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Geotechnical News December 2011
THE GROUTLINE
Introduction
The risks to a grouting project in
karst are manifold. These include the
potential for delays and cost overruns
due to difficulty in drilling, reduced
productivity during grouting for
various reasons, material overruns,
and equipment problems; as well as
the risk of failure of the process to
achieve the required objectives, or to
provide long term performance. Risks
can be effectively managed if they are
adequately understood. Conversely,
the risk to the project is greatest when
uncertainty is greatest. The combination
of all of the above uncertainties taken
together with normal project risks (i.e.
labor issues, material pricing, weather,
etc.) will reveal the total project risk.
This paper will deal exclusively with
the risks associated specifically to
karst, since the other normal project
risks are routinely managed and
covered elsewhere in the literature.
Uncertainty in Karst Grouting
Risk analysis has been applied to
subsidence risk in karst (Kaufmann,
2008; Doctor et al., 2008, Perlow, 2008,
Zisman, 2008, etc). Most of this type of
work has been focused on development
risk and not on identifying the specific
risk of karst features being present at
any specific location, though similar
approaches can be used, provided
sufficient site specific data is available.
There are a number of categories
of uncertainty associated with the
planning and execution of a grouting
program that must be addressed and
managed for successful grouting
outcome. These can be divided into
site uncertainties, methodological
uncertainty and temporal behavior
uncertainty.
Site Uncertainties
For the purposes of this discussion,
the term site uncertainties is defined as
those unknown conditions present in
the subsurface of the site that will affect
the grouting performance and outcome.
Site uncertainties would affect the rate
and difficulty of drilling grout holes,
the number of grout holes and injection
locations, the quantity and distribution
of grout in the subsurface and the
overall effectiveness of the grouting
program. The site uncertainties include
site geologic variability, formational
structural variability (i.e. the occurrence
of fractures, folds or other features), the
degree of weathering and karstification,
the maturity of the karst, the presence of
infilling, caves, etc., as well as, the depth
and condition of soil overburden and the
geo-hydrologic conditions.
Geological uncertainty relates to
the nature of the formation as defined
in geologic terms. That is the type of
rock, rock material properties. This oc-
curs where the nature of the geologic
formation is either not known, or poor-
ly defined. An example of this would
be where a formation is mapped that
consists of alternating beds of differing
rock types, without a defined sequence
Figure 2. Isolated karst conduit in otherwise intact rock.
