general, larger holes are required. Upper groutable zones may be clogged
or partially clogged by cuttings from lower strata. The pressure required to
pump grout through the relatively small opening in the packer may exceed
the pressure needed to inject the grout into the rock.
d.
Circuit
Grouting.
( 2 ) Disadvantages. If the packer is set near the top of hole, the entire
hole must be grouted at a pressure low enough to prevent lifting of surface
rock. If the packer is set several feet below the surface, the upper part of
the rock is ungrouted. A large hole must be provided to the depth of the
packer to permit installation of a packer large enough to accommodate both
injection and return grout lines. Excessive time is required to assemble and
disassemble grout pipe in the hole. Flush-joint pipe is needed to reduce loss
of grout pipe since the packer must be left in the hole until the pressure" on
the grout dissipates.
e. Combining Methods. No large grouting job is likely to be completed
using--only one grouting method in the strictest sense of the definitions. F o r
example, if during the drilling of a hole for stop grouting-, the drill water is
lost, drilling is stopped immediately and the hole is grouted. In such cases
it can be said that stop grouting is done by stages. In stage grouting if the
upper rock is so fractured that it cannot be sealed well enough to withstand
the higher pressures desired for the lower zones, it may be necessary to
grout the lower zones through a packer set below the fractured rock. This
again combines stage grouting and stop grouting. If a badly fractured upper
zone extends over a considerable area, treatment may be by a grid' of shal-
low holes grouted by the series- or stage-grouting methods to form a
grouted-rock blanket before continuing with the lower zones in this area by
stop, stage, or series grouting. Specifications should be flexible enough to
permit the use of the method or methods best suited to whatever situation is
encountered and should provide means of compensating the contractor for the
w o r k performed.
f. Selection of Method. Stage grouting and stop grouting are the two
most common methods of grouting in the United States. Service records
show that effective results can be obtained by either method. If grouting is
delaying another construction operation and time is an important factor, stop
grouting should be given serious consideration. If higher pressures are
needed in lower zones of the grout hole than near the top, stop grouting is the
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