Geotechnical News • December 2015
43
THE GROUT LINE
damage due to too higher pressures,
as is sometimes erroneously done,
becomes therefore superfluous. Fol-
lowing the GIN concept, the grout
takes near the surface or gallery, where
the fissures generally tend to be rather
open, automatically increase, while the
pressure remains rather low. At depth,
on the other hand, the openings are
generally smaller so that less grout is
absorbed. As shown in Figure 2, the
grout path in this latter cases (grout
paths 3 & 4) is steep reaching quickly
higher pressures. Therefore, respecting
this self-adaptive nature of GIN grout-
ing, once a certain maximum pressure
is defined, it should be kept constant.
Changing systematically the maxi-
mum pressure in function of depth
does not only unnecessarily compli-
cate the whole grouting procedure,
but it also carries the risk of stopping
grouting before the natural equilib-
rium is actually reached, resulting in
an incomplete execution of the works.
The only zone where a certain pres-
sure limitation might be acceptable is
the upper 5 m, in order to avoid grout
break-out to the surface, especially if
grouting is not performed through a
concrete slab or similar. To ensure an
efficient grout result along the entire
borehole length, it is common practice
to increase in addition
In this respect, it is recalled that the
adequacy of the selected maximum
grouting pressure can be best con-
firmed by several representative grout-
ing test sections.
Maximum grout take (target
volume)
The maximum grout take does actu-
ally not present an absolute stop crite-
rion. It rather defines a decision point
on whether to
à
à
Continue grouting
à
à
Terminate grouting
à
à
Pause grouting and restart later
after setting of grout
à
à
Abandon the hole & drill an-
other one nearby
à
à
Modify the grout mix
In contrast to the grouting intensity
number and the maximum pressure,
this parameter is mainly defined
considering economical rather than
physical aspects. A rough indication
of commonly chosen maximum grout
takes, V
max
, for certain grouting inten-
sities is given in Figure 3.
Mix design
One of the key aspects of the GIN
concept is the use of a single stable
grout mix. The mix should be for-
mulated to achieve the specified
performance criteria as efficiently as
possible (i.e. the minimum number
of boreholes, the minimum number
of injection phases, and the optimum
injection rate throughout each individ-
ual injection). Its selection and design
is based upon a thorough understand-
ing of the site rock conditions, includ-
ing fissure widths. It stands to reason
that one of the most important aspects
actually limiting the groutability is
the maximum cement grain size rela-
tive to the fissure width. As a general
rule, for a fissure to be groutable, its
aperture should be at least three times
the maximum grain size of the cement.
Finally, the mix is also of low water-
cement ratio to ensure both long-term
strength and durability, and the avoid-
Figure 2. Grouting paths for different fissure openings, illustrating the self-
adaptive nature of GIN grouting.
Figure 3. Typical range of GIN values, as well as corresponding maximum
pressures and volumes.
