Scour Monitoring Study Based On Distributed Strain Sensing Technology

In the water conservancy, water supply and sewerage, dam protection, bridges, offshore platforms and other engineering projects, it is quiet common that the structure is damaged by the waster scour. In most cases the water scour reduces the strength of the structure’s foundation, which brings out potential hazard in the structure. The scour is one of the main causes of bridge failures, each year many bridges failed due to scouring during flood. It is not easily discernible because it is hidden under the channel flow. The scour depth is the most critical factor for structure safety. Thus, the monitoring of scour depth is essential. Over the years, a variety of sensors have been developed for detection of scour depth. On the basis of current research in scouring and in the distributed long-gauge FBG sensing technology, a new and simple type of scour sensor is developed in this article and the performance of the sensor on monitoring scour depth have been tested and verified in terms of changes in strain and frequency of the sensor. The main research contents are as follows:(1)The sensing principle of fiber Bragg grating sensing technology, the production and packaging process of long-gauge FBG sensor are briefly introduced. And the basic sensing properties of packaged sensors have been tested. Packaged long-gauge FBG sensors can be applied to harsh working conditions and have both static and dynamic test properties.(2)With the sensing principle, which is based on the long-gauge FBG sensing technology, the new and simple scour sensor is developed. When the scour depth detection concept is based on measuring the change of strain of a straight FRP rod, which is embedded in the riverbed, a long-gauge FBG sensing probe arrays must be pasted on the straight FRP rod. Water flow makes the strain of the straight FRP rod change dramatically between the interface of the riverbed and water. The new developed scour sensor can determine the height of the interface of the riverbed and water by wavelength characteristics of FBG sensor above and below the riverbed, thereby obtain the scour depth. When the scour depth detection concept is based on measuring the change of natural frequency of a straight FRP rod, which is embedded in the riverbed, only a single long-gauge FBG sensor must be pasted on straight FRP rod. After the scour of the riverbed, the exposed straight FRP rod from riverbed section will increase, leading to the change in the vibration characteristics of the straight rod such as natural frequencies. The new developed scour sensor can not only determine the occurrence of scouring, but also monitor the scour depth by calculating the natural frequency of the straight rod through measuring the time signal of the FBG sensor.(3)Through deduction and derivation, the article obtains a method of solving natural frequency of underwater structure considering fluid-structure interaction, and this method is based on added mass. ANSYS is also used to solving the natural frequency of underwater structure, and a law has been obtained in the article:water has a great influence on the natural frequency of underwater structure, the natural frequency underwater is lower than in the air. However, it is the same trend in the air that the relation of the straight rod’s natural frequency underwater with the cantilever length.(4)The new developed scour sensor is tested trough flume experiments simulating the scouring process on riverbed. The riverbed scour is simulated by making water plowing pass the flume, which is filled with fine granular sand and monitored by the new developed sensor. The experimental study has showed that the new developed scour sensor worked well both through measuring the changes in stain and frequency.