Optimization of early seepage detection in embankments using a distributed temperature system based on fiber optic sensing : doctoral dissertation
Optimizacija postopka zgodnjega zaznavanja pojava precejanja vode v nasipih na podlagi spremljanja temperaturnega polja z optičnimi kabli : doktorska disertacija

english

doktorska dela

2023

Built Environment   civil engineering   doctoral thesis   embankment   optical fiber DTS   seepage   soil   temperature

Univerza v Ljubljani, Fakulteta za gradbeništvo in geodezijo
(Obvezni izvod spletne publikacije)

Seepage is the most important indicator in the safety assessment of embankments which requires continuous monitoring to maintain structural integrity. Punctual monitoring techniques for seepage in embankments can only provide limited information on the process. As the thermal behavior of embankments is affected by seepage, a continuous temperature measurement system can be established in embankments to identify the possible seepage flow. With the advancement of fiber optic measurement technology, measurement equipment is available that allows distributed temperature sensing. Optical fiber Distributed Temperature Sensor (DTS) was developed to measure the ambient temperature along the entire length of the fiber optic cable at an arbitrary time interval. There are two techniques to perform this measurement: passive measurement technique and active measurement technique, which, unlike passive measurement, require an additional power supply. The passive measurement technique measures the natural temperature of the soil and is therefore suitable for long-term temperature monitoring in embankments. In this thesis two objectives were set: first, that it is possible to detect the onset of seepage in embankments with a DTS fiber optic cable measurement system based on the passive measurement technique, and second, that the correct distribution of the DTS fiber optic cable system increases the ability to detect the onset of seepage in embankments at an early stage. The experimental part of the task was carried out in two phases. In the first part, a small-scale experimental model was developed to simulate the seepage phenomenon in embankments. The soil material was replaced in the model by quartz sand with three different granulometric compositions. The hydraulic and thermal properties of the materials were obtained from laboratory tests or the relevant literature. In the second part, a largerscale experiment was carried out under laboratory conditions, using only one type of silica sand. By simulating the seepage process under laboratory conditions, it has been shown that the DTS optical cable measurement system can detect any small temperature changes in the sand layer as a result of the seepage phenomenon. The system can detect the start of the seepage process in the initial stages, as well as monitor the progress of the seepage process, even in cases where there are small temperature differences between the soil and the seepage water. The numerical analysis was conducted for both the seepage process and the heat transfer progression in soil. A clear relation between the variation in the degree of saturation and temperature measurements was observed visually in experiments and in the numerical simulation. The application of passive DTS for seepage detection in embankments was validated. The efficiency of DTS application in seepage monitoring strongly depends on the optical fiber installation approach, calibration technique, temperature data interpretation, and post-processing. To achieve the accuracy of the measurement capture, a special method of laying the fiber-optic cable with loops was used, based on the configuration of the sensory ring, which allows precise point-to-point measurement capture. Some guidelines for optimization of optical fiber DTS application were provided by a comprehensive review of different techniques to interpret temperature data in terms of seepage information. Additionally, different methods for the calibration of DTS data were reviewed. Various techniques both in calibration and interpretation were classified and their comparison was provided that can be used for a more efficient DTS employment in seepage detection. A method was also suggested for the interpretation of DTS data based on the comparison of numerical results with the obtained temperature measurements in embankments.

12.05.2023