Fiber Optic Strain Gage Installation at the DUSEL Site

Collaborators at Montana Tech and the University of Wisconsin-Madison plan to use fiber optic strain gages to monitor rock deformation at the Deep Underground Science and Engineering Laboratory (DUSEL) site in Lead, SD. This new fiber optic technology, accurate to 1-microstrain, has been used in engineering applications, but this is the first installation in hard rock underground. The goal is to study the performance of the new technology and make use of the resulting rock deformation data. Distributed and local strain measurements will inform the design and construction of large caverns and contribute knowledge to several geomechanics, hydrology, and geoengineering studies at DUSEL. Installations are planned for the 4100 and 4850 levels. The first installation, in July 2009, is on the 4100 level in the Precambrian Poorman formation. In the installation area, the Poorman formation consists of a strongly foliated and lineated mica schist with garnet porphyroblasts and disseminated pyrrhotite along bedding planes. Quartz veins oblique to foliation are boudinaged. The area also includes several joint sets oblique to the foliation and our sensors. The gages installed in July 2009 are six Micron Optics os3600 temperature-compensated strain gages, 1-m long. The photo shows a triplet, a sensor array of three mutually perpendicular sensors. Each triplet has two surface mounted gages and one embedded gage. Mounting brackets for the surface gages are welded to the heads of 5-ft deep DYWIDAG rock bolts. The embedded gages are grouted into 5-ft deep reamed holes drilled 10 degrees downdip. A portable laser interrogator, Micron Optics si325, collected data during installation confirming the operation of each gage. An industrial laser & computer in an environmental box, Micron Optics sm130 controlled by an sp130, will be installed in October 2009. It will be connected to the 4100 level AC power and Ethernet and will continuously record data. The online data will be controlled and accessed remotely. Fiber Optic strain gage triplet: 2 surface gages, 1 embedded gage. 4100 Level. July 2009.