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highs+~all nvning is initiated in each seam, it is recommended that barriers be left • <br />more frequently, say every 10 openings, until design performance can be evaluated. <br />At lower cover depths and higher mining heights, the 1.5 safety factor criterion <br />was sometimes satisfied by web pillars with width height ratios less than 0.~. Because <br />the Mazk-Bieniawski formula was derived from pillars with w/h ratios greater than 0.~; <br />NSA feels that some adjustment is necessary. The calculated pillar width was averaged <br />with one that has a w/h ratio of 0.5, and this is reflected in the design charts and tables <br />provided. Sti]], pillars with w/h ratios of less than 0.5 should be used with caution. NSA <br />recommends that if design guidelines call for a ti~eb pillar with a w/h ratio <OS, <br />panel ~~~idth should be halved (5 openings to start in a seam, ]0 for normal <br />operations). This practice should be followed until the performance of web pillars with <br />]ow w/h ratios can be evaluated. <br />5.0 NUMERICAL MODELING ANALI'SIS <br />The empirical method used for the web and barrier pillar design has been <br />confirmed by mining experience in a wide variety of mining types and geological <br />conditions. However, it does not account for properties of the rock mass, multiple seam <br />interaction, or roof/floor stability. Since al] of these factors are important to the proposed <br />highwal] mining at Seneca, numerical modeling was applied to 1) confirm the stability of <br />pillaz layouts developed using the design curves provided, 2) explore the effects of seam • <br />interaction between the Wolf Creek and Sage Creek Seams. 3) test the robustness of the <br />designs against cascading pillar failure. and 4) examine roof and floor stability. The <br />modeling approaches used were LAMODEL, anon-linear boundary element method for <br />examining in-seam pillar behavior; and UDEC, adistinct-element code for examining the <br />interaction and stability of the floor; seam, and roof in two dimensions. Pillar designs for <br />analysis in each study area were developed using the empirical design curves; as <br />explained below. <br />5.1 Design Procedure/De~~elopment of Modeling Cases <br />To optimize coal recovery, NSA recommends that web pillars be laid out on a <br />panel-by-panel basis as mining proceeds. This will allow chanties in the seam model <br />that result from mining experience to be incorporated into the mining plan. Accordingly, <br />modeling scenarios were developed by choosing asub-area out of each study area with a <br />width approximating a panel wid[h (20 openings) and a depth from the highwal] equal to <br />the planned penetration depth (].000 ft). Sub-areas were chosen to represent typical <br />portions of the Seneca IIR% and Yoast areas. Also, each panel should be designed 1'or <br />the greatest mining height anticipated. <br />For example; for the }l%adge Seam in Seneca Ill',%. the typical maximum mining <br />height is ]0 ft. For a particular panel; the Wadge cover typically ranges from 30 ft to 330 <br />ft, but designing for the maximum cover is, in NSA`s opinion; overly conservative. • <br />Seneca Coal Company' 14 NSA Eneinrerin~. lnc. <br />Hiehu~all Mine Design Repon June 2G0:'. <br />