(2) D i s a d v a n t a g e s . The principal disadvantage of stage grouting is the
e v e r - p r e s e n t danger of lifting or heaving the rock when grouting without a
heavy confining load. This causes grout waste and may seriously damage the
rock and/or any superjacent structure. Lifting occurs when grout at com-
paratively high pressures is actually injected into and displaces rock near
the surface. Thinly bedded, horizontally stratified rocks are easily lifted.
To prevent lifting such rocks, it is sometimes necessary to use pipes sev-
eral feet long or to grout all but the first stage through a packer set in a
reamed-out hole at a depth of several feet. In the first instance the upper
few feet of rock are not grouted; in the second case, one of the advantages of
stage grouting is lost since a packer must be set for each stage of grouting.
A second major disadvantage of stage grouting, as compared with stop grout-
ing, is its higher costs. A drill must be moved to and set up over each grout
hole at least once for each zone in the hole and grout lines must be connected
to the hole equally often. Both items add time and money costs to the job.
Connections to grout holes are usually pay items; more are required for
stage grouting. Labor is expended and grout is wasted for each stage of
grout hole cleaned before deepening. If the cleanout is made prematurely,
grout injected into the rock may flow back into the hole and be wasted also.
-- . Series Grouting.
b
( 1 ) Advantages. The advantages given above for stage grouting (except
"the last listed) apply also to series grouting. Other advantages of series
grouting are that all grouting is done from a new hole in freshly exposed rock
(this provides for a maximum exposure of groutable voids) and grout injected
into the rock is not lost by poorly timed cleanouts as in stage grouting.
(2) D i s a d v a n t a g e s . The major disadvantages of stage grouting, i.e.,
danger of lifting and increased expenditure of time and money, apply to se-
ries grouting also. The increased amount of drilling makes series grouting
the most expensive of the methods described.
--. Stop Grouting.
c
( 1 ) Advantages. The stop-grouting method is the quickest and least
costly method of grouting. This is primarily because of the time and labor
saved by not having to move drills and grout lines repeatedly to and from the
same hole. Grouting through a packer set at depth provides positive knowl-
edge that grout under high pressure is not being injected into lightly loaded
rock near the top of the hole, as may be the case in other methods of grout-
ing. Stop grouting is the least likely to produce undiscovered lifting and
resulting grout waste. Stop grouting is particularly well adapted to situa-
tions that require the highest pressures in the deepest zones.
( 2 ) Disadvantages. P a c k e r s are sometimes hard to seat. This results
in loss of time and may result in the loss of the packer by blowout. In
36
Previous Page
