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Distributed fiber optic shape sensing along shotcrete tunnel linings: Methodology, field applications, and monitoring results

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Deformation monitoring and structural reliability assessment are key components in modern conventional tunneling. The state-of-the-art monitoring design is usually based on displacement measurements of geodetic targets using total stations paired with pointwise geotechnical sensors inside the tunnel lining. In recent years, distributed fiber optic sensing (DFOS) has become more popular in tunneling applications. DFOS measurements basically deliver internal strain and temperature distributions, but no direct relation to the tunnel shape’s behavior. This paper introduces a novel sensing and evaluation concept, which combines DFOS strain measurements and geodetic displacement readings for distributed shape assessment along curved structures, such as tunnel cross-sections. The designed system was implemented into shotcrete tunnel cross-sections as well as shaft linings and enables the determination of displacement profiles with high spatial resolution in the range of centimeters. Evaluations of continuous monitoring campaigns over several weeks as well as epoch-wise measurements performed by different DFOS sensing units in combination with stochastic analysis demonstrate the high potential of the developed approach and its capability to extend traditional monitoring methods in tunneling.
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Journal of Civil Structural Health Monitoring (2021) 11:337–350
https://doi.org/10.1007/s13349-020-00455-8
123
ORIGINAL PAPER
Distributed fiber optic shape sensing alongshotcrete tunnel linings:
Methodology, field applications, andmonitoring results
ChristophM.Monsberger1 · WernerLienhart1
Received: 30 July 2020 / Revised: 3 November 2020 / Accepted: 24 November 2020 / Published online: 27 January 2021
© The Author(s) 2021
Abstract
Deformation monitoring and structural reliability assessment are key components in modern conventional tunneling. The
state-of-the-art monitoring design is usually based on displacement measurements of geodetic targets using total stations
paired with pointwise geotechnical sensors inside the tunnel lining. In recent years, distributed fiber optic sensing (DFOS)
has become more popular in tunneling applications. DFOS measurements basically deliver internal strain and temperature
distributions, but no direct relation to the tunnel shape’s behavior. This paper introduces a novel sensing and evaluation
concept, which combines DFOS strain measurements and geodetic displacement readings for distributed shape assessment
along curved structures, such as tunnel cross-sections. The designed system was implemented into shotcrete tunnel cross-
sections as well as shaft linings and enables the determination of displacement profiles with high spatial resolution in the
range of centimeters. Evaluations of continuous monitoring campaigns over several weeks as well as epoch-wise measure-
ments performed by different DFOS sensing units in combination with stochastic analysis demonstrate the high potential of
the developed approach and its capability to extend traditional monitoring methods in tunneling.
Keywords Distributed fiber optic sensors· Shape sensing· Shotcrete tunnel lining· Displacement sensing· Field
applications· Tunneling
1 Introduction
The design of excavation and supporting methods in modern
tunneling is usually based on geotechnical monitoring and
reliable data interpretation to enable an assessment of the
structural integrity and, finally, to guarantee a safe construc-
tion and operation. State-of-the-art monitoring approaches
utilize displacement measurements of geodetic targets at the
inner surface of the tunnel using total stations [22, 24, 25],
which are however time-consuming and always require a line
of sight between the instrument and the measured object.
Therefore, the measurements might interfere with the regular
tunnel construction work, involve risks for the surveying
team, and can cause construction delays every time they are
performed. Electrical sensors, e.g., vibrating wire sensors
[23] or extensometers [2], may be installed in addition to
3D monitoring targets inside the shotcrete lining to provide
continuous insitu measurements. Nevertheless, the number
of sensors inside the lining is limited due to practical reasons
as each electrical sensor needs its own connecting cable to
the data logger, and hence, information can only be obtained
at particular locations of the lining.
Distributed fiber optic sensors (DFOS) are advantageous
as the cable itself acts as the sensitive element and distrib-
uted measurements can be performed along the entire sens-
ing fiber. Only one lead-in cable is necessary to realize a
large number of monitoring points, that which significantly
reduces the installation effort to gather strain values in the
tunnel lining with a high spatial resolution. The DFOS uti-
lization might be, however, limited, if major impairments
with huge crack widths become prevalent along the lining
or yielding elements are used to control large deforma-
tions. Although the sensing cable installation procedure is
critical due to the harsh tunnel environment, DFOS have
already been successfully implemented inside shotcrete tun-
nel linings. These existing installations are mainly focused
on investigations of mechanical stress as a result of creep-
age, shrinkage, and/or rock pressure [9, 19, 28], as well as
* Christoph M. Monsberger
christoph.monsberger@tugraz.at
1 Institute ofEngineering Geodesy andMeasurement Systems,
Graz University ofTechnology, Steyrergasse 30, 8010Graz,
Austria
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338 Journal of Civil Structural Health Monitoring (2021) 11:337–350
123
convergence analysis [4, 10], but do not deliver concepts
for fully distributed shape analysis along the lining. This
paper introduces a distributed fiber optic shape sensing and
evaluation approach, which utilizes DFOS strain measure-
ments along different sensing layers in combination with
pointwise displacement readings for fully distributed shape
assessment along curved structures, such as tunnels. The
developed system was implemented into shotcrete tunnel
cross-sections as well as shaft linings at a railway tunnel cur-
rently under construction and extends conventional geodetic
readings, which are carried out anyhow. The installations
were interrogated by different DFOS sensing units based
on Rayleigh and Brillouin scattering, whose basic charac-
teristics are described in the following (sect.2). Moreover,
the designed shape sensing algorithm and its capabilities by
means of stochastic analysis (sect.3) are presented. Results
of continuous monitoring campaigns and evaluations of
epoch-wise follow-up measurements are shown and dif-
ferent evaluation setups are discussed (sect.4). Finally, the
outcomes are concluded and an outlook on future research
aspects is given (sect.5).
2 Distributed ber optic monitoring system
andpractical aspects
Distributed fiber optic sensing systems use natural scatter-
ing of optical signals during the forward propagation along
the sensing fiber. Small parts of these intensity losses are
backscattering effects, whose spectral characteristics carry
information about geometrical, physical, or chemical quan-
tities. Backscattering effects can be basically divided into
linear (Rayleigh) and non-linear (Raman and Brillouin) scat-
tering. Raman-based systems are only sensitive to tempera-
ture, whereas Rayleigh as well as Brillouin instruments are
sensitive to both, strain and temperature changes [8]. Their
capabilities regarding spatial resolution and measurement
accuracy are however significantly different, which is why,
the applications presented in this paper were partially inter-
rogated by two different sensing units to assess potential
impacts of the DFOS characteristics on the shape sensing
approach.
The used Rayleigh backscattering system OBR 4600
[14] from Luna Innovations Inc. enables distributed strain
sensing with high spatial resolution of some millimeters, a
measurement precision in the range of 1
μ
m/m, and a meas-
urement frequency of about 0.1 Hz. The usual sensing range
is limited to 70m, but can be extended up to 2km using
specially developed software components. This, however,
results in limitations of the feasible measurement resolution
of 4.1
μ
m/m using a spatial resolution of 3cm according to
the manufacturer. These specifications can be confirmed by
IGMS (Institute of Engineering Geodesy and Measurement
Systems at Graz University of Technology) experiences in
practical environment.
Unlike Rayleigh sensing units, Brillouin interrogators
provide measurements over tens of kilometers, though with
limitations in the measurement capabilities and, typically,
significantly longer measurement times of several minutes.
Based on the BOFDA (Brillouin Optical Frequency Domain
Analysis) technique, the fTB 5020 from FibrisTerre GmbH
(Germany) allows monitoring over 25km with a spatial
resolution of 0.5m [5]. Under geotechnical conditions, a
measurement precision between 2 and 10
μ
m/m can be usu-
ally achieved depending on the sensing network.
In addition to the interrogation unit itself, the reliability
and robustness of the DFOS cable are highly relevant to
guarantee the integrity of the optical sensing fiber in harsh
tunnel environment. Different manufactures offer such cables
especially developed for strain monitoring in geotechnical
applications, e.g., [26]. These can protect the optical fiber by
a metal tube or even by a special steel armoring. The cables
show good resistance against mechanical impacts and their
suitability could be proven by various successful installa-
tions at different construction sites, e.g., [13, 16, 28]. The
outer surface of selected cables is also structured to provide
a solid connection with the surrounding shotcrete material.
Since Rayleigh and Brillouin systems are strain and tem-
perature sensitive, appropriate temperature compensation is
important in practical applications. For that reason, specially
designed temperature sensing cables [27] or sensing cables
in loose tubes are typically installed parallel to the strain
sensing cable to numerically correct the temperature impact.
