higher pressure grout injected into lower horizons. The final spacing of
grout holes necessary to accomplish this will depend on the nature and ori-
entation of the groutable openings in the rock, on the orientation of the grout
holes, and on the grouting operations. In general, the more numerous the
groutable openings, the more closely spaced the holes must be. Holes on
2- or 3-ft centers may be required in badly broken rock.
g. Inclined Grout Holes. In jointed rock, holes should be drilled to
intersect the maximum number of joints practicable. , This may require di-
rectional drilling. If all the joints dip at angles less than 45 deg, vertical
grout holes will be entirely satisfactory. On the other hand if joints are ver-
tical or almost vertical and the holes are vertical, grouting must be done on
spacings of a few inches to obtain the same degree of coverage possible with
properly inclined holes on 5-ft centers. In practice, holes are usually 'not
inclined more than 30 deg from the vertical because greater inclinations
bring increased drilling costs which offset the savings accruing from fewer
holes and wider spacings. The shortest seepage path through the grout cur-
tain is along the joint most nearly normal to it. Therefore, to construct a
grout curtain to control seepage with inclined grout holes, the holes should
be inclined along the plane of the curtain, if the pattern of jointing is at all
f a v o r a b l e . This provides for the greatest number of intersections of joints
trending normal to the curtain. If more than one line of inclined grout holes
is needed to construct the curtain, better coverage of joints trending normal
to it can be obtained by staggering the holes in adjacent rows. Holes should
not be staggered if the joints cross the curtain diagonally.
h . Drill Water Loss. Observations of the drill water during drilling
operations can provide much information on the rock encountered by the
d r i l l . The cuttings carried by the water provide information on the type and
color of the rock. Fluctuations in the quantity of the returning water are in-
dictive of rock permeability. An abrupt change in the amount of water re-
turning to the surface usually signifies that the drill has reached a perme-
able horizon. If all the drill water flows into this permeable zone, all the
cuttings produced by the drill will be carried into it also. If drilling is con-
tinued, it is possible that the opening will become so clogged with cuttings
that the drill water cannot enter it and will again vent from the top of the
hole. In such fashion, openings of appreciable size can be lost to grouting
but still remain hazards from the seepage standpoint since there is no as-
surance that water percolating through the rock will not remove the cuttings
by piping. Therefore, to avoid clogging major groutable openings with cut-
tings, drilling should be stopped when all the water is lost, and the hole
grouted. If there is sudden appreciable gain in water, drilling is also usu-
ally stopped and the hole grouted. This is done, not because of the possibil-
ity of plugging the permeable zone with cuttings, but because an opportunity
is afforded to treat a groutable void of significant size on an individual basis.
The same reason would be sufficient for grouting after a sudden water loss
if the possibility of clogging with cuttings did not exist. If the drill rods do
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