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31-04-1994 11~34gM FP.OM SUIMIITT OFFICE • TO 1303428546'3 P.02 RE-EXAMINATION OF THE PIT WALL DESIGN AND STABILITY, t•1P:1 SPEC-AGG PIT P.obert H. Merrill January 1994 INTRODUCTION: Tliis examination an3 report was prepared at the request of t9r. John Hickman, Land Manager, [4estern/Mobile, and summarizes the factors leading to slope stability analyses to determine safe, sta- ble, slope walls at the Specification ?ggregate (Spec-Agg) Pit operated by idobile Premix. in Jef~'erson County a short dist+ance south of Golden, Colorado. The purpose of the report is to resp- ond to a part of a b7inerals Inspection Report prepared by t7r. William York-Feirn following an inspection made September tl, 1993. In his report ha, in effect, suggested that Mobile Premix :should perform a slope stability analyses of the bench walls under the added load of the backfill material and prove that the bas}:fill is stable; lie was also critical that the report "Rock StruCttsre Ex- aminations of the Slope Wall Stability Pre9lctions" (Merrill 1981) "does not include any hard data concerning rack stability, shear strength, evaluation of individual bench stability or detezmina- tion of the safety factor for potentially fsilure surfaces on benches or quarry walls". In this response there is a discussion of some of the past and present methods of slope stability analyses and the uncertainties that are faced in making the necessary assuptions required to quan- tify a so called "factor of safety" for slope walls. Conversel;~, the request for identification of failure surfaces can be addressed with a much higher degree of certainty. GENERAL CONSIDERATIONS: During the period from 1962 to 1967 the U.S. Bureau of !tines and the Kehnecott Copper CorporaL•ion jointly performed the first, full- scale design and test of a safe slope wall. in this test an intEns- ive examination of the geologic structure (strike and dip of faults, fractures joints or any other visual discontinuity or defect in the solid rock mass), a regional and surface measurement of the rock stress, and laboratory test of confined rock specimens was made to evaluate the safety factor of the designed slope wall prior to the steepening of an existing 45-degree slope wall to a wall thtit was from GO- to 70-degress. During the test instrumentation was install- ed in tunnels mined into the Slope wall to measure changes :Cn stress, strain, displacement and seismic response. The results of tl7e pre- mining tests showed that the constrained specimens broke along ex- isting defects in the rock mass and at substantially higher values than the measured rock stress in the walls. Somo of the specimens broke a values of 350- to 900-psi, which was about 1.5 timers higher than the stress predicted by the analysis of the stress around the hemispherical dish in a plate. The structure analyses disclosed high angle fractures so the so called "design" of the pit wens that the pit wall o~ould be safe if the high angle fractures were not exposed idaylighted) and the ratio of the working stress to the