The temperature-corrected raw measurement quantity
(i.e., wavelength, frequency, or intensity) must be finally
converted into strain. The parameters of the sensor charac-
teristic curve often vary depending on the cable type as well
as different batches of the same cable in some cases. Cable
calibration is therefore essential to achieve accurate sensing
results. IGMS developed a unique calibration facility [29]
which enables highly precise, fully automatic calibration of
strain sensors with lengths of up to 30 m under stable labora-
tory conditions. Individual calibrations are carried out prior
to field applications to provide reliable conversion param-
eters for the used cable. Further information on the design
of different strain sensing cables from Solifos AG and cor-
responding strain calibration results are presented in [17].
3 Shape sensing algorithm
3.1 Functional model
DFOS systems deliver distributed strain (and temperature) pro-
files along the longitudinal axis of the installed sensing fiber.
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339Journal of Civil Structural Health Monitoring (2021) 11:337–350
123
Multidimensional information can however also be captured
if two or more fibers are parallelly aligned along the structure.
This setup enables the determination of curvature profiles in
case of bending orthogonal to the DFOS sensing direction.
It is known from elastic bending theory (e.g., [15]) that the
deflection w at one specific sensing element s along an object
can be described by
where M is the bending moment, E the modulus of elastic-
ity, and
Iy
the moment of inertia at the observed position.
The deflection w can also be expressed by the segment’s
curvature
𝜅
and, therefore, by the bending radius R. The
relation of the bending radius and the measured strain along
the outer layer
𝜖out
and the inner layer
𝜖in
in combination with
the distance between the fibers d enables a direct curvature
acquisition from the DFOS measurements:
Beside influencing shear stresses, the concrete object might
also be affected by longitudinal stresses due to shrinkage,
creepage, or temperature-induced expansion. These effects
are taken into account by the longitudinal strain, which is
equal to the mean strain value of both sensing layers
𝜖
:
To obtain displacements orthogonal to the sensing direc-
tion, the distributed curvature values can be numerically
double-integrated based on the difference equation method,
as already introduced by [21]. The relation between the cur-
vature
𝜅
at the i-th position along an object and the deflec-
tion value w is described by
where h is the distance between the sensing points and,
therefore, equal to the spatial resolution of the DFOS sys-
tem. The linear functional model is given by
(1)
(s)=
EI
=
𝜅(s)
(2)
𝜅
(s)= 1
R(s)
=
𝜖
out
(s)−𝜖
in
(s)
d(s)
.
(3)
𝜅
(s)=
𝜖
out
(s)−𝜖
in
(s)
d(s)
(
1
+𝜖(s))
(4)
𝜖
=
1
2
(
𝜖out +𝜖in
).
(5)
𝜅
i=
1
h
2
(wi+12wi+wi1)
,
where
𝛴𝜅𝜅
describes the stochastic of the observations. A
least-square adjustment based on the Gauß–Markov model
can be finally used to derive the estimated deflection values
̂w
:
The absolute position and orientation of the object is how-
ever unknown. The normal equation matrix N has therefore
a rank deficiency of 2 and cannot be inverted without fur-
ther information. For linear structures in geotechnics, this
boundary-value problem is commonly solved using the can-
tilever beam approximation, where the starting point (i.e.,
the bottom point) and its orientation is assumed to be fixed.
For curved structures like tunnel linings, two sensing
cables may be installed in circumferential direction, but
with different distances to the center, see Fig.1a. This
results in slightly different sensing segment lengths d(s)
along the outer and inner layer, which can be taken into
account by the installation radii of the different layers [18].
Nevertheless, the values resulting from Eq.2 only repre-
sent the curvature change due to shear stresses acting on
the single sensing segment, i.e., stresses orthogonal to the
(6)
(7)
𝛴
𝜅𝜅 =
𝜎
2
k1
0
00
0𝜎2
k2
00
⋮ ⋮ ⋱⋮
00𝜎2
kn1
0
000𝜎2
kn
(8)
y
x
εout
εin
sensing
segment
(a) (b)
φ
d
ε
in
±κ
±κ
ε
mean
ε
out
Fig. 1 Shape sensing principle: aSchematic representation of cross-
section profile; bdetail of one single sensing segment along the lining
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340 Journal of Civil Structural Health Monitoring (2021) 11:337–350
123
tangent to the lining. Considering the curved initial geom-
etry, the curvature impact within the two-dimensional
coordinate system can be rewritten and expressed by:
where
𝜑
is the orientation of the sensing segment relative to
horizontal coordinate axis x (Fig.1b). This geometry param-
eter can be initially retrieved from the planning model. The
numerical integration process is performed individually
for each coordinate direction. In contrast to the cantilever
approximation, the boundary-value problem can be solved
by extending the functional model with additional obser-
vations, e.g., pointwise displacements of geodetic targets
(
𝛥xTS
,𝛥yTS
)
recorded by total stations at the j-th position of
the fiber optic installation along the lining:
The number of geodetic points is basically variable, but must
be at least 2 to solve the boundary-value problem. Using
more than two supporting points provides an estimation with
redundancy, which enables an assessment of the correctness
of the functional and the stochastical model.
(9)
[
𝜅
x
i
𝜅y
i]
=
[
sin 𝜑i
cos 𝜑
i]
𝜅i
,
(10)
𝜅x
1
𝜅x
n
𝛥xTS
j
=
1
h2
2
h2
1
h20⋯⋯⋯ 0
⋮ ⋱⋱⋱⋱⋱⋱
0⋯⋯⋯⋯ 1
h2
2
h2
1
h2
a0a1⋯⋯⋯⋯anan+1
⋮⋮⋮⋮⋮⋮⋮ ⋮
𝛥x0
𝛥x1
𝛥xn+1
(11)
𝜅
y
1
𝜅y
n
𝛥yTS
j
=
1
h2
2
h2
1
h20⋯⋯⋯ 0
⋮ ⋱⋱⋱⋱⋱⋱
0⋯⋯⋯⋯ 1
h2
2
h2
1
h2
a0a1⋯⋯⋯⋯anan+1
⋮⋮⋮⋮⋮⋮⋮ ⋮
𝛥y0
𝛥y1
𝛥yn+1
(12)
𝛴
x
𝜅𝜅
=
𝜎
2
𝜅1
00
⋮ ⋱⋮ ⋮⋱
0𝜎2
𝜅n
0
00𝜎2
x1
000
(13)
𝛴
y
𝜅𝜅
=
𝜎
2
𝜅1
00
⋮ ⋱⋮ ⋮⋱
0𝜎2
𝜅n
0
00𝜎2
y1
000
(14)
a
i=
{
1i=
j
0ij
.
The derived curvature values
𝜅x
i
and
𝜅y
i
are always related to
the geometry of the lining. The workflow can hence be under-
stood as an iterative approach, see Fig.2, where the lining’s
geometry (
𝜑
) is continuously updated. This evaluation proce-
dure is performed as long as the total sum of squares (TSS) of
the coordinate differences between the current and the previ-
ous iteration is decreasing. The coordinates in both directions
can be finally determined by adding the estimated differential
supplements
𝛥xi
and
𝛥yi
to the initial model shape.
3.2 Stochastic analysis
It is obvious that measurements from different sensing tech-
nologies are recorded with different stochastics. Appropriate
weighting of the different observation types is therefore essen-
tial to guarantee the suitability of the estimation model.
Geodetic measurements in tunneling are usually performed
with modern total stations with a distance measurement pre-
cision of 1mm for prisms or 3mm for bi-reflex targets and
a standard deviation of 1” (=0.3 mgon) for angle readings
[12]. These specifications typically result in standard devia-
tions between 1 and 5mm for displacements in both coor-
dinate directions depending on the target type as well as the
measurement configuration.
The curvature values are represented by a combination of
different measurement quantities, i.e., the DFOS strain meas-
urement
𝜖
, the distance between the fibers d and the orienta-
tion of the sensing segment
𝜑
(cf. Eq.2 and 9). Their standard
deviation in both coordinate directions can be derived using
variance propagation:
(15)
𝜎
2
𝜅x=
𝜕𝜅x
𝜕𝜖
2
𝜎2
𝜖+
𝜕𝜅x
𝜕d
2
𝜎2
d+
𝜕𝜅x
𝜕𝜑
2
𝜎2
𝜑
(16)
𝜎
2
𝜅y=
𝜕𝜅y
𝜕𝜖
2
𝜎2
𝜖+
𝜕𝜅y
𝜕
d
2
𝜎2
d+
𝜕𝜅y
𝜕𝜑
2
𝜎2
𝜑
.
