Application of Distributed Fiber Optic Sensing for Subsurface Levee Monitoring

Distributed fiber optic sensing (DFOS) offers the ability to create a continuous profile of strain and temperature measurements along an installed fiber optic cable. This technology has been successfully applied to a range of geotechnical applications, including recent advances in direct embedment in soil through installation in geotechnical boreholes. In this paper, the effectiveness of subsurface DFOS fiber optic strain and temperature monitoring to measure deformations and temperature changes is explored as a levee monitoring tool. The hypothesis is that vertical soil displacement and temperature changes in the subsurface can be measured and correlated with river water levels and uplift below the clay foundation. A field trial was performed to investigate this phenomenon, comprising of the installation of four vertical fiber optic arrays at a test site in Black Hawk, Louisiana, on a section of levee along the Mississippi River. A sand boil had previously developed in this area and was remediated using a combination of a gravel pad and a series of relief wells. Despite the remediation, the levee is still being actively monitored. A fiber optic monitoring system has been developed at U.C. Berkeley and deployed on site to provide readings of the long-term subsurface strain and temperature fluctuations along the landside of the levee. This data is supplemented by piezometric readings within the borehole arrays and monitoring of the river levels at a nearby station. The preliminary results have shown that concentrated tensile strains can be observed at the interface of the clay foundation and the underlying sand layer, correlating to fluctuations in the river level and foundation water pressures. This research demonstrates that DFOS can be used to detect small fluctuations in subsurface movements and temperature, as well as act as a tool for monitoring seepage and the potential for detecting issues prior to sand boil development.