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Geophysical ahead investigation methods / Seismic methods Featured

Using underground space more and more
implies improvements to the economic
feasibility of underground construction.
Common practice nowadays is a
geotechnical site investigation that is carried
out in order to enable a geotechnical and
environmental assessment of the ground
conditions. geological and hydro-geological
conditions are key factors during the
planning of and budgeting for a project and
its subsequent viability.
There is a strategy for site investigation at
various stages of a project. This phased
strategy consists of investigations for
feasibility studies and the preliminary
and detailed design and of investigations
during the construction phase. Among
other purposes, these investigations are
carried out to determine the 3-dimensional
geotechnical and hydrogeological model
during the design studies and validate using
face mapping, investigations ahead of the
tunnel face (e.g. probe drilling, geophysics),
TBM performance data, etc. [1].
Today, machine driven tunnelling has to
provide high performance, normally to
justify the high initial investment costs.
However, geological anomalies and risks
can massively reduce tunnelling rates over
weeks or months, and sometimes bring a
project to a complete stop. Conventional
methods, such as probe drilling, generally
cover a range of 30 to 40 metres ahead
of the tunnel face. They are useful but its
execution can reduce the daily advance
performance of a TBM because excavation
must stop during these periods. In addition,
exploratory drilling only provides a selective
record of the ground conditions unless
multidirectional probing is adopted ahead of
the face creating further time delays.
From its origin in oil and mineral resources
exploration, investigations ahead of the
tunnel face by means of geophysical
methods entered into the field of tunnelling
in Europe in the early 1990s [2] followed by
many applications on Japanese tunnelling
sites throughout the 1990s [3] [4]. Since
then, commercial systems became
available on the market and the number
of applications of geophysical methods
increased. In addition, a systematic use of
such systems began in some regions of
the world (Japan and China) where intense
tunnelling activities were going on. Hence,
geophysical methods increasingly became
an essential part of the risk management
process over the last 20 years. The
tunnelling industry has already identified the
potential of these usually non-destructive
methods that valuably contributes to the
assessment of the ground conditions and
to the provision of interpretative reporting.
This guideline gives an overview of existing
geophysical methods and technologies
ahead of the tunnel face. In its present
version, it focuses on seismic methods
and describes technical features and
case studies of these methods. It further
suggests requirements to be included in
tender specifications for the described
investigation systems for tunnelling projects.
Seismic reflection imaging is the most
effective prediction method because of
its large prediction range, high resolution
and ease of application on a tunnel
construction site. In particular, when using
the information of the full seismic wave field
propagating through the ground, seismic
properties such as seismic velocities
and their derived elastical parameters
like Poisson’s ratio or stiffness present
valuable information to characterise the
ground. However, geophysics deals
with more methods than just seismics.
There are electro-magnetical, electrical
and gravimetrical methods, which are
being used more and more in tunnelling.
Certainly all of them can contribute by
providing further information on the ground
conditions.
This guideline has been written to orientate
tunnel designers, contractors and owners
towards understanding the benefits
and limitations of the currently available
technology for seismic investigations during
tunnelling that is already used and has a
proven record.

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