Civil Engineering Reference
In-Depth Information
In order to interpret the results of direct and indirect methods, the results from the first
phase of investigation like historical research, aerial photos, geological documentation
and other knowledge should also be considered.
Tunnel with pilot heading.
The investigation stages and the objectives of investigation
are the same as for tunnelling without a pilot heading. One advantage of a pilot heading is
that its smaller cross-section has to make it easier in principle to overcome any unexpected
karst features than the larger section of the final tunnel.
It is advantageous to place the pilot heading inside the section of the final tunnel. Con-
sidering the small size of a pilot heading and the resolution of the geophysical process,
which decreases with increasing distance, and the costs which rise over-proportionately
with increasing investigation depth, it should be considered in each case whether the pilot
heading can already deliver sufficiently accurate investigation results for the final struc-
ture. Experience shows that investigation and treatment performed from the pilot head-
ing cannot rule out further undiscovered anomalies being found when the main tunnel is
driven. This requires additional investigation and treatment measures as the main tunnel
advances.
In cases of heavy karstification, which demand correspondingly extensive ground im-
provement measures, economic considerations could also argue against complete advance
ground improvement from the pilot heading, if for example extensive drilling and grouting
works have to be carried out in the restricted space of the pilot heading.
4.5
Monitoring and evidence-gathering measures for tunnelling
beneath buildings and transport infrastructure
4.5.1 General
Tunnelling underneath buildings and roads and railway lines is subject to additional moni-
toring requirements and normally demands a survey of the existing situation to secure
evidence. The condition of buildings and transport infrastructure is mostly determined by
an independent expert before the start of the works to gather evidence and after comple-
tion. In order to restrict damage, the effect of tunnelling on buildings is monitored and
immediate measures are undertaken when the deformations or forces specified in advance
are exceeded. The following section describes the procedure proposed by N. Klawa [109].
4.5.2 Monitoring and evidence-gathering measures
The condition of the building is established in a protocol with exact recording of existing
cracks and damage (drawings, photos). Patches of plaster are applied to larger cracks or
joints to indicate movement. The gathering of evidence and monitoring of building set-
tlement during the driving of underground tunnels is normally performed with levelling
bolts on corners of buildings above the tunnel and adjacent buildings, possibly also with
one or more intermediate points on walls. Settlement of the ground surface between build-
ings (along and across the tunnel axis) is also surveyed, since the movement of buildings
and the ground surface are not identical. From the difference between the settlement of
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