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Lyons Sandstone Quarry Mr. Richard Spots Slope Stability Analyses mater and Earth Technologies 1.0 INTRODUCTION This project consists of a Sandstone Quarry operated by the Cemex, Lyons Plant ( Cemex), near Lyons, Colorado. The sandstone is extracted for use as a silica additive in the cement manufacturing process. The Sandstone Quarry is currently operating render a Limited Impact (I 10) Mining and Reclamation Permit administered by the Colorado Division of Reclamation, Mining and Safety (DRMS). The location of the Cemex Sandstone Quarry is shown on Figure 1. Engineering Analytics, Inc. (EA) previously performed a slope stability analysis for this project in August of 2013 (EA, 2013). In March of 2014 we were contacted to provide this analysis which includes revised pit slopes and wall configurations. The scope of work included completion of slope stability analyses of the critical closure slope for the final, proposed quarry. 2.0 SLOPE STABILITY EVALUATION EA has completed this slope stability analysis of the revised final proposed quarry slopes to evaluate the long term stability of the quarry at closure. Figures 2 and 3 show the existing quarry topography and the proposed topography, respectively. The slope stability analysis was based on the cross - section labeled AW located on Figure 3. EA modeled the intact rock mass shear strength/normal stress relationship using the Mohr - Coulomb criterion. The slope stability input parameters for the rock fill are friction angle (❑) and cohesion (e). Friction angle and cohesion values were derived from published generalized Hoek -Brown parameters for the jointed rock mass criterion (Hoek et al., 2002; Hoek 2007). These parameters included the Unconfined Compressive Strength (UCS), Geological Strength Index (GSI), a material constant (mi), and a disturbance factor (D) as defined by Marinos and Hoek (2000). A UCS value for sandstone was selected from West (1995). The slope stability analysis was completed using a unit weight of 140 psf, cohesion of 6,445 psf and a friction angle of 46.9'. Slope stability analyses were conducted using the commercially available geotechnical modeling software, GeoStudio SLOPEW 2012, developed by Geo -Slope International Ltd. SLOPE/W is a two - dimensional slope stability analysis program that analyzes the stability of a slope by various methods of slices. Both static and pseudostatic analyses were completed for the critical slope. Pseudostatic analysis, simulates seismic forces in terms of horizontal acceleration expressed as a coefficient (or percent) of gravity (g). Pseudostatic analyses for the quarry incorporated a horizontal acceleration coefficient equal to 0.05 g. This represents a design earthquake with a 10% probability of exceedance in 250 years (U.S. Geological Survey, 1990). It is common practice to reduce the peak ground acceleration (PGA) by a factor of 0.33 to 0.50 according to research conducted by the U.S. Army Corps of Engineers (Hynes - Griffen and Franklin, 1984). This reduction in horizontal acceleration is justified for earth and rock structures because damage to these types of structures results from sustained ground acceleration. However, EA March 21, 2014 1 Engineering Analytics, Inc.