2018-10-24_PERMIT FILE - M2018055 (6)

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E Slope Stability Analysis June 29,2018 Page 2 excavation occurred in the past. Vegetation consists of patchy grass, randomly dispersed clusters of sage bushes, and a variety of small and large trees. The Eagle River is less than one-half mile north of the site. 4.0 SITE GEOLOGY According to Tweto et al.(1978),the site is situated within Pleistocene-age unconsolidated gravel including stream,terrace, and outwash gravels deposited by the Eagle River.Although Tweto describes this material as "gravel," our investigation, as well as investigations by others (as noted below) discovered substantial amounts of cobble-and small boulder-sized clasts.The majority of clasts in this unit consist of igneous and metamorphic rock types such as granite,granodiorite,gneiss, schist,quartzite, and basalt, although there is also some fine- to medium-grained red sandstone. The gravel is commonly well-sorted, rounded to well- rounded, and imbricated. The bedrock underlying the site is the Pennsylvanian-age Eagle Valley Evaporite.This unit is composed of gypsum, anhydrite, and interbedded siltstone with minor dolomite and shale. It assumes a distinguishing popcorn-like texture on exposure surfaces formed as a result of wetting and drying cycles. Dissolution voids and subsequent subsidence are commonly associated with this unit. Furthermore, settlement of surficial material may result from hydrocompaction and/or piping of underlying evaporitic rock. Although Tweto et al. (1978) indicates that the unit thickness of the Eagle Valley Evaporite is indeterminate, Lidke (2002) states that the distribution of the unit in the Geologic map of the Eagle quadrangle, Eagle County, Colorado suggests that the total thickness may be as much as 2000 meters. A dense alkali basalt known as the Basalt of Bimodal Suite is present approximately one and three miles north and northwest, respectively, of the site. Locally,this unit reaches thicknesses of 200 feet. It contains interbedded tuffs and conglomerates.The Weber Sandstone and the Maroon Formation are present approx- imately two miles southwest of the site. See Figure A.2 for a geologic map of the site. 5.0 SITE INVESTIGATIONS 5.1 D A Smith Drilling Company (DASD) Drilling/Gravel Exploration DASD was hired by Roaring Fork Resources to perform what they refer to as a drilling/gravel exploration at the site in 2000.This exploration consisted of four-inch diameter traditional auger drilling in four differ- ent locations, which are unknown at this time. Generally, DASD logged a five- to seven-foot soil layer followed by a two-to four-foot gravel layer underlain by a four-to seven-foot cobble layer and subsequent boulder layer of indeterminate thickness. The cobbles reportedly ranged in diameter from three to six inches. Due to the nature of traditional auger drilling, DASD was not able to drill to desired depths and reached refusal when what they described as "boulders"were encountered. 5.2 Deere and Ault Consultants, Inc. (D&A) Drilling Program D&A reportedly implemented a drilling program from January 15-21, 2008,consisting of seven boreholes advanced with a Boart Longyear DeltaBase540 rig using ODEX methods. Figure A.3 shows the location of each borehole. D&A identified four main material types including 1)topsoil,very moist,non-plastic; 2) silty sand (SM, SP), loose to medium dense, moist, non- to slightly plastic, locally with gravel, locally poorly graded; 3) sandy gravel (GM, GP, GC), dense to very dense, moist, locally silty or clayey; and 4) gypsum, hard, slightly to moderately weathered, slightly to heavily cemented, dry to slightly moist, non- plastic. The thickness of topsoil ranged from 0 to 1 foot. Silty sand was reportedly up to 14-feet thick on the western portion of the site, but boreholes on the eastern portion of the site only contained a maximum silty sand thickness of 4 feet. Only two out of seven boreholes revealed sandy gravel thicknesses bound by