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March 7998 Sktmp Ln~estigation oral Mitigation Report ~ Page 6 4.0 FIELD INVESTIGATION 4.1 PHYSIOGRAPHY AND LOCAL GEOLOGY The slump is located in the southern end of the permit area. It is approximately 100 feet downhill from the top of a ridgeline that runs roughly east-west near the edge of the former disturbance area. The slope where the slump is located faces to the nonh-northwest (350 degrees azimuth) and has an average dip of approximately 10 degrees. The scarp at the head of the slump dips approximately 30 degrees; the slope of the main portion of the slump dips approximately 10 degrees; and the toe of the slump dips approximately 40 degrees. There is also a zone between the head scarp and the main portion of the slump that is roughly horizontal. The slump is approximately 100 feet wide in the downhill direction and 150 Eeet wide across the hill and is asymmetrically shaped, elongated obliquely to the dip of the hillside, as shown in Exhibit I-I. Based on its surface expression, the slump does not appear to be more than 8 to 10 feet thick near its center, which is consistent with the trench and borehole observations. Bedrock within the vicinity of the slump was encountered at approximately seven feet below ground surface (bgs) consisting of completely weathered to unweathered siltstone. Apparent fill material was present above that with about 1 to 154 feet of organic topsoil. The structural attitude of the bedrock bedding beneath the slump, as measured in an outcrop on the south side of the adjacent ridge, is N90W, 8N (i.e., dipping 8 degrees to the north, roughly parallel to the hillside in the vicinity of the slump), which is consistent with the regional geology. 4.2 TRENCHING On September 3, 1997, a trench was excavated along the axis of the slump. The trench was approximately 3 feet wide, 7 feet long and 9 feet deep. Due to the depth of the trench, it was not possible from a safety standpoint for the MWA geologist to get into the trench and directly describe the trench walls. The apparent stratigraphy of the pit was as follows: • 0 to approximately 1 to l Ik feet bgs -dark brown organic topsoil; • llk to 7 or 8 feet bgs -brown gravely fill material wi[h cobble-sized pieces of flagstone in the upper three to four feet; and, • 7 to 9 feet bgs -dark brown to dark gray silt or weathered siltstone with evidence of calcium carbonate and sulfur precipitation. At 9 feet bgs, the excavator met with refusal due to a medium gray, finely laminated, strongly consolidated siltstone. Some pieces of the bedrock from 7 to 9 feet bgs contained gouging and slickensides. These results are generally consistent with what was observed in the boreholes, as described in the following section. Montgomery Watson Americas ' R O. Box 774018 "Steamboat Springs, Colorado • 80477 (97Gf 879-6260 ~Y"\P]!\SlvmQ Ni,gu~on\Sbmp Rrynn 1: )1NA~/v