Fig. 2 Basic determination workflow of the designed DFOS shape
sensing approach
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341Journal of Civil Structural Health Monitoring (2021) 11:337–350
123
According to the manufactures of the used sensing units
(see Sect.2), the DFOS strain readings can be performed
with a standard deviation between 2 and 10
μ
m/m. The
distance between the fibers is basically determined by the
planning model, which, however, does not exactly rep-
resent the actual installation in most cases due to practi-
cal reasons on-site. To analyze typical variations between
model and realization, the positions of the DFOS cables
at one selected construction site were captured by reflec-
torless total station measurements before the respective
shotcrete layer was applied (see Fig.3a). From the interpo-
lated cable routes along both installation layers, the DFOS
cable spacing can be continuously derived in circumfer-
ential direction. The resulting profile (Fig.3b) along the
lining depicts deviations to the mean value of up to 10cm.
Although the mean value itself is basically in accordance
with the planning model (
dmodel
=17cm), these variations
with a standard deviation of 4.1cm must be considered as
an essential part of the combined curvature’s measurement
uncertainty.
Analogous to the cable spacing, the initial geometry of
the lining is also retrieved from the planning model. Laser
scans may be carried out after the shotcrete lining is con-
structed to investigate the excavation accuracy (Fig.3c). The
orientation angles of the differential sensing segments in cir-
cumferential direction derived from the model and the laser
scan of the observed cross-section are shown in Fig.3d. This
comparison delivers variations with a standard deviation of
3.41
, which should be also incorporated for thorough vari-
ance propagation.
The appropriate combination of all affecting measure-
ment quantities enables a simulation of the achievable stand-
ard deviation of the resulting displacement profiles in both
coordinate directions, see Fig.4. The analysis was done
using different measurement uncertainties for the geodetic
displacement observations as well as different specifica-
tions for the DFOS strain readings according to Sect.2. The
results show that the standard deviation basically increases
from the tunnel crown to the side walls. This seems logical,
since the integration process is less well-controlled at the
-100
-50
0
50
100
φ [°]
0246810 12 14 16
circumference length [m]
σ = 3.41°
φ
scan
model
0246810 12 14 16
circumference length [m]
0.10
0.15
0.20
0.25
0.30
cable spacing [m]
d = 17.3 cm
σ = 4.1 cm
d
scan
model
(a) (c)
(b) (d)
T1
T2
T3
T4
T5
Fig. 3 Stochastic analysis of constructed cross-section: a DFOS
cables along inner and outer shotcrete layers from total station meas-
urements; b derived distance between installed sensing cable layers
in circumferential direction; claser scan of excavation compared to
planning model and positions of geodetic targets (
T1
-
T5
); (d)orient a-
tion of single segments in circumferential direction derived from laser
scan and planning model
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342 Journal of Civil Structural Health Monitoring (2021) 11:337–350
123
outside due to the setup of supporting points (c.f. locations
of total station targets in Fig.3). While the geodetic readings
have major influence, different DFOS instrument capabilities
depict only a small effect on the precision of the resulting
displacement profiles. This is why, strain profiles measured
by Brillouin sensing units, typically with lower measure-
ment precision, but significantly longer sensing range, may
be also appropriate to determine capable displacement dis-
tributions. The standard deviation is similar for both coor-
dinate directions with small deviations in the central area.
It is obvious that particular orientations tend the curvature
value to 0 (e.g., approx. 90
for
𝜅x
). The curvature uncertain-
ties of these positions have significantly lower influence on
the estimation, which, therefore, provides a better result in
x-direction at the tunnel crown area.
4 Field applications andmonitoring results
As part of the European TEN-T Network Corridor, the Sem-
mering Base Tunnel (SBT) is one of the main railway infra-
structure projects currently under construction in Europe.
The original 150-year-old railway track crosses the mountain
ridge with small curvature radii and large height gradients
and, therefore, the train speed is low. The two tunnel tubes,
with a total length of 27.3km each, will be part of a high-
speed rail connection, which will reduce the traveling time
between Austria’s capital Vienna and the second largest city
Graz by about 30% in the future. The optimized track routing
through the tunnel additionally enables significantly better
capabilities for rail goods traffic.
As discussed in [6], the geological conditions along the
tunnel track are challenging and most parts are being exca-
vated by conventional tunneling based on the New Austrian
Tunneling Method (NATM). This requires extended moni-
toring of the tunnel construction itself as well as of criti-
cal infrastructure nearby. DFOS monitoring systems were
installed by IGMS at each construction lot (Fig.5) to assess
the structural integrity of individual construction parts and,
finally, to increase the work safety on-site. These installa-
tions include monitoring of conventional tunnel cross-sec-
tions [19, 28] and shaft linings [13] (SBT 1.1), reinforced
earth structures [20] (SBT 2.1) as well as pipelines [11]
(SBT 3.1).
4.1 Conventional tunnel cross‑sections
The outbreak in conventional tunneling based on the NATM
is performed in different, well-defined sequences, which
enables the rock to support itself. Both instrumented cross-
sections presented in this publication were constructed in
two steps: first, the upper part of the tunnel (so-called top/
heading) was excavated and supported with two shotcrete
Fig. 4 Standard deviation of resulting displacement profiles in lateral
direction (solid) and height (dotted)
Fig. 5 Semmering Base Tunnel: project overview and IGMS monitoring sites (based on [6])
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343Journal of Civil Structural Health Monitoring (2021) 11:337–350
123
layers. DFOS sensing cables were installed in different con-
figurations along the supporting wire meshes of both layers
using cable ties. Their routing was retrieved by reflector-
less total station measurements before the shotcrete was
applied, which guarantees an exact spatial allocation of the
cables within the cross-section for data analysis (Fig.6a). In
practice, this may also be an appropriate solution for future
installations performed by workers on-site without IGMS
support, since these measurements can be carried out by the
surveyor on-site. About 5 days later, the lower part (so-called
bench/invert) was removed and the lining ring was closed.
Wagner etal. (2020) give detailed information on the DFOS
concept and installation inside the tunnel [28].
DFOS monitoring was started immediately after the
installation and was continuously performed over several
weeks, while the further tunnel excavation continued. The
first instrumented cross-section was interrogated by a Ray-
leigh sensing unit, which can provide a spatial resolution
of 3cm over measurement ranges up to 2km (cf. Sect.2).
Figure6b shows the strain profiles along both DFOS cables
about 5 days after the installation. Both layers basically
depict negative strain due to the interacting rock pressure as
well as shrinkage and creepage effects. Differences between
the layers at the tunnel shoulders and the side walls indicate
bending along the lining, which is confirmed by the derived
curvature changes (Fig.6c). The observed behavior seems
logical, since the bench/invert section was excavated and
supported about 5 days after the installation of the top/head-
ing. The entire top/heading section therefore moves down-
wards before the support and the lining is bent due to the
resistance of the bench/invert.
Distributed displacements along the cross-section can be
determined by combining the DFOS curvature profiles with
displacement readings of five geodetic targets using the algo-
rithm presented in Sect.3. The derived displacement profile
in Fig.6d shows good agreement to the pointwise geodetic
observation, even if this behavior is partly implied by the
correlation within the sensing algorithm. The correctness
of the functional as well as the stochastical model is how-
ever additionally confirmed by the estimation’s redundancy,
which delivers an a-posteriori variance factor
̂𝜎 2
of about
1.06.
The DFOS displacement profile can be estimated for each
geodetic measurement epoch (usually once a day) to analyze
typical deviations between the different sensing techniques
over time. Figure7 depicts the coordinate residuals at the
supporting point locations over the first 24 days of continu-
ous monitoring. The first measurement of both sensing tech-
niques exactly at the same time is only available about 12 h
after the initial DFOS measurement, which is why the dis-
played curves are referenced to this second geodetic epoch
after construction. The results present deviations of about
±3mm in x-direction and ±4mm in y-direction. These are
20.03.2017 12:00
(approx. 128 h after installation)
rock
bolts
50.0 mm
(d)
0.005 [1/m]
20.03.2017 12:00
(approx. 128 h after installation)
rock
bolts
(c)
1000 μm/m
20.03.2017 12:00
(approx. 128 h after installation)
rock
bolts
(b)
(a)
inner
layer
outer
layer
Fig. 6 Distributed displacement sensing along instrumented shotcrete
tunnel cross-section: ainstallation overview captured by laser scan;
b measured strain profiles along inner (green) and outer (yellow)
shotcrete layer; c derived curvature values; d displacement curves
derived from DFOS profiles and pointwise geodetic measurements
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344 Journal of Civil Structural Health Monitoring (2021) 11:337–350
123
in accordance with the 3-
𝜎
range of the theoretical analy-
sis (cf. Fig.4) and confirm the capabilities of the designed
approach.
