2006-04-19_REPORT - M1977004 - Laserfiche WebLink

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April 14, 2006 4.O MONITORING AND ANNUAL REPORTING 4.1 ~ Slope Movement Vector Analysis y monuments have been installed on the east and south walls of the North Pit to monitor al slope movement. For clarity, the area east of the pit lake and south of Northing 113,500 is :d to as the south wall of the North Pit; and the area east of the pit- lake and north of Northing DO is referred to as the east wall of the North Pit. This differentiation is for ease in discussing movement and does not imply that there is a kinematic distinguishing feature at this location. original group of monitoring points was installed in 1994 with additional monitoring points fished in 1996, 1997, 1998, and 1999. Since 2004, 24 new monuments have been installed. ment locations are provided on the Slope Displacement Vectors drawing presented in idix A. Surveys are usually conducted as close to one year apart as possible, allowing the itude of displacement to be reported as a velocity in terms of fUyear. Surveys of all accessible monuments were conducted on October 15, 2005 with the previous survey performed on September 9, 2004. The surveys are actually conducted over several consecutive days with the median date used to represent the data set. Of the 85 points surveyed in 2005, only 15 had (magnitudes that were greater than survey error, as shown on Table 1. By comparison, 17 points had magnitudes greater than survey error reported in 2004. Velocity vectors were calculated for the 15 points and are depicted with green circular symbols and velocity vectors on the Slope Displacement Vectors drawing provided as Appendix A. Points with a displacement less than the survey error are shown in gray on the map. • Monitoring point MP-63 is located on the East Wall of the South Pit. The displacement indicated at MP-63 for the 2005 yeady interval is 0.40 ft. Similar displacement was been observed in this area in last years (2004) report. The area was inspected during the 2005 season. A small area of slope displacement was noted in the vicinity of MP-63. It appears that a former area of slope instability was re-activated. Displacement at the headscarp is on the order of approximately 1.5 ft. Additional monitoring points were installed, and the first (baseline) survey of the points took place during the 2005 survey. The area of slope displacement has been documented, and will be better defined with current and future survey data. Based on performance of similar slopes at the site, it is reasonable to assume that displacement will be minimal (less than 0.5 ft per year), and should reach aquasi- equilibrium. Although the consequences of complete slope failure would pose a very low risk to human safety, the slope should be monitored visually, in addition to annual survey monitoring. All of the total displacement values with the exception of MP-63 are less than or equal to 0.24 fUyear. The low magnitude of slope displacement data suggests that the North Pit slopes have stabilized to less than 0.3 Wyear. As was noted last year, as the number of monitoring points that exhibit slope displacement decrease with time, the remaining points tend to cluster in the northeast corner of the North Pit, and at the south wall of the North Pit. The northeast comer of the North Pit is the location where the two rapid, large-scale slope displacements took place in 1983. The south wall Hof the North Pit has historically been the second most active part of the North Pit, with the northeast corner being the most active. The south wall of the North Pit also had a rapid, large-scale slope displacement event in 1983. The clustering of slope movement in these two areas, and the lack of slope displacement anywhere else in the North Pit, with the exception of MP-63 in the South Pit, suggests that the North Pit slopes are becoming more stable. The increased degree of stability is likely related to the pit regrading work done in 1996.