2023-06-28_ENFORCEMENT - M1982121 (2)

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Table 2. Leadville Limestone and Interbed Strength Parameters Material Parameter Cohesion (psf) Friction Unit Weight Angle(deg) (pcf) Empirical 5,000 35 150 Leadville Limestone Post- 10,000 35 150 Backanalysis Empirical 40 25 150 Interbed Material Post- Backanalysis 550 25 130 East Face Stability Slope stability results of the East Face based on modeling of the above conditions indicate a factor of safety of 1.2 for the south facing highwall. This factor of safety is along a failure plane angle of 30 degrees which correlates to bedding dip of the soft interbed material. A tension crack was inserted as a release plane for the planar slide that correlates to the secondary joint set (mean set plane 45°; 055) mapped in the field on the East face.This joint set is perceived as the release plane for the West face 2023 ground event that can be seen in Photo 2 (Appendix A). Critically, water pressure was deterministically modeled as 30%filled with peak pressure at the tension crack base. Sensitivity analysis shows the factor of safety is particularly sensitive to water level assumptions. For any rock mass there is the possibility of large-scale, random joints with a low strength such as from weathering, historic sliding, or clay infilling. If such a joint or several joints exist and if these joints have a disadvantageous orientation and location, then there could be a large-scale slope instability. However, field observations by KUE did not reveal any such joints beyond those previously identified. ROCKFALL MODELING Rockfall modeling was performed on three transects along the East face that are representative of the varying geologic and topographic conditions (Figure 1). The three slope geometries were created from LiDAR data provided by RMRA. Modeling was performed using the computer program Rockfall v.8.004 by RocScience that simulates the bounce paths of rock blocks down a slope, and calculates block velocities, end points and kinetic energies at user specified points along the slope. The rockfall simulation uses coefficient of restitution (both normal and tangential) parameters to model the loss of kinetic energy between the rockfall block and ground surface at the point of impact. Based on the site reconnaissance, two slope materials were identified: limestone headwall and Limestone Scree/ Blast pile. A mean value was assigned for each property with a normal distribution of standard deviation. Similar to the slope stability analyses, input values for normal restitution, tangential restitution, dynamic friction Page. 7 535 16th STREET,SUITE 620 1 DENVER,CO 80202 1 (303)732-3692 1 WWW.KILDUFFUNDERGROUND.COM