Validation of Fiber-Optic Strain-Sensing Cable for Deep Underground Application

The laboratory tests presented here are in preparation for rock deformation monitoring at the Deep Underground Science and Engineering Laboratory (DUSEL). These tests validate the strain-sensing capability of fiber optic cable. In contrast to traditional, point-measurement sensors, this Distributed Strain and Temperature (DST) cable provides continuous data along its length, up to several kilometers. The fiber optic cable was subjected to increasing tensile strain in two different test setups. The first setup checked for slip between the glass fiber and the outer layers of the cable. The second setup proved that the experiment design is adequate for comparing recorded strain with analytical strain. The first test setup held a 3.5-meter cable between two clamps with screws that allowed fine increases in cable length. These changes in length were measured with a digital caliper and were used to calculate incremental strain values. The data sets for caliper-measured strain and recorded strain plotted as parallel straight lines, indicating no slip between the fiber and the cable’s outer layers. Our lines also matched the manufacturer’s slope and intercept within 10%. The second test setup approximated an ideally deflected beam. The concrete beam had a 3-meter cable resting on top, attached at each end. An analytical solution provided strain values as a function of beam thickness and radius of curvature. The beam was incrementally bent by placing it on supporting arches of increasing center height. The arches were cut to various radii for center deflections of 0 - 5 inches. For each radius, there was a reasonable comparison between the recorded strain and analytical strain. Ongoing understanding and application of the DST system may improve that comparison. The ultimate goal is to develop a robust and practical attachment method that does not allow slip between the cable and the rock. Continuing lab work will test various methods of attaching the entire cable length onto concrete beams. As additional validation, point-measurements of strain will be provided by electric foil gages and fiber optic gages. Future testing will be conducted outdoors on large concrete blocks and rock outcrops, in preparation for underground deployment at the DUSEL site. Second Test Setup: Concrete beam supported by a circular arch