48
Geotechnical News • December 2015
THE GROUT LINE
additional boreholes do not neces-
sarily need to be drilled to full depth.
Instead, their optimum depth should
be selected based on the grout-
ing results of adjacent boreholes at
certain depth intervals. This simple
design consideration shows how, by
proper integration of the observational
method within the grouting procedure,
the full benefit of the self-adaptive
nature of the GIN concept can be
gained, thereby achieving a complete,
efficient, cost-effective, and safe
grouting job.
Acceptable final permeability
Before defining an acceptable final
permeability for a grouting job, one
should first think about what might
actually be the consequence of the
seepage and/or leakage caused by it.
There should be a clear differentiation
between seepage, which is defined
as interstitial movement of water in
the foundation, or the abutments,
and leakage, which is flow of water
through holes or cracks.
Taking a closer look, it quickly
becomes clear that foundation perme-
ability may directly affect the stability
of the structures to varying degrees,
mainly depending on the dam type and
height. Whilst for rock fill dams, for
example, a certain amount of leak-
age is common and is rather of little
relevance, for concrete dams, in par-
ticular if they are large, the same leaks
might already significantly impair
their safety.
This distinction was already recog-
nized by Lugeon in 1933, when he
came up with first indications for
allowable foundation permeabilities.
He suggested a limiting Lugeon value
of 3 for small dams and a Lu < 1 for
large dams, respectively. Based on
subsequent experience and critical
expert reviews, this concept has been
further refined over time, in particular
focusing on the actual warranty for
grouting. Today, engineers commonly
refer to the guidelines proposed by
Houlsby [3], which can be summa-
rized as indicated in Table 2. In the
same table also some typical ranges
for allowable hydraulic gradient allo-
cated to different dam types are given.
It is obvious that the highest hydraulic
gradients in the rock mass occur in the
contact zone at the dam foundation.
In the treated zone they diminish with
increasing distance from the dam rock
mass contact surface at the foundation.
Both, the recommended Lugeon and
typical allowable hydraulic gradients
as listed in Table 2 refer therefore to
the zone close to the dam rock mass
interface in the central foundation part.
With depth corresponding less strin-
gent values (i.e. higher Lugeon and
lower gradients) might be acceptable.
These values are obviously intended
for guidance only and their appropri-
ateness must be reviewed and veri-
fied individually for each project in
terms of the project-specific risks. To
arrive at an appropriate value, It is
important to identify the possibility of
encountering particular features and
peculiarities of the site by means of
thorough geological and hydrogeo-
logical investigations, and to evaluate
their influence on the permeability on
a short and long term. If permeability
and geological conditions on one site
are highly variable, certain generaliza-
tions are necessary.
Relevance of additional testing -
pre-injection and post-injection
The determination of permeabilities
is essential both to justify the need for
grouting, and to evaluate the success
of the works executed. Thus, water
pressure tests should be performed
in exploratory primary holes before
grouting and in check holes after com-
pletion of grouting in a certain section.
These tests are required to compare
the initial and the final permeabilities
of the rock mass and to assess in this
way the grout efficiency and success,
respectively.
On the other hand, the execution of
pre-injection water pressure tests in
individual grout stages during the
grouting programme, is not generally
necessary, and might negatively affect
the already treated rock mass. In addi-
tion, such tests during the injection
works may not be representative, since
there is no direct and/or consistent
relation between the penetration of
grout and that of water in a rock mass.
As shown in Figure 7, a unique wide
crack (A) may give the same Lugeon
value as a high frequency of fine joints
(B), while due to its binghamian rheol-
ogy as well as the maximum cement
grain size, the actual grout take might
be much lower in the latter case.
Figure 7. Difference in Lugeon values and grout takes for different fissures.
