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~~ ~ - SubTerra, Inc. Twentymile Coal Company 4/7/00 Northern Mining District Rockfall Hazards Evaluation 1. INTRODUCTION • . This report presents the results of our Rockfall Hazards Evaluation at Cyprus Twentymile's Foidel Creek mine near Oak Creek Colorado. This phase evaluates potential hazards associated with longwall undermining of an outcrop of the Twentymile Sandstone adjacent to Coun Road 33, Panel 12-Right in the Northern Mining District wdl extract approximate-rSb eet of coal from beneath portions of the outcropping sandstone potentially causing failure, release of boulders from the failure zone(s), and hazards on Routt County Road 33 whose alignment is located at the base of the slope beneath the outcrop. The objectives of this report are to describe the field data collection and rockfall hazard assessment procedures, present the results of the rockfall hazards analysis, and rovide recommendat' ns for mitt ation measures. This report is the second in a series of reports by SubTerra, Inc., addressing rockfall azards and mitigation measures associated with the Foidel Creek mine. The initial, Phase I report (SubTerra, Inc., 1994) addressed potential impacts from panels in the southwest mining district on County Road 27. Much of the information in the Phase 1 report is included herein for completeness. 1.1 Background In May, 1994, an area of the Twentymile Sandstone cliff failed over Panel 6 in the western mining district. This area was East of the original, Phase 1 study area and was not previously evaluated due to the distance of the road from the cliff and generally favorable toreslope topography. However, data from this event were used to develop input parameters required for the Colorado Rockfall Simulation Program (CRSP) that was used in the Rockfall Hazards •~.~_ evaluations. The Phase I study eval fining in the southwest mining district. As a result of this study, approximately 3,000 feet of 20 feet deep trench was constructed between County Road 27 and the Twentymile cliff to intercept and contain boulders. In addition, a 1,500 foot section of the county road was re-located towards the south to enable construction of the remedial measures. A short section of the cliff subsequently failed during undermining and approximately twenty boulders, weighing between 10 and 900 tons were stopped by the trench. The Foidel Creek Mine is planning to extract coal from the Northern Mining District (NMD) at the Foidel Creek mine. This district will utilize 1000-foot wide longwall panels mined on 1,200 foot centers at a depth of approximately 9_o eet below the surface within the study area. Panel 12- i ht, the first Ion wall anel in the Northern Minin District, will undermine an outcrop o~Fit e - Twentymile sandstone cliff located adjacent to outt County Road 33, in Routt County, Colorado. The layout of longwall panels that will affect stability of the Twentymile outcrop is shown in Figure 1. Predictions indicate that the outcrop will be subject to up to 30-in of subsidence. 1.2 Scope Of Work The scope of work was defined by the experiences of the Phase I rockfall hazards analysis and mitigation at the Foidel Creek Mine and by the requirements of the Colorado Rockfall Simulation Program, Version 3, CRSP3 that was written by the Colorado Transportation Institute in 1993 (CTI, 1993). • - The overall approach was subdivided into 3 tasks. Fieldwork was confined to source area characterization and toreslope characterization as the remaining CRSP3 model calibration data 1 PN: 9915