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s s ~ t:lo~ M _2co5 - n'7/ December 24, 2005 Gregory D. Lazear 20508 Brimstone Rd. d~ ~~ZD~ Cedaredge, Colorado 81413 (970)856-6810 Mr. Steve Shuey Division of Minerals and Geology Grand Junction, Colorado Subject: Benson Brothers Proposed Red Shale Mine in Coalby Valley, Delta County, Colorado Dear Mr. Shuey, I am a geophysicist (M.S. Geophysics, University of Arizona, 1978) with 25 years of experience in seismology including; 1) engineering seismology used for mapping highway road cuts, slope failures in open pit copper mines (Butte Montana and Tucson, Arizona), and refraction profiling of placer gold deposits (Alaska Range); 2) exploration seismology data acquisition for oil and gas; and 3) seismic imaging research on three-dimensional imaging algorithms for computer processing of seismic exploration data. I worked for Conoco Inc. for 20 years and retired as a Senior Research Scientist in 1996. I have strong concerns regarding stability of the Rollins sandstone cliff that borders the south property line of the Benson Brothers proposed Red Shale Mine. The county has allowed Benson Brothers to remove the top 50 ft. of the ridge bordering their south property line, and lying directly above the Rollins sandstone cliff. In my opinion, the operation of heavy equipment on that ridge, including ripping of bedrock by bulldozers and shovels, poses a high risk of rock fall from the Rollins cliffs that would endanger the homes, lives, and property of three residents near the base of the cliffs. This concern results from an analysis of natural fractures in the Rollins sandstone. Fifty- seven measurements were made on joint sets in the Giff atwve the homes of Larry Chick and Janet Croissant. A Rose Diagram of the strike of the fractures (Figure 1) shows dominant trends with azimuth angles of 45, 95, 115, and 130 degrees. Dips associated with these fractures are all to the south and have values of 10, 20, 40, and 70-80 degrees, as shown in Figure 2. The orientation of these fracture planes relative to the cliff face creates a very unstable condition in which rock slides and rock falls can easily occur due to the lack of northward restraining dips. Figure 3 illustrates how the orientation of the joint sets produces broken wedges that can readily come out of the face. Figure 4 illustrates how the dip of the fractures is also favorable for land slides and rock falls from the cliff face. Evidence that rock falls occasionally occur includes a documented account of a fall within the last ten years (photo documented by Janet Croissant), and a distribution of large boulders from the slope below the cliffs to the vicinity of the buildings and home of Janet Croissant. There are several rock slabs poised above Janet's house that are of concern. One is an overhanging block about 6 ft. by 8 ft. that has open fractures on all three sides along the cliff and appears to be held in place by a few small chock stones. Another rock slab 30 ft. high and 8 ft. thick appears to be highly undermined with open fractures. Both slabs lie above basal slopes that would not stop a rock of this size from rolling down the slope toward out buildings and homes. I feel that due to the nature of the fractures, low frequency vibrations generated by heavy equipment on the ridge above the cliffs could dislodge large rocks from the cliff. If Benson Brothers are permitted to proceed by the State, ground motion monitors should be placed above and below the cliff to determine actual ground displacement that results from various activities, including rock crushing and ripping of bedrock by bulldozers. This data should be interpreted by a specialist in seismology and slope stability. Monitoring should be done by an independent agency