2018-05-22_PERMIT FILE - M2018027 (4)

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Mr. David W. Hunt May 13, 2018 Page 3 • Oil&Gas Facilities surrounding the site(Anadarko) • WCR 25.5& ROW(Weld County Public Works) • Highway 85(CDOT) • Rail Road (Union Pacific) • Suncor Gas Line • Private Irrigation Appurtenances(owned by land owner) • Overhead Power Line(North, South, East&West Boundary,XCEL) • Power Line(Private) • 34-6 Well Head (Kerr-McGee/Anadarko) • 23-6 Well Head (Kerr-McGee/Anadarko) • 33-6A Well Head (Kerr-McGee/Anadarko) • 25-6 Well Head (Kerr-McGee/Anadarko) • 24-6(DIR)Well Head(Kerr-McGee/Anadarko) • 43-6(DIR)Well Head(Kerr-McGee/Anadarko) • Telephone,west, south and by Hwy 85(Century Link) • Telephone/Fiber by Rail Road ROW, east side(Level 3 Communications) STABILITY ANALYSES Recently, Division of Reclamation and Mining Safety(DRMS)staff drafted a policy regarding stability analyses of neighboring structures. The draft summarizes adequate factors of safety(FOS)for non-critical and critical structures. The structures around the DHF mine are,for the most part,considered critical structures. Discussions the author of the memo, Mr. Tim Cazier, indicate the FOS will be adopted by the MLRB. The FOS are for both static and seismic(from an earthquake) stability analyses. For generalized strength assumptions and critical structures, an FOS of 1.5 is considered sufficient for static conditions and an FOS of 1.3 is considered suitable for seismic conditions. The stability of structures within 200 feet of the proposed mining limits was evaluated at four sections under anticipated loading conditions around the perimeter of the site as discussed below. The computer program XSTABL was used for the analysis. The method for selecting the critical failure surface for each analyzed loading condition is the following. The Modified Bishop's Method of Analysis is used to find the critical failure surface by randomly searching with 20 termination points and 20 initiation points(400 failure circles)with 7 foot line segments over a broad range of the slope surface and at the structure in question. This procedure is repeated over different initiation and termination locations until the most critical factor of safety failure surface is identified. The range is narrowed and 20 initiation points and 20 termination points (400 failure circles)with 7 foot line segments for the final run of 400 circles to determine the lowest factor of safety. Therefore, prior to submitting the final stability run, at least 800 failure surfaces were analyzed to determine the lowest factor of safety. Both static stability under anticipated mining conditions and seismic stability under peak ground acceleration loads were performed. Seismic loading was obtained from the U.S.G.S. Unified Hazard Tool. Review of the Hazard Tool indicated a maximum horizontal acceleration of 0.084g with a return period of 2,475 years for the site. The four cross section locations were selected and analyzed as described below. The locations are shown on Figure 1. ► Section 1: This section is on the west side of the main cell and considers a tall highwall at a point where the mine limit is 30 feet from a gas line. This is the most critical section as it marks the point where a tall highwall (deepest bedrock)and closest structure coincide. Everywhere else around the site,the highwall is shorter when structures are within 30 feet of the mine limit. In other areas, structures are greater than 30 feet from the mine limits. ► Section 2: This section is on the east side of the main cell and shows the case where the soils contain some clay and are slightly weaker. In this section,the highwall is approximately 29 feet from the slurry wall and