The continuous construction process in conventional tun-
neling requires fixed installation equipment (air ventilation
system, electricity supply, etc.) and heavy tunnel machinery,
which can restrict the field of view to geodetic targets. Con-
sequently, some monitoring points of selected cross-sections
may be partially unavailable for displacement measurements
by total stations. The DFOS-based estimation can be per-
formed using only a selected number of supporting points
(min. 2) to overcome these limitations and to provide dis-
placements along the entire top/heading section.
The estimated displacement profiles of different set-
ups (utilized supporting points, respectively, marked in
red) are shown in Fig.8. Estimations with uniformly dis-
tributed supporting points (Fig.8, top-left and top-right)
depict a very good agreement with residuals smaller than
3mm to the reference profile (dotted red line), which rep-
resent the estimation result with all supporting points (cf.
Fig.6d). Unilateral configurations with three geodetic
points (Fig.8, bottom-left) might be the most common
limitation in tunneling. Even with this non-uniform sup-
porting arrangement, the DFOS approach can deliver
displacement profiles with maximum deviations of about
Fig. 7 Coordinate residuals at supporting point locations over first 24
days of continuous monitoring
20.03.2017 12:00
(approx. 128 h after installation)
rock
bolts
50.0 mm
20.03.2017 12:00
(approx. 128 h after installation)
rock
bolts
50.0 mm
20.03.2017 12:00
(approx. 128 h after installation)
rock
bolts
50.0 mm
20.03.2017 12:00
(approx. 128 h after installation)
rock
bolts
50.0 mm
Fig. 8 Displacement profile estimation with different supporting point setups (used supporting points marked in color) (color figure online)
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
345Journal of Civil Structural Health Monitoring (2021) 11:337–350
123
3.5mm to the geodetic readings. This can be advanta-
geous, especially if one side is blocked by tunnel infra-
structure over longer periods.
Limitations of the DFOS-based estimation become
visible if only two supporting points at one tunnel side
are used (Fig.8, bottom-right). Using this configuration,
uncertainties of the curvature profiles might lead to a pro-
gressive error propagation starting from the supporting
points, which finally result in large deviations at the oppo-
site tunnel side.
Geodetic monitoring in conventional tunneling involves
significant risks for the surveying team on-site. Since the
instrument is often positioned in the middle of the tunnel
axis to obtain an optimal measurement setup, surveyors
must always be attentive not to be overlooked by work-
ers driving heavy tunnel machinery. Tragically, disastrous
working accidents cannot be ruled out completely [1]. For
that reason, every monitoring system which may reduce
the physical human presence inside the tunnel is a real
benefit.
It is obvious that the DFOS approach also requires dis-
placement readings to solve the boundary-value problem
of the double integration. However, if geodetic readings
are not available over longer periods of time, the displace-
ments at the supporting point locations may be estimated
from the recorded DFOS strain values. The approximated
strain–displacement relation can be defined linearly by a
minimum of two arbitrary measurement epochs i and j to
predict the displacement value at k-th epoch
where
𝜖
represents the mean strain at the geodetic target
position. Using more than two measurement epochs might
optimize the prediction, but requires more presence of the
surveyor inside the tunnel.
For concept proofing, the displacements at the instru-
mented cross-section after 175 h were predicted from the
readings about 12 and 36 h after the installation to support
the DFOS-based estimation. The results in Fig.9a demon-
strate that the prediction method can provide displacement
profiles with maximum deviations of about 5mm to the
exact solution. The prediction was subsequently performed
for all DFOS epochs of the continuous monitoring campaign
(Fig.9b) to analyze the method’s long-term suitability. The
supporting points over the first 175 h are derived from the
geodetic readings on the first and second day after construc-
tion (indicated with I in Fig.9b). After this point in time,
the bench/invert of the cross-section was already excavated,
supported as well as refilled, which essentially changes the
deformation behavior. For that reason, the prediction model
is updated with two displacement observations for all fol-
lowing monitoring epochs, see II in Fig.9b. The comparison
between the DFOS-based estimations and the geodetic read-
ings basically depict good agreement for all target positions
with a mean deviation of about 1.2mm in x-direction and
0.9mm in y-direction over the entire monitoring period. The
(17)
𝛥xpredict
k
𝛥ypredict
k
=
𝛥xTS
j
𝛥xTS
i
𝛥yTS
j
𝛥yTS
i
𝜖k
𝜖j𝜖i
,
(a)
50.0 mm
22.03.2017 11:00
(approx. 175 h after installation)
20/03
25/03
30/03
04/04
15/03
-10
0
-20
-30
-40
disp
[mm]
height
40
60
20
0
-20
-40
-60
disp
[mm]
lateral
09/04
14/04
19/04
(b)
III
LeftBot
Top
RightTop
RightBot
Geodetic
LeftTop
Fig. 9 Displacement profile estimation based on supporting point pre-
diction: across-sectional displacement profile; (b) displacement val-
ues at supporting point locations derived from DFOS curvature pro-
files over 35 days of continuous monitoring compared to pointwise
geodetic measurements
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
346 Journal of Civil Structural Health Monitoring (2021) 11:337–350
123
maximum deviation of about 5mm can be observed at the
left-sided targets shortly before the update of the support-
ing points about 175 h after installation. The displacement
profile of this epoch is already displayed in Fig.9a, which,
therefore, represents the estimation with the highest devia-
tions to geodetic observations. Although this displacement
accuracy might be insufficient for high-precise geotechnical
monitoring applications, the resulting information certainly
allows general conclusions on the deformation behavior
with significantly lower presence of the surveyor inside the
tunnel, in this example, 4 instead of 35 daily monitoring
epochs. Moreover, distributed displacement profiles can also
be determined for DFOS epochs without simultaneous geo-
detic measurements, which further extends the capabilities
of the DFOS-based approach.
Even if the used Rayleigh sensing unit provides strain
profiles with high spatial resolution, the sensing range is
restricted with a maximum of 2km. Especially in tun-
neling applications, sensing over longer distances can be
advantageous to monitor numerous cross-sections using
only one interrogation unit, which is placed at a protected
place, preferably outside the tunnel. Brillouin sensing sys-
tems usually enable measurements over tens of kilometers,
but are limited in the spatial resolution and the measure-
ment precision. To evaluate potential effects on the cross-
sectional strain and subsequently derived displacement
profiles, the DFOS system was installed within another
cross-section at the same construction lot and continuous
measurements were performed using a BOFDA sensing
unit (see Sect.2). Further information on the installation
as well as strain and temperature monitoring results may
be found in [3].
The derived displacement curve of one selected epoch
about 82 h after the installation is shown in Fig.10a. The
estimation was supported by seven geodetic targets along
the cross-section. Their displacements are in accordance
with the shape derived from the BOFDA measurements,
whose estimation redundancy delivers an a-posteriori vari-
ance factor
̂𝜎 2
of about 1.33. This also confirms the cor-
rectness of the statistical model at a significance level of
95%.
Analogous to Fig.7, the DFOS displacement profiles
can be estimated at each geodetic measurement epoch to
analyze the typical variations between the different sens-
ing approaches. The coordinate residuals at all seven geo-
detic target positions over the first 15 days of continuous
monitoring are displayed in Fig.10b. The different curves
were referenced to the second geodetic epoch about 10
h after installation, where simultaneous results of both
technologies are available for the first time. The derived
deviations are within a range of about ±5mm in both
coordinate directions, except for the right-bottom target,
and comparable to Rayleigh sensing results. The higher
number of supporting points could, however, also be used
for data snooping to detect and eliminate potential outli-
ers in the curvature profiles or the supporting point dis-
placements. This would potentially further optimize the
estimation result.
08/02
11/02
14/02
17/02
05/02
4
8
0
-4
-8
Δ
[mm]
height
4
8
0
-4
-8
Δ
[mm]
lateral
08.02.2018 07:00
(approx. 82.5 h after installation)
50.0 mm
(a)
LeftBot
LeftMid
LeftTop
Top
RightTop
RightMid
RightBot
(b)
Fig. 10 Displacement profile estimation from BOFDA measurements: across-sectional displacement profile; bcoordinate residuals at support-
ing point locations over first 15 days of continuous monitoring
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
347Journal of Civil Structural Health Monitoring (2021) 11:337–350
123
4.2 Tunnel shaft linings
Intermediate headings with shaft constructions are widely
used in modern conventional tunneling to shorten construc-
tion times. The SBT project includes shafts at three con-
struction lots with depths of up to 400m [6]. As shown
in Fig.11a, the Göstritz intermediate access as part of the
SBT 1.1 (Fig.5) requires a complex construction system.
