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Geotechnical News December 2011
THE GROUTLINE
The reader will notice the absence
of ‘filling voids’ as an objective. This
is because such an objective must be
defined as to why it is desired to fill the
voids. Such an objective is incomplete
since it implies no clear endpoint and
even if completed, may not yield the
desired result.
For example, one may be willing
to invest the huge sum necessary to
completely fill all of the voids within
a given area, but there is no assurance
that doing this will prevent sinkholes,
form and impermeable barrier, or pro-
vide long term structural support for
foundations. Often voids are filled or
partially filled with loose sediments
and mud that can erode after grouting
and undo the work of the grouting pro-
gram. The complication with such an
objective is that there is no practical
way find and verify that all voids have
been filled; and complete filling of all
voids is seldom needed for satisfactory
performance. One must view the filling
of subsurface openings as part of the
solution and not the end goal.
One must set goals that reflect real
measurable performance and that de-
fine the purpose of the subsurface
improvement. One of the key reasons
for grouting failure is using a grouting
method where it is inappropriate. One
must be careful not to presume the so-
lution when setting the goal. In some
cases, the most effective solution might
be to excavate from the surface and
complete the improvement with con-
crete and compacted fill with no grout-
ing at all. Often times, a combination
of methods will be most appropriate
and these may include more than one
type of grouting, or grouting in combi-
nation with another measure.
Tactical Planning
Once the overarching goal has been
defined, the focus can shift to the
tactical level, where methods and
mechanisms are reviewed to select
the appropriate approach. In grouting,
there is always more than one way to
skin the proverbial cat. One can fill a
void with anything using a variety of
methods, from dumping gravel into
a hole, to pumping through a tube á
manchette and everything in between.
The trick is to fill the hole with the
right something in an economical way
to achieve the project goal.
The first real step is identifying the
type, frequency, and orientation of the
voids in question and assessing what im-
pact they will have on the end site use;
and then to examine the distributaion sta-
tistically to determine the probability of
encountering a void that would adversely
affect the end use within the project foot-
print. Once that has been established, it
becomes a question of determining what
approach will decrease this probability to
acceptable levels. This then becomes the
tactical objective. The presence, absence,
and flow of water in voids will also affect
the grouting method selection. High ve-
locity flows may require special methods
(Warner, 2008; Bruce et al., 1998).
With this understanding together
with knowledge of drilling and grout-
ing technology and achievable perfor-
mance, one can conduct an economic
analysis weighing the relative cost of
drilling and grouting using various
methods. Where voids are to be filled,
the first job is to drill casings into them.
This is where knowledge of the orienta-
tion and distribution of voids is critical.
The grout holes must be spaced such
that grout will travel the amount need-
ed to effectively treat the area required.
Structural applications are often
simpler, because high strength is easier
to achieve than low permeability. For
structural applications, it may not be
necessary for 100% coverage, since
high strength limited mobility grout
can create structural columns within
the rock mass that may be sufficient
without filling of intermediate voids.
An example of this approach is includ-
ed in Berry et al (2001).
For a hydraulic cutoff, it will be
necessary to completely seal all hy-
draulic connections across the barrier
location. This can be complicated by
the presence of sediments within the
rock voids that can be difficult to dis-
Figure 7. Vertical holes in karst can be ineffective orienting grout holes can
increase success.
