Geotechnical News December 2011
63
THE GROUTLINE
place or permeate. In such instances,
the hole spacing must be close enough
to permit overlap of grout injection be-
tween holes.
The orientation of grout holes should
be selected to maximize the intersec-
tion of karst zones. This may involve
steeply angled holes where steeply
dipping or vertical features are pres-
ent or vertical holes, where horizontal
or horizontally connected features are
present. Grout holes should be orient-
ed across faulted zones or other areas
within a project site where additional
karst features would be expected.
The materials and methodology of
grouting can be selected based on eco-
nomics and performance. The effects
of groundwater, where present, must be
considered to prevent dilution and loss
of grout effectiveness. A limited mo-
bility grout should be selected where
displacement and/or compaction of
sediments is required, or where it is de-
sirable to limit filling voids to specific
areas without significant lateral spread
of the grout. For very large voids, grav-
ity filling with a concrete mix may
be appropriate followed by second-
ary grouting with a finer or more fluid
grout mix to seal remaining openings.
The key in successful planning is
to anticipate variability. Even though
large interconnected voids may not
have been encountered, it is essential to
have a plan to address them whenever
grouting in karst. Identify volume alert
levels so that the grouting plan may be
changed to limit the loss of large vol-
umes of grout. If grouting with a high
mobility grout, be prepared to change
to a limited mobility grout or other ap-
propriate method, should an unantici-
pated large take occur.
Managing the Drilling
The drilling should be used as an
investigative tool as well as a means to
make grout injections. All holes should
be logged and evaluated to verify that
conditions are as anticipated and are
appropriate for the methods planned.
Automated drilling equipment that
records down-pressure, torque, and
depth can effectively communicate
drilling conditions in real time without
the delays and labor required for hand
logging. Have a plan of action to adapt
to changing conditions. For example, if
it is anticipated to grout small fractures,
and large cavities are discovered during
drilling the notifications to the engineer
and owner, must be immediate so that
an evaluation can be made as to whether
and how to proceed with handling this
new condition.
The drilling should attempt, to the
extent practical, to assess whether
voids in rock are soil filled and con-
tinuous. The continuity of voids is
often observable as lost circulation of
drilling fluid (air or water) appearing in
adjacent holes and should be recorded
and reported where it occurs. The con-
ditions in each hole should be evalu-
ated by the project engineer prior to the
grouting. A hole should never be termi-
nated in a void without direction from
the engineer, since it may be desirable
to deepen the hole and it will shave cost
to do this while the rig is already pres-
ent than to have to move it back into
place later.
In karst it is not uncommon to en-
counter rock drops that bind the drill
casing or for the casing to become
wedged due to drift of the drill string
on sloping rock surfaces. In these in-
stances, it may be of value to change
the orientation of the boreholes. The
boreholes should be oriented to be as
close to perpendicular to the feature
surfaces as possible. This can reduce
the potential for casing drift and make
it less likely for sections of rock to fall
at an angle to the drill string.
Managing the Grouting
The actual injection of the grout may
or may not achieve the desired result.
It is essential to closely monitor and
interpret the observed behaviors during
grouting to assess whether the grouting
is likely to meet the project objective.
While the cost for engineering
observation during the grouting is
often considered excessive, the cost for
a failure of the grouting or for later re-
grouting the site will be considerably
higher. The engineer in the field must
have a clear understanding of the
subsurface conditions, what the grout is
expected to do in the ground, and what
the overall objective of the grouting is,
to be able to make good decisions.
Monitoring of the grout properties
is essential to interpreting the grouting
records. The viscosity, and thixotropy
of the grout will directly determine
grout behavior. Low viscosity grouts
will penetrate fine openings and travel
farther than higher viscosity or limited
mobility grouts under the same pres-
sures and rates of injection. The grout
material properties, both wet and in the
hardened state, must be consistent with
the planned injection procedures and
controls, and with the final objective of
the grouting.
Refusal criteria must be established
to permit effective grouting while
maintaining adequate control. The dan-
ger of causing damage with the grout-
ing increases directly with the volume
and pressure of grout injected. So,
refusal criteria should include provi-
Figure 8. Grouting in Karst Design Cycle.
