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Geotechnical Responses to Cazier's Comments <br />March 11, 2011 Memorandum <br />bearing capacity). If this is the intention, the workplan must indicate how these supported <br />soils will be compacted as well as how the load bearing soils will be compacted. <br />Response #2 <br />The steel set footings will bear on rock. The geotechnical engineer will observe footing <br />excavations during construction to confirm rock is exposed in footing excavations. To <br />estimate the factor of safety for bearing pressures of 16,000 lbs/ftz of footings bearing on <br />rock, we used the rock strength range obtained from the Portal Support Calculations Table <br />(Exhibit 6.5-2, page 3), and Terzaghi's bearing capacity factors and equation. The results <br />are presented on the following table. <br />Footing y cohesion H BC Factors Bearing (psf) <br />width <br />(ft) depth <br />(ft) Pcf Kpa psi psf deg. Nc Nq Ny Qu Qw F.S. <br />1.5 1.5 120 100 14.50 2088 15 12.9 4.4 2.5 27,958 16,000 1.75 <br />1.5 1.5 120 200 29.01 4177 20 17.7 7.4 5 75,714 16,000 4.73 <br />1.5 1.5 120 100 14.50 2088 20 17.7 7.4 5 38,748 16,000 2.42 <br />1.5 1.5 120 200 29.01 4177 15 12.9 4.4 2.5 54,900 16,000 3.43 <br />As shown, factors of safety across the range of conservatively obtained strength values <br />range from 1.75 to over 4.7. <br />Comment <br />3. On page 3 of the GSE (Highwalls), the application states, "Potential geologic hazards will be <br />evaluated once site access is granted... " This statement contradicts the statement made on <br />page 2, referenced in Item I above. Please clarify the applicant's certainty as to whether <br />geologic hazards exist or not relative to highwalls and portals. Also, see Item 7 below. <br />Response #3 <br />The statement on page 2 was made with reference to slopes at 1.5:1 or flatter. The referenced <br />statement is more general in that it references all construction on steep slopes. <br />Comment <br />4. Page 3, Bench Stability: The last paragraph states "Existing mine benches were created in <br />the early 1900's with no signs of failure or instability. " The Division is concerned that if <br />footings for the proposed steel sets is constructed on top of historic fill, that additional <br />settling may occur due to the increased load and potentially compromising the safe operation <br />of the mill. Please provide either a demonstration of the stability of the historic fill (if <br />structures are to be founded on it) or avoid placing structural supports on any historic fill. <br />Also, see Item 6 below. <br />5/25/2011 <br />2