Two horizontal access tunnels with a total length of more
than 1km each were built with a massive cavern at the end,
which subsequently enabled the construction of two vertical
shafts with a total depth of approximately 240m to reach the
planned altitude of the future railway line.
Exploration drillings revealed very challenging geologi-
cal conditions for the shaft constructions, which is why an
extended monitoring program was set up to detect any degra-
dation of the structural stability of the linings. Conventional
geodetic measurements of shaft walls using total stations are
very difficult because of very steep, almost vertical sightings
as well as water intrusion at the shaft floor. Furthermore, the
shaft construction must always be completely paused during
the time-consuming measurements, which, therefore, delay
the construction process as a whole. After completion of the
shaft construction itself and the installation of corresponding
infrastructure, geodetic measurements additionally become
almost impossible due to the limited field of view.
To overcome these limitations, DFOS cables were
embedded into five selected shaft cross-sections based on
the geological conditions to measure distributed strain and
temperature profiles in circumferential direction of the shot-
crete linings. An instrumentation along both shotcrete layers
also enables an assessment of potential curvature changes.
The cable routing was recorded by total station measure-
ments before shotcreting to ensure the exact position along
the lining. These measurements as well as the installation
itself were challenging due to small working space inside the
shaft as well as permanent water intrusion (Fig.11b). The
sensing cables of all cross-sections were guided from water-
proof connection boxes at the cross-section locations to an
instrument box at the shaft head, from where measurements
can be carried out without any interference of the regular
construction.
Contrary to partial excavations in conventional tunneling,
the continuous construction of tunnel shaft linings enables
an installation along the entire cross-section at the same
time and can provide a closed ring system along both sens-
ing layers. Based on this configuration, the boundary-value
problem of the DFOS-based estimation can be solved by
assuming that the displacement value and its gradient at
the starting point must be equivalent to the last integration
position by extending the functional model with constraints
instead of pointwise displacement readings (cf. Eq.10
to14). These constraints can be realized in various ways,
e.g., by pseudo-observations:
The extension allows an estimation of relative displacement
profiles along the shaft lining free of external observations.
(18)
𝜅x
1
𝜅x
n
0
0
=
1
h2
2
h2
1
h20⋯⋯⋯ 0
⋮ ⋱⋱⋱⋱⋱⋱
0⋯⋯⋯⋯ 1
h2
2
h2
1
h2
10⋯⋯⋯⋯ 01
21 0 ⋯⋯⋯ 01
𝛥x0
𝛥x1
𝛥xn+1
(19)
𝜅y
1
𝜅y
n
0
0
=
1
h2
2
h2
1
h20⋯⋯⋯ 0
⋮ ⋱⋱⋱⋱⋱⋱
0⋯⋯⋯⋯ 1
h2
2
h2
1
h2
10⋯⋯⋯⋯ 01
21 0 ⋯⋯⋯ 01
𝛥y0
𝛥y1
𝛥yn+1
.
(a) (b)
access tunnel
shaft02 shaft01
s02-185m
s02-204m
s02-228m
s01-177m
s01-229m
instrument
box
Fig. 11 Tunnel shaft lining monitoring at Göstritz access point: aschematic representation of construction site (based on [7]) and DFOS moni-
toring setup; bvertical view down the shaft during sensor installation [13]
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
348 Journal of Civil Structural Health Monitoring (2021) 11:337–350
123
Although rigid-body motions of the linings remain
unknown, this procedure can be very valuable to obtain the
shaft’s deformation behavior without any interruption of the
shaft construction.
The initial measurement of the instrumented shaft lin-
ings was taken immediately after the installation using the
BOFDA interrogator. Up to now, follow-up monitoring has
been conducted epoch-wise on request of the geotechnical
engineer on-site. Relative displacement profiles may be
derived for each monitoring epoch based on the DFOS strain
measurements, which are detailedly introduced in [13].
The evaluated profiles of two monitoring epochs along
one selected cross-section (shaft02, 228m) are shown in
Fig.12a. These present only small displacements within a
range of about ±13mm for both epochs, but allow conclu-
sions on the cross-sectional deformation behavior and its
progress: the first measurement already displays a squeezed
shape orientated to the right-bottom side with small mag-
nitude, which has significantly further developed about 171
days after installation.
To verify the DFOS-based approach (blue), the deforma-
tion shape can also be approximated by an ellipse based on
the geodetic displacements measured by total station (red).
The individual profiles of both technologies must however
be reduced by their respective mean value, since the DFOS
approach depicts only relative deformation within the lining.
The orientation of the geodetic ellipse represents the defor-
mation process well, which confirms the above concluded
assumption, although the estimation shows lower deforma-
tion magnitudes. Numerical deviations between the sensing
techniques at the geodetic target positions are within a range
of some millimeters (Fig.12b) and, therefore, only slightly
lower than the total deformation amount. The differences
generally increase over time, which might be related to the
increasing sighting steepness due to the further shaft sinking
process. This usually results in higher measurement uncer-
tainties for geodetic monitoring. The DFOS-based approach
can therefore be a valuable substitute to capture the relative
deformations profiles along shaft linings without physical
access of the surveyor or delays of the construction process.
5 Conclusion
This paper introduced an innovative shape sensing and
evaluation approach, which enables fully distributed shape
assessment along curved structures, such as tunnel cross-
sections, based on distributed fiber optic sensing combined
with geodetic displacement readings. The designed system
was installed inside conventional tunnel cross-sections as
well as shaft linings and interrogated by different DFOS
sensing units based on Rayleigh and Brillouin scattering.
The special setup of the used sensing cable in combination
with appropriate installation techniques can enable success-
ful installations with survival rates of more than 95%, even
in the harsh tunnel environment.
The sensing concept is related to the double integration
of distributed curvature values derived from DFOS strain
measurements along two layers in well-known arrange-
ment along the structure. Stochastic analysis could show
10/18
11/18
01/19
02/19
12/18
03/19
09/18
5
10
0
-5
-10
Δ
[mm]
height
5
10
0
-5
-10
Δ
[mm]
lateral
P01
P02
P03
P04
(b)
18.02.2019 14:08
(171 days after installation)
20.0 mm
P01
P02
P03
P04
24.10.2018 17:48
(64 days after installation)
20.0 mm
P01
P02
P03
P04
(a)
Fig. 12 Shaft monitoring results (s02-228m): a cross-sectional displacement profiles estimated from fiber optic sensing (bright/blue) and dis-
placement ellipse derived from geodetic measurements (dark/red); bcoordinate residuals at geodetic target locations (color figure online)
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
349Journal of Civil Structural Health Monitoring (2021) 11:337–350
123
that the curvature’s measurement uncertainty is not only
related to the DFOS strain measurements, but also to the
distance between the fibers and the accuracy of the geo-
technical planing model. The standard deviation of the
estimated displacement profile also strongly depends on
the geodetic measurement precision.
Evaluations of continuous monitoring of tunnel cross-
sections demonstrate that the distributed displacement
shape can be assessed without any gaps along the entire
top/heading section. The results depict maximum devia-
tions of about 4mm to displacements measured by total
stations at the geodetic target position, which confirms the
stochastic analysis. Additionally, the deformation behavior
might be also captured by laser scanning in future appli-
cations to independently verify the derived displacement
shape. The DFOS-based approach is also capable to pre-
dict supporting information based on the measured strain
values. It could be shown that the displacement profiles
can be determined with mean deviations of about 1mm
in both coordinate directions at the supporting point loca-
tions by using only 4 instead of 35 geodetic measurement
epochs. This significantly reduces the surveyor’s physical
presence inside the tunnel.
The closed ring system along tunnel shaft linings ena-
bles an estimation of relative displacement profiles, even
without external observations. The resulting shape allows
conclusions on the deformation behavior of the instru-
mented shaft cross-section, whose deformation progress
and orientation could be verified by evaluations of an
ellipse estimated from pointwise geodetic displacements.
The deformation behavior along the instrumented cross-
sections is mostly homogeneous and the deformation’s
magnitude is small, especially at the tunnel shafts. Long-
term monitoring of the installations is currently being per-
formed to detect and quantify potential structural integrity
anomalies. The outcomes will further prove the suitability
of the designed shape sensing approach and will also give
information on the long-term behavior of the DFOS tunnel
installations.
