Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 173

the maximum edge stress in the inner lining of more than 10 % compared to a loca-
tion without karst features. From the effects that have already been described and the
selected limit line, it is possible to derive the size of the caves or voids that have to
be investigated depending on the distance from the tunnel (Fig. 4-43) and thus the
specification for the site investigation in order to ensure permanent structural and
operational safety of the tunnel.
Figure 4-43 Specification
for the investigation of
karst features / anomalies.
Investigation concept. The investigation required to provide an adequate description of
the ground is carried out in stages [181] (Fig. 4-44), with the investigation before the start
of construction and the investigation before the construction of the inner lining being dif-
ferentiated. The objectives of investigation are based on this concept:
The first phase of investigation before the start of construction, and the tunnelling
concept derived from it, enable the tunnel to be constructed safely. Using field in-
vestigations, mapping of karst morphology and historical research, scenarios are pro-
duced of relevant dangers to the construction works. In order to secure the tunnel
drive against caves in the immediate surroundings of the tunnel, angled holes are
drilled (Fig. 4-45).
It can be sensible for shallow tunnels to use geophysical methods coupled with drilling in
the first phase of investigation in order to provide full coverage for the geological model.
These investigations serve to better determine the model of the rock mass or improve it
further.