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Geotechnical News • March 2014
45
THE GROUT LINE
Norwegian grouting strategies
Same or not in Sweden
Emphasis is on the injection of a limited number of stable,
micro-cement grout mixes with a water:cement ratio ranging
between 0.5 and 0.9 and containing additives to increase
penetration and control stability, workability and gel time.
The grout is mixed by a high-speed colloidal mixer.
Stable grouts, a water:cement ratio ranging between 0.8 and
1.0, are commonly being used today in Sweden.
Adequate and relevant use of ‘stop criteria’ in terms of the
amount injected as well as the grouting pressure applied in
order to optimise grouting efficiency and prevent the spread
of ‘wild’ grout.
The stop criteria based on pressure is common. Limiting
volume per borehole is always used. The time criteria have
been used in shorter trial sections in some few tunnels so far.
The testing is adapted for an eventual use in the progress of
the tunnel. So far it seems promising.
Adaptations of the grouting pattern, grout mixes and injec-
tion procedures are carried out by a skilled grouting crew
to best suit the conditions that are being encountered on a
shift-by-shift basis.
A predetermined pattern based on prognosis are used and
followed.
The adaptations are made in a systematic manner and
remain loyal to the agreed grouting strategy and stop
criteria. Therefore, the agreed strategy and criteria need to
anticipate and accommodate a wide range of conditions at
the outset. The strategy and criteria are then actively refined
with due responsiveness to the actual experience being
gained on site.
Similar in Sweden.
Records are maintained of the drilling and grouting details,
engineering geological characteristics of the rock mass and
groundwater inflows from probe holes, control holes and
the excavated tunnel in order to provide a sound basis for
subsequent decision making.
Same in Sweden.
Pre-grouting is preferred rather than post-grouting.
Same in Sweden.
Table 3. Comparison of two grouting procedures adopted in Norway (Bærum tunnel) and Sweden (Nygård tunnel).
Bærum tunnel
Nygård tunnel
Excavation year
2010 Completion
2008 Completion
Tunnel cross-section
approx. 100sqm
approx. 125sqm
Circumference
approx. 42m
approx. 20 - 50 m
Rock quality
Shale, schist and limestone, mixed with
volcanic intrusions
“normally Swedish rock”, i.e. gneiss
of granitic composition, RQD between
75 – 90, 2 joint sets plus irregular
joints and a number of small weakness
zones
Number of holes
63 holes in the crown + 10 holes in the
face, giving a total of 73 holes
50 holes in the crown and zero in the
face
Hole spacing at start
0.67 metres
1.5 m in the roof and 2.0 in the walls
and bottom
Cover length
23.5 metres
24 meters
Overlap between fans
approx. 8 metres
6 meters
Stipulated inflow criterion
4 litre/minute and 100 metres
2 - 5 litre/minute and 100 metres