38
Geotechnical News June 2011
THE GROUTLINE
sion drilling was then synonymous
with the use of air flush, which
many (but not all) did recognize as
detrimental to fissure cleanliness
and amenability to grout. (The age
old debate about rotary versus per-
cussion drilling as being more suit-
able for grout holes was wrongly
focused: it should have been water
versus air.)
• The concept of a “one row curtain,”
except notably under the cores of
embankment dams, where even
then the shallowest possible excuse
was taken to revert to one row.
• The use of relatively low grout pres-
sures, resulting from the recurrent
specification to provide “constant”
pressures which therefore meant
the use of progressive cavity pumps
(“Moynos”) as opposed to higher
pressure piston or ram pumps.
• The use of “thin” grouts (with ex-
cessive water:cement ratios often
well in excess of 10 by weight – al-
though typically mixes were mea-
sured by volume). Such mixes of
course were easy to pump due to
their low apparent viscosity, but
naturally had extremely high bleed
values and horrible pressure filtra-
tion resistance. These mixes were
allied with a fundamental distrust/
unawareness of the benefits of ad-
ditives (except for calcium chlo-
ride in “taker” situations) although,
latterly, the use of bentonite was
entertained and ongoing though
somewhat misguided experimen-
tation with superplasticizers was
conducted in certain quarters.
• Curtains were grouted until a cer-
tain cement refusal was obtained
(e.g., 1 bag per foot) as opposed to
a measured residual permeability.
This is, however, a charitable view:
often the grouting was discontin-
ued when the budget was expend-
ed and, in the aftermath when the
underseepage became of alarming
quantities, the cry was made that
“the grouting didn’t work!” The
general result (Weaver and Bruce,
2007) of these deficiencies was
either a) a poor travel of grout in
the ground, leading to the drilling
of families of higher order holes
at ridiculously close centers (e.g.,
1 foot at Chickamauga Dam, TN),
or b) uncontrollable flow of “thin”
grouts into karstic voids or similar
major features.
It is somewhat of a testament to
the enlightened, the lucky, and the
meticulous that so many of the curtains
constructed in the period from the
1920’s to the early 1980’s in particular
appear to have actually functioned
adequately given the restrictions, the
misconceptions and the prescriptions.
Uncharitable views would have it
that such curtains may not have been
needed at all, from a dam performance
or safety viewpoint, and that the curtain
was inserted by rote and by paradigm.
On the other hand, the fact that so
many of our dams have now been
remediated, or are facing remediation
as a result of an ineffective, incomplete
and/or deteriorating grout curtain, does
lead us back to the inescapable fact that
the “old ways” in retrospect contained
major flaws in their workings. One
definition of the word “insanity” is
to continue to do the same thing even
when it has been repeatedly proved to
fail or to be wrong. To persist with, or
revert to, the “old” ways of grouting
dam foundations is an example of this
definition.
Current Principles (“The New”)
There had arrived in theNorthAmerican
scene by the mid-1990’s a potent
mixture of knowledge and opportunity.
As arguably first articulated at a
Grouting Seminar
in
Toronto,
ON in 1989,
but
certainly
emphasized
to
the cogniscenti
in New Orleans
in 1992, the
world of dam
grouting in North
America
had
begun to change
d r a m a t i c a l l y.
This statement
is made with all
due recognition
of Dr. Wally
Baker who, some
years before, had instigated an advance
into new technical fields, but an
advance which proved economically
unsustainable in the face of prevalent
contracting and procurement vehicles
of the time.
Of particular significance was a pa-
per by DePaoli et al. (1992) which, in a
deceptively understated way, explained
quite clearly the critical control and
importance of pressure filtration coeffi-
cient over the effective travel of grouts
into fissures, and hence their efficiency
in generating low and durable residual
rock mass permeability. As described
in Weaver and Bruce (2007), pressure
filtration can be conceived as follows:
“The injection of particulate grouts
into small apertures is similar to press-
ing the grout against a filter material:
depending on the formulation of the
grout, water can be expelled from the
grout in motion, leading to the devel-
opment of cementitious filter cake at
the borehole wall. With more time, the
cake blocks off the entrance to the ap-
erture and so renders the aperture inac-
cessible to further injection via that av-
enue. This tendency of the grout to lose
water during injection is quantified by
the term
pressure filtration coefficient
(Kpf)…”
“To enhance the penetrability of a
grout, a low-pressure filtration coeffi-
cient that minimizes the increase in ap-
parent viscosity (Figure 1) is required.
The general relationship between the
two vital parameters of cohesion and
pressure filtration coefficient is shown
Figure 1. Rheological behavior of typical Binghamian fluids
(modified after Mongilardi and Tornaghi, 1986).
