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

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All cut and fill slopes and all foundations have been designed to be stable under the geologic and soil conditions at the Rock School Sodium Lease property. No pond dam heights are over ten feet in height, no pond surface areas are over 20 acres, and no pond volumes are over 100 acre feet; therefore, all ponds are in compliance with the rules of the Colorado Division of Dam Safety. Any construction in flood plains will be designed to withstand the 100 year flood as required by stipulation in the Sodium Lease Renewal with the Bureau of Land Management. The three streams located near the Lease area (Corral Gulch, Stake Springs Draw and Yellow Creek) over the lengths of their channels are intermittent. The channels are not known to have frequent, excessive flooding which could create hazardous conditions. U-2 SOLUTION MINING CAVITY STABILITY Hardy and Goodrich (July, 1996) conducted a cavern stability evaluation for the Rock School Lease Project. This evaluation is included as a confidential document in this Application. Thermomechanical stress analysis were conducted to evaluate cavern stability. Cavity temperatures were assumed up to 300°F. Cavern growth was assumed to occur at an approximate rate of 32 foot increase in diameter per year. Due to the low permeability/porosity of the strata, the rock was assumed to be unsaturated and fluid pressures within the cavern exert a boundary pressure provided by the weight of a column of water to the surface. For single cavern stability: • Evaluation of the structural stability of 125 foot diameter single caverns indicate that cavern walls and roof will be stable during solution mining and after cooling. In fact, thermomechanical analysis indicate that cavern roof and wall stability increase after cooling. • Only small zones of shear failure were predicted for the 125 foot diameter cavern near the cavern roof/wall intersection. Analysis of fallout of the zone of failed material did not indicate a propensity for propagation of failure further into the roof. • During solution mining some slabbing of the cavern wall will occur. Depths of predicted slabbing are approximately 8 feet for the 125 foot diameter cavern. However, this material cannot fall out, because of the amount of insolubles that will nearly fill the caverns. If the failed material cannot fall out, propagation of failure further into the pillar is unlikely after mining. • After cooling, the thickness of the zone of failure in the wall is diminished suggesting that failure will not propagate after cooling. Based on the analytical results, it appears that cooling will promote long-term stability. Upon cooling of the brine, nahcolite should precipitate out and tend to reheal fracturing and promote stability. G:\LMM78545.002\Reclamation\Reciamexh.DOC U-2