Validation and implementation of a new method for monitoring in situ strain and temperature in rock masses using fiber-optically instrumented rock strain and temperature strips

• New fiber-optic sensor for monitoring strain and temperature in rock masses.
• We utilize fiber optic Fiber Bragg Grating gages accurate to 1 με and 0.1 °C.
• The tool is embedded in an intact rock mass providing real-time in situ monitoring.
• Our monitoring tool records six strain and temperature data points over ~200 cm.
• Experiments validate that the tool is coupled to and more compliant than the rock.

Fiber-optically instrumented rock strain and temperature strips (FROSTS) are a new method for monitoring in situ strain and temperature in intact rock masses. FROSTS are an alternative to other techniques such as borehole extensometers or instrumented rock bolts for monitoring strain around underground openings. FROSTS have six fiber-optic strain and temperature gages installed at 30 cm intervals along specially designed strips of 304-stainless steel and are embedded into the rock mass. FROSTS are modified Rock Strain Strips (ROSS). If pretensioned prior to installation, the FROSTS can measure both shortening and elongation. Two FROSTS were pretensioned and installed on the 4100 level (~1250 m below the surface) of the Sanford Underground Research Facility (SURF) in Lead, SD. Data from the initial monitoring of these FROSTS record strain in the rock mass due to the closure of microcracks and fractures created during drilling for installation and then stabilization of the FROSTS sensors. The fiber-optic temperature sensors on the FROSTS also record a detailed thermal gradient of increasing temperature with depth into the rock mass. Laboratory experiments show that the FROSTS accurately measure strain, are more compliant than the rock mass they are embedded into, and thoroughly couple to the rock mass to ensure accurate readings. FROSTS provide an accurate and detailed record of in situ strain and temperature in rock masses and are useful for the structural health monitoring of underground spaces.