2011-04-18_REVISION - M1981185 (20)

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Response to CN-01 Preliminary Adequacy Issues for a 112(d) Application 6.5 Geotechnical Stability • Chief Portal Stability o The Chief Portal is a pre-existing structure built before 1926. The Portal was stabled in 2009 and the bench leading to the portal was graded to remove debris at the entrance. No additional material was removed from the headcut above the adit. The stability of the road has not been adversely affected by the activities related to the Chief Portal repair activities. (3) Where there is the potential for off-site impacts due to failure of any geologic structure or constructed earthen facility, which may be caused by mining or reclamation activities, the Applicant shall demonstrate through appropriate geotechnical and stability analyses that off-site areas will be protected with appropriate factors of safety incorporated into the analysis. The minimum acceptable safety factors will be subject to approval by the Office, on a case by-case basis, depending upon the degree of certainty of soil or rock strength determinations utilized in the stability analysis, depending upon the consequences associated with a potential failure, and depending upon the potential for seismic activity at each site. There are no areas adjacent to the permit area where potential off-site stability impacts could occur. The closest adjacent properties considered in the assessment parallel the access road were conducted on the new access road. The private owner property boundary is approximately 80 feet from the new access road cut slope. Based on geotechnical assessments presented in 6.5 (1), and (2) the cut slope is deemed to be competent and geotechnically stable. (4) At sites where blasting is part of the proposed mining or reclamation plan, the Applicant shall demonstrate through appropriate blasting, vibration, geotechnical, and structural engineering analyses, that off-site areas will not be adversely affected by blasting. The effects of blasting on adjacent structures have been conducted by Mr. Terry Morris (Colorado Registered Professional Engineer #28457). Mr. Morris signed and sealed report is included as attachment F where he concludes "using 100 pounds of explosive per blast (even without the benefit of delay timing) falls well under the predicted 1,100 pounds of explosive which could be detonated with no expected structural damage. Predicting surface air and ground blasting vibration is based on research information published on March 8,1993 by US Office of Surface Mining (OSM). GSM's developed the "scaled distance factor (SDF)" which is a conservative and accepted approach to estimating ground vibration without the use of seismographs For example, if a structure is locate from 0 to 300 feet from a blast site, the "scaled distance factor" would 50._ If the blast site is located from 301 to 5,000 feet away, the SDF would be 55 and for blasts where the structure is greater than 5001 feet away would have a SDF of 65. Using the following OSM blasting can be used to predict potential impacts. May Day Idaho Mine Complex 112(d) Permit Application Revised: April 14, 2011 4