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LANDSLIDE MITIGATION IN THE VICINITY OF ROADS AND HIGHWAYS USING FIBER OPTIC MONITORING SYSTEM

Among the main threats to roads integrity are continuous mass and vibration loads induced by moving vehicles (especially trucks when moving at high speed), cycles of freezing-thawing and other actions. One of the significant risks leading to disastrous consequences those are hard to forecast are ground movements occurring in the vicinity of highways and roadways.
Ground movements can arise from many sources, such as ground instability caused by unstable slopes, geological hazards, ground water erosion, instant climate change accompanied by periodic freeze-thaw processes which cause heave and settlement problems or due to intense mineworks and construction activities. Construction of bridges near highways is also posing additional adverse impacts to structural health of an infrastructure such as change to river bed geometry and/or ground water flows, etc. When road routes are crossing such critical zones structural health monitoring (embankment, river level, road) offers the best solution to ensure their safety and integrity.

Fig.1. Landslide in Taiwan

 

 

Landslide monitoring sets challenging engineering tasks that can be solved by means of different diagnostic and control tools. The main difficulties arise during construction of large structures such as roads, tunnels, buildings in landslide zones. Once in use, these structures are subjected to evolving patterns of load distribution inside the landslide mass. Key methods of landslide control are listed below:

1. Inclinometric survey


Principle of the method:

Inclinometer probe is installed in a vertical borehole that passes through suspected zones of ground movements into stable ground and is used to survey the borehole. During the survey, inclination parameters at multiple certain locations of the borehole are measured along the borehole. The method is used to obtain a completed measurement of the inclination and azimuth of a location in the borehole. Inclinometer probe is run into the borehole to measure tilt of the casing (borehole). When the inclination and azimuth angles are known, the actual measured depth can be determined and the inclinometer probe path enables to calculate the displacements of the borehole that can be applied for monitoring of the lateral movements in landslide territories. It is assumed that using this method the bottom end of casing must be static (as it is lower than sliding plane of a landslide). Therefore proper selection of the borehole depth requires preliminary survey data.

Features:
A) subsurface method allowing determination of landslide slide path.
B) high-priced.

Measurement accuracy: 0.1 – 0.01°C.


Fig.2. Inclinometer probe for well measurement.

 

2. Geodetic shooting

Principle of the method:

Method is based on tacheometric survey performed in highlands with a tacheometric gauge combining standard theodolite and laser distance meter. Three parameters are obtained with tacheometric telescope – direction, distance (north coordinates) and deviation of the measured object in relation to position of the tacheometer.

Features:
A) Surface control method
B) Open space is required, poor performance in highland tree areas;
C) Poor accuracy of landslide control, insufficient for high-quality monitoring. Accuracy improvement requires complex inventive solutions.

Distance measurement accuracy: ~ 2 mm + 2 х 10-6 * (distance)
Angle measurement accuracy: ~ 5''

 
 

A)

 

B)

Fig. 3. Measurements performed with theodolites.

 

3. Automated geodetic survey

Principle of the method:

Laser is placed on a static section of the slope, whilst the controlled section under potential threat is equipped with laser targets. Periodic scanning of distances to the targets is performed with the interrogator.

Features:


А) Surface control method;
B) Open space is required, poor performance in highland tree areas;
С) Poor accuracy of landslide control, insufficient for high-quality;
D) Distance between laser and targets doesn’t exceed 1 km in clear weather; rainy, foggy, snowy weather pose significant limitations to this method.
Measurement accuracy: ~ 1-10 mm.

 

Fig.4. Coordinate measuring by automated laser distance meters.

 

 
   

A)

 

B)

 

Fig. 5. Automated equipment for geodetic measurements: laser distance meter (A), receiver (target) (B).

 

4. GPS-transducers

 

Principle of the method:

 

A GPS-transducer is installed on a landslide area. Coordinates of the transducer are periodically transmitted though communication satellites.

 

Features:

A) Surface control method;

B) System accuracy is not enough for detecting all types of landslides;

C) Proper performance requires correct positioning of three satellites (system’s accuracy changes depending on geographical position);

D) GPS-transducers can be easily blanketed by signal rejection systems;

E)  GPS-transducers accuracy is seriously deteriorated with longer distance from central base.

