Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 167

excavation - prestressed with 30 kN. Its anchor force increased as a result of the excava-
tion of the second phase by 55 to 85 kN, and the opening of the third partial excavation led
to a further increase of load to altogether 93 kN. Anchor II, which was set and prestressed
in the second phase, took an additional load of 33 kN, which increased in the subsequent
fourth phase of excavation once again by 7 to altogether about 70 kN. Anchor III, after an
initial decrease of loading, also reached its maximum load of 40 kN in the fourth phase,
while anchor I experienced a continuously decreasing load in this phase. The load on
the other anchors only started to decrease after the invert was closed; the final state was
then reached after about 22 d: anchor 1 was the highest loaded with 30 kN more than the
prestressing force, altogether 60 kN, anchor II maintained approximately its pre-stressing
force of 30 kN and the force in anchor III decreased to 10 kN.
Figure 4-36 Log of anchor forces on the site of the Engelberg Base Tunnel
(autobahn Heilbronn - Stuttgart).
4.3.4.2 Checking of anchor forces with mechanical measurement anchors
While the anchor force can be measured with a sensor at the anchor head, mechanical
measurement anchors can determine the function of the anchor in detail, with the curve of
loading against depth and time. They correspond to the SN anchors used for pattern bolt-
ing and have the same cross-sectional area with the difference that they are hollow in the
middle; inside their hollow shafts, measurement rods are fixed to the anchor shaft at vari-
ous depths. The length changes between the fixed points and the anchor head are measured
(Fig. 4-37). Mechanical measurement anchor 1 contains several partial lengths I to IV,
with extensometer points 4 to 7 fixed at their ends in the internal bore. The extensometer
rods 8 to 11 from the extensometer points to the measurement stop 2 permit the measure-
ment of the change of the partial length under load and thus the determination of the force
acting there [87]. The difference of the displacements between the individual fixed points
is a measure of the loading of the anchor in the individual sections; these figures can be
converted to stress since the E modulus is known. It has to be considered that the results
can be falsified, for example by bending. The 2 nd derivation of the curve of the displace-
ments gives the shear transfer between anchor and rock mass, from which the action of the
anchor can be estimated.