Geotechnical News June 2011
39
THE GROUTLINE
in Figure 2. Whereas cohesion was
traditionally minimized in simple ce-
ment–water grouts by using extremely
high w:c ratios (Albritton 1982), such
mixes have high Kpf values, which
severely curtail their penetrability.
However, by using much lower water
contents (typically less than 1.5 weight
by volume) and combinations of stabi-
lizing and plasticizing admixtures and
additives (including bentonite, silica
fume, and Welan Gum), grouts of low
viscosity (less than 60 seconds Marsh),
low cohesion, minimal bleed, and ex-
cellent Kpf values (less than 0.02 min–
1/2) can be produced. DePaoli et al.
(1992) found that even under moder-
ate injection pressures, such balanced,
stabilized grouts provided enhanced
penetrability and performance via the
following:
• an increased radius of travel;
• a more efficient sealing ability as a
result of the improved penetrability
and the lower permeability of the
mix;
• a high volumetric yield, with uni-
formly filled voids; and
• a higher erosion resistance because
of improved mechanical strength
for a given cement content.”
Of course, it must be acknowledged
that other factors will impact curtain
effectiveness, but never in the
U.S. literature before 1992 was the
significance (or even concept) of
“pressure filtration” mentioned in
conjunction with
rock grouting.
It is only fair
to separate from
the comparison
between “old” and “new” those ele-
ments which are, by invention and
technology, the exclusive privilege of
the “new.” Much has been written and
rightly so, about the tremendously ben-
eficial effect that the use of computer-
based systems have had on the col-
lection, processing, interpretation and
display of data from the field (Dreese
et al., 2003). No reputable grouting
project of any significant scale or im-
portance does now not have such a ca-
pability, feeding news back into a cen-
tral “mission control” (Photograph 1),
and back into the Project Executive’s
desk in head office, as well. The best of
these systems can now integrate all the
drilling and water testing data, as well
as the grouting data, to compliment and
compare with the historical site inves-
tigation data (and original grouting in-
formation) which may be available on
any particular project. Given the power
of this knowledge, curtains can be con-
structed to engineered standards with
a degree of reliability and confidence
which was unthinkable under old re-
gimes.
Another child of the new age is the
Optical or Accoustic Televiewer, an ex-
tremely acute and reliable instrument
which basically provides a “flat core”
of a preexisting hole (Photograph 2).
With this capability, the borehole wall
conditions of drill holes — formed
“destructively” without the expense of
core drilling — can be closely scruti-
nized, and compared with results from
permeability tests and grout injections.
This is an extremely important diag-
nostic tool, and represents a compat-
ibility far beyond the grainy, boring
images hitherto provided by down-the-
hole video cameras.
Returning to a comparison of “old”
and “new” concepts, the fundamental
change in attitudes towards mix de-
signs and mix properties has already
been discussed: it is one absolutely
vital component in the revolution.
However, even today, the author finds
specifications — or worse, projects —
where the grout mix design comprises
three components at best, and mixes
are changed from “thin” to “thick” by
changing from water:cement ratios
of 3:1 to 0.8:1, or 0.6:1 in the case of
“gulpers.” This is simply inexcusable
and not acceptable given the state of
knowledge which currently exists and
is freely available on this subject.
Other areas of important distinction
in contemporary grout curtain design
and construction may be summarized
as follows:
• Curtain Geometry:
Curtains must
have, as a minimum, 2 rows of
holes, which extend, wherever fea-
sible technically, into a confining
layer. They are not simply installed
to a target depth below ground
surface. Also, the holes in each
row are inclined say 15º off verti-
cal. The inclination of each row of
holes is in the opposite direction,
thereby producing a “criss cross”
effect, assured to intercept all fis-
Figure 2. Historical path of development from unstable mixes
to contemporary balanced multi-component mixes (modified
after De Paoli et al., 1992).
Photograph 1. (Courtesy of ACT and Gannett Fleming, Inc.).
