GLÖTZL Gesellschaft für Baumeßtechnik mbH Forlenweg 11 D ‐ 76287 Rheinstetten Tel. + 49(0)721 5166 0 Fax + 49(0)721 5166 30 [email protected]www.gloetzl.com Geotechnical Monitoring of Construction Activities by Distributed Fiber Optic Sensors Embedded in Geotextiles European Workshop on Structural Health Monitoring/ 5th Edition June 28 ‐ July 2, 2010/ Sorrento, Italy Authors: Joachim Schneider‐Gloetzl, Gloetzl GmbH, Rheinstetten, Germany Rainer Gloetzl, RG Research, Ettlingen, Germany Katerina Krebber, Sascha Liehr, Mario Wendt, Aleksander Wosniok, BAM, Federal Institute for Materials Research and Testing, Berlin, Germany
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GLÖTZL Gesellschaft für Baumeßtechnik mbH Forlenweg 11 D ‐ 76287 Rheinstetten Tel. + 49(0)721 5166 0 Fax + 49(0)721 5166 30 [email protected] www.gloetzl.com
Geotechnical Monitoring of Construction Activities by Distributed Fiber Optic Sensors Embedded in Geotextiles European Workshop on Structural Health Monitoring/ 5th Edition
June 28 ‐ July 2, 2010/ Sorrento, Italy
Authors:
Joachim Schneider‐Gloetzl, Gloetzl GmbH, Rheinstetten, Germany
Rainer Gloetzl, RG Research, Ettlingen, Germany
Katerina Krebber, Sascha Liehr, Mario Wendt, Aleksander Wosniok,
BAM, Federal Institute for Materials Research and Testing, Berlin, Germany
The planning and design of monitoring systems for civil engineering applications typically focus on structures as they are
directly subjected to live loads, are most exposed to weathering and are the most visible to the public. As such, the
monitoring of structures above the ground is beginning to mature and applications in the field are emerging. An area not
yet researched in depth, however, is the monitoring of foundations and earthworks. Similar to the monitoring of
structures such as buildings and bridges, in‐service data from foundations, retaining walls, and earthworks can be utilized
to validate design assumptions, control construction operations, assist with life‐cycle maintenance and management
actions, and provide alert to extreme events. Particular to substructure and foundation elements is the possibility of their
reuse when superstructures are replaced. In such cases, their load history and performance over time can be critical in
deciding whether or not they can be reused or must be reconstructed. Also motivating the collection of data for
substructures, foundations, and earthworks is the need to calibrate consistent reliability levels across each component of
complex structures (e.g. the foundation, abutments, and superstructure in the case of a bridge).
Traditional measuring systems include pressure cells, piezometers, and inclinometers which produce point
measurements. A new tool designed for distributed measurements is sensor‐embedded reinforcing textiles. These smart
textiles provide in‐service data using fiber optic sensors while they perform their traditional role of soil stabilization,
reinforcement, or separation. This paper highlights the state of the art in geotechnical monitoring with special emphasis
on smart geotechnical textiles. Applications and case studies for these materials are provided.
Geotechnical Measurement
The application of geotechnical measuring instruments is commonly used for dam constructions or buildings which are
accompanied with similar impact to the surrounding area. The building of all complex structures below and above ground
are solved by static analysis which bases on input data from soil examination and laboratory tests of soil samples. For a
continuous control of the construction efforts and its related consequences the monitoring concepts mostly concentrate
on geodetic measurements. Because of always new construction methods the geotechnical measurements gain more and
more in importance. The information which can be extracted from the underground can be used to validate the static
computing method and the soil mechanic model. Furthermore the underground measurements allow a monitoring
during the lifetime. This offers the possibility to recognize a change of structural loading and can lead to reduced
maintenance costs by reparation at the beginning of damage. For example is it standard in inner‐city tunnel projects to
monitor building settlements during the under‐ride of a tunnel boring machine. In the moment a settlement is occurring
a countering injection is executed until the settlement is compensated.
Figure 1. Landslide monitoring with geodetic and geotechnical system
State of Art
Depending on the nature of the underground and the kind of project different physical values are measured. In general
the monitoring includes the measurement of settlement, inclination, deformation, earth and pore water pressure as well
as temperature and vibration. Each parameter can be measured by different sensing techniques which are selected in
consideration of measuring frequency, accuracy and price demand. This variety of requirements leads to a wide range of
solutions from simple mechanical instruments to wireless operating electrical sensors. Also the measuring frequency
knows no bounds ‐ from single static measurements up‐to dynamic measurements with high clock rate.
Figure 2. Monitoring example (building and excavation)
New Techniques
The next step in a modern risk management is the integration of monitoring solutions in the reinforcement concept. In
the frame of several German projects and the European research program POLYTECT a breakthrough was done with the
successful integration of sensing optical fibers – silica and polymer optical fibers ‐ in geotextiles. This combi‐mesh is
designed for geotechnical applications to monitor dikes and creeping slopes or to control new build earthwork structures
with critical subsurface conditions. Advantages of the new technology are an easy installation and a distributed sensing
method. This means that a deformation along the fiber is detected in size and location. Sensing techniques based on
OTDR (optical time‐domain reflectometry) in polymer optical fibers (POF) and Brillouin scattering in silica fibers have been
used to measured distributed mechanical deformation (strain) in geotechnical structures. Recently, the first real field tests
have been successfully conducted.
