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Ms. Connie Davis January 4, 001 Page 2 Inc Post Piney Creek Alluvium. The alluvium over the remainder of the site is mapped as Piney Creek Alluvium. Cotton's (1978) mapping indicates the bedrock underlying the alluvium is most likely clayey sandstone ;and/or sandy claystone of the upper transition member of the Pierre Shale. However, the mapping and sandstone bluffs northeast of the site indicate sandstone, possibly the Fox Hills Sandstone, is nearby, :md may also underlie alluvium at the site. Aggregate Industries, Inc. provided us with summary logs of 43 exploratory borings and five test pits drilled and excavated at the site. Review of the logs indicates the general subsurface profile at [he site consists of approximately 1.5 to 11 feet of silt and clay topsoil and overburden overlying approximately four to 16 feet of sand and gravel on top of bedrock. From a geotechnical standpoint, the sand and gravel will form most of the mine slopes. These soils are generally strong and stable, particularly when dewatered. STRUCTURES WITHIN 200 FEET OF DISTURBED AREAS Aggregate Industries provided us with a list of permanent man-made structures within 200 feet of the disturbed ;area of the proposed mine. The structures on the list include: Natural gas, electric, and telephone lines running under the Peschel driveway 30 feet from the proposed mine; Two water lines running on the east side of County Line Road 50 feet from the proposed mine; County Line Road 50 feet from the proposed mine; and The Peschel residence located 150 feet from the proposed mine. STABILITY ANALYSIS We performed stability analyses in order to evaluate the potential for damage to permanent man-made structures. The most critical mining situation potentially affecting permanent man-made structures will be the 0.5:1 (horizontal to vertical) perimeter mine highwall located 30 feet away from utilities under the Peschel driveway. We reviewed the summary logs of Aggregate Industries' borings and found that the most critical section along the Peschel driveway is at Boring 27 where approximately seven feet of overburden overlies approximately 14 feet of sand and gravel on tap of bedrock. We modeled this 21- foot high highwall with a 0.5:1 (horizontal to vertical) slope and dewatered conditions using the XSTABL computer slope stability program. No laboratory soil strength analyses were performed. The soil strength parameters inputted into the program are typical values for the anticipated soils based on our experience, and are listed in the following table: Total Unit Saturated Unit Effective Effective Friction Material Weight (pcf) Weight (pci) Cohesion (pc-f) Angle (degrees) Overburden Clay 114.0 126.0 200.0 28 Sanc and Gravel 130.0 137.0 0 35 Clayey Sandstone/Sandy 124.0 134.0 100.0 30 claystone