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MITIGATION MEASURES TO TUNNEL STRUCTURES WITH FIBER OPTIC MONITORING SYSTEM

General description

Fiber-optic geotechnical monitoring system (FOGMS) includes 3 basic components: analyzer (or interrogator), distributed (uninterrupted) fiber-optic sensor and a dedicated software (Fig.1). The analyzer is a powerful diagnostic instrument for distributed measurement of strain and temperature over 65 km per channel, allowing measurement of thousands of locations by means of a single sensing cable. Use of internal optical switch featuring 2 measuring channels allows uninterrupted measurement of up to 135 km in two opposite directions from the analyzer. The number of measurement channels can be further extended by using an external multiple optical switch module and star topology. Once an external switch is connected to the analyzer, up to 21 sensors can be used for distributed sensing.

Distributed sensor is an uninterrupted fiber-optic cable that is often custom-designed for a specific application and environment. Each millimeter of sensor is used as a sensitive element offering a great alternative to numerous point sensors. Taking into account high spatial resolution of the analyzer each 50 centimeters of sensor can be considered as an individual point sensor, therefore 50 centimeter sensor section is equivalent to 100 000 point sensors. Fiber-optic distributed sensors are fully passive and do not require connection to power supply. One sensor is usually connected to the analyzer from one or two ends and is adjacent to the entire length of a monitoring facility. Optical fiber integrated in sensor design is sensitive to various external parameters (temperature, elongation/compression, acoustic pressure, etc.) by changing its optical properties. Thus, fiber-optic geotechnical monitoring system offers a wide range of performances and suitability for different applications.

The system automatically controls an extended facility in every section with an installed sensor providing accurate measurement capabilities in real time. The superior sensing technique and operation flexibility makes FOGMS a unique and unrivalled solution to most demanding applications. For the details of system’s operation principles and advantages of fiber-optic sensors, please refer to Section “Resources”.


A)  

B)

 

C)

Fig. 1. FOSGTM includes a set of components which consist of:

A) Analyzer unit and supporting equipment mounted in a 19” rack cabinet; B) Fiber-optic sensor; C) Dedicated software to be installed in a server room.

Monitoring of the tunnel crown

Continuous monitoring of the tunnel crown is an essential part of reliable tunneling practice. It focuses on controlling tunnel stability and elimination of destructive effects of the surrounding environments, such as:
Decreasing strength of reinforced-concrete structure over time (drying shrinkage of concrete, scouring, elastic shortening of metal structures, etc.);
Settlement of tunnel surrounding grounds due to water pressure (scouring);
Exposure to vibration due to crossing railroad trains.


To reach the appropriate level of confidence in the automatic and continuous health monitoring of the tunnel the system provides dedicated software algorithms, that can be divided to 3 modules:

Standard SW, representing data in graphical and tabular formats that is obtained by fiber optic sensors installed at different parts of the structure. Such SW is capable to provide evolution of structural behavior (strain profile of one or another section as a function of time or load applied to the structure);
Numerical analysis SW which allows modeling of the structural behavior of the monitored object. This SW provides structural strain calculation (which is important in terms of forecasting its behavior, for example, by identification and localization of cracks, weakening of structure, etc.). Moreover, SW based on mathematical analysis provides tracking of important parameters and evaluation of an overall condition of the structure.
Statistic analysis SW is used to analyze dependence of strain from periodic and rapidly changing external processes (such as transport traffic).

Physical parameters of the tunnel crown are controlled with fiber optic sensors installed inside of the crown surface. Monitoring system provides uninterrupted data acquisition at certain section of the tunnel where sensors are installed. Thus, the observed longitudinal strain induced to the crown may reach 0.05 mm.


Fig. 2. Example of continuous fiber optic sensors layout

 

In addition to long-term uninterrupted monitoring of tunnel condition fiber optic sensors have shown excellent performance by controlling initial behavior of a newly raised construction. Such systems being embedded in concrete during construction stage can provide valuable and accurate data concerning initial expansion of concrete due to heating (alkali-aggregate expansion), shrinkage of concrete under wet and dry cycles and external loads that cause strain.

