PERMFILE139786 - Laserfiche WebLink

Home Browse Search

SubTerra, Inc. Twentymile Coal Company 5/23/99 Northern Mining District Rockfall Hazards Evaluation ' were previously collected during 1994. Task 2 included characterization of slope • geomorphology and identification of the size for the block/boulder to be used in the rockfall t analysis. The scope of work for Task 3 included selecting sections for rockfall analysis, setting up the CRSP3 data files, performing the analyses, and summarizing the results in this report. ' 1.3 Report Contents The results of the study are presented in five sections. Section 2 contains a summary of the Phase I work, Section 3 describes the derivation of the CRSP3 model parameters used in the NMD analysis, Section re kfall hazards assessment and recommendations for mitigation, and Section 5 contains references ci e m e xt. 2. SUMMARY OF the PRIOR PHASE I RESULTS ' This section presents a summary of the prior Phase I rockfall hazard assessment project completed by SubTerra, Inc. in October, 1994. Much of the information in the Phase I report is included herein for completeness. 2.1 Slope Morphology ' In the 1994 investigation, morphological features were recorded by SubTerra personnel to supplement Twentymile Coal Company's 1-inch to 200-feet topographical maps and to aid in M developing CRSP3 model parameters. Features of interest included upper and lower slope breaks, cliff base and crest lines, and man-made features such as leveled areas, drainage ditches, and CAT roads. ' The general distribution of the surficial materials appeared to be related to the slope breaks, however, they were mapped only in sufficient detail to enable appropriate rebound parameters to be assigned in the CRSP3 models. These surficial materials consisted predominantly of talus and colluvium. Talus appeared to be restricted to the upper part of the slope, and included some buried or partially buried material. The colluvium is considered to be a widespread deposit consisting mainly of fine sand or silty fine sand with a variable amount of sandstone pebbles, cobbles, and boulders. In areas where the marine shale horizon is close to the surface, the coiluvial matrix contained a higher proportion of sift and clay sized material. 2.2 Structural Regimes And Boulder Characteristics The Twentymile sandstone outcrop mapped in the 1994-study area had a massive, unjointed ' appearance for significant lengths. However, the 1994 cliff failures, in response to undermining by Panel 6, confirmed that significant jointing was, in fact, present. This jointing resulted in the formation of a variety of block sizes, some of which were very large. Closer inspection ' revealed that jointing was present in the outcrop, although weathering or surface patina tended to obscure these features in many areas. ' 2.2.1 Scanline Survev • Although the intent was to base the design block size primarily on the boulder catalog, it was ' deemed necessary to make an estimate of the block size distribution based on jointing in the ' 2 PN: 9915