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SubTerra, Inc. Twentymile Coal Company 5!23/99 <br />Northern Mining District Rockfall Hazards Evaluation <br />Plates 4 and 5 show the slope profile above the county road and the immediate foreslope that <br />contains a majority of released blocks. Plate 6 is a close-up of the source area showing the <br />• 'slab-like' characteristic of released blocks. This shape, in combination with a relatively low <br />release height, minimizes the potential for released blocks to move very far from the outcrop. <br />Finally, Plate 7 illustrates the undercutting action that leads to the formation and subsequent <br />release of blocks. <br />Inspection of the upper outcrop area (upper left in Plate 7) indicated the presence of a small set <br />of rock blocks that could detach and drop approximately 2-ft. The design block size was <br />selected based on bed thickness and the spacing of joints in this area. An 'equivalent' disk, with <br />a diameter of 4-ft and a thickness of 5-ft was subsequently used in the CRSP3 analyses. <br />4. ROCKFALL HAZARD EVALUATION and MITIGATION <br />Three Sections (Drawings 2 through 4) were developed from the topographic data and Section <br />C was selected as the critical section due to its relative proximity to the county road. Four final <br />rockfall simulations were performed using the zones illustrated in Drawing 5. Two simulations <br />were made using a surface roughness for the immediate foreslope of 50% of the design block <br />diameter while varying the release height between 2-ft (SECC2) and 4-ft (SECC1). Two <br />simulations were also made using a surface roughness for the immediate foreslope of 33% of <br />~, the design block diameter while varying the release height between 2-ft (SECC4) and 4-ft <br />(SECC3). The results of these simulations are presented in Appendix A. <br />~•. <br />The results illustrate that release height is less important than surface roughness in determining <br />the potential for failing blocks tom reacFi"fFie county road. TILe results also indicate that released <br />i blooms are ~ o reach the county road when they are not retaine on a imme 9T€-'-" <br />foreslo e. <br />The above discussion and analysis results rely explicitly on the assumption that the outcrop will <br />fail and that blocks will detach and drop onto the immediate foreslope with a nominal horizontal <br />velocity (about 2-mph). In reality, there is very low probability that blocks will detach from the <br />outcrop face. By analogy, the area of detached blocks in Phase 1 was associated with a zone <br />of complete undermining where the cliff was subject to cyclic tensile-compressive-tensile forces. <br />In comparison, the outcrop in the Northern Mining District will be subject to less than 30-in of <br />subsidence and tensile strains. In addition, the combination of favorable structure (bedding dips <br />gently into the slope) and rough foreslope conditions is expected to prevent blocks from <br />"~ traveling beyond-the-irmmediate~orestope-- ------_ <br />Nevertheless, in recognition of Twentymile Coal Company's conservative approach to risk <br />/ management, we recommend that an loose or potential source area blocks be restrained or <br />( removed prior to undermining. This may require temporary closure of County oad 33 for short <br />'~ periods of time. We do not believe that remedial measures similar to those implemented for the <br />~.\ hase 1 study area will be necessary if the above mendation is implemented -- <br />5. REFERENCES <br />CTI, 1993. Colorado Rockfall Simulation Program (CRSP), Users Manual for Version 3.0, <br />Colorado Transportation Institute, December 1993. <br />• Pfeiffer, T.J. and Bowen, T.D. "Computer Simulation of Rockfalls", Bull. Of Assoc. of <br />Engineering Geologists, Vol. 26 1989. <br />7 PN: 9915 <br />