Great care is required throughout the entire construction process to ensure that the concrete has the desired properties and is compatible with other materials. So, an ultimate benefit offered by those systems is possibility to control adhesion of materials with different properties and lifetime (rock-concrete, brick-shotcrete, concrete-steel, new concrete-old concrete, etc.). Knowing the proper mix and selecting suitable concrete materials can significantly reduce the formation of cracks and self-strain effect.

Monitoring of tunnel inflammation


Uninterrupted fiber optic sensors are also used to prevent tunnel inflammation by detecting local temperature increases along the tunnel length. The connection between the measurement unit and sensors can be ensured by standard telecommunication cable. The measurement unit is connected to a sensor end and placed either inside of the monitored tunnel, or outside in the control room. Eventually the monitoring system provides continuous records of temperature profiles distributed along the whole length of the tunnel. The processing of temperature profiles is based on special alarm identification algorithms (detection thresholds), enabling localization of a critical event with 1 m accuracy. It detects abnormal events by comparing recorded temperature profiles with temperature thresholds that can be assigned to each section of the tunnel. When the real-time data exceeds the threshold, then alarm is generated. Over 40 km tunnel section can be controlled by permanent monitoring system.


Fig. 3. Example of continuous fiber optic temperature sensors layout.

 

 

Fig. 4. Different designs of continuous fiber optic sensors

 

 

Fig.5. Fire distribution profile: artificial inflammability profile obtained by two sensors – installed on the ceiling (on the left) and on the wall (on the right).

 

Compared to traditional fire alarm signalization systems this monitoring system brings increased reliability and versatility. Exceptional fire detection techniques offer fast recognition, identification, analysis and localization capabilities of different fire types within 1 meter. The system is fully insensitive to wind exposure as it relies on heat-driven convection and heat emission. Additionally to high sensitivity fiber optic sensor is a rigid instrument for long-term monitoring due to its ability to withstand extreme temperatures up to 1000°C (special design cable). An inflammation detection system installed in the tunnel offers insightful information which is sent to the operator for timely mitigating measures and thus effective prevention of your asset.
The system features special algorithms for precise events detection and classification. It is especially vital for long distance monitoring when cable route is laid through different temperature zones. For example, entries and exits of the tunnel are more sensitive to daily and seasonal temperature changes that inner part of the tunnel. This feature is facilitated by assigning certain sections of tunnel to several types of alarms; the system continuously compares the real condition of the tunnel to the predefined alarm thresholds eliminating the occurrence of false alarms.


Features and benefits of fiber optic sensing cables:

Expected lifetime 30 years;
Standard or specialty cables (incl. high temperature resistant up to 1000 0Ń);
Fully passive device, requires no power and law maintenance;
Resistant to dirt, dust, corrosion, evaporation of organic particles, EMI and radiation, high temperatures.

Leakage monitoring in tunnel crown

Prevention and localization of leaking in a tunnel crown is not an easy task. In the wake of recently reported incidents in Russian and European subways the need for monitoring and warning systems in tunnels has become evident. For example, an accident in St-Petersburg subway occurred as a result of water flood between stations “Lesnaya” and “Ploshad Muzgestva”. It was discovered that the accident was caused by a raised washout section of bearing horizon waters which resulted in loaded soil and occurrence of cracks in tunnel lining. Another flooding threat example has occurred in Moscow subway between stations “Tzaricino”-“Orekhovo” due to damaged hydro insulation.
Proven by the world transport accidents study standard approaches of leaking monitoring based on point sensors are generally ineffective. Among today’s available sensing technologies, leakage detection using distributed fiber optic sensors can be a comprehensive solution for continuous real-time monitoring of transport facilities. The implementation of this technology will increase transport system performance by timely detection and mitigating risks of transportation accidents.


 

 

 

   

 

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