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Rock Mechanics Analyses: Upon reviewing data from the vibration study, and learning that equipment generated vibrations operating at distances farther than 300 feet from the Rollins Sandstone would create very low intensity motions at relatively high frequencies averaging more than 35 Hz and less than 13 Hz, it was reasonably concluded that there is no need to study the structural condition of the rock bluff because impacts of normal weathering effects, temperature changes and perched hydrostatic water pressure in vertical joints within the bluff are much more likely to trigger rock falls that will certainly occur at some future time, whether mining occurs or not. It is well documented in scores of technical papers and geotechnical references that rock falls are commonly triggered by perched hydrostatic water in vertical joints or cracks. In perspective, at an expected maximum particle velocity of 0.05 in/s, the theoretical peak pressure exerted by compressive vibration waves at a velocity of 8,000 ft/s, in sandstone with an average density of 143.5 ]b/ft , and a modulus of elasticity of 6.5 x 106 psi, would be around 3.41 psi [0.05 / (8,000 x 12) x 6.5 x 106]. Perched water with a height of 25 feet creates a static pressure of 10.38 psi, which is three times greater than the peak theoretical pressure created by the expected peak vibrations. Gregory D. Lazear Letter -April 11, 2006: In this letter Mr. Lazear creates a hypothetical case whereby it assumed that a normal acceleration of 0.165g is required to keep a block of rock in place. A sinusoidal motion calculation is then used to predict acceleration, which is then doubled because differential acceleration of the block and main rock bluff are assumed. This whole argument whereby static forces like gravity are compared to dynamic forces is not appropriate and will almost always lead one to false conclusions regarding the stability of rock slopes exposed to low intensity vibrations. A very good reference describing the fallacy of these calculations is explicitly explained in "Explosive Engineering, Construction Vibrations, and Geotechnology" authored by Lewis L. Oriard. I quote from Page 125: Several basic concepts about stability analysis are attractive in their mathematical con- venience and have found their way into contracts, specifications, even regulations. The most common are those known as pseudostatic methods of analysis, based on the assumption that even dynamic forces such as blasting can be equated to static forces like gravity if we include a mod~ing factor (as opposed to dynamic forces like seismic waves). In some cases, the modifying factor has not even been included. The acceleration of the dynamic force (such as blasting) is either predicted or measured, then assumed to be applied to the slope as a continuing or steady horizontal acceleration. Unfortunately, this concept is physically inaccurate and leads to a false conclusion of slope failure in many situations which are quite safe. It is not appropriate to apply acceleration as a criterion for a wide range of frequencies, and certainly not as a static force. REVEY Associates, Inc. Page 4 of 13 4/19/06