2015-02-26_REPORT - C1982056 (3)

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Rockfall hazard risk analysis and mitigation in an area of coal mine subsidence Chris D. Breeds President, SubTerra, Inc., Seattle, WA Rick Mills Environmental Manager, Twentymile Coal Company, Steamboat, CO ABSTRACT: This paper describes the site characterization and design of rockfall remedial measures for a two mile length of the Twentymile Cliff subject to longwall undermining. A focused approach to data collection and interpretation is presented along with details of the failures that actually occurred and the performance of the remedial measures. 1. INTRODUCTION This paper documents an evaluation of a rockfall hazard assessment associated with longwall under- mining of the Twentymile sandstone cliff at Cyprus - Amax's Foidel Creek mine near Oak Creek, Colorado. The study area incorporates a section of County Road 27 and associated infrastructure that was located from 300 to 1200 feet horizontally from the base of the cliff. 1.1 Background The layout of longwall panels that could affect the stability of the Twentymile sandstone cliff is shown in Drawing 1. Predicted subsidence limits indicated that the cliff would be affected by mine workings below Zones 1 to 8 and 13 to 14. Maximum strains and displacements would be experienced in Zones 1 to 5, Zone 8, and Zones 13 to 14. In May, 1994, an area of the Twentymile sandstone cliff failed over Longwall Panel 6. This area was Northeast of the limits of the study area and was not previously evaluated due to the distance of the road from the cliff and the generally favorable foreslope topography. The locations of the two major failure areas and boulders that emanated from each area are shown on Drawing 1. 1.2 Approach The approach was primarily defined by the requirements of the Colorado Rockfall Simulation Program (CRSP), that was chosen to model boulder travel from the cliff towards the road, and previous experience. The overall approach was subdivided into 4 tasks Task 1: Field Work: Work under this task included source area characterization, foreslope characteriza- tion, and collecting Colorado Rockfall Simulation Program (CRSP) model calibration data. Task 2: Data Collation and Interpretation: Task 2 included preparation of a morphology map and boulder catalog. The boulder catalog contains location and dimensional data for boulders located on the foreslope. Task 3: Model Calibration and Analysis: The scope of work for Task 3 included: • Selecting representative sections for morpho- logical zones and structural domains. • Setting up the CRSP data files for each section. • Calibrating the CRSP model. • Running the CRSP model for each section and plotting the results on a contoured plan. Contours were drawn for the 0, 5, and 50 percent passing zones (these contours are analogous to sieve analysis where the 50% passing contour represents points on each section where the CRSP analysis indicates that 50% of the modeled boulders have passed). • Concurrently verifying model parameters based on the boulder catalog. • Preparation of a hazard map showing cliff and foreslope contours, position of road and major structures.