Acknowledgements The authors thank the Austrian Federal Railways
(ÖBB-INFRA), namely Johannes Fleckl-Ernst, Michaela Haberler-
Weber, Frank Klais, Tobias Schachinger, Petra Wolf, as well as Ger-
hard Gobiet (SBT project leader) for the opportunity to realize various
DFOS monitoring applications at the Semmering Base Tunnel pro-
ject. We also would like to acknowledge all other project partners,
especially the geotechnical surveying team ARGE GTM SBT1.1 (VSP
Stolitzka & Partner Ziviltechniker GmbH and DI Dr. Karl Strobl) and
the Institute of Rock Mechanics and Tunnelling of Graz University of
Technology (Michael Henzinger, Alexander Kluckner, Wulf Schubert,
Lukas Wagner). Last, but not least, special thanks to the IGMS team
members (Peter Bauer, Fabian Buchmayer, Dietmar Denkmaier, Slaven
Kalenjuk, and Madeleine Winkler) for their valuable efforts during the
sensor installations.
Funding Open Access funding provided by Graz University of
Technology.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of
interest.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article’s Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.
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... This would finally lead to additional interpretation possibilities for the geotechnical engineer on-site. A corresponding evaluation concept for distributed shape sensing based on DFOS strain sensing supported by point-wise geodetic measurements is presented in [11]. ...
... Contrary to the point-wise VWS measurements, DFOS is capable to provide distributed curvature values along the large parts of the lining. These distributed curvature profiles can be finally used to derive the full cross-sectional shape profile analogous to sensing in longitudinal direction [11]. The authors emphasizes that the monitoring results presented in this paper are limited to the shotcrete lining. ...
Article
Full-text available
Structural integrity assessment is essential in modern tunneling to ensure safe construction works. State-of-the-art monitoring approaches like displacement readings of geodetic prisms are often limited in the spatial as well as the temporal measurement resolution, which is why potential safety hazards might be overlooked. This paper introduces a large-scale distributed fiber optic sensing (DFOS) network inside the tunnel lining of a highway tunnel currently under construction in Austria. The tunnel construction site faces challenging geological conditions with loose rock excavation near to the surface with minimal covering. Fiber optic sensing cables were installed along both tunnel tubes to autonomously monitor 13 cross-sections of the primary shotcrete lining, about 220 m of the tunnel in longitudinal direction and 10 cross-sections of the secondary inner lining. Measurements are continuously evaluated and autonomously transferred to the geotechnical engineer on-site for further analysis. While the construction works are ongoing, alerts are additionally sent out automatically, if pre-defined thresholds are exceeded. The paper outcomes demonstrate that the innovative DFOS system immediately responds to structural modifications and, indeed, increases safety at the construction site.
... This can be especially advantageous if more than two supporting points can be utilized to provide redundancy and handling of potential erroneous data in practical applications on-site. Using DFOS in combination with pointwise displacement readings, e.g., from total station measurements, can provide an integrated approach, which allows fully distributed shape assessment along the civil engineering structures [52]. ...
... For closed ring system, it can be assumed that the displacement value and its gradient at the starting point is equivalent to the last integration position by extending the functional model with constraints instead of pointwise displacement readings. More detailed information on the functional model and the updating process is given in [52]. It is obvious that displacements in the plane of the x-axis are minimal for measurements at the test rig due to the loading orientation (cf. Figure 17b). ...
Article
Full-text available
Civil structural health monitoring (CSHM) has become significantly more important within the last decades due to rapidly growing construction volume worldwide as well as aging infrastructure and longer service lifetimes of the structures. The utilization of distributed fiber optic sensing (DFOS) allows the assessment of strain and temperature distributions continuously along the installed sensing fiber and is widely used for testing of concrete structures to detect and quantify local deficiencies like cracks. Relations to the curvature and bending behavior are however mostly excluded. This paper presents a comprehensive study of different approaches for distributed fiber optic shape sensing of concrete structures. Different DFOS sensors and installation techniques were tested within load tests of concrete beams as well as real-scale tunnel lining segments, where the installations were interrogated using fully-distributed sensing units as well as by fiber Bragg grating interrogators. The results point out significant deviations between the capabilities of the different sensing systems, but demonstrate that DFOS can enable highly reliable shape sensing of concrete structures, if the system is appropriately designed depending on the CSHM application.
... [16][17][18] In addition to applications in civil engineering, this technique is gaining traction in other disciplines such as geotechnics, 19 hydrology, 20 and tunneling. 21 Unlike Discussion on this paper must be submitted within two months of the print publication. The discussion will then be published in print, along with the authors' closure, if any, approximately nine months after the print publication. ...
Article
Full-text available
Strain and temperature measurements on reinforced concrete structures with Rayleigh-based fiber optic sensors (FOS) promise dense data networks of crucial structural parameters. In this context, the measurement of temperature and mechanical strain is invariably intertwined, precipitating in a frequency shift recorded via FOS. Consecutive experiments were carried out on reinforced concrete beams under mechanical, thermal, and thermo-mechanical loading. Basic analysis of fiber optics equations indicates the sensitivities toward both influences. These are quantified and juxtaposed in experiments, first separately and subsequently combined. As concerns temperature measurement, the slightest tensile forces exerted onto the FOS may engender distortions of several degrees Celsius. Conversely, strain measurements are affected by temperature changes to a lesser degree. Nevertheless, the level of strain to be sensed and the severity of corrupting temperature shifts must be carefully weighted. The article raises awareness for the coupling of temperature and strain and enables the practitioner to identify and assess perturbations.
... Fiber optic cables with rugged jackets and reinforcing fibers were adopted for high mechanical protection [59,60], as depicted in Fig. 2. Fibers are glued within metal tubes or multiple fibers are twisted and combined to provide a high mechanical strength that is essential in many geotechnical applications such as tunnelling [61,62]. The strain transfer analysis in this family of fiber optic cables is not the focus of this review because it is important to consider potential slippage between the jacket and fibers as well as the interaction between different fibers in the fiber optic cable in case of twisted fibers. ...
Article
Strain transfer phenomenon in distributed fiber optic sensors (DFOS) has shown significant effects on sensor survival and measurement of strain distributions as well as detection and quantification of cracks in health monitoring and condition assessment of civil infrastructure. This review aims to establish a holistic understanding on the strain transfer effect for measurement using DFOS. The reviewed contents cover the fundamental mechanisms, influencing factors, practical solutions, and applications of strain transfer models. Both forward and inverse strain transfer analysis of DFOS are elaborated. Challenges and opportunities of strain transfer analysis for DFOS are discussed. This review shows that the forward and inverse strain transfer analysis are capable of accurately determining the strain distributions and cracks in host structures subjected to arbitrary strain fields. The clarification of the strain transfer effect will facilitate the applications of fiber optic sensors in civil infrastructure.
... Sometimes, they are embedded in shield tunnel linings beforehand to perform stress measurements (Huang et al., 2014;Bursi et al., 2016;Lai et al., 2016;Cheng et al., 2017). DFOS strain measurements can also be combined with geodetic displacement readings to obtain better measuring reliability (Monsberger and Lienhart, 2021). Working as a fiber optic nervous sensing (FONS) system of the tunnel structure, DFOS allows a comprehensive understanding of the overall deformation characteristics of curved tunnels both in construction and operation. ...
Article
With the accelerated urbanization and population growth in the Yangtze River Delta of China, numerous metro tunnels are in operation or under construction, and adjacent deep excavation activities are frequently encountered. For curved shield tunnels, the impact of excavation-induced ground movements is more complicated due to their asymmetric shape. This paper presents the monitoring results of a curved shield tunnel in clayey soil in Suzhou, China, which were captured by a fiber optic nervous sensing system. This system utilized Brillouin optical frequency domain analysis technology to monitor the distribution of longitudinal and circumferential strains of tunnel linings induced by adjacent excavation. The results show that the tunnel linings were mainly subjected to bending deformations along the tunnel alignment. Maximum compressive strains were observed below the tunnel springline, and their absolute values were higher than those of maximum tensile strains measured at the tunnel crown, distorting the circular tunnel into a rotated oval/ellipsoid. Based on the monitoring results, two kinematic models reflecting the spatial relationship between movements of tunnel segments and measurements of strain sensing cables are proposed. Furthermore, a mechanical method is proposed to convert strain measurements into radial displacements of tunnel linings and the structural health condition of the tunnel is evaluated using longitudinal and circumferential risk indexes. The conclusions drawn in this study provide improved insight into the deformation pattern and health condition of curved shield tunnels subjected to adjacent excavations.
