Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 133

Spacing. The following spacings of the simple monitoring sections can be seen as a rule
of thumb and altered to suit specific local conditions:
Stable rock

a ≈ 30 m

Unstable rock a < 20 m

Squeezing rock a < 10 m

In special cases, for example when unsymmetrical loading can be expected or the displace-
ments are too high, additional instruments or extra sections may be necessary (Fig. 4-2).
Location and extent of displacement measurements. The spatial position of selected
points on the outer support layer is determined absolutely in three dimensions by optical
surveying. These measurements can be used to derive settlements, convergences, diver-
gences and displacements along the tunnel.
Survey points are installed at a distance of less than 1 m from the face before the next
round and surveyed (zero position). Subsequently, the points are normally surveyed at
least once a day for the first few days. Near fault zones or in the event of heavy deforma-
tion, the interval is shortened. When a number of successive measurements show decreas-
ing rates of deformation, the measurement interval can be lengthened in agreement with
the site supervision.
The vertical movements of the points at the feet of the top heading cannot be directly
surveyed since the points have to be about 0.8 to 1.0 m above the invert in order to be
recorded by instruments. If large displacements occur at the feet of the top heading, it is
therefore often better to measure the force at the foot of the top heading with load cells.

Extent of anchor measurements. The forces in anchors are recorded at regular spacings
of 100 to 200 m. These measurements should not be confused with the separate require-
ment to check the load-bearing capacity of anchors.

4.2.2.2 Principal monitoring sections
The main purpose of principal monitoring sections (Fig. 4-3) is to check the input data for
the design calculations (rock mass parameters and assumed loadings) and the suitability of
the calculation model, and if appropriate quantitatively determine any necessary corrections.
It should be possible from this data to derive the loading on the temporary support and the re-
sulting displacements and stresses in the surrounding rock mass, globally and in their actual
magnitude. Principal monitoring sections should thus be provided at the start of the scope
of applicability of each structural verification. It is possible to check the measurements by
investigating the compatibility of measured loadings, displacements and stresses.
Measurements. The following individual measurements are normally carried out:
- Horizontal and diagonal convergences.
- Settlement of the crown; for appropriate cases of bench excavation, also the settlement
of the foot points.
- Displacement of the surrounding rock mass.
- Radial pressures on the outer support layer.
- Stress in the outer support layer and the steel arches.
- Loading of the anchors.