Report of Data Pertinent to Lake San Cristobal

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-3- t 10 COO The only information available describing the subsurface soils in the vicinity of the lake and outflow channel was contained in a report by Chen- Northern, Inc. The report was used in design of a replacement bridge across the outflow channel about 100 feet downstream of the lake. In the subsurface investigation, three borings were completed to a maximum depth of about 30' below the existing bed of the outflow channel of Lake San Cristobal. Boring No. 1 was located on the right abutment, boring No. 2 in the streambed, and boring No. 3 on the left abutment. It should be noted the Slumgullion slide mass reaches depths in excess of 100 feet in this location. Although it was reasonable to assume the Slumgullion slide was inactive at the bridge site, movement has been documented further upslope. The following is an excerpt of the most pertinent information from the report describing the subsurface soils at the bridge site. "The subsurface profile encountered in the borings was quite variable consisting of interlayered clay, sand and silty gravel extending the maximum depth studied, 40 feet. Sandy to very sandy clay was encountered in two of the borings in layers ranging from 8 to 10 feet thick. Penetration resistance values ranged from 9 to 29 blows per foot which classifies as having a consistency of stiff to very stiff. Swell- consolidation tests indicate that the soil is compressible. Moisture contents of the clay range from 34.0% to 39.8 %. Atterberg limits tests indicate the clay is highly plastic. I nterlayered gravel and clay was encountered in Borings 1 and 2 in layers ranging from 4 to 26 feet thick. This layer contained scattered to numerous cobble size material. The material is medium dense. Penetration resistance values ranged from 12 to 30 blows per foot. Silty, sandy gravel containing scattered to numerous cobbles was encountered in Borings 2 and 3 in layers ranging from 3 to 18 feet thick. Penetration resistance values range from 9 to 35 blows per foot. Materials having penetration resistance values in this range are classified as loose to medium dense.