The leak detection of rockfill dams is currently hindered by spatial and temporal randomness and wide monitoring range. The spatial resolution of fiber Bragg grating (FBG) temperature sensing technology is related to the distance between measuring points. As a result, the number of measuring points should be increased to ensure that the precise location of the leak is detected. However, this leads to a higher monitoring cost. Consequently, it is difficult to promote and apply this technology to effectively monitor rockfill dam leakage. In this paper, a practical mobile distributed monitoring system with dual-tubes is used by combining the FBG sensing system and hydrothermal cycling system. This dual-tube structure is composed of an outer polyethylene of raised temperature resistance heating pipe, an inner polytetrafluoroethylene tube, and a FBG sensor string, among which, the FBG sensor string can be dragged freely in the internal tube to change the position of the measuring points and improve the spatial resolution. In order to test the effectiveness of the system, the large-scale model test of concentrated leakage in 13 working conditions is carried out by identifying the location, quantity, and leakage rate of leakage passage. Based on Newton's law of cooling, the leakage state is identified using the seepage identification index ζ v that was confirmed according to the cooling curve. Results suggested that the monitoring system shows high sensitivity and can improve the spatial resolution with limited measuring points, and thus better locate the leakage area. In addition, the seepage identification index ζ v correlated well with the leakage rate qualitatively.
Purpose The spatial resolution of seepage monitoring methods based on fiber Bragg grating (FBG) temperature sensing technology is limited by the distance between measurement points. Improving the spatial resolution for a given number of measurement points is a prerequisite for popularizing this technology in the seepage monitoring of rockfill dams. The purpose of this paper is to address this problem. Design/methodology/approach This paper proposes a mobile-distributed seepage monitoring method based on the FBG-hydrothermal cycling seepage monitoring system. In this method, the positions of the measurement points are changed by freely dragging the FBG sensing cluster within the inner tube of a dual-tube structure, consisting of an inner polytetrafluoroethylene tube and outer polyethylene of raised temperature resistance heating tube. Findings A seepage velocity calibration test was carried out using the improved monitoring system. The results showed that under a constant seepage velocity, the use of the dual-tube structure enables faster cooling, and the cooling rate accelerates with an increase in the diameter of the inner tube. The use of the dual-tube structure can improve the sensitivity of the seepage evaluation index ζv to the seepage velocity. When the inner diameter increases, ζv becomes more sensitive to the seepage velocity. Originality/value A mobile-distributed seepage monitoring method based on FBG sensing technology is proposed in which the FBG sensors are not fixed. Instead, the positions of the measurement points are changed to improve the spatial resolution. Meanwhile, the use of the dual-tube structure in the presented monitoring system can improve its sensitivity.
Seepage monitoring is an important element in geotechnical engineering. This article proposed a new integrated system for seepage monitoring composed of a fiber Bragg grating (FBG) sensing system and a water-heating cycling system. The boiler is used as the heating equipment in the integrated system. The heated water is distributed to each heating pipeline for cyclic heating through the water separator and water collector. The FBG temperature sensors are preburied in the heating pipeline to monitor the water temperature in real time. Recognizing the correlation between the temperature field and the seepage field, we proposed to fit coefficient ξv according to the cooling curve and used it as an index to identify the seepage state. We conducted this numerical simulation to analyze the heat transfer process of the heat source in the porous media. We carried out the calibration experiments of seepage velocity using the FBG-sensing heating system in the porous medium with four different gradations. Our results showed that the temperature gradient decreased over time, indicating that the primary way the heat source was transferred was through the convective heat transfer caused by the seepage. Therefore, the coefficient ξv could be used as the seepage identification index. On the basis of our calibration experiments, we obtained the fitting formulas of ξv and the seepage velocity in four kinds of porous mediums. The formulas can be used for the inversion of seepage velocity. The experimental results proved that ξv was unrelated to the initial cooling temperature. This finding showed that the influence of an uneven temperature distribution along the heating pipeline on monitoring results could be ignored.
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