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1.0 INTRODUCTION AND SCOPE 1.1 INTRODUCTION NaTec Minerals, Inc. (NMI) proposes to solution mine crystalline nahcolite from the Boies Bed in the Piceance Creek Basin, as described in the Environmental Impact Statement (EIS). As part of their permitting requirements, a mine plan must be presented annually outlining their plans for the next year and their 30-year mine plan. In 1991, NMI proposes to develop and monitora single cylindrical cavity of maximum possible dimensions and a long prismatic cavity, both in the Boies Bed. For the 30-yeaz mine plan, the operational design dimensions of the cavems and remnant pillars between them must be established. The maximum extent of caving after abandonment and pillar stability between cavems are of concern for the 30-year mine plan. Accordingly, NMI has engaged J. F. T. Agapito & Associates, Inc. (JFTA) to review the mine plan to assess the extent of roof failure and/or caving that could be expected during or after mining, and the adequacy of the pillars separating the solution cavities. The most recent analysis (JFTA, August, 1988) analyzed a number of potential fallout • zone geometries above 26 ft high by 58 ft wide long caverns with vertical walls to establish a stable configuration that would provide an estimate of the maximum probable extent of roof . caving. Analyses were done for both 46 and 60 ft pillaz widths. The results of this study indicated that failure could extend 19 ft into the roof of the cavems with the wider pillars. The mining height was not tt significant factor in cavern stability, and investigation of a larger height ~ was recommended. The present srudy was performed at the request of NMI to expand the earlier studies to incorporate the site-sptxific data developed in recent investigations and to incorporate recent fmdings by the EPA regarding the impact of mining on the lower aquifer. The variables differing from earlier analyses include increasing the length of the long cavems from 600 ft up to 1500 ft and the evaluation of cylindrical caverns mined from a single vertical production well. The new site-specific database developed from the coring of hole NaTec 26-88-1 has become available. The steps undertaken to accomplish these tasks included: • review of geologic and rock mechanics data and planned dissolution procedures, n U J. F. T. Agapito & Associates, Inc. 1 __ ~ k- , ...~.a.c:r ate:. - - _ .~._ ,ss- _y_