Civil Engineering Reference
In-Depth Information
1
1
a)
b)
3
2
2
3
9
8
14
10
15
11
4
5
4
5
6
12
13
7
Figure 7.10
Examples of a measuring profile a) for a crown and, b) a double
track tunnel excavated with side wall drifts
invert, five monitoring points are installed in the crown (Figure 7.10a), and
one or two points on each side in the bench (not shown). The monitoring
points are numbered so as to distinguish them from each other. It is common
to number the highest monitoring point in the middle of the crown as
number '1'. Usually numbering continues on the left-hand side with even
numbers, and on the right-hand side with odd numbers (line of vision in
the direction of excavation). The invert is not usually monitored because
it is covered with backfill that provides a track for the construction equip-
ment. For information on monitoring the invert see section 8.1 on the case
history of the Eggetunnel.
Readings are taken by a laser theodolite, which digitally stores the three-
dimensional coordinates of each target: vertical, horizontal and longitudinal.
After collecting the readings, these data are transferred to a computer for
further processing. From the change in coordinates, displacements in all
three directions can be calculated and the deformation behaviour of the
structure can be derived (this is discussed later in this section).
It is important to have a few stable reference points at the beginning of
the measurements, which are not affected by the tunnel construction. The
accuracy of the laser theodolite measurement should be
1 mm or better
for ideal measuring conditions. The accuracy can go down to
3 mm or
worse in difficult conditions, e.g. dust or large temperature differences
inside the tunnel. This can happen for example, if the monitoring is done
from a large cross section into a small one with a significantly higher tem-
perature due to the hydrating of the sprayed concrete or if the ventilation
is shut down for maintenance.
The digital technique of using a laser theodolite has widely replaced
measuring with a tape extensometer. The latter is a special steel tape, usually
of 20 m or 30 m in length. With a tape extensometer distances instead of
coordinates are measured. This means that immediate information on
