Instrument selection, installation, and analysis of data for the spent fuel mine-by, Nevada Test Site, Climax Stock

During the time period of February to April, 1979, twelve rod extensometers and twenty-two convergence measurement points were installed, calibrated and monitored in support of the spent fuel mine in granite. Readings taken during heading and bench advance shows good instrument stability, with little or no anchor creep or slippage. Repeat calibrations indicate excellent repeatability. Measurement points within the heater drifts indicate little closure. Convergence pins within the spent fuel drift were subjected to significant blast damage that resulted in a discontinuous record. A numerical analysis of the stresses and displacements of the rock mass as a result of the mine-by was performed. Two methods, finite element and displacement discontinuity, were used to model the mine-by. Comparison of the actual to predicted displacements show good agreement for the 33{sup 0} and 50{sup 0} extensometers for a rock mass modulus of 3 to 5 x 10{sup 6} psi and Poisson`s ratio of .2. The horizontal extensometers however indicate a convergence of anchor and collar (divergence predicted) and the IRAD stressmeters installed within the pillar indicate a significant reduction in vertical compression during mining of the heading. A simple reduction of pillar modulus will not account for the observed stress and displacement changes. Varying the ratio of vertical to horizontal stress ratio over the range .8 to 1.25 also did not account for observed stresses and displacements. It is concluded that the displacements and stresses are a result of block motion or joint slippage within the pillars. This is primarily the result of the small dimensions of the pillars in relation to the spent fuel and heater drifts. This joint slippage can account for the formation of a stable stress arch around the openings and thus a relaxation of the pillar