REV103200 - Laserfiche WebLink

Home Browse Search

~® CTL/THOMPSON, INC. CONSULTING GEOTECHNICAL AND MATERIALS ENGINEERS February 14, 1992 Colowyo Coal Company. 5731 State Highway 13 Meeker, CO 81641 Attention: Mr. Jim Kiger Subject: Supplemental Slope Stability Analysis West Pit Excess Spoil Pile Colowyo Mine Moffat County, Colorado Job No. 18,349 Gentlemen: We have performed additional slope stability analysis to confirm stability of the West Pit Spoil Pile. These analyses supplement those in our previous report (Job No. 16,516, dated March 1, 1991). This letter contains laboratory test results, the results of our analyses, and conclusions regarding the stability of the West Pit Pile. We were provided a sample of clay Colowyo employees obtained from the general area of TH-1 from the previous investigation, This sample represents the clay fill which is to be compacted to at least 90% of maximum modified Proctor dry density below the toe of the spoil pile. A Proctor compaction test (ASTM 1557-78A) indicated a maximum dry density of 119 pcf and an optimum moisture content of 12.5% for the clay (Fig. 1). We performed a consolidated, undrained triaxial test with pore pressure measurements on a sample of clay which was remolded to 90°,6 of maximum dry density. Figure 2 shows the triaxial test results. The friction angle (~) and the cohesion intercept (c) were calculated from this data as 28° and 650 psf; respectively. A stability analysis was executed using the Geoslope "Stahl" computer program from Geocomp, Inc. This analysis was run with the soil properties determined for the various soils in this investigation and the previous investigation. The soil profile considered in this analysis was representative of the design. The analysis assumed natural clays at the toe of the pile would be removed and replaced with compacted clay fill. The spoil was then placed above the clay fill layer. Groundwater was assumed at the top of the clay fill. This profile exhibited a computed factor of safety of 2.0. Figure 3 shows the profile and the ten most critical failure surfaces with the most critical denoted by the number 1. 971 wE5-:2TH .:VBdUE DENVER CCLCRnD0502C- ~?p379350::7