Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 158

4.3.1.7 Sliding micrometer measurements
The sliding micrometer is a highly precise instrument for measuring strain. The device
is used for the complete determination of axial displacement components along straight
lines in rock, concrete or soil. The high degree of precision is based on the bracing of the
portable probe in the measuring marks on the ball-and-cone principle.
Installation, function. Metallic measuring marks, which are connected to each other
with a plastic protection pipe, are fixed by grouting in a borehole of about 100 mm
diameter (Fig. 4-24). To perform a measurement, the probe is inserted at the operating
rod in the protection pipe stepwise to the measuring marks, which are spaced at 1 m
from each other. The two ball-shaped measuring heads at the end of the probe pass
through the measuring marks (sliding position). By turning it through 45° and pulling
the operating rod or the cable, the probe is braced with the measuring heads between
two adjacent measuring marks (measurement position). In vertical or heavily inclined
measuring tubes at depths of up to 50 m, the probe is brought into the measuring posi-
tion and braced with the aid of the operating rod alone, or with a reel if the depth is more
than 50 m. With horizontal or slightly inclined tubes, straight distances of up to 100 m
can be measured without a reel.
The extremely high setting accuracy of ± 10 -6 m in the calibration device, or ± 2 × 10 -6 m
in place in the measuring tube, is achieved through the exact setting of the probe in the cut-
outs (ball-cone). With regard to strain, the device has a measurement sensitivity of 10 -6 m
and the measurement range is 20 mm.
Fig. 4-25 shows the monitoring of ground loosening with a sliding micrometer through
the example of an urban rail tunnel in Dortmund. The change of direction of the displace-
ments after the tunnel has passed the measuring point can be easily recognised. During the
arrival of the tunnel, negative displacements are measured in the direction of the borehole,
or compression of the ground (left-hand branch), while after the passage of the tunnel,
positive displacements are measured in the direction of the borehole, or pulling (right-
hand branch).
Figure 4-24 Sliding micrometer for the determination of the axial displacement component in the
direction of the borehole. Section through the measuring probe in the borehole (Solexperts) .