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Geotechnical News • March 2014
www.geotechnicalnews.com
THE GROUT LINE
determining the minimum stress com-
ponent by doing hydraulic fracturing
of the rock mass, thus creating a new
crack in virgin material, or by jacking
existing ones. By high pressure means
here such as 100 bars or more and by
low means static water pressure plus
a compensation of 1-10 bars. Is there
any explicit description on what pres-
sure is the correct to be applied in any
given circumstances; Can it be that
several different techniques, such as
different pressures can be applied giv-
ing the same predetermined outcome?
Would it be a matter of functional
requirements as a result of the grout-
ing efforts? Or is it simply governed
by a number of other aspects that are
culturally based?
The primary objective is to make the
tunnel tight enough for its purpose. In
addition this article will include two
more objectives to have in mind when
planning and executing rock mass
grouting; namely that grouting is all
about water control - not water proof-
ing, with the focus on prevention rather
than cure suggesting pre-excavation
grouting being preferred to post-grout-
ing. This paper discusses the differ-
ences between the grouting strategies
in Sweden and Norway aiming at con-
cluding whether these are fundamen-
tals of cultural differences, contractual
practices, and construction methods or
have any other factual causes.
Background
Grouting pressure in Sweden -
development
From the early 1900s until the 1970s a
comprehensive expansion of dams and
hydroelectric power projects was car-
ried out in Sweden. In these projects,
sealing of the rock mass was normally
done by cement grouting. The results
of the grouting was controlled by
Lugeon measurements in control hole
that would achieve a certain tightness,
normally 0.1 to 0.3 Lugeon.Tradition-
ally for these grouting works, grouting
was performed with low pressure,
i.e. 5-15 Bar in overpressure, and a
long grouting time (Vattenfall, 1968).
In connection with the urban tunnel
expansions in the 1970s and 80’s, with
low rock overburden and requirements
of the groundwater levels, rules of
thumb was developed and experience
based knowledge for grouting tech-
niques. A common rule of thumb was
that the overpressure would be about
half the rock cover but with a maxi-
mum of 20 bars in overpressure.
During the 90’s, the development of
theoretical consideration started of the
applied pressure and its cases to the
bedrock and grouting effect. Different
criteria for how large pressures should
be applied were presented and some
conclusions were that is all depended
on the stress situation and rock quality.
Further, field tests were made using
higher pressures in some tunnels in
Sweden. The experiences showed that
this was not beneficial to the sealing
effect. A criterion that was developed
for the maximum allowed grouting
pressure, Δp, to avoid a global “jack-
ing” or uplift, ie:
Δp = 3
ρ
rock
gd
rock
— 2
ρ
water
Where
ρ
rock
is the density of rock,
g
is
earth acceleration, d
rock
is the depth of
the overburden rock mass and p
water
is
the ground water pressure. Typically
for a tunnel in granite (
ρ
=2650 kg/
m
3
) at 100 m depth and a ground water
pressure of 80 meters the maximum
overpressure is around 60 bars. There
is another rule of thumb saying that
the maximum overpressure is 3 times
the vertical stress. With the same
parameters as above gives a maxi-
mum overpressure of some 80 bars.
Research studies started in the 2000s
to show how the fractures behave dur-
ing high pressures. Issues which were
highlighted in the surrounding rock
and fractures were the deformations,
blocking of the grout spreading, stiff-
ness changes and follows a progressive
process. The recommendation was to
control the pressure in real time, ie not
to a fixed pressure criterion.
In Sweden today, the determination
of the pressure is usually based on
the traditional rules of thumb and
sometimes on the criteria to avoid
“jacking’’, ie, the pressure varies
substantially depending on the own
experience. With current research on
grouting, the focus is on the grout
spread and the development follows
the environmental principles such as
not overdoing material consumption
if one can handle a situation with less
material applied.
Table 1.Typical grout pressure
applied for Swedish infrastructure
tunnels in urban areas
Rock cover
Max grout pres-
sure in holes
0 – 5m
5-15 bar
5 – 15m
15-25bar
15 – 50 m
25-35bar
Grouting pressure in Norway -
development
The use of high pressure grouting has
shown to be effective in good rock
mass conditions and in situations with
rather impervious rock. It also requires
knowledge of the minimum in-situ
stress component, as this is considered
an upper boundary of the grout pres-
sure to avoid uncontrolled hydraulic
jacking or splitting of the rock mass.
The use of grouting pressure reaching
as high as 100 bar has become quite
common in conjunction with Ordinary
Portland Cement (OPC). By utiliz-
ing micro fine cement (MC fulfilling
Blaine size of not greater than 650 m²/
kg) a more moderate grout pressure
of 50 to 70 bars above actual water
head may be applied. It requires that
strict compliance to the stop criteria
is executed throughout the work. In
extreme cases hydraulic fracturing can
be applied to improve the effective-
ness of the grouting, which implies
that the grouting is actually exceeding
the minimum in-situ stress component.
However, in poor rock mass condi-
tions caution must be undertaken to
avoid a too high grout pressure, which
could cause a lengthy and consuming