AAI procured, installed, and monitored geotechnical instrumentation for measuring rock stress and deformation adjacent to the first longwall panel at the BHP Billiton San Juan underground coal mine. Instrumentation included 160 encapsulated borehole pressure cells (BPCs), 14 hydraulic load cells installed on cable bolts, 14 sonic probe extensometers installed in 20-foot holes with 20 magnetic anchors, 31 triple-anchor tell-tales, and 45 roof-to-floor convergence stations. AAI’s pneumatic Longyear CP-65 drilled NX core holes 20 feet into the roof and 10 feet into the floor at 5 locations. All drilling equipment was permissible for use in return airways.

A second instrumentation campaign was later completed using AAI’s SD-1500 electric–hydraulic rig and CP-65 pneumatic drill. Installed instruments included 44 BPCs with fiber-optic pressure transducers, 3 BPCs with vibrating-wire transducers, and 11 four-anchor multiple position borehole extensometers (MPBXs) equipped with vibrating wire linear transducers. Fiber-optic cables were routed through a vertical borehole to the surface and connected to Fiso DMI-32 data loggers. MPBX vibrating-wire transducers were connected to a Campbell Scientific CR-1000 data logger inside the mine and programmed with Geokon MultiLogger software. NIOSH-developed permissible data logger units were installed by AAI and field tested for the first time at the San Juan mine in 2010.

As part of a separate program, AAI installed three downhole time domain reflectometry (TDR) cables for monitoring subsurface caving and ground subsidence over the second longwall panel at the San Juan Mine. Three 5-inch-diameter boreholes were drilled from surface to the longwall horizon and 0.75-inch-diameter coaxial cables grouted in over the length of the holes. Specially designed stainless steel cable ends (designed by AAI and Northwest Machine Works Inc. of Grand Junction, Colorado) were used to facilitate reliable cable placement and borehole grouting. A portable, self-contained data acquisition system was constructed consisting of a Campbell Scientific, Inc. TDR100 time domain reflectometer, a CR10X-2M data logger, a BP24 12-volt rechargeable battery with a CH100 12-volt charger/regulator, and a MSX20 20-watt solar panel. The components were installed inside a weather-tight ENCTDR100 16-inch × 18-inch enclosure and attached to a protective frame. TDR waveforms, consisting of 2,048 sample points each, were recorded every 30 minutes as the longwall face mined past each cable location. Analysis of the waveforms provided a detailed, high-resolution record of strata separation and upward cave propagation that was ultimately useful for geotechnical mine design and gob gas reservoir characterization.


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