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The minability by seam of the various areas is summarized in Figures 29 through 33. • <br />Although not reflected in the tables or figures; another issue with respect to the <br />Sage Creek Seam is the minimum mining height of 50 in. In order to mine with a <br />sidedip, mining height is sacrificed to ]eve] the machine and still stay in-seam. Sidedips <br />in excess of about 7° would mean that the corners of the cut would be out of seam. This <br />would be the case in at ]east two of the six Saae Creek areas shown in Table 5 to be <br />minable. <br />The results in Tables ~ and 6 assume the standard 36-in lift cylinders that are <br />currently on the PM. Stewart 1`9yers of SHM indicated that the machine could be fitted <br />with 60-in lift cylinders without involving a major rebuild. This would increase the <br />maximum practical sidedip to 1~.6°, and would significantly increase the number of areas <br />minable (from 20 out of 38 areas at Seneca ]1VV to 28. and from 15 out of 23 areas at <br />Yoast to 18). However, the limitation of 7° for the Sage Creek would be unaffected. <br />Although in many areas the Wadge and Wolf Creek Seam heights exceed the <br />maximum cutting height of the PM (10.5 ft), it is NSA's opinion that multiple-lift <br />mining. with a stable septum between lifts; is impractical with the PM at Seneca. SHM <br />personnel have experience taking multiple lifts; but onh~ where fill or removal of material <br />is used to create a "second bench" for the machine. Another possibility, that of ramping <br />up into the top part of the seam, withdrawing from the hole; and going back to mine the <br />lower part of the seam from the same machine position; has not been field tested by • <br />SHM. Taking multiple lifts to create a single, tall opening may be possible by excavating <br />the bench. Even though this is likely impractical operationally. this option is reflected in <br />the design curves discussed later. However, in the modeling analyses described later, the <br />maximum mining height considered was 10.5 ft. <br />3.3 Roof Stability <br />An initial assessment of roof stability and unsupported stand time was made using <br />a combination of the CSIR Rock Mass Rating (RI\4R) and the NGI Q system (Jaeger & <br />Cook; 1976). A further examination of roof (and floor) stability was made using UDEC <br />numerical modeling. as described in Section 5.3. <br />The RMR and Q systems are rating systems that relate rock mass quality to case <br />history databases; including unsupported span; stand time; and support requirements. <br />Originally developed for the tunneling industry. they provide a means of comparing roof <br />stability from one area to another. <br />Because the type of data required for the Q synem was generally not available, <br />the RMR was used for the basic rating, with the t~~%o systems compared using the <br />relationship: <br />Q- ef(RMR-44)/91 (1) <br />Seneca Coal Company' 1 O NSA Engineering. loc. <br />Hi~hwall Mine IY_sign Repon lone?003 <br />