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April21, 2020 <br />Page I <br />INTRODUCTION <br />As requested by Trapper Mining, Inc. (Trapper), Agapito Associates, Inc. (AAI) has <br />completed a geotechnical evaluation of the proposed spoil pile to be constructed to the southwest <br />of the Nighthawk (N)-Strike Pit (Figure 1). The pile will store approximately 7.6 million cubic <br />yards of spoil over a 3,500-foot (ft) X 1,500-ft surface footprint and will have a maximum height <br />of approximately 140 ft. The pile will provide temporary storage for the spoils generated in the <br />N-Strike Pit over a time period of 4-6 years. The pit will be sequentially mined in four blocks, <br />progressing west to east. Upon completion of mining and highwall mining within the N-Strike Pit, <br />the spoils will be returned to backfill the pit. The foundation material under the proposed spoil pile <br />footprint is predominantly decades -old spoils placed on the I -Seam floor with thicknesses of <br />approximately 30 ft towards the south and close 100 ft to the north. The old I -Seam mining cuts <br />are shown in Figure 1. A small portion of the spoil pile will be placed on the native ground. The <br />current topography at the site dips in a general northerly direction, which includes both old spoil <br />and no -spoil zones. The spoil pile will be constructed gradually using incremental bench heights <br />of 25 ft, from north to south. The overall slope of the pile is approximately 27 degrees (°). Given <br />the sloping nature of the terrain and the presence of old/weathered spoils in the foundation, a global <br />stability analysis was necessary to assess integrity of the spoils over the storage period in support <br />of Trapper's ground control plan. <br />The objectives of the study were as follows: <br />1. Characterize the foundation spoil strength parameters <br />2. Assess global stability of the proposed spoil piles, through <br />a. Numerical modeling of slope stability <br />b. Foundation stability analysis <br />3. Recommend remedial measures, as necessary <br />Development of Spoil Properties <br />As the first step towards assessing the stability of the spoil pile, AAI asked Trapper to <br />collect foundation spoil samples through trenching from within the proposed spoil pile footprint. <br />Three sampling trenches were excavated at three different locations along the approximate north — <br />south centerline of the proposed pile (Figure 1) and grab samples were collected from 1 to 4 ft of <br />trench depth. Foundation materials encountered in the upper 1 ft of depth were discarded, as they <br />were likely to have experienced excessive weathering. Representative spoil samples were directed <br />to Advanced Terra Testing (ATT) for material characterizations pertinent to the stability analysis. <br />Particle size analysis (screens only), moisture content, Atterberg index (plastic and liquid limits), <br />and large-scale direct shear (LSDS) tests were performed on the collected spoil samples. <br />Particle size distribution analysis indicated that the tested spoil material ranged from sandy <br />silt to clayey sand, although in reality, the spoils are likely to be silty to clayey sand with a generous <br />percentage of gravels/boulders, which were removed during the sampling. The moisture content <br />of the spoils varied between 6 percent (%) and 12%. The plasticity index, the difference between <br />the liquid and plastic limits, varied from 0 to 15, indicating relatively low plasticity of the fine <br />Agapito Associates, Inc. <br />