SGER: Distributed Measurements of Temperature, Moisture and Strain for Civil Engineering Applications in Subsurgace Using a Fiber Optic Sensor

Lehigh University, Bethlehem PA

Investigators

Abstract

This SGER project is for the development of a new fiber optic sensor assembly for continuous monitoring of temperature, moisture and strain over extended subsurface environments, such as road embankments, wall back-fills, and landfills. At present, the cost of fiber optic sensors is prohibitively expensive for large-scale applications in such Civil Engineering facilities. The proposed sensor is based on Stimulated Brillouin Scattering (SBS) phenomenon to achieve spatially distributed measurements. It meets the necessary requirements of subsurface monitoring by measuring selected parameters at different locations with only a single fiber, rendering it a relatively low cost tool. The objectives of this project are to construct a prototype SBS sensor system, and evaluate its functions in the laboratory. The system will consist of a sensing element and a signal generator/analyzer. The signal generator uses a single laser source that is modulated through an electro-optical modulator to generate both pump- and probe- lightwaves. The sensing element will be developed using standard optical fibers. Two bare fibers will be used to measure strain and temperature as calibrated to their signal variation. For moisture sensing, a third fiber will be coated intermittently with a hydrogel material along its length. A semi-rigid and permeable composite polymeric fiber braid jacket will be used to encase the hydrogel-coated optical fiber. The jacket will provide protection to the fiber as well as a constrained space for the hydrogel. The swelling of the hydrogel in a constrained space around the optical fiber is expected to induce axial strain on the encapsulated fiber. The magnitude of the axial strain will be dependent upon the degree of swelling and, in turn, to the degree of available moisture in the surrounding. Bench-scale laboratory tests will be conducted to test and verify the spatially distributed sensing capability of the SBS sensors. The strain sensing experiments will be conducted by using simple pulley and weight assemblies. The moisture and temperature sensing will be tested using test chambers instrumented with heating elements, thermocouples, humidifiers and humidity sensors to control and measure the temperature and humidity. In all the experiments, the variation of the sensor signals will be mapped to the variation of the actual measurements. The prototype sensors will be tested and evaluated for coupling effects of temperature, moisture and strain when all three parameters may induce signal variation at the same time. The final set of tests will be conducted by embedding the prototype sensors into a bench-scale test box filled with layers of soil at different moisture contents, representing near real subsurface environments. The proposed sensor assembly and its methodology is envisioned to significantly advance the state-of-art in monitoring of civil infrastructure systems in the subsurface with highly precise measurement capability over extended area and depth at a relatively low cost. This project will involve the collaboration of investigators with diverse expertise from two institutions. The graduate students will be exposed to multi-discipline subjects and research experience.