Geotechnical News December 2011
61
THE GROUTLINE
gation must penetrate rock sufficiently
to assure that sound bedrock has in fact
been reached, or that the condition of
the rock meets the requirements for
support or conductivity needed for the
intended site use. This can be a signifi-
cant risk factor where the intent is to
grout the soil overburden and takes are
grossly under estimated where grout
can migrate into undiscovered open-
ings in the rock below.
The number of borings/corings/test
pits must be sufficient to be statisti-
cally significant relative the variability
of the conditions. In order to do this,
there must be good understanding of
the geologic conditions. It is impera-
tive to have some idea as to whether
the surface of the rock is highly vari-
able and pinnacled and at what spacing
karst features might be expected. If the
surface of the rock is pinnacled with
large pinnacles spaced 10 m apart, or
with tall narrow pinnacles 3 m apart,
or if it is a relatively uniform flat rock
surface with irregularly spaced vertical
karst conduits, a different exploration
is necessary. The objective of the in-
vestigation is not necessarily to iden-
tify and locate every karst opening, but
should be sufficient to project the quan-
tity, type and size of openings such that
an appropriate grout hole spacing can
be defined.
Where the exploration reveals con-
ditions inconsistent with the geologic
assessment, the geologic interpreta-
tion should be revisited and revised to
provide the best picture of the subsur-
face. Where the purpose of grouting is
to control permeability, or where high
velocity flows can be expected, wells
should be included with appropriate
measurements made. Where grouting
in rock is required, it is necessary to as-
sess whether voids detected by coring
contain soil. This can be done using a
small diameter split spoon that can be
inserted through wireline coring drill
stem.
Additional exploration tools such as
borehole video, borehole geophysics,
and other geophysical investigation
methods should be used where appro-
priate. When using geophysics it is es-
sential to understand the resolution and
limitations of the methods used. Most
methods cannot identify small voids
more than a few meters deep. Even
large voids can be difficult to identify
at depth. When using electrical and
compression wave methods it is es-
sential to understand that the absence
or presence of water can radically al-
ter the result. All geophysical methods
must be ground truthed using boring,
and/or coring.
The Karst Assessment includes
evaluation of all of the data and con-
clusions from the Geologic Assess-
ment, Hydrogeologic Assessment, and
Subsurface Exploration in concert to
develop a unified model of conditions
that fits all of the data and site condi-
tions. To do this effectively requires a
geotechnical engineer with a thorough
understanding of geology, hydrogeol-
ogy and karst processes. This model is
used to design the grouting program.
Controlling Methodological
Uncertainties
The effectiveness of a grouting
methodology in karst is inextricably
linked to site conditions. Without
a proper understanding of the site
conditions, it is impossible to select
the correct method and approach
to mitigation. That said, there
are measures that can be taken to
improve the prospect of obtaining as
a satisfactory result and reducing cost
and schedule. The key issues come
down to control:
• Control of materials
• Control of drilling
• Control of the grouting process
These control measures together
with a clear focus on the goals of the
grouting program and an understand-
ing of the mechanism by which the
goals are to be achieved will provide
a higher likelihood of a successful
outcome. A summary of karst grout-
ing approaches can be found in Warner
(2004) and Byle (2001).
Grouting Goal Setting
The materials and methods used in
grouting in karst must be selected to
provide the performance desired. The
performance must be carefully defined
so that success can be measured. Some
big picture goals may include:
• Creating and impermeable barrier
• Stopping seepage
• Preventing sinkholes
• Stabilizing an active sinkhole
• Providing a sound structural base
for foundations
Figure 6. Karst grouting approaches.
