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-z- (2) The report also analyses the required barrier pillar requirements between adjacent mined panels. By analogy, in the case of workings designed to prevent subsidence, the mine design technique can be applied to the external barrier pillar requirements adjoining the permit area boundary. However, confusion might arise concerning the concept of the external boundary pillar. Tne consultant's projected 50' width for barrier pillars is intended to prevent subsidence by supporting the overlying trioutary load areas above adjacent mined panels. If the operator intends to allow subsidence to occur, by utilizing the more complete retreat approach, subsidence would propagate along the angle of draw upwards and outwards toward the boundary of the permit area. Therefore, if we are to preclude subsidence occurrence outside the permit area boundary, the external boundary pillars are determined geometrically, rather than by the strength stability requirements included within P1r. Abel's analytical calculations. In fact, the required barrier pillar width would be calculated as the product of the tangent of the angle of draw (angle of draw measured as angle from vertical) multiplied by the overburden depth. Assuming an angle of draw of 20.50, the required external barrier pillar width for a 900' overburden depth would be 336.4', and for a 1,200' overburden depth would reach 448.7' in width. (3) The report's analytical underground mine riorking design recommendations are: (a) If pillar yield is not to occur, in order to minimize subsidence, 30' x 80' pillars could be developed on advance (52% extraction) and the pillars could be slabbed to 16' x 80' during retreat (75% total extraction) without pillar failure. Within the report three different slabbed pillar widths are presented, 14', 15' and 16'. I have selected 16' for reference in this memo for purposes of conservancy. (b) If pillar failure (yield) and associated surface subsidence are acceptable, retreat pillars could be slabbed to 10' x 80' (84% total extraction), which should allow maximum recovery of coal, in combination with an appropriate route support technique, followed by roof and pillar failure. (c) The present 200' wide barrier pillar between mined panels could be safely reduced to 40' under overburden depths of up to 910' and to 50' at greater planned mining depths up to 1,200'. (4) The consultant's barrier pillar recommendations relate, however, to the currently utilized conventional 50' x 60' pillar configuration. The consultant should evaluate the barrier pillar stability implications if the recommended amended 16' x 80' final slabbed retreat pillar configuration is utilized.