Research Grant for Outstanding Researchers
Abstract: Distributed Fiber Optic Sensing for Monitoring Underground StructuresReturn to previous page
By Kenichi Soga, University of California at Berkeley, US
The critical deterioration of civil infrastructure has driven the search for new methods of rehabilitation and repair by incorporating sensors and developing remote systems that would allow monitoring and diagnosis of possible problems occurring. It is envisaged that structures will eventually be able to monitor themselves and inform owners of their state. These smart structures have unusual abilities: they can sense a change in temperature, pressure, or strain; diagnose a problem; and initiate an appropriate action in order to preserve structural integrity and continue to perform their intended functions. Sensors measure the state of the actual ambient conditions. If the sensor signals differ from the nominal conditions, the rehabilitation action can be taken. The application of smart structures for buildings is a rapidly growing area of research. There are a number of benefits to smart structural technologies; the most obvious one is the increased safety levels they can provide to cope with adjacent new constructions and with natural disasters such as climate change, flood warnings and earthquakes. Furthermore, these technologies will also be able to reduce costs associated with end-of-life structures. However, in order to exploit this technology, there is a need to know how load develops, how it is distributed and what factors need to be understood in case of changes in the loading conditions.
There has been a rapid development in the area of smart structures over the last decade thanks to innovation in sensor/actuator design and fabrication, fiber optics, micro-electro-mechanical sensors (MEMS) and other electronic devices, signal processing and control, and wireless sensors and sensor networks. Structural integration of fiber optic sensing systems represents a new branch of engineering which involves the unique marriage of: fiber optics, optoelectronics and composite material science. Optical fiber sensors have a number of advantages over their electrical counterparts. The transmission of light down an optical fiber is an established technique in optical communications for carrying information and is the primary candidate for resident sensing systems. Fiber optic sensing techniques have been developed as part of aerospace research because of its use in monitoring aeronautical and space structures composed of advanced materials. This technology can be transferred to the field of civil engineering to provide new opportunities in sensing and smart structures.
Design limits are frequently based on strain developing in the structure. Although strain measurement is well established, current practice has until recently been restricted to measurement of point-wise strains by means of vibrating wire (VWSG) or metal foil strain gauges and more recently by fiber optics utilizing Fiber Bragg Grating (FBG) technology. When instrumenting building components such as columns or beams where the strain distribution is merely a function of the end conditions and applied loading, point sensors are suitable to define the complete strain profile. However, where structures interact with soil (e.g. underground infrastructure such as foundations, tunnels or pipelines) or indeed in the case of a soil structure (road or dam embankments), the state of the structure is not fully understood unless the complete in situ strain regime is known. In the context of monitoring strain in underground structures, capturing the continuous strain profile is often invaluable to pinpoint localized problem areas such as joint rotations, deformations and non-uniformly distributed soil-structure interaction loads. In this paper, cases that utilized distributed fiber optic sensing for monitoring the performance of underground structures are presented. The novel aspects of this technology lies in the fact that tens of kilometers of fiber can be sensed at once for continuous distributed strain measurement, providing relatively cheap but highly effective monitoring systems. The system utilizes standard low cost fiber optics and the strain resolution can go down to 2 micro strains. The distributed measurement nature of this technology clearly differentiates from the other discrete point-wise strain measurement technologies. The aim of this paper is to demonstrate the importance of distributed strain measurements to monitor the performance of underground structures. Using the distributed strain data, the performance of underground structures that require rehabilitation, repair and reuse is shown.
Kenichi Soga is Chancellor’s Professor at the University of California, Berkeley. He obtained his BEng and MEng from Kyoto University in Japan and PhD from the University of California at Berkeley. He was Professor of Civil Engineering at the University of Cambridge before joining UC Berkeley in 2016. He has published more than 350 journal and conference papers and is co-author of “Fundamentals of Soil Behavior, 3rd edition” with Professor James K Mitchell.
His current research activities are Infrastructure sensing, Performance based design and maintenance of underground structures, Energy geotechnics, and Geotechnics from micro to macro. He is a founding member of the Cambridge Centre for Smart Infrastructure and Construction (CSIC) at the University of Cambridge and led the sensor and data analysis group. He is a Fellow of the UK Royal Academy of Engineering and a Fellow of the Institution of Civil Engineers. He is recipient of awards including George Stephenson Medal and Telford Gold Medal from the Institution of Civil Engineers and Walter L. Huber Civil Engineering Research Prize from the American Society of Civil Engineers.