soil load, may
limit the maximum safe grouting pressure. If in doubt, a structural engineer
should be consulted. When a packer is used in a grouting operation, the in-
spector should be aware of the possibility that the gage may be reflecting the
pressure required to force the grout through the orifice in the packer rather
than the pressure needed to inject it into the rock. This condition will not
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