Development of fibre optic based sensors for critical road, railway, port, bridge infrastructure

Roads, ports, bridges and pipelines are critical infrastructure that underpin the economies of Australia and Indonesia. Ageing and degradation can affect the integrity and performance of infrastructure, as can increased use and climate change. Threats to infrastructure include frequent and more intense floods, increased heavy road traffic, ground liquefaction during earthquakes, increased corrosion due to moisture fluctuation and water pressure surges in pipelines. Failure of these assets can lead to lost revenue due to downtime and, in some cases, greater financial loss for repair and remediation.

Both practical experience and literature indicate that integrated structural health monitoring can significantly improve asset management and safety. Such monitoring often requires considerable planning, especially in deploying sensors, setting up alarm systems, post-processing of signals and data, and cost-efficient analysis. There are many structural health monitoring techniques available commercially, but there is no perfect solution for monitoring large and long civil structures. This is because most sensors are best suited to localised monitoring, where the weak point or monitoring region is known. With infrastructure like pipes, railways and bridges, identifying weak points is a challenge. There is therefore a need to develop advanced sensing technology to perform real-time, continuous, distributed and permanent integrity monitoring for ageing infrastructure.

Over the years, distributed fibre optic sensors have gained attention. Their potential to be used in structural health monitoring has been successfully demonstrated, especially in structures such as rail tracks, pavements, tunnels, harbours, slopes, bridges and pipelines. The biggest advantage of distributed sensing over conventional sensing is the ability to monitor thousands of points over a single fibre spanning kilometres. Building on existing strengths, this project focused on developing novel fibre optic sensors for application in monitoring critical infrastructure, and on studying the practicality of these applications.

This project was broken up into three major tasks:

• The development of a sensor to measure water pressure in changing conditions.
• The development of sensors to be used under roads.
• A review of the use of these fibre optics sensors in monitoring a range of infrastructure types.

From existing literature, almost all fibre optic sensor deployment methods have required the fibre to be attached to or embedded in the structure. This restricts the application of fibre optic sensors to new and smart pipelines. A challenge remains for the use of fibre sensors for the assessment of existing, buried and old pipelines. In the first task, an optical fibre sensor package was designed with the intention of providing ‘contactless’ deployment to continuously monitor water pressure and detect leaks via anomalous vibration. A prototype package with multiple optical fibre sensors was constructed at Monash University and tested in in the civil engineering laboratory. In this study, the quantification of leak size with both detection methods deployed in the optical fibre sensors package was possible. The test also showed possibility of correlating the acquired information to minimise a false positive alarm.

The sensor package was also successfully duplicated and arranged in series forming a quasi-distributed sensor. The results showed that the proposed sensor deployment method significantly improved the sensitivity and spatial resolution of the monitoring to detect even small anomalies. Development of the quasi-distributed optical fibre-based pressure sensors and signal processing is ongoing, with the aim of improving accuracy and sensitivity.

A review was conducted on the use of optical fibre sensors to monitor pavements. Pavements are fundamental transport infrastructure that sustain vehicular and human traffic. The review found that to place fibre sensors under road, regardless of the monitoring technique, a number of factors must be considered in the planning stage. These include fibre protection, location of the fibre, orientation of and quantity of fibre, deployment method, and operating conditions. The ability of optical fibre sensors to detect and pinpoint the location of cracks in pavement has been successfully demonstrated. A novel application of fibre optics sensors to the evaluation of the flexural behaviour of pavement material was developed.

Findings

This project has developed a novel application of fibre optic sensors with the potential to be practical for monitoring leaky buried pipeline. The results obtained from the prototype constructed at Monash University show the capacity of the sensors to detect leaks and assess condition of pipeline. Nevertheless, the prototype will need to be fine-tuned. Once signal processing of the data is developed, it could assist asset managers to better manage buried pipeline, and even help predict the remaining lifespan of the pipe.

This project found an innovative application of fibre optic sensors for designing pavement. It showed that the sensors are capable of measuring the flexural properties (initially difficult to obtain) of pavement material. Knowing the flexural properties of the pavement material may lead to a paradigm shift in design. The results have been presented to pavement utilities in Australia, and some are willing to be involved in developing this concept.