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Geotechnical News • March 2014
47
THE GROUT LINE
high grout pressure without the risk
that any opening of fractures occurs,
the opposite also applies. With a high
vertical stress and low horizontal
stress, perpendicular fractures towards
the tunnel will have higher potential
of opening up. A categorization of
tunnels is made to compare the applied
ground water pressures, see Figure 2.
Mountain tunnels
. Often a high verti-
cal stress confining any horizontal
fracture planes but opens fracture
sets vertical to the tunnel drift. The
horizontal stress can be lower than the
vertical one which opens up fractures
parallel to the tunnel. High overburden
of rock minimizes the risk for spread-
ing of grout to the surface.
Forest
tunnels
and sub-sea tunnels
. Drainage
of wet areas is often a risk. Lower in-
situ rock stresses increases the risk of
opening the fractures during grouting
producing a larger spread of grout.
Risk for grout to enter the wet areas or
streams.
Urban tunnels
. Have low vertical
stresses in-situ and often relatively
high horizontal stresses. The low verti-
cal stress produces the risk of uplift-
ing. The high in-situ horizontal stress
confines parallel fractures with the
tunnel with obvious risk for spreading
grout to the surface. Strict demands on
water ingress.
The targeted ingress to tunnels will
govern the grouting procedure. A strict
demand requires often a more thor-
ough design on grouting, where not
so strict demands requires often only
a drill pattern with a the type of grout
and a borehole layout. Below follows
a list of how the targets can be set.
Commonly used figures in Norwegian
tunnels for typical Limit Residual
Inflow Rate (LRIR):
• 2 – 10 liters/minute/100m of tunnel
length – Urban tunnels in sensitive
areas
• 10 – 30 liters/minute/100m of tun-
nel length– Subsea/Forest tunnels
• > 30 liters/minute/100m of tunnel
length– Mountain tunnels with no
specific requirements
The same differentiating for Swedish
tunnels would look like:
• 0.5-2 liters/minute/ 100m of tunnel
length- Urban tunnels
• 2-10 liters/minute/ 100m of tunnel
length- Forest tunnels
• About 10 liters/minute/ 100m of
tunnel length- Mountain tunnels.
NB! not common for Swedish
tunnels
From the comparison we can see that
the stipulations on water ingress in
tunnels vary quite a bit across the
border between Norway and Swe-
den. This can be due to the historic
experiences described earlier where
focus in Sweden has been on grouting
research whilst in Norway the empiri-
cal approach has governed. And the
research in itself can attract discus-
sions and focus to the projects with
more demands and requirements as a
consequence.
These LRIR-values are determined
based on investigating the sensitiv-
ity of surface structures and habitats
(biotypes) on ground water lowering.
The LRIR value may vary from one
section of a tunnel to the next, pending
on the sensitivity of identified struc-
tures or habitats on the surface above
the tunnel. It must be noted that LRIR
is not the key element in rock mass
grouting whilst tunneling the key issue
is how sensitive surface structures and
habitats are to ground water lower-
ing. The LRIR-value is just a way to
indirectly measure such sensitivity
through an assessed impact on the
ground water in conjunction with the
inflow rate to the tunnel. A possible
scenario would be that that no impact
occurs on surface even in situations
with an increased inflow to the tunnel,
and in such case the LRIR must be
revised.
In urban tunnels with a low specified
LRIR a systematically pre-grouting
regime should be applied to secure a
desired result. As a rule of thumb in
Norway:
• 2 – 15 liters/minute/100m => sys-
tematic pre-grouting
• > 15 liters/minute/100m => pre-
grouting initiated by measured
water inflow in probe holes
In Sweden are all urban tunnels
done with systematic pre-grouting
and the requirements are < 10 liters/
minute/100m.
A summary of pressure and stipulated
ingress of water into tunnel from dif-
ferent tunnels in Sweden and Norway
is found in the tables below (table 4,
table 5 and table 6) below. The sum-
mary below is divided as much as
possible to the three types of charac-
teristics of the tunnel. The focus is on
the grout pressures and the stipulated
ingress requirements. The values in
table below are “mainly”, i.e. some
sections in the present project could
have other values.
Table 4. Mountain Tunnels
Project
Depth (m)
Final pressure (MPa)
Stipulated ingress
Äspö, TASS, Sweden
450
10
1 l/min/100 m
Äspö TASP, Sweden
420
8
25 l/min/100 m
Namntall, Sweden
100
5.5
12 l/min/100 m
Bragernes, Norway
100
9
8-30 l/min/100 m
Lunner, Norway
130
5
4-20 l/min/100 m