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': SUBSIDENCE PROTECTION AND t10tIITOR[11G PLArI Protection tlethods While subsidence is expected to have a very limited effect on the sur- face flows of the Try Fork basin (particularly spring runoff)Horse Creek and South Prong, ARCO recognizes the need to protect these streams from subsidence effects. It is therefore ARC O's intention to protect the channels of the Dry Fork, Horse Creek and South Prong from subsidence effects by creating a safety zone under .t he channel. Mining in this safety zone will leave 50 percent of the coal in place as protection pillars to support the channels. Protection pillars amounting to 50 percent of the coal are felt adequate to prevent sub- sidence and should remain stable indefinitely. This is consistent with Pennsylvania statutes (enacted in 1966) concerning subsidence in populated areas. In these areas subsidence must be prevented entirely and therefore the state requires that 50 percent of the coal must be left in place as protection pillars. The width of the safety zone will be determined by projecting a 69.5 degree angle of draw measured from the horizontal downa+ard from the creek bed to the coal seam (Figure 2). This means that the width of the safety zone in the coal unit will increase as the depth of over- burden increases. The 69.5 degree angle of draw was based on measure- ments by C. R. Dunrud in the Somerset Area. In fact this angle of draw is more conservative than Pennsylvania statutes, which require a 75 degree angle of draw measured from the horizontal. Using the 69.5 degree angle of draw the width of the safety zone in the coal zone will vary from 40 feet in the shallow overburden areas (do~+nstream sections) to 640 feet in thick overburden areas (up <.;tream areas). Additional evidence for the adequacy of a 50 percent limited miniry zone comes from a report by Allen and Paone of the U. S. Bureau of ~4ines who state: "The potential for sucsidence is yreatast where the r1 tl,J (10)