Basic HTML Version
46
Geotechnical News • March 2014
www.geotechnicalnews.com
THE GROUT LINE
water yields a synthesis of what has
been accomplished regarding the
“tightness” of tunnels.
The geology may affect significantly
the performance and results of a grout-
ing scheme, therefore a brief overview
of the Norwegian and Swedish geo-
logical conditions will be provided.
Norway and Sweden forms part of a
Precambrian shield and are consid-
ered typical ‘Hard Rock’ environment
with respect to tunnelling. In Norway,
approximately two thirds of the coun-
try is covered by Precambrian rocks
(older than 545 million years), and for
Sweden the figure is some 9/10. The
most common rock type of these old
rocks is the gneiss, other rock types
from this era are granite, gabbro and
quartzite. The oldest rocks is found in
the north east part in Sweden where
the rocks are 2,6 billion years or older,
whilst in Norway the oldest rock
is found along the west coast. The
younger rocks that are formed by an
over thrust where older sediments are
metamorphosed and now overlaying
the Precambrian rocks. This mountain
ridge is called the Caledonian moun-
tains and found in hilly areas in west
of Sweden and covering almost one
third of Norway, particularly in Mid-
Norway. The rock types are mostly
shale, gneiss and schist with varying
degree of metamorphose. In the outer
rim of the mountain in Sweden the
Precambrian rocks are found but with
a high degree of metamorphose. The
youngest rocks found are sedimentary
rocks, with almost no metamorphose,
has an age younger than 500 million
year. These are found in southern
parts of Sweden, the island Öland and
Gotland around the Lake Vättern and
Siljan. The rock types are mainly shale
and limestone. A similar province is
found in the Oslo region but with an
extra touch of magmatic intrusive
which make the Oslo region a rather
complicated geological province both
with respect to the number of types of
rocks and to large deformation zones
due to the intrusions.
The Precambrian shield as whole
is heavily fractured due to the long
period of geological and tectonic
events. The rock mass is also severely
cut by various types and generations
of discontinuities, from cracks and
joints to zones containing totally disin-
tegrated material. It is common that at
least three main fracture sets are found
when mapping and not seldom even
four and up to totally crushed mate-
rial can be found. Through a stretch of
some 100 meters it is common to find
at least a few larger fracture zones,
sometimes with clay fillings making
the progress of a tunnel difficult, both
mechanically and hydrogeologically.
One positive thing is though that, at
present, the province is tectonically
stable for all practical aspects related
to tunnelling work.
The rock mass is consequently a
very typical jointed aquifer where
water occurs along the most perme-
able discontinuities and with a high
ground water level tunnelling works
are normally taking place in saturated
conditions with the risk of disturbing
the ground water balance imposing
consequential damages.
Characteristics of tunnels
To compare different grouting con-
cepts with focus on pressure one has
to have in mind that there are different
groundwater pressures acting in the
rock. When grouting towards a high
groundwater pressure one must apply
a high total grouting pressure. The
total pressure, P
t
(the one set on the
pump) is then the sum of the over-
pressure, ΔP and the ground water
pressure, P
w
. Besides the ground water
pressure the rock stresses are essential.
The overall principle for any type of
underground project should be based
on the environmental consideration
like the best available technology and
minimize resources in every step of
building process. This can be nar-
rowed down to use the most environ-
mental friendly grout and use as little
as possible but still seals the tunnel.
To summarize the chain of events
using high pressure is something like
this; with opening of fractures, if it
happens, increase the amount of grout
and increases the time to reach a stop
pressure. Time is one of the most
costly ingredients in grouting works in
Sweden which coincides well with the
conditions in Norway. From this point
of view the “problem” lies in much
spent resources. The more evident
factors could be that grout enters the
surface with pollution effects as a
consequence or a heave of the ground
with uplift of building or infrastructure
as a risk. Both the vertical and hori-
zontal stress governs if the fractures
will open and potentially cause a
stress problem in the tunnel or not. If
both stresses are high, one can apply
Figure 2. Schematic drawing of the three main types of tunnels.