1999-04-26_PERMIT FILE - M1999051 (2)

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0 0 • The same conclusions apply to the structural stability of a 225 foot diameter single cavern as do apply to the 125 foot diameter single cavern. Results of analyses for the 225 foot diameter cavern suggested wall slabbing and a small zone of shear failure in the roof near the roof/wall intersection. These zones of failure were very similar to the results from the 125 foot diameter cavern. • The results suggest that cavern roof and wall stability are insensitive to diameter variations for the range of cavern diameters analyzed (125 feet to 225 feet) and exemplifies the stabilizing effect the fluid pressure and residual solids within the cavern have on cavern stability. The stability of multiple caverns also was evaluated to assess pillar stability within the well fields. Multiple cavern stability was analyzed for the proposed six well field (125 foot to 225 foot diameter caverns on 350 foot spacings). A 330 foot barrier pillar was assumed separating the well fields. • Evaluation of the structural stability of the proposed six well layout on 350 foot centers indicate that pillars between caverns will remain stable during and after solution mining for cavern diameters ranging from 125 foot to 225 ft. Although analyses predict pillar core temperatures will approach the injection temperature of 250OF for the 225 foot diameter cavern, the confinement provided by the fluid pressures within the caverns stabilizes pillar material preventing failure. As was true in the single cavern stability evaluation, cooling of the cavern appears to enhance stability and promote long-term stability. • The 330 foot barrier pillars are structurally stable for the geometries analyzed (125 foot to 225 foot diameter caverns on 350 foot centers). Load transfer to the barrier pillars would not become significant unless yielding or pillar failure occurred within the well field. Hardy and Goodrich (July, 1996) also concluded that measurable subsidence over the proposed six well field (125 foot to 225 foot diameter caverns on 350 foot centers) is unlikely due to the anticipated stability of the pillars during production and enhanced stability after cooling. Given the stability demonstrated in the single cavern and multiple cavern analyses, negligible, if any, deformation is anticipated at the Dissolution Surface, Mahogany Zone, or on the ground surface. Based on these results, no impacts are to be expected on the overlying aquifer systems. The above appears conservative based on mathematical calculations by PB-KBB, Inc. (January, 1999) that cavities with 300 foot spacing between the centers of the cavities will have stability and subsidence characteristics similar to the 350 foot centers evaluated by Hardy and Goodrich. G:\LMW178545.002\Reclamation%Reclamexh.000 U-3