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relatively steep slope of the end-dumped material to the final fill configuration having <br />a slope of 1 vertical to 3 horizontal with intermediate terraces at each 100-foot eleva- <br />lion as shown on Figure 1. <br />The discussion which follows on the design and stability of the spoil fill in <br />Streeter Gulch is prompted by the promulgation of interim regulations of the Office of <br />Surface Mining in December 1977 which in Section 715.15 state: <br />"(b)(7) Spoil shall be transported and placed in a controlled <br />manner concurrently compacted as specified by the regulatory <br />authority in lifts that are less than 4 feet thick in order to <br />achieve the densities designed to ensure mass stability, to <br />prevent mass movement, to avoid contamination of thg rock <br />underdrains and to prevent formation of voids. ." a <br />Some of the material which follows has been taken from earlier reports submitted <br />by the Colowyo Coal Compony to the U. S. Department of Interior Task Force appointed <br />in July 1977 to review the mining plan, to the Mining Enforcement and Safety Adminis- <br />tration, and from other technical reports prepared for the Colowyo Coal Compony. <br />Design Considerations <br />The general geology of the area is characterized by alternating beds of light- <br />colored sandstone and darker colored sandy and silty and carbonaceous shales. These <br />rocks which dip slightly to the north form the overburden above the uppermost coal seam <br />and between the individual coal seams, are jointed and weathered to some depth and <br />merge into the sandy and silty residual soils at the surface. The upper areas and walls <br />of Streeter Canyon are mantled by a thin layer of residual soil, and in some areas out- <br />crops of harder rocks are exposed in near vertical bluffs. Along the south side of the <br />canyon and at several areas along the north abutment, clinker-like materials, formed <br />by the burning of underlying coal beds, are exposed. Among the more important con- <br />siderations sought to be achieved in the design of the spoil fill and in the planning of <br />fill placement operations in Streeter Canyon are the following: <br />a Federal Register, Vol. 42, No. 239, of December 13, 1977. <br />2 <br />