Geotechnical News December 2011
59
THE GROUTLINE
or where bedrock mapping is incom-
plete. Bedrock mapping is often in-
complete where it is overlain by a thick
mantle, or where surface geomorphol-
ogy is not residual, such as where the
karst stratum is overlain by a alluvium,
glacial deposits, or other such soils that
would mask the presence and nature
of the underlying bedrock. This can
obscure contacts between formations.
Another instance of geologic uncer-
tainty would be where complex fault-
ing or folding results in local disruption
of the regional geology that may not be
completely mapped.
Structural uncertainty refers to
uncertainty related to the geologic
structure. This includes location and
condition of joints, faults, as well as,
voids in the bedrock formation which
comprise the secondary porosity of the
formation. Structural geology informs
the search for voids, since solution is
typically more pronounced in areas of
higher transmissibility where rock is
fractured or broken and along discon-
tinuities such as unconforming geo-
logic contacts. Resolving or reducing
structural uncertainty probably has the
greatest impact on setting up the drill-
ing patterns for a grouting program.
Hydrologic uncertainty arises from
complexity of groundwater flow in karst.
The impact of groundwater hydrology
on the grouting can be profound. Inject-
ing grout above the water table, below
the water table or into flowing water will
have quite different results that can seri-
ously affect both the quantities of mate-
rials needed and the performance of the
completed grouting.
Geomorphology is the study of the
processes, characteristics and configu-
ration and evolution of rocks and land
forms. It is important to know what
stage of the geomorphologic process
the formation is in. Karstification is
a geomorphologic process involving
many stages from the initial dissolu-
tion of rock minerals, and formation
weathering, to the erosion and infilling
of voids, to the ultimate decomposi-
tion of the rock matrix. Understanding
this process in a particular formation is
necessary to assess whether voids are
active conduits, plugged paleo-karst,
or something in between. An excellent
discussion of karst conditions and their
formation is included in Waltham et al
(2005) and White (1988).
Methodological Uncertainty
The application of this methodology
used in grouting poses its own
uncertainties. Whether a low or
high mobility grout is selected;
whether displacement by compaction
grouting, replacement by jet grouting,
displacement by fracture grouting, or
void filling. Void filling would seem
to be a relatively simple process, but
it can be complicated where the voids
are interconnected, where flowing
water is present, the voids are partially
filled with soil, or other factors.
Grouting controls and materials can
radically affect the performance of
the grout. If the geology, structure
and geomorphology are sufficiently
understood, the grouting method and
materials should be selected to provide
effective performance. The method
must allow versatility to accommodate
uncertainties and adapt to varying
subsurface conditions.
The temporal factor is one that is
often ignored in grouting solutions.
The passage of time will allow natural
processes to continue that can alter the
long term performance of the grouted
formation. Erosion and transport of
sediments around the grout may contin-
ue, albeit at a slower pace, but can un-
dermine the effectiveness of the grout
by opening new passages through pre-
viously plugged conduits, or through
soils surrounding the hardened grout
(Lolcama, 2009). To be effective, the
grouting design and execution must
consider the full consequence of the
Figure 4. Results of grouting 20 ft into
top of rock in karst with battered holes.
Figure 3. Karst conduits can be connected or isolated, soil filled, air filled or water
filled.
