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• Nature of the Experimental Practice Colowyo is presently constructing a permanent fill in Streeter Draw. The spoil placed in these fills will not be required to achieve approximate original contour as shown on the Post- mining Topography Map (Map 19) and the Fill Design -Experimental Practice Map (Map 30). Colowyo has undertaken geotechnical engineering studies to demonstrate that spoil pile fills in this area can be constructed with 100 foot lifrs, and no durable rock underdrains and still meet or exceed the public safety and environmental protection standards intended by regulation. See Exhibit I2, Experimental Practice Information. Use of the Experimental Practice The alternative method of excess spoil pile construction at the Colowyo operation provides an opportunity to advance technologies of mine fill construction beyond the stringency of existing design criteria to assist in the formulation of a systematic approach to tailor the design standards found in regulations for specific conditions found in the arid West. The proven effectiveness of the practice will be an economical savings not only to Colowyo, but ultimately to the consumer. Environmental Protection The location of the excess spoil pile in Streeter Draw is shown on the Existing Structures - • South Map (Map 22). The fill structure has been designed so that drainage will not be directed over the outslope and all surface water runoff from the area above the fill will be diverted into designated diversion structures as shown on the Hydrology Map -South (Map 12) and the Fill Design - Experimental Practice Map (Map 30). Runoff will be directed to the sedimentation pond located at the base of the fill. The remaining fill to be placed in Streeter Fill will be placed in lifrs not to exceed 100 feet. Detailed geotechnical work has been performed on the Streeter Fill to evaluate and 2.06-6