Emma Niemeläinen, Markku Juvankoski, Tommi Kaartinen, Jutta Laine-Ylijoki, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Henna Punkkinen & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, FINLAND.
Physical monitoring: Optical fibre sensing
Optical fibre sensing is currently one of the most interesting methods used for physical monitoring. Optical fibre sensing technology is relatively new and accurate method widely applied in different applications, such as soil, structure and e.g. building monitoring. It can be used for indirect monitoring of long-time stabilities of a mine dam, waste rock pile and/or their surroundings by measuring temperature and strain changes inside the structure. Also other properties, such as vibration, turbidity or pH can be detected depending on application. Technology is based on back scattering of light in optical fibre that is installed in the dam/pile foundation or a toe. (NRC 2009, ICOLD 2014, Habel & Krebber 2011, Optimon [no date].)
Distributed fibre optic temperature sensing (DTS) is a useful technique when an accurate temperature measurements and/or a large amount of measuring points are required. The measurement results form a temperature field of a dam/pile which indicates the flow field of seepage water.
Thermal measurements can be automatic, and either passively or actively performed. In the passive method the flow field is formed and the system detects and quantifies anomalies. The variety of external temperature (reservoir water, air, seasonal changes) affects to the temperature of seepage water and has to be taken into account (Figure 1). The active method detects the presence and movement of water accurately after the input external heat is induced. The existence of moisture around the cable is indicated and thermal response is analysed. Active measurements can be carried out periodically. (ICOLD 2014, Habel & Krebber 2011.)
Figure 1 Calculated temperatures on January 1 and July 2 for a 60 m high intact dam (left) and a dam with a 2 m height leakage zone and 50 times higher hydraulic conductivity (right). As boundary conditions, a sinusoidal water and air temperature variation were assumed (water – mean temperature of 10°C and an amplitude of 9°C, and air – mean temperature of 10°C and an amplitude of 9°C). (ICOLD 2014.)
Distributed fibre optic temperature and strain sensing (DTSS) is similar to thermal monitoring technology and used for strain monitoring. Optical fibre sensing is a good supporting detection method for movement monitoring. Relatively movements can be detected at an early stage, if measurements are continuously (“early warning system”) or regularly repeated. However, for the evaluation of the absolute movement values, other methods should be used. (ICOLD 2014.)
The optical technologies are often combined with remote monitoring methods. Case examples of the use of optical methods are presented by:
Zhu et al (2011): Safety Inspection Strategy for Earth Embankment Dams using Fully Distributed Sensing. http://www.sciencedirect.com/science/article/pii/S1877705811001081
Johansson & Sjödahl (2004): Downstream Seepage Detection using Temperature Measurements and Visual Inspection – Monitoring Experiences from Røsvatn Field Test Dam and Large Embankment Dams in Sweden. http://www.sensornet.co.uk/images/PDF/Oslo_2004-_Downstream_Seepage_Detection_using_Temperature_Me.pdf
Table 1 presents advantages and disadvantages linked to the use of optical fibres.
|Accurate, rapid detection and interpretation, wide detection range
||High demands for cable materials: stability, durability, resistance to strain and loads (especially in soft, consolidating soils)
|Autonomous monitoring, can be remotely operated
Fibre optic sensors can be made of glass (silica) or plastic. Polymer optical fibres, POF, are elastic and able to strain up to 40 %, so they are suitable for consolidating embankment structures. Their ength can be over 10 km. The use of silica cables is restricted when the strain is over 1 %. Cables may be also be integrated into a geosynthetics or a geotextile, which is said to improve the detecting accuracy (Djicker et al. 2011). Geosynthetics also increases the stability of the structure at the same time. (Habel & Krebber 2011, Djicker et al. 2011.)
Erosion, lightning strokes and chemically aggressive environments do not disturb the functioning of cables. (ICOLD 2014, Habel & Krebber 2011.)
One optic fibre can include several sensor chains and the measurement accuracy is high; precision is ± 0.5 °C. Measuring range in embankments is up to + 30 °C. Also spatial resolution is good, around 1.0 m. Strain measurement interval is approximately 1 m, even 0.02 % strain can be detected. Length of standard cable system may be over 10 km. If the length is longer, i. e. up to 30 km, the resolution decreases. (ICOLD 2014, Beck et al. 2010, Djicker et al. 2011.)
There are several commercial systems available (for example Sensornet, http://www.sensornet.co.uk/industries/environmental/dams-dykes-levees.html, http://www.sensornet.co.uk/images/PDF/download95dd.pdf). Optical fibre measurement systems are generally independent and remotely controlled. Data is recorded automatically (ICOLD 2014). Professional personnel is needed for data interpretation.
The optic fibre system is installed into predefined sections of a structure, such as drainage systems, downstream toes, slopes, or appurtenant structures. The system may contain either bare cables or fibres that are integrated into geosynthetic structure. Installation has to be “loose”, allowing the cables to move together with the structure. Installation is possible either during construction or afterwards. Temperature calibration is required after the installation is made. (ICOLD 2014.)
Beck, Y. L., Khan, A. A., Cunat, P., Guidoux, C., Artières, O., Mars, J. and Fry, J. J. 2010: Thermal Monitoring of Embankment Dams by Fiber Optics. 8th ICOLD European Club Symposium 2010, Austria.
Dijcker, R., Van Der Wijk, M., Artières, O., Dortland, G. and Lostumbo, J. 2011: Geotextile Enabled Smart Monitoring Solutions for Safe and Effective Management of Tailings and Waste Sites – Two case studies: Volgermeerpolder (The Netherlands) and Suncor (Canada). Proceedings Tailings and Mine Waste 2011, Vancouver.
Habel, W. R. and Krebber, K. 2011: Fiber-Optic Sensor Applications in Civil and Geotechnical Engineering. Photonic Sensors (2011) Vol. 1, No. 3: 268–280.
International Commission on Large Dams (ICOLD) 2014: Dam surveillance guide. ICOLD Bulletin Preprint 158.
Johansson, S. & Sjödahl, P. 2004: Downstream Seepage Detection using Temperature Measurements and Visual Inspection – Monitoring Experiences from Røsvatn Field Test Dam and Large Embankment Dams in Sweden.
National Research Council (2007): Assessment of the Performance of Engineered Waste Containment Barriers. The National Academies Press, 2007, Washington, DC. 121 pp.
Optimon (no date): Maapatojen suotovesien valvonta. Brochure. http://www.optimon.fi/ Read 3.2.2014
Zhu, P., Leng, Y. B., Zhou, Y. and Jiang, G. L. (2011): Safety Inspection Strategy for Earth Embankment Dams using Fully Distributed Sensing. 2nd International Science, Social Science, Engineering and Energy Conference 2010: Engineering Science and Management. Procedia Engineering 8 (2011) 520–526, Elsevier. 7 pp.