Design and application of fiber grating sensor in safety monitoring of Shuibuya concrete dam

introduction

Dams require large investments and have high benefits. They play a vital role in the development of the national economy and are closely related to people's lives. Once a large reservoir fails, the loss of life and property is huge. This paper analyzes and experimentally studies the temperature/strain sensing characteristics of fiber Bragg gratings; explores its deployment process and the technology used in dam construction and long-term strain monitoring.

1. Fiber Bragg Grating Strain Sensing Characteristics

Fiber Bragg grating sensing technology achieves absolute measurement of strain and temperature of the measured structure by detecting the reflection or transmission wavelength spectrum of the grating written inside the optical fiber. Its sensing principle is shown in Figure 1. The reflection or transmission wavelength spectrum of the fiber Bragg grating mainly depends on the grating period L and the effective refractive index neff of the reverse coupling mode. Any physical process that changes these two parameters will cause the drift of the reflection or transmission wavelength, that is, DlB="2neff"·DL (1)

Figure 1. Fiber Bragg grating sensing principle

Among all the external factors that cause the wavelength drift of the grating Bragg, the most direct one is the strain parameter, because whether the grating is stretched or compressed, it will inevitably cause the grating period L to change, and the elastic-photoelectric effect of the optical fiber itself makes the effective refractive index neff also change with the change of the external stress state, which provides the most basic physical properties for the optical fiber strain sensor made of fiber Bragg grating.

2. Fiber Bragg Grating Temperature Sensing Experiment

2.1 Test equipment and materials

This paper uses the fiber Bragg grating strain sensor produced by the Fiber Optic Sensing Technology Center of Wuhan University of Technology. The connector uses a universal fiber FC/APC jumper head. The Bragg center wavelength identification system uses the FBG-IS (Fiber Bragg Grating —Interrogation System) fiber grating demodulator produced by Micron Optics, USA. The instrument demodulates the center wavelength of the Bragg reflection spectrum based on the FP (Fabry-Perrot) interference principle, with a wavelength resolution of 1pm, a scanning range of 1283-1312nm, and a scanning frequency of 50Hz.

2.2 Strain sensing test principle and result analysis

When leakage occurs somewhere in the seam around the reservoir dam, the water there will penetrate into the dam through the gap on the surface of the dam and flow along the gap inside the dam. During the flow of water, the temperature of the dam changes. The location of the leakage point can be determined by detecting this temperature change using a fiber grating temperature sensor. Multiple fiber grating temperature sensors can form a distributed sensing network to monitor the seepage of the seam around the entire dam.

The plane diagram of this test model is shown in Figure 2. The fiber Bragg grating sensor is laid under the test water tank. The water tank uses soil and rocks near the dam body to try to get close to the actual effect. The cross-sectional diagram of the test model is shown in Figure 3. The fiber Bragg grating strain sensor with a central wavelength of 1296.5 and 1298.5nm is pasted on the heating cable with 102 adhesive, and a high-precision resistance strain gauge is arranged at the corresponding position and loaded by weights.

Figure 2 Schematic diagram of the test model

Figure 3 Schematic diagram of the test model cross section

This simulation test selected three seepage points for secondary water test to observe the temperature change process before and after the probe water test at the seepage point. The test data is recorded in Figure 4. Among them, 2-1; 2-2; 2-3; 2-4; 2-5 are the temperature change curves reflected by the Bragg gratings at different positions on a single-mode optical fiber. [page]

Figure 4 Temperature of test point of channel 2#

It can be seen from the test results that when seepage occurs, the correlation coefficient between the central wavelength and temperature is very high due to temperature changes, and there is no hysteresis phenomenon. They have a good linear relationship. It proves that the fiber Bragg grating is an ideal temperature sensing element; this is in good agreement with the theoretical analysis results. Moreover, the sensor has good consistency and is very effective in large-area patch measurement, realizing the measurement of engineering structures.

3. Composition of Fiber Bragg Grating Health Monitoring System

The optical fiber intelligent health monitoring system is mainly composed of the following three parts: optical fiber sensor system, signal transmission and acquisition system, and data processing and monitoring system.

Figure 5 Fiber Bragg Grating Sensor (FBG) Bridge Structure Monitoring System

During monitoring, the real-time status signal of the dam measured by the optical fiber sensor is sent to the monitoring center through the signal transmission and acquisition system for corresponding processing and judgment, so as to evaluate the health status of the dam. If the key health parameters monitored exceed the set threshold, the relevant management agency will be notified through instant information so that corresponding emergency measures can be taken.

4. Application of Fiber Bragg Grating Sensors in Safety Monitoring of Concrete Panel Dam Project

Shuibuya Dam is the world's highest concrete face rockfill dam. Many of its engineering and technical problems are global challenges. To explore and solve these problems, a large number of advanced technologies and processes are needed. In terms of dam safety monitoring, after comprehensive comparison, it was finally decided to use the fiber Bragg grating seepage (temperature)/panel strain monitoring system with independent intellectual property rights of Wuhan University of Technology. This system is a complete safety monitoring system with modern monitoring and management level, which strengthens accident detection and mitigation measures, and fully reflects the characteristics of reliable operation, timely response, and accurate monitoring.

5. Conclusion

This paper successfully applied fiber Bragg grating temperature sensors and strain sensors to the construction monitoring of dams, accumulating valuable experience for further engineering applications. The monitoring results show that fiber Bragg grating strain sensors have superior sensing performance, especially in terms of long-term stability, and are very suitable for the long-term monitoring of dams, bridges and other projects. Fiber Bragg grating strain sensors can effectively monitor the construction process of dams, and have great application potential and prospects in realizing the monitoring of the entire life cycle of dams, long-term health monitoring and safety assessment of dams.

References:

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