coarser ingredients omitted from the grout to properly seal the smaller
v o i d s . Mine maps should be used, if available, to reduce the number of holes
needed to inject the grout. Observation holes should be used to check the
distribution of grout from various injection points. If mine maps are not
available and the size and orientation of haulageways and room spacings can-
not be determined, coverage can be obtained by drilling on a grid pattern. If
the mine workings extend beyond the boundaries of the area requiring treat-
ment, bulkheads of thick grout should be constructed in all mine tunnels
crossing the perimeter of the area to prevent the spread of grout beyond
limits of usefulness. Large solution cavities, like mines, can be grouted
with a coarse grout if sufficiently free from debris and muck. Since grout is
unlikely to displace an appreciable amount of solution-channel filling, it may
be necessary to provide access to the cavities and manually clean them prior
to backfilling with concrete or grout. Cleaning is particularly important if
seepage control is the purpose of the treatment.
f. Order of Drilling and Grouting. For grout curtains, holes are ini-
tially--drilled on rather widely spaced centers usually ranging from 20 to
40 ft. These holes are referred to as primary holes and are grouted before
any intermediate holes are drilled. Intermediate holes are located by
splitting the intervals between adjoining holes; the first intermediates are
midway between primary holes and the second intermediates are halfway be-
tween primary and first intermediate holes. Spacings between holes are
split in this fashion until the grout consumption indicates the rock to be sat-
isfactorily tight. All holes of an intermediate set in any section of the grout
curtain are grouted before the next set of intermediates is drilled. Although
primary holes are most often drilled on 20-ft centers, other spacings are
equally acceptable. If grout frequently breaks from one primary hole to
another, an increase in the primary spacing is indicated. If experience in
apparently similar conditions suggests that a final spacing of between 5 and
10 ft will be satisfactory, a primary spacing of 30 ft may be in order since it
will break down to 7.5 ft with the second set of intermediates. As the split-
spacing technique reduces the intervals between grout holes, the average
grout consumption per linear foot of hole should also become smaller. If the
final spacings in a grout curtain constructed in rock that contains no large
cavities are 5 ft or less, the total grout take for neat portland-cement grout
is likely to average less than 0.5 cu ft of cement per linear foot of hole. In
blanket grouting an area to serve as the foundation for a structure, it is well
to arrange operations so that the final grouting in every section is done
through intermediate holes drilled between rows of previously grouted holes.
This limits the travel of grout in the last holes and permits maximum pres-
sure to be applied to all openings encountered. If the area to be consolidated
is not bounded by natural barriers to grout travel, consideration should be
given to establishing such a barrier by grouting a row of holes around the
perimeter of the area before any other grouting is done. If the blanket-
grouted area is to serve as the capping zone for deeper grouting, it must be
tightened sufficiently by grouting to prevent appreciable penetration by the
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