Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 148

4.3.1 Deformation measurement
Continuous observation of the behaviour of a tunnel at every phase of construction through
simple and reliable measurement of displacements is an essential aid to safe and economic
construction. The observation of deformation and displacements is particularly significant
because these can be relatively simply measured and interpreted. From deformations caused
by changes of construction states and their behaviour with time, it is possible to reach con-
clusions about the properties of the rock mass, the development of ground pressure and the
extent of surface settlement. The information has a quantitative character and can provide a
sound basis for important decisions. Measurements also often fulfil a control function, with
possible collapses being detected before being noticed visually to enable immediate action
to reduce damage, for example by strengthening or propping the support.
4.3.1.1 Geodetic surveying
Considered on its own, geodetic surveying is time-consuming in performance and evalu-
ation. A considerable part of this time, however, is already necessary for regular tunnel
surveying as the tunnel advances, so the additional time taken to determine deformation
is not particularly long.
Measurements. Geodetic instruments can be used to determine the alteration of the loca-
tion of points on the support relative to other points on the support. The absolute level of
points in the tunnel above sea level can also be determined by levelling from bolts, which
have been checked once or more by levelling from known benchmarks. In this way it is
possible to record the absolute heave or settlement of support elements.
The situation with the vertical location of foot points is similar. When more than one adja-
cent tunnel is excavated, not only the change of the horizontal diameter of the first tunnel
during the excavation of the adjacent tunnels is important but also the change of vertical
position of the right- and left-hand benches.
Measurements of settlement on the surface (terrain or buildings) also belong to geodetic
surveying and are performed by precision levelling.
4.3.1.2 Convergence measurements
Convergence measurements determine the relative displacement between measurement
points situated at the perimeter of the cavity. An example of the layout of the measured
distances shown in Fig. 4-11. The measuring of diagonals can cause considerable obstruc-
tion to construction operations.
Measurements. Immediately after the exposure of the perimeter of the cavity, convergence
measurement bolts are set at various points and a first reading is measured (zero measurement).
The measurement is repeated, initially at short intervals and later as the support becomes effec-
tive at longer intervals according to the specified measurement programme. The measurement
of diagonals is often impossible without considerable obstruction of construction operations.
Fig. 4-11 shows as an example a diagram of a classic convergence measurement.
Convergences are measured with special measuring tapes, which are constantly tensioned
to a reproducible degree. According to the quality of the measuring tape and the type of