... The study and application of DOFS has not been limited to laboratory experiments and several cases exist where this sensing technology has been successfully deployed and applied on-site for the monitoring of real structures. Some examples of real-world applications are the Sarajevo bridge [28], the precast tunnel lining of the L9 subway line in Barcelona [29], the shotcrete lining in the Sammering Base tunnel [30] or the monitoring of driven ductile piles [31], to mention just a few. Despite the successful implementation of DOFS systems in real structures, the existing applications are often either limited to the measurement of strains or unable to be validated against complementary systems or carried out in a discontinuous way and/or for a short period of time. ...
Article
Full-text available
This paper explores the performance of distributed optical fiber sensors based on Rayleigh backscattering for the monitoring of strains in reinforced concrete elements subjected to different types of long-term external loading. In particular, the reliability and accuracy of robust fiber optic cables with an inner steel tube and an external protective polymeric cladding were investigated through a series of laboratory experiments involving large-scale reinforced concrete beams subjected to either sustained deflection or cyclic loading for 96 days. The unmatched spatial resolution of the strain measurements provided by the sensors allows for a level of detail that leads to new insights in the understanding of the structural behavior of reinforced concrete specimens. Moreover , the accuracy and stability of the sensors enabled the monitoring of subtle strain variations, both in the short-term due to changes of the external load and in the long-term due to time-dependent effects such as creep. Moreover, a comparison with Digital Image Correlation measurements revealed that the strain measurements and the calculation of deflection and crack widths derived thereof remain accurate over time. Therefore, the study concluded that this type of fiber optic has great potential to be used in real long-term monitoring applications in reinforced concrete structures .
... By means of the distributed fibre optic technique, it is possible to detect local damages [22], such as leaks or unsealing [23], that are usually related to changes in temperature. Another aspect is the mechanical loading caused by the ground movements. ...
Article
Full-text available
Due to the low costs of distributed optical fibre sensors (DFOS) and the possibility of their direct integration within layered composite members, DFOS technology has considerable potential in structural health monitoring of linear underground infrastructures. Often, it is challenging to truly simulate the actual ground conditions at all construction stages. Thus, reliable measurements are required to adjust the model and verify theoretical calculations. The article presents a new approach to monitor displacements and strains in Glass Fiber Reinforced Polymer (GFRP) collectors and pipelines using DFOS. The research verifies the effectiveness of the proposed monitoring solution for health monitoring of composite pipelines. Optical fibres were installed over the circumference of a composite tubular pipe, both on the internal and external surfaces, while loaded externally. Analysis of strain profiles allowed for calculating the actual displacements (shape) of the pipe within its cross-section plane using the Trapezoidal method. The accuracy of proposed approach was positively verified both with reference spot displacement transducer as well as numerical simulations using finite element method (FEM). DFOS could obtain a comprehensive view of structural deformations, including both strains and displacements under externally applied load. The knowledge gained during research will be ultimately used for renovating existing collectors.
Article
Full-text available
The increasing demand for civil infrastructures, the aging of existing assets, and the strengthening of safety and liability laws have led to the inclusion of structural health monitoring (SHM) techniques into the structural management process. With the latest developments in the sensors field and computational power, real-scale SHM systems’ deployment has become logistically and economically feasible. However, it is still challenging to perform a quantitative evaluation of the structural condition based on measured data. The paper addresses recent efforts to associate measured observations with an identification of local stiffness reduction as a global parameter for damage onset and growth. It proposes a hybrid methodology for model updating and damage identification. The proposed methodology is built on data feature extraction using the principal component analysis (PCA), finite element (FE) simulation, and Monte Carlo simulation to quantify the extent of local damage of a 60-year-old prestressed concrete bridge. The methodology allows a sensor-specific quantification of the local stiffness reduction and makes it possible to focus succeeding bridge inspection, recalculation, and repair works on these areas. Even more, the monitoring in combination with the FE model and proposed methodology provides continuous information on developing stiffness reduction and the acuteness of rehabilitation measures.
Article
Tunnel deformation monitoring is an important process for ensuring the safety of the tunnel structure. This study presents a method for sensing tunnel cross-section deformation based on distributed fiber optic sensing and a neural network. To verify the feasibility of the proposed method, a laboratory test is conducted on a tunnel model made of polymethyl methacrylate. The test results confirm that the proposed method can accurately obtain the deformation of the tunnel structure, with a maximum deviation of 5.7%. Based on this result, a field application trial is conducted in a section of an actual subway tunnel to further verify the effectiveness of the method. The field observation result confirms the feasibility of applying the proposed method to tunnel cross-section deformation monitoring.
Article
Cross-sectional deformation is a crucial index for evaluating the stability and safety of shield tunnels. Monitoring the cross-sectional deformation has been an important content in assessing the health condition of tunnels. For this reason, an optical-electrical co-sensing tape (OECST) by embedding distributed optical fiber sensor (DOFS) and coaxial cable F-P interferometer sensors (CCFPI) into a high elastic polyurethane tape has been proposed in this paper. It can simultaneously measure the low-strain area of structure with high accuracy by DOFS and large-strain area with relatively lower accuracy by CCFPI. The conceptual and geometric design, fabrication method, and sensing mechanism have been discussed. A laboratory test has also been conducted to validate that the OECST can provide reliable strain measurement. Subsequently, an algorithm has been proposed to measure the cross-sectional deformation of shield tunnels by using OECST strain measurements. The cross-sectional deformation has discontinuous features with large deformation of joints (dislocation, opening, closing, etc.) and small deformation of segments. The former can be obtained by performing geometrical analysis on rough strain measurements of the CCFPI in OECST. The latter can be derived by implementing the strain–displacement conversion on the accurate distributed strain measured by the DOFS-S in OECST. The method has been investigated experimentally. The findings from the study have demonstrated that the proposed OECST can measure the cross-sectional deformation of shield tunnels with a satisfactory accuracy, which can be used to assess the health condition of tunnels.
Article
Full-text available
Efficient and economic foundations are essential to ensure the long-term integrity of structures. Driven ductile piles offer a safe and quick solution for foundations, which can be individually customized to changing soil conditions. Geotechnical load tests on a small subset of piles can be performed at large construction sites to examine the bearing capacity for optimization purposes. Arising deformations during these statical tests are usually measured using electrical sensors at the top, which, however, do not deliver information about the stress distribution along the pile. This paper presents a fiber optic monitoring approach, which provides distributed strain profiles with a spatial resolution of up to 10 mm along driven ductile piles. The high measurement resolution of about \(1~{\mu}m/m\) enables the detection of local effects in the load transfer from the pile to the surrounding grout and soil. The critical sensor installation on-site as well as results of various field applications with pile lengths of up to 25 m are presented. Verification measurements at the pile’s head and internal measurements of strain gauges prove the suitability of the developed monitoring approach and demonstrate the high potential of distributed fiber optic sensing for applications in soil mechanics.
Article
Full-text available
This paper introduces the successful implementation of a fibre-optic sensing system for direct and distributed strain measurement within the shotcrete lining of a conventional tunnel drive. The shotcrete lining of the top-heading and the invert are equipped with two layers (rock side and cavity side) of fibre-optic sensing cables installed in circumferential and longitudinal direction. All cables are measured autonomously for several weeks to capture the strain evolution inside the lining from the day of the construction to a posterior, well-hardened state. An additional follow-up measurement is conducted 2 months afterwards. The measurements enable an assessment of the strain distribution inside the lining with a spatial resolution in the range of some centimetres and a measurement resolution of up to 1 µm/m. Besides the conventional monitoring targets used for displacement recordings, measurement equipment like strain gages and pressure cells are also installed in the cross-section under investigation. Back-calculated strain from absolute displacements and the readings from the strain gages show good agreement with the results of the conducted fibre-optic measurements and verify the suitability of the used system.
Conference Paper
In modern tunneling, the assessment of the structural integrity is usually based on displacement measurements of the tunnel lining using total stations or on readings of electrical strain gages embedded inside the liner. However, these technologies only deliver information on particular points rather than a complete picture of the linings’ deformation. This paper presents a fiber optic monitoring concept, which enables distributed strain measurements with a spatial resolution in the range of some centimeters. The system was installed in two layers of a shot-crete liner at a conventionally driven tunnel and continuous monitoring was performed over several weeks. The recorded strain profiles were used to determine the distributed curvature changes along the lining. Comparisons to evaluations of conventional geodetic measurements prove the derived results and show that DFOS systems not only allows capturing large-scale de-formations of the liner but also identifying local anomalies.
