64
Geotechnical News December 2011
THE GROUTLINE
sions to prevent high pressures applied
to large volume injections. Additional
criteria relating to the effectiveness of
the grouting should be selected to iden-
tify when the resistance to the grouting
(as measured by pressure) is consistent
with the degree of penetration desired
for the grout. Setting target values of
grout injection in soil is helpful, where
displacement and compaction are pos-
sible; but seldom appropriate in rock,
unless the conditions of the karst are
well understood and known to have a
well defined distribution of openings
for the grout to enter.
Setting criteria for changing grout
type or consistency may also be of
value. In some situations, compaction
grouting, permeation grouting, and fill-
ing of open voids may be required at a
single location at various depths within
a hole. Anticipating the different mech-
anisms for grouting and establishing
criteria for each is an important way
to control grouting overruns and poor
performance. Such a criteria must con-
tain a diagnostic component such that
the performance of one type of grout-
ing is used to assess the need for an-
other. In such instances, it is common
to start with a relatively fluid grout and
step up to increasingly stiff or lower
mobility grouts as various criteria are
reached for volume and pressure.
All refusal criteria must include
a rate of injection. The rate of injec-
tion will affect the pressure measured.
Higher rates of injections will produce
higher grouting pressures and may lead
to early refusal due to viscosity effects
and line pressures that may not be re-
flective of the ground conditions.
Closing the Loop
Once the planning is complete, controls
are established and the work begins,
it is essential to establish a feedback
loop (Figure 8) whereby the additional
information obtained by drilling and
grouting is evaluated and employed to
supplement the initial assumptions and
conditions. This constant flow of new
information will reveal conditions not
identified in the original investigation.
This is primarily due to the increased
frequency of soundings over what can
practically be accomplished during the
initial investigations. Hole spacings
are always closer in the execution
of the grouting program than during
the investigation and this additional
information can only be neglected to
one’s own detriment.
Likewise, the grouting records will
indicate how effectively grout pene-
trates features identified during the hole
drilling. This can provide useful infor-
mation about the continuity of open-
ings and infilling that may not have
been detected. There may be cause to
use a different grouting approach to de-
termine if it can be more effective or to
reveal additional information about the
formation. For example, if limited mo-
bility grouting is encountering repeated
refusal with low volumes in identified
voids, it may be worth attempting a
more fluid grout to assess whether
there is low or higher permeability in-
filling. It may also be worth consider-
ing using a borehole camera to visually
assess the conditions prior to grouting.
It is essential to do this in real time.
There is no point in injecting 150
points to depths of 10’s of meters only
to determine at the end of the work,
that there was no real benefit and an-
other approach is necessary. Such cases
often lead to conflict among owners,
engineers and contractors over who is
