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• samples was 2607 psi. This is significantly lower than the 7000 psi indicated by earlier tests <br />from Dowell Schlumberger and may reflect the influence of inhomogeneities and bedding <br />~ separations in the core tested. The rock mass strength for the Boies Bed could then be 30 to <br />40% of the laboratory strength, dependine on the RMR as discussed in Section 3. <br />The venical stress on the pillars, o,, can be estimated using the tributary areas method <br />a-=6,1(1-e) , <br />where e is the extraction ratio and 6,,, the in situ vemcal stress. <br />(6) <br />The pillar strength depends on the rock mass strength and the shape and size of the pillar. <br />For flat pillars, ones with a low height-to-width ratio, the pillar strength can be higher than the <br />rock mass strength becau:;e of the confining stress in the interior of the pillar. <br />Conventional pillar:strength formulas are difficult to apply to this situation because of the <br />trapezoidal shape of the pillars and the fluid pressure conditions in the cavity and surrounding <br />rock. <br />• To evaluate the stability of the pillars, atwo-dimensional plane strain model of the cav- <br />~ ern(pillaz geometry was analyzed using the same material properties and operational prtessure <br />~. <br />~ conditions as for the cylindrical cavern stability analysis. Figtue 12a shows the geometry. The <br />lines of symmetry through the center of the cavity and pillar allow simulation of an extensive <br />array of cavities. The georetry shown has a trapezoidal pillar with 30 ft width at the top and <br />78 ft width at the bottom. Figure 12b shows the stress vectors and Figtues 13a and 13b, the <br />safety factor contours using suength parameters typical of the pillar (13a) and roof (13b). Both <br />~ safety factor plots use RMR=tiO rock propemes, typical of the expected conditions based on <br />the NaTec 26-88-1 core. Notice that there is a small zone of indicated failure in the wall of the <br />pillar (13a), but chat a yield zone above the pillar is predicted. <br />The zone of yield above the pillaz is of concern and could indicate potential pillarpunching <br />type failure, if such failure were extensive involving long lengths of the cavern and multiple <br />rows of pillars, some vertical movement of overlying strata could rt:sult. <br />The spacing of caverns, thus, should be such to enstue that the top of the pillars be grea[er <br />than 30 ft. Because of the potential for lateral curvature of the long horizontal holes and the <br /> <br />J. F. T. Agapit0 & Associates, Inc. 27 <br />