Article
en In modern tunnelling, deformation monitoring is an important component to ensure a safe construction. It is state of the art to measure displacements at the inner side of the tunnel lining using total stations. In addition, pointwise geotechnical sensors, e.g. electric strain gauges, may be installed in geological fault zones, which, however, do not deliver a complete picture of the internal deformations. The Institute of Engineering Geodesy and Measurement Systems (Graz University of Technology) supported by the Austrian Federal Railways (ÖBB‐Infrastruktur AG, SAE Fachbereich Bautechnik/Tunnelbau) developed a fibre optic sensing system, which realizes thousands of measurement points inside the tunnel lining. The distributed measurements can be used to assess the in‐situ strain behaviour as well as to localize failures (e.g. cracks) in the lining. This paper reports about the calibration of the fibre optic system under well‐known laboratory conditions and the practical utilization of the system in mechanized and conventional tunnelling. The results demonstrate the high potential of distributed fibre optic systems and their capability especially in the operational phase to extend classical measurement methods in tunnelling projects. Abstract de Im Zuge der Errichtung von Tunnelbauwerken erweisen sich zuverlässige Überwachungsmessungen als essentieller Bestandteil, um sichere Vortriebsarbeiten garantieren zu können. Die Erfassung von Verschiebungen entlang der Innenseite der Tunnelschale erfolgt standardmäßig mit Totalstationen. Zusätzlich werden in geologischen Störzonen spezielle Messquerschnitte mit geotechnischen Sensoren wie z.B. elektrische Dehnungsgeber hergestellt. Jedoch liefern derartige Sensorsysteme lediglich punktuelle Messwerte, wodurch kein vollständiges Bild der internen Auslastung der Tunnelschale entsteht. Zur In‐situ‐Deformationsanalyse im Tunnelbau wurde vom Institut für Ingenieurgeodäsie und Messsysteme der TU Graz unterstützt von der ÖBB‐Infrastruktur AG (SAE Fachbereich Bautechnik/Tunnelbau) ein verteiltes faseroptisches Messsystem entwickelt. Aus den Dehnungsmessungen resultieren tausende Messstellen entlang einer einzelnen Messfaser im Inneren der Tunnelschale, die eine flächenhafte Beurteilung der Auslastung ermöglichen. Darüber hinaus können Überbeanspruchungen der Tunnelschale (z.B. Risse) detektiert werden. Dieser Beitrag erläutert die Kalibrierung des faseroptischen Gesamtsystems unter Laborbedingungen sowie die praktische Anwendung unter realen Umgebungsbedingungen im maschinellen und konventionellen Vortrieb. Anhand der Resultate zeigt sich, dass verteilte faseroptische Messsysteme großes Potenzial für Überwachungsmessungen in Tunnelprojekten insbesondere auch in der Betriebsphase bieten und wertvolle Informationen in Kombination mit klassischen Methoden der Ingenieurgeodäsie abgeleitet werden können.
Book
Das Buch enthält eine umfassende Einführung in die traditionell als Festigkeitslehre bezeichnete Fachdisziplin Technische Mechanik deformierbarer fester Körper. Nach wesentlichen mathematischen Grundlagen dieses Fachgebietes werden die folgenden Teilgebiete behandelt: - Grundlagen der Elastizitätstheorie - Prinzipien der virtuellen Arbeiten - Energieprinzipien - Lineare Stabtheorie - Lineare Theorie ebener Flächentragwerke - Stabilitätsprobleme - Anstrengungshypothesen - Anelastisches Werkstoffverhalten - Fließgelenktheorie I. Ordnung für Stäbe - Grundlagen der Plastizitätstheorie einschließlich der Traglastsätze - Grundlagen der Bruchmechanik - Näherungsmethoden einschließlich der Methode der finiten Elemente und der Randelementemethode - Experimentelle Methoden Anhand zahlreicher vollständig ausgearbeiteter Beispiele wird die Leistungsfähigkeit analytischer, numerischer und experimenteller Methoden der Festigkeitslehre zur Lösung technischer Aufgaben demonstriert. Neu an der 5. Auflage ist das mit Grundlagen der Bruchmechanik betitelte Kapitel. Die Zielgruppen Studierende des Bauingenieurwesens, des Maschinenbaus und der Mechatronik an Technischen Universitäten sowie in der Praxis tätige Ingenieure Die Autoren Herbert A. Mang Institut für Mechanik der Werkstoffe und Strukturen, Technische Universität Wien Günter Hofstetter Institut für Grundlagen der Technischen Wissenschaften, Arbeitsbereich für Festigkeitslehre und Baustatik, Universität Innsbruck
Article
en With the start of construction on the third tunnelling contract, work has now started on all sections of the Semmering Base Tunnel. The Tunnel Grautschenhof contract is a challenging construction project with numerous unusual problems. Complex ground conditions make extensive grouting necessary. Numerous constraints above‐ground, like high‐pressure gas pipelines require monitoring and protection measures. Since the start in May 2016, two shafts, the first of three caverns and the first metres of the running tunnels have already been driven. In parallel to this, the works above ground to create two site facilities areas are now largely complete. Abstract de Mit dem dritten in Bau gegangenen Tunnelbaulos haben die Arbeiten in allen Abschnitten des Semmering‐Basistunnels begonnen. Der Tunnel Grautschenhof ist ein herausforderndes Bauprojekt mit einer Vielzahl nicht alltäglicher Problemstellungen. Komplexe Baugrundverhältnisse machen umfangreiche Injektionsmaßnahmen erforderlich. Zahlreiche übertägige Zwangspunkte wie Hochdruckgasleitungen erfordern Monitoring‐ und Schutzmaßnahmen. Seit Baubeginn im Mai 2016 wurden zwei Schächte, die erste von drei Kavernen sowie bereits die ersten Vortriebsmeter der Streckenröhren aufgefahren. Parallel dazu konnten die übertägigen Arbeiten auf zwei Baustelleneinrichtungsflächen weitgehend fertiggestellt werden.
Article
en The Semmering Base Tunnel (SBT) is about 27.3 km long and is being driven from the portal at Gloggnitz and from three intermediate construction accesses in Göstritz, Fröschnitzgraben and Grautschenhof. The main components of the tunnel system are the two single‐track running tunnels, cross passages at a maximum spacing of 500 m and an emergency station in the middle tunnel section, with two shafts about 400 m deep for ventilation and extraction in case of an incident. For organisational, scheduling and topographical reasons, the tunnel is divided into three construction contracts. The eastern contract section SBT1.1 ”Tunnel Gloggnitz“ has been under construction since mid 2015. Construction started on contract section SBT2.1 ”Tunnel Fröschnitzgraben“ at the start of 2014. The western contract section SBT3.1 ”Tunnel Grautschenhof“ has been under construction since May 2016. Abstract de Der ca. 27,3 km lange Semmering‐Basistunnel (SBT) wird vom Portal Gloggnitz aus und über die drei Zwischenangriffe Göstritz, Fröschnitzgraben und Grautschenhof aufgefahren. Die Hauptbestandteile des gesamten Tunnelsystems sind zwei eingleisige Tunnelröhren, Querschläge mit einem Abstand von maximal 500 m und eine Nothaltestelle im mittleren Tunnelabschnitt mit zwei ca. 400 m tiefen Schächten zur Be‐ und Entlüftung im Ereignisfall. Aus organisatorischen, terminlichen und topographischen Gründen ist der Tunnel in drei Baulose unterteilt. Das östlichste Baulos SBT1.1 „Tunnel Gloggnitz” befindet sich seit Mitte 2015 in Bau. Der Baubeginn im mittleren Baulos SBT2.1 „Tunnel Fröschnitzgraben” erfolgte Anfang 2014. Das westlichste Baulos SBT3.1 „Tunnel Grautschenhof” wird seit Mai 2016 gebaut.
Book
This book explains physical principles, unique benefits, broad categories, implementation aspects, and performance criteria of distributed optical fiber sensors (DOFS). For each kind of sensor, the book highlights industrial applications, which range from oil and gas production to power line monitoring, plant and process engineering, environmental monitoring, industrial fire and leakage detection, and so on. The text also includes a discussion of such key areas as backscattering, launched power limitations, and receiver sensitivity, as well as a concise historical account of the field’s development. Note from the author: I have had several requests for a full-text version: however, the copyright belongs to the publisher so I cannot provide this. However, the book may be obtained from https://www.crcpress.com/An-Introduction-to-Distributed-Optical-Fibre-Sensors/Hartog/p/book/9781138082694 and from a number of bookshops.