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RAILROAD GEOHAZARD MITIGATION USING 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.

Necessity of railway monitoring


Russian railroad beds and superstructures become subjected to critical strain conditions every year. Large strains can result in serious accidents when loss of life, damage to transport vehicle, railroad beds, nearby assets, cargos, disruptions in train schedule and environmental consequences, such as soil and water body contamination are caused.

Continuous monitoring of railroad bed condition at critical sections of the railway track (such as bridges, tunnels, landslide areas, rockfalls, soils subjected to erosion, etc.) integrated to railway traffic management system is essential to guarantee appropriate operational reliability of the railway transport. Railway monitoring based on fiber optic technologies enables to:

provide timely reinforcement and repair of the railroad bed in critical zones;
forecast the dynamics of railroad bed behavior during operation and maintenance of railway system;
optimize newly built and reconstructured railway assets through statistical analysis of railroad bed behavior under different conditions (soil type, ballast type, load intensity, etc.) based on accumulated experience;
When control of a railway equipment is concerned a periodic monitoring becomes the best solution. All stated above appears even more relevant in terms of new high speed railway routes which are supposed to have minimized tolerances and deviations from standard values of railroad bed and equipment during operation.

This section is devoted to railroad bed monitoring (for details of structural bridge and tunnel monitoring please refer to the respective site sections).

Ground temperature control of railroad bed foundation

For railways erected on the ground surface in permafrost areas it is crucially important to know ground temperature of the railroad bed foundation. Controlling temperature range between -3°C to 0°C is of particular importance because thawing and further transition of soil from permafrost to thermokarst take place within this interval. The process of ground transition from permafrost to thermokarst is usually associated with sudden decrease in bearing capacity of soil. Opposite seasonal transition to a frozen ground may induce frost heave. Thus control of ground temperature allows to make an early assessment (early forecast) of structural behavior of the supporting grounds and railroad bed, ensuring accurate and timely prediction and mitigation of hazardous processes.

The complete monitoring solution provides uninterrupted control of the entire monitoring section where fiber-optic sensor is installed. High spatial resolution measurements offer accurate localization of the critical event along 1-2 m railway section with temperature resolution of ~ 1°Ń.

 

 

Ground / embankment movement detection

Any thawing, soil heave, landslide and other soil-related geologic processes are recognized as major threats to an engineering construction. Additional elongation/compression induced on the sensing cable is usually caused by geohazards or ground movements and can be used for calculation and localization of strain events. As shown in Fig.3 below, one sees that the original section d of sensing cable is submitted to a constant strain ɛ, whereas the rest of the cable remains strain-free. The cable elongationd depends on the lateral displacement L and the strain ɛ is simply given by:

 

 

By transforming the formula, we obtain:

 

where L / d ratio provides information about the magnitude of sensing cable displacement.

 

 

Fig. 3.  Schematic representation of elongation/compression induced on fiber-optic sensing cable caused by a landslide or other geohazard.

 


Different options for installing fiber-optic sensors along the railroad bed are shown on the scheme below:

Train localization using fiber optics

Strain sensor used for control of possible vertical movements of railroad embankment caused by thawing, soil thieves, landslides, etc. can also respond to train movement. The fiber optic monitoring system features accurate localization of a train within 1-2 m (by static measurements, i.e. at the moment when cyclic measurement is performed). Measuring frequency can be varied from a few seconds to 10s minutes depending on a specific application.


 

 

   

 

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