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Geotechnical News June 2011
THE GROUTLINE
sure sets, especially those verti-
cally oriented. The zone between
these “outer rows,” typically about
10 feet wide, is then available for
additional “tightening” holes, per-
haps using special or different
grouting materials, and for drilling
and testing Verification Borings
which are installed to demonstrate
the residual permeability achieved
by the curtain.
• Residual Permeability:
The pur-
pose of a grout curtain is to stop
water flowing through the rock
mass. Therefore, its acceptability
as an engineered structure must be
verified by measuring its residual
permeability — to water, not some
arbitrary limiting grout take. (As
described above, an inappropriate
grout will have premature refusal
in certain fissures, while not reduc-
ing the permeability of the ground
further away.) This test is best done
in cored (or Optilogged) holes, us-
ing multipressure Lugeon Tests as
first described by Houlsby (1976).
• Declaring the Target Residual
Permeability:
Residual permeabil-
ity is the goal which must be de-
clared as part of the design by the
Engineer and which therefore must
be satisfied by the Contractor. A
grout curtain truly now is a “Quan-
titatively Engineered” structure
(Wilson and Dreese, 2003), created
by real-time control of subsurface
construction processes. This “mea-
sure of success” will vary from
project to project, as articulated by,
for example, Houlsby (1990), but
is vital to declare and essential to
satisfy.
• Stage Refusal
: Each and every
stage should now be brought to a
virtually total refusal. When view-
ing the grouting process on the
computer monitor, this means an
Apparent Lugeon Value of practi-
cally zero for each stage (i.e., the
(stable) grout is used as a test fluid
in the same way as water is). In
reality, this means that the stage
in question is consuming grout at
less than 0.1 gpm over a period of,
say, 5 minutes, at target pressure.
More lax refusal criteria will result
in incompletely and inefficiently
grouted stages, and so higher than
desirable residual permeabilities in
the rock mass.
• Drilling Methods and Concepts:
Water is the drilling and flushing
medium of choice in rock masses.
Whether the drill-
ing is done by
percussive meth-
ods (top hole, or
water-powered
down- t he -ho l e
hammer) or ro-
tary
methods
(which tend now
to be less com-
petitive and have
greater
devia-
tions) is techni-
cally
immate-
rial. Also, the
development of
commercially vi-
able rotary-sonic
systems (Bruce
and Davis, 2005)
has provided a
method
which
has entirely satis-
fied federal regu-
lations (USACE, 1997) for drilling
through existing embankment dams
without fear of hydro or pneumatic
fracture. In this regard, it is also
the case that innovative contractors
can devise other conforming over-
burden drilling systems which are
equally protective of embankment
fills (Photograph 3).
In all drilling operations, the recording
of drilling parameters (e.g., rate of
penetration, flush characteristics,
torque and so on) has been regularized
by developing automatic recorders as
opposed to relying on drillers or junior
field engineers: the overall rise in the
quality and usefulness of these data has
been predictably spectacular.
• Specifications and Contractor
Procurement Processes
: Specifi-
cations are no longer so prescrip-
tive (“yes: we do need the head of
the contractor as well as his arms”)
and so all contracts are not let on
the low bid basis, although to do
otherwise is still not permissible
for many organizations, especially
in the public sector. Grouting con-
tractors are being hired, correctly,
based on their skills and experi-
ence and not just their capability
of calculating a low price. There is
absolutely no doubt that this “Best
Value” approach has raised techni-
cal standards across the board and
has, interestingly, honed the com-
petitive instincts of all competent
contractors: all this is to the inesti-
mable benefit of the projects them-
selves. Further insight on specifi-
cations is provided in Bruce and
Dreese (2010).
Retrogressive Principles (i.e.,
“The Retrogressive”)
The fact is that certain engineers in
North America have become involved
in projects where a certain expertise
in curtain grouting is needed and
where, for commercial reasons, they
have chosen to go their own way,
and/or to “reinvent the wheel.” The
uncomfortable truth is that they
are either not aware of the “new”
approach, or do not have the technical
background to be able to differentiate
its value in comparison with older
Photograph 2. (Courtesy of ACT and Gannett Fleming, Inc.).
