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Subsidence Evaluation For Exhibit 608 South of Divide Mining Area • The alluvium in Dry Fork and its tributaries and in the main fork (north fork) of Lick Creek is estimated to have a similaz thickness range in Deep Creek and Sylvester Gulch. Although the overburden thickness range of 375 to 800 feet is less than that beneath Deep Creek and