Figure 3. Geotextile with integrated polymer fiber
Monitoring of geotechnical structures using distributed POF OTDR sensors embedded in geotextiles
A field test in an open brown coal pit has been carried out near Belchatow, Poland using POF equipped geotextiles and the
OTDR technique. The test was initiated, organized and supervised by Gloetzl Baumesstechnik GmbH, Germany in close
cooperation with Budokop, Poland and the owner of the coal pit. A sensor‐equipped geogrid was installed directly on top
of a creeping slope. The 10 m long geogrid was manufactured by Alpe Adria Textil, Italy and comprised one standard
PMMA POF. Fig. 4 shows the installation of the sensor textile on top of the slope. It is covered with a 10 cm thick sand
layer. The textile is installed with the POF sensor bridging the cleft perpendicular to the opening. The geogrid was installed
in a slightly corrugated way simulating realistic installation conditions. Measurements were conducted before and after
installation.
Fig. 5 (left) shows the OTDR traces of the sensor fiber section in the middle of the textile where the fiber bridges the cleft.
The figure clearly shows backscatter increase due to strain in the fiber at the position where the cleft was expected. The
magnitude of the backscatter increase relative to the reference measurement is shown in Fig. 5 (right). The first three
measurements after installation show a steadily increasing strain signal extending over a length of about 7 m. The shape
of the backscatter signal of the last two measurements indicates that the strain distribution along the textile is not
symmetric. The high peak at about 35 m is caused by a very high and confined strain in the sensor fiber and textile. The
magnitude of backscatter increase corresponds to a maximum strain in the fiber of more than 10 %. Such high strain
values can only be measured by POF sensors. Silica fiber‐based sensor systems would have failed at a strain exceeding
about 1 %.
Due to the gradual increase of cleft width, the overlying textile and therefore the sensor fiber change their absolute
length. By evaluating the relative shift of the reflection peaks at both ends of the textile‐integrated fiber, the values of the
total elongation of the fiber sensor indicating the width of the cleft was obtained. Fig. 6 shows a relative linear increase of
the POF length with time. The measurements indicate that the creep velocity of the slope was constant during the time of
observation with an average rate of about 2 mm per day. Further field tests in the open mine in Belchatow are being
conducted at present in order to provide more information about the geotechnical processes.
Figure 4. Installation of a geogrid containing POF in an open brown coal pit near Belchatow, Poland (left) and a cleft of the creeping slope (right).
Figure 5. OTDR traces of the sensor fiber at the position of the cleft (left) and change of the sensor signal relative to the reference measurement (right).
Figure 6. Total elongation of the POF sensor obtained by peak shift evaluation.
Monitoring of geotechnical structures using distributed Brillouin sensors embedded in geotextiles:
The focus of the performed tests using the Brillouin‐based sensing technique by means of a commercial measurement
instrument operating in the time domain (BOTDA) was on the detection of geophysical activities in dam structures. The
method using Brillouin backscattered light in silica optical fiber is based on the distributed measurement of the frequency
shift of the Brillouin light (Brillouin‐frequency) caused by strain and temperature changes applied to the fiber.
In order to detect soil displacement, a non‐woven geotextile mat manufactured by STFI, Chemnitz of a length of 17.5 m
with integrated single‐mode silica fibers as sensors was embedded in an earth‐filled dam in Myczkowce (Fig.7), Poland 3
years ago. The sensors in the form of standard optical cables were integrated into the geotextile using the warp‐knitting
technique.
Fig. 8 shows the distribution of the Brillouin frequency of the fibers embedded in the geotextile measured 3 years after
installation. Field tests were conducted in April and August 2009. Each Brillouin trace corresponds to the Brillouin
frequency measured on two sensing fibers connected by fusion splices at the far end of the geomat. Due to higher
environmental/soil temperatures during the field test conducted in August, the Brillouin trace of this measurement
features higher values of the Brillouin frequency. In the fiber sections between 205 m and 240 m where the geomat was
embedded in the soil, a mechanical load is assumed which results in a change of the recorded Brillouin frequency in these
fiber sections. The origin of this effect should be investigated by further systematic measurements and a deep analysis of
the geotechnical processes. However, this was to our knowledge the first real field test demonstrating the use of a
distributed Brillouin sensor embedded in geotextiles for monitoring of geotechnical structures.
Figure 7. The earth‐filled dam in Myczkowce (left) and the construction site with the sensor‐based non‐woven geotextile before embedding into the soil
and 3 years later (right).
Figure 8. Brillouin measurements on the sensor‐based geotextile 3 years after embedding into the soil. The section between 205 m and 240 m illustrates
Brillouin frequency shift distribution of two interconnected sensing fibers embedded in the geotextile.
Summary and Outlook
In civil engineering the geotechnical monitoring is gaining in significance and the used measuring systems are in steady
development. This paper presented two novel techniques using distributed fiber optic sensors embedded in geotextiles.
Both measuring solutions (OTDR in polymer optical fiber and Brillouin frequency in silica optical fiber) provide the
detection of local deformation in position and size. This particular feature means a new quality in geotechnical monitoring
and permits a wide range of applications aimed to the needs of precise damage detection, structural analysis and early
recognition of structural mutation.
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