best suited method. Examples of the latter are reservoir rims, dam abut-
ments, mine shafts or other similar deep excavations, and underground
structures grouted from the surface. In some instances portions of grout
holes must be drilled through rock above the horizons requiring treatment.
Since grouting the upper rock is unnecessary, stop grouting is well adapted
to this situation. If sufficient rock overlies the grouting horizon, it may be
possible to grout the entire hole with one stop and with only low or gravity
pressure at the collar of the hole. If the surface rock in the grouting area
is thinly bedded and has a nearly horizontal attitude, stop grouting is the best
method o avoid lifting. A stage of grouting is always required if the drill
water is lost before the hole reaches final depth. Stage grouting should be
used to prevent natural muds formed by drill cuttings from shales or similar
rocks from filling or obstructing groutable openings at higher horizons. If it
is desirable or necessary to consolidate the upper rock before proceeding
with grouting at depth, stage or series grouting is indicated. If it is desired
to grout the foundation of an existing structure at pressures comparable to
the load imposed by the structure, series or stage grouting should be used,
especially if the upper part of the foundation is known to contain groutable
v o i d s . In this case great care must be exercised to avoid lifting and tilting
the structure. The danger of lifting is less if the rock is massive or medium
bedded, if the joints are at high angles, or if the strata are steeply dipping.
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