Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 156

- If on the other hand I > A and or B > M , then the load-bearing ring already has failure
deformation. At failure deformation, volume increases occur because the broken pieces
fracture further and the broken material takes up a larger volume than the original
compact rock. The volume increase at failure deformation is associated with the loss of
load-bearing capacity of the ring (loosening). The location and type of failure deforma-
tion can be read from the extensometer measurements, since each extensometer gives
the displacements occurring in its sector ( H , L , O , W ).
4.3.1.6 Inclinometer / deflectometer measurements
In order to measure displacements transverse to a borehole, deflectometers are used, also
called inclinometers. Transducers, which are generally connected to each other by a tight
wire and measure the relative displacement at right angles to the axis in a polygonal trav-
erse, are set at defined spacings into a lined borehole. In this way, they can deliver the
components of the movement of the rock mass, which are not detected by extensometers,
which measure changes of length along the axis of the hole (Fig. 4-21).
Figure 4-21 Deflectom-
eter to measure displace-
ments transverse to a
borehole.
A simple deflectometer (Fig. 4-22) consists of a head piece 1, a measuring element 2 and
an end piece 3, which are each mounted at three points inside the casing of the borehole 4
and connected to each other with spacing tubes 5, which have universal joints. In the three
measuring elements are precision blades 6 for the exact support of the tight measuring
wire 7. In the central measuring element 2, an electrical inductive transducer with actuat-
ing motor 8 is mounted so that it can measure changes of the length of the measuring wire
in one direction both sides of the blades. If, for example, displacements occur along the
length head piece - measuring element or measuring element - end piece, then this move-
ment is transferred to the borehole casing and thus to an arm of the deflectometer, and then
displayed by the inductive transducer.
The chain deflectometer is an extended form of the simple deflectometer and can contain
up to eight members connected in a row.
Installation. Deflectometers are installed in the borehole at the head piece with a guide rod
and can be reused after the completion of measurements. The diameter of the device is rela-
tively large due to its electrical components. Measured lengths of up to 60 m are possible,
and the accuracy is 0.1 to 0.01 mm according to the spacing of the measuring elements.