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II ~ 4.3 INFLUENCE OF CAVERN LENGTH ON STABILITY <br />The current EIS for the NMI property identifies long prismatic solution cavities developed <br />from long horizontalboreholes. Thecavemshapeistrapezoidalwithwallsinclinedat45degrees <br />from the vertical, a flat floor approximately 12 ft wide, and a roof span of 58 f[. The cavems <br />would be developed from production wells 300 to 600 ft apar[ with well spacings between the <br />cavities of 72 ft. <br />Recent horizontal drilling technology allows increased spacing between the production <br />wells without impactinc; [he resource recovery. Spacing of wells between 1000 and 1500 ft <br />appears to be technically feasible. NMI has asked that we assess the structural stability impacts <br />of increasing the length of the solution cavities from 600 to up to 1500 ft. <br />The stability of cavems is influenced by the span of the cavem and the length-to-width <br />~ ratio. Short caverns with low length-to-width ratios aze more stable than long cavems. However, <br />i this stabilizing effect is only noticeable when the length-to-width ratio is less than approximately <br />~ 3. For lazger length-to-width ratios, the stability is controlled only by the width of the cavern. <br />• For example, in concrete slab design, if the length-to-width ratio is greater than 2, the slab is <br />designed as if a beam. <br />r, <br />a '- For NMI solution mining, the EIS plan has alength-to-width ratio of 5.2 to 10.3 for 300 <br />to 600 ft long caverns. The proposed plans would increase this ratio to up to 25.6. <br />The longer cavems will expose more roof material increasing the potential to intersect <br />poor quality rock in a given cavem. However, this effect does not strictly reduce the stability <br />of the cavem. The potential to intersect poor quality roof rock is a function of the total quantity <br />of resource removed and the in situ variability of the roof rock. If zones of poor roof rock are <br />intersected in shorter Caverns, stability will be equally impacted. <br />4.4 PILLAR STAIILITY <br />Pillarstability focuses on the spacing ofcavities and the strength of the crystalline nahcolite <br />in the remnants in the Boies Bed after solution mining. Strength tests of the crystalline nahcolite <br />were completed on six samples of core from the NaTec 26-88-1 core hole, four of which were <br />from the Boies Bed. One of these samples was tested with a confining pressure of 1000 psi, <br />• the others were uniaxial tests. The average strength of the 3-1 /2 in. diametercrystalline nahcolite <br />1 <br />J. F. T. Agapito & Associates, Inc. 26 <br /> <br />