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assumptions, roof fallout is predicted. It is less likely that the roof will fall out during devel- <br />opment because the roof span is less and the roof material is crystalline nahcolite: amore <br />homogeneous material than the nahcolitic oil shale. <br />4.2 EXTENT OF ROOF FALLOUT <br />With both the 100 and 125 ft caverns indicating zones of yield in the roof, some roof <br />fallout is to be expected. The extent of that roof fallout can be estimated by removing material <br />from the model until a stable geometry is established. This procedure was followed for the <br />100 ft diameter cavity u~ evaluate the probable extent of roof fallout using the wors[ case (roof <br />strength parameters (RMR=40%o). <br />Figure 10 shows the geometries modeled in the development of the stable opening <br />• geometry. The final geometry analyzed shows roof fallout to above the tubber bed but <br />approximately 17 ft below the dissolution surface. Figure 11 shows the effective stress vectors <br />and the safety factors for the final geometry analyzed for the operational fluid pressure distri- <br />bution. This shows some zones of predicted yielding over the pillaz wall, but noeffective tension <br />in the immediate roof. The nonlinear analyses results are also shown (Figure 11C) for this <br />geometry and indicate drat although some additional yielding will occur in the immediate roof, <br />~ ~ no additional failure is indicated above the pillaz wall. <br />Ultimately, when the fluid pressure in the cavern is in equilibrium with the lower aquifer, <br />failure may progress in those areas where the rock quality is poor (RMR=40). In the majority <br />of the areas, the roof should stabilize at or below the configtuadon shown in Figure 10. <br />Bulking of the failed roof will eventually limit the extent of vertical fallout and collapse <br />of the overlying strata Tests conducted by IRI, the lessee, indicated a bulking factor of 1.3 <br />with an extraction height within the Boies Bed of 24 ft and an insoluble percentage of 20% and <br />an effective extraction height in the trapezoidal cylindrical cavity of 18.8 ft. The formula <br />predicting the height, H, of caving is <br /> <br />where k is the bulking factor and /:, the effective extraction height. <br />This Cormula predicts a maximum height of fallout of 63 ft. This would place the maximum <br />• height of caving at 1862 ft below the surface, 81 ft below the R63 zone, and 280 ft below the <br />B-Groove. <br />J. F. T. Agapito & Associates, Inc. 23 <br /> <br />