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Mr. Ed. Baker April 18, 2003 Page 3 Q 6.2: How much subsidence warns the potential collapse of the cavity that could extend into the R-6 Zone? A: Loss of return of cable signal, or flooding of the cable, will be considered an indication that movement has occurred and that collapse, to that point, is likely. If one cable fails above the end of another cable, then cable failure of the fast cable will be considered a premature cable failure. Potential failure into the R-6 will be possible after TDR failure is confirmed in the third lowest cable, that is, above the end of the second cable located at 75 ft above the Dissolution Surface. This will imply that cable failure has occurred between 75 and 95 ft above the Dissolution Surface, but not into the R-6 located 113 ft above the Dissolution Surface. The TDR cables are expected to fail at, or closely above, the interface between the "cave zone" and the "fractured zone." Earlier studies by Agapito Associates (1995)2 indicate that the cave zone could extend up to the L-5/R-6 contact and that vertical hydraulic conductivity above the cave zone is predicted to be undetectable. In the AAI (2002)3 report, an equivalent mine height of 37 ft resulted in a prediction of cave of 51 ft above the Dissolution Surface for medium hazd rock types appropriate for the mazlstones in the L-5. The mine height of 37 ft was selected based on mining of all the nahcolite from the Boise Bed to the Dissolution Surface, and no bulking of the insoluble mazlstone interbedded with the nahcolite. The fractured zone was predicted to extend up to 171 ft above the Dissolution Surface. The change in hydraulic conductivity in the fractured zone will be gradational from the cave zone up to the top of the fractured zone. The change in hydraulic conductivity is expected to be undetectable at the top of the fractured zone. The TDR cables will detect fracture opening or shearing of 0.4 to 0.8 inches. Such fracture opening or shearing is expected to modify the hydraulic conductivity of the surrounding rock mass. However, the groundwater flow patterns will not be modified because of the neutral pressure of the cavern relative to the overlying aquifers. The fracture opening, or sheazing sufficient to damage the TDR cable, is anticipated to be at the top of the cave zone or partially into the lower portion of the fractured zone. Caving from the cavern is expected to be gradual after some threshold width of cavern is exceeded. Hence, the cave development should be detected, first in the lowest TDR. This will give some warning that caving or significant deformation has begun. The operator may elect to slow mining or to direct mining to other portions of the cavern once caving has been detected. Once a cable is ruptured, water flooding will render that cable useless for future TDR monitoring, so monitoring will be limited to the higher cables. Agapito Associates, Inc. (1995), "An Evaluation of Impacts of High-Extraction Solution Mining on Overlying Aquifers and Minability of Oil Shale," Report to White River Nahcolite, Inc., November 22nd. 3 Agapito Associates, Inc. (2002), "Evaluation of Geotechnical Impacts-Pane12 Solution Mining," Report to White River Nahcolite, Inc., April 8th. Agapito Associates,