Distributed Fiber Optic Sensing for Deep Foundation and Soil Vertical Strain Monitoring

Geotechnical engineering, more so than any other branch of engineering, relies on measurement for the design and execution of our works. The natural materials that form the basis of geotechnical engineering design cannot be specified in a factory – they were formed through geologic processes and are already in place, awaiting discovery by the engineer. To characterize their properties, the materials must be measured. The execution and performance of the design must also be measured if it is to be confirmed to conform to the quality and behavior expected by the engineering team. Since the subsurface structures cannot be directly observed, we rely on instrumentation to measure their behavior.Distributed Fiber Optic Sensing (DFOS) refers to a range of technologies that enable the measurement of physical phenomenon along a fiber optic cable. DFOS relies upon the fiber optic cable as both the sensor as well as the data conduit. By carefully interrogating changes in light that is sent through the fiber optic cable, measurements such as strain and temperature can be taken in a continuous readout along the cable. This technology has been in development for several decades and has been adopted for applications within civil engineering, including geotechnical engineering. The unique ability to generate continuous measurement profiles offers a significant benefit over conventional point-based instrumentation, particularly in applications where the strain and temperature fields are changing in non-linear and unpredictable ways. This research explores the use of DFOS for the monitoring of strain and temperature in deep foundations during loading and vertical soil displacements. These applications are particularly suited to the benefits that DFOS offers, specifically the continuous strain readout that is produced by fiber optic monitoring. Best practices for installation and monitoring were developed, and these advancements were applied on a series of field projects to ascertain their performance and gain insight into the behavior of the monitored assets. The applications included monitoring of strain development in deep foundations during post grouting, monitoring of strain in deep foundations during top-down load testing, monitoring of blanket uplift on a river levee, and monitoring of vertical ground deformations during surcharge loading of a soft clay site. In each case, the author developed novel improvements for the design, installation, monitoring, and interpretation of DFOS data to provide new insight into the geomaterial and geostructure behavior. In the application of strain measurements in deep foundations, three major improvements and new findings were explored and verified. For the installation phase, the current state of practice of pre-tensioning strain cables prior to concreting was replaced with installation with slack removed but without any additional pre-tension. The encapsulation in concrete was proved to be enough to fully transfer compressive strains within the foundation element to the fiber optic sensing cable through validation data using conventional point-based strain measurement. During the data processing, installation of 4 vertical strain cables over the standard 1 or 2 verticals in the literature allowed for identification and analysis of non-uniform strain distribution across the pile at single depths. This variation has often been ignored or discarded in favor of averaging the strain measurements across the pile, missing real non-idealized pile behavior during loading, or incorrectly discarding valid data. Finally, lab and field testing was performed to capture and quantify the effect of fiber optic analyzer architecture on the processed strain readout when the strain along the sensing fibers is not constant during the reading interval. This effect has not been identified or explored and was shown to have a quantifiable and correctable impact on the data, as well as highlighting an additional performance metric to consider when selecting a fiber optic analyzer for a particular application. In the application of DFOS for soil vertical strain monitoring, two improvements on the state of practice were developed. The first is the detailed exploration and demonstration of the benefit of continuous strain measurements over point-based measurements in the identification and quantification of local subsurface strain. The highly local nature of these strains, as well as the judgement required in the placement of point-based measurement points, result in DFOS offering a clear advantage over the standard state of practice. The necessity for temperature compensation of subsurface DFOS strain measurements was also explored and quantified. The current state of practice is to treat the subsurface temperature as stable, however measurements show that surface temperature fluctuations can impact the soil temperature to significant depths and must be corrected prior to integration of the distributed strain profiles to calculate cumulative surface displacement.

Advisor: Soga, Kenichi