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1 • • <br />MEMORANDUM <br />August 19, 1999 - 6 - 993 - 2057 <br />' the lower salt zone will be detected by the presence of NaC1 in the solutions. Since this is <br />detrimental to the nahcolite mining process immediate action would be expected to control <br />this. <br />' Uncontrolled solutioning of the sidewalls could lead to coalescence of adjacent cavities or <br />to excessive loss in pillar width and hence potential pillar collapse. The hazard associated <br />' with coalescence depends upon the extent of this connection. Early in the process <br />coalescence might lead to (at most) a channel between cavities in the TI zone. This would <br />be of minimal thickness, and would probably lead to collapse of the overlying rock. <br />' However collapse into such a narrow channel, combined with bulking of the rock, would <br />result in minimal upward extension of the collapsed zone and little, if any, subsidence <br />effects. Later in the process coalescence could occur over a larger channel, however again <br />rubblization and bulking of any collapsed rock would provide support and limit the <br />potential subsidence hazard. The likelihood of occurrence of this coalescence is judged to <br />' be very low at the pilot scale, where the pillar separating the cavities is 125 to 225 ft. wide. <br />This hazard combined with the very low likelihood is judged to lead to an overall risk <br />assessment of very low. <br />' If excessive pillar solutioning were to occur this could lead to an increase in stress on the <br />pillars. If these stresses were high enough to cause pillar collapse, the resulting subsidence <br />' hazard could be low to high depending on the extent of the collapse. However the pillars at <br />the pilot scale are quite wide, and the pillars receive lateral support from the process fluid <br />pressure and from the rubblized insolubles in the cavities. The likelihood of this <br />' mechanism is therefore considered negligible, as is the overall risk assessment. <br />One other cavity related failure mechanism has been identified. This involves the <br />' connection between the cavity and overlying aquifers along a fluid conducting fault or <br />fracture. This could lead to a negligible to high hazard in terms of groundwater effects, <br />however the likelihood of occurrence is judged to be negligible. This negligible likelihood <br />is assigned because of the absence of any evidence of a conductive fault at this time, and <br />the facts that any fault significant enough to lead to a connection would likely be identified <br />while drilling the process holes, and that any loss of fluid would be detected by change in <br />volume of the fluid cushion. Potential mitigation is discussed in Section 9. <br />1 8.2 Well Mechanisms <br />Two potential failure mechanisms have been identified in connection with the wells. The <br />first of these is the loss of the fluid cushion due to leakage along the cement bond, and the <br />resulting upward solutioning of a channel along the casing to the overlying aquifers. This <br />could result in a negligible to high groundwater hazard, however the likelihood is judged to <br />' be negligible since a poor cement bond would be detected by the initial well testing (MIT <br />and well log) and loss of the fluid cushion would show in the volume monitoring. The <br />overall risk is therefore judged to be negligible. <br />\ \ GAI_DEN1\ PROJECTS \WP\99 \2057\20577 M1.DOC Golder Associates <br />