 

Measurement accuracy: 3-5 m (using correction signal of the earth station with up to 1 km ~ 1 cm).


Fig. 6. Coordinate measurement with GPS-transducers

 

 

 

A)

 

B)

Fig.7. GPS-transducers.

 

5. Fiber optic geotechnical monitoring system

Principle of the method:

Trench excavations with depth of ~30-50 cm are carried out, then uninterrupted fiber optic sensor (sensing cable) is embedded into the ground. After the sensors have been installed backfill takes place. The ground-embedded sensor is subjected to elongation/compression (strain) caused by ground movements which change the parameters of probe signal. Transmitted along the ground sensor, the location and amount of strain is calculated by the analyzer connected to one or two sensor ends.

A) Surface and in-depth method*;
B) Distributed method (sensor is a fully passive element with unlimited lengths options from several meters to hundreds of kilometers). Example: 500 m sensor can replace ~ 1 000 point sensors that are installed each 50 cm along the sensing path;
C) The best accuracy compared to all available landslide control methods;
D) Unattended continuous monitoring offers highest performance 24/7/365. Up to 50 km sensor can be controlled with one analyzer. Complete remote monitoring capabilities allowing operators to leave displacement to field only for periodic inspections and physical works.

* The method allows to recycle boreholes used for inclinometer surveys. When significant displacements take place the inclinometer borehole is induced by strain and can be assumed ineffective for further surveys. But fiber optic monitoring system with an installed strain sensor enables to extend operational lifetime of the borehole providing important data about strain evolutions at different depths.

Measurement accuracy: from 0.05 mm / 1 m !!! (depending on sensor installation geometry).

   

A)

B)

     

C)

Fig.8. Installation of Fiber Optic Geotechnical Monitoring System (FOGMS): sensors embedded sin a trench (A, B) and ground anchor attached to the cable (C).

 

 

Taking into account a brief review of a number of landslide detection technologies, the most efficient method of landslide risk mitigation is the combination of FOGMS and inclinometer system. Using a few inclinometer casings a sectional area and a slide path pattern of a landslide cab be identified. FOGMS provides best possibilities to identify landslide borders and record velocity evolution at different sections along the entire structure that is controlled. As previously stated, when large ground movements and significant strains are induced to the inclinometer boreholes they can be used for fiber optic sensing with FOGMS. Thus, FOGMS is an effective solution for continuous in-depth landslide control.
Besides, combination of two different systems will benefit in sharing and checking the aquisited data significantly enhancing the reliability and quality of monitoring (this can be fulfilled by interconnection utilizing a point-to-point link between the systems).

Fig.14. Schematic representation of FOGMS and inclinometer system combination for an advanced landslide control.

 

 

It should be noted that strain FOGMS sensors for ground movements feature an important design property: an increased fiber reinforcement provides worse sensitivity (measurement accuracy). The ideal measurement accuracy is achieved with a bare fiber without any coating. Accordingly, for landslide monitoring 2 different approaches can be followed :

1. Using a low-protected, but very accurate sensor at the stage of construction survey. Such sensor will quickly demonstrate the landslide “path” enabling to take timely corrective measures if any hazardous processes are detected (rerouting the construction avoiding critical areas or strengthening the structure, etc.).
2. Using less sensitive strain sensors for ground control provided that they have good resistance to tensile and longitudinal strains, rodents, etc. and guarantee a long-term uninterrupted operation with no breakups and deterioration.

In some cases, if there is a road section presenting a particular risk of ground movements the strain sensor can be embedded into the road. Thus, a better understanding of structural condition of the road can be provided. However, ground movements usually arise at an earlier stage as the road structure is more rigid than the soil and is resistant to movements to some point. Therefore having more information about ground movement-induced risks in the early stages of the project development can be beneficial for early prediction and mitigating actions.

 
Fig. 15,16. Asphalt-embedded strain sensor

 

6. 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 “Technical Notes”.
«Resources».

 


A)  

B)

 

C)

Fig. 17. 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.

 

 

 

 

 

   

 

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