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-3- <br />N - 20 + 4T = O.1D <br />• where N - width of the barrier pillnr, T - thickness of pillar, <br />and D - thickness of overburden. Therefore, with T - 10 ft <br />and D - 480 ft. <br />N = 20 + 4(10) + 0.1(480) 108 ft.~110 ft. <br />The existing barrier pillar is 360 feet wide, 3.3 times the <br />width of [he calculated barrier pillar required. <br />To date, northern Coal Company has mined under <br />overburden in excess of 1,000 feet thick in the Rienau 02 Mine, <br />using standard solid pillars of 80 feet by 60 feet on 100 feet <br />by 80 feet centers with no detectable convergence or pillar <br />failure. Also, both 1st and 2nd East Sections are Life of <br />Mine Entries. That is, these main entries will be maintained <br />for the life of the Rienau C2 Mine for ventilation and haul- <br />age purposes. Therefore, there will be no retreat or second <br />mining in these entries. <br />Assuring worse case data, the following factors <br />have been deterrined: <br />(a) Entry pillars under the protected area <br /> have a factor of safety of 7.3. <br />• (b) Entry pillars adjacent to the protected <br /> area have a safety factor of 6.8. <br />(c) The Barrier Pillar between 1st and 2nd <br /> East is 3 times larger than required. <br />(d) Cirgin coal is adjacent to the northern <br /> edge of the protected area. <br />(e) There will be no retreat or second mining <br /> in 1st or 2nd East Sections. <br />(f) 1st and 2nd East Sections will be main- <br /> tained for Life of Mine_ <br />Considering the above factors, it can be safely <br />stated that there will be no disturbance [o the surface or <br />overlying strata in the protected surface area due to con- <br />vergence or pillar failure as mining ad-:ances. <br />n <br />