Role of the interface between distributed fibre optic strain sensor and soil in ground deformation measurement

Geohazards are threats of a geological, geotechnical, hydrological or seismic/tectonic nature that can potentially damage pipelines and other infrastructure. Depending on the physiographic setting of a particular pipeline, a broad range of geohazards may be possible along the pipeline route. However, only a limited number of geohazards such as landslides, fault displacement, mining-induced subsidence, liquefaction-induced lateral spreading, and hydrological scour, which can result in permanent ground deformation or exposure of the pipeline to direct impact, typically represent credible threats to pipeline integrity. Identifying potential geohazard occurrences and estimating the likely severity of each occurrence in relation to pipeline integrity is an integral part of geohazard management, and overall risk management of pipelines. Methods for identifying and assessing the potential likelihood and severity of geohazards vary significantly, from purely expert judgment-based approaches relying largely on visual observations of geomorphology to analytically-intense methods incorporating phenomenological or mechanistic models and data from monitoring and field characterization. Each of these methods can be used to assess hazard and risk associated with specific geohazards in terms of qualitative, semi-quantitative, or quantitative expressions as long as uncertainty and assumptions are understood and communicated as part of the assessment. Engineering judgment is highlighted as an essential component to varying degrees of each geohazard assessment approach.

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Fibre optic Bragg grating strain sensor used to monitor the respiratory spectrum

Fourteenth International Conference on Optical Fiber Sensors ◽

10.1117/12.2302314 ◽

2000 ◽

Author(s):

Gunther Wehrle

Keyword(s):

Strain Sensor ◽

Bragg Grating ◽

Fibre Optic

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Leak detection on water pipelines in unsaturated ground by discrete fibre optic sensing

Structural Health Monitoring ◽

10.1177/1475921719881979 ◽

2019 ◽

Vol 19 (4) ◽

pp. 1219-1236 ◽

Cited By ~ 2

Author(s):

Schalk Willem Jacobsz ◽

Sebastian Ingo Jahnke

Keyword(s):

Unsaturated Soil ◽

Ground Deformation ◽

Bragg Gratings ◽

Leak Detection ◽

Bragg Grating ◽

Fibre Bragg Grating ◽

Fibre Optic ◽

Temperature Changes ◽

Water Leak ◽

Fibre Bragg Gratings

The article describes a study using discrete fibre optic sensing as a means of leak detection on water distribution pipes installed in unsaturated ground. A short length of pipe fitted with artificial leak points was installed, to which a fibre optic cable with fibre Bragg gratings was attached. An optical fibre with fibre Bragg gratings was also installed in the ground parallel to but separate from the pipe. Thermistors were installed at selected locations to measure temperature changes independent of strain. It was found that a simulated water leak resulted in clearly detectable temperature changes and thermally induced fibre Bragg grating wavelength changes in the ground around the pipe. However, significantly larger deformation-induced fibre Bragg grating wavelength changes were measured on the pipe walls and also in the initially unsaturated ground in response to leaks. A wetting front originating from a water leak propagating through unsaturated soil is associated with significant effective stress changes because the infiltrating water alters the ambient matric suction in the soil. This effective stress change is associated with significant ground deformation resulting in a fibre Bragg grating response which significantly exceeds the thermal response associated with (usually) colder water leaking into unsaturated soil. The study illustrates advantages of measuring ground deformation-induced fibre Bragg grating wavelength changes over pure temperature changes as an efficient means of leak detection in unsaturated ground. However, due to the limited number of fibre Bragg gratings that can be monitored along a single optical fibre, a leak detection system suitable for practical implementation should be based on distributed fibre optic strain sensing, an aspect requiring further research.

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