Analyses of Slope Failures and Development of a Reclamation Plan, Pikeview Quarry, Colorado Springs, Colorado

The Pikeview Quarry is a limestone aggregate quarry located northwest of Colorado Springs, Colorado. The sedimentary units in the quarry, which contain low-strength clay-bearing seams, have been geologically uplifted by the underlying Pikes Peak Granite, resulting in steep dips toward the east (downslope). Large-scale landslides occurring in December 2008 and September 2009 led to the suspension of mining activity at the quarry. Prior to the slope failures, mining was occurring at the base of the highwall, which undercut the sedimentary beds and left the highwall unsupported. Engineering Analytics, Inc. and Telesto Solutions, Inc. were retained to determine the causes of the slope failure and the extent to which the mining practices contributed to the failures. The investigation consisted of data review, field investigations, laboratory testing, and slope stability modeling. Three-dimensional model simulations were performed to evaluate various mining scenarios including 0, 25, 50, and 100 percent excavation of the toe below the highwall. For the models with 0, 25, and 50 percent of the toe excavated, the computed displacement values were 1.6, 1.9, and 3.6 ft, respectively. In the final model with 100 percent of the toe excavated, as was the case prior to the failures, the computed displacement was 13.6 ft, which agreed well with the observed displacement into the excavation. It was concluded that the removal of too much material at the base of the highwall without removing the sedimentary rocks above it was also a cause of the landslides. A conceptual reclamation plan was developed that addressed slope stabilization, revegetation, additional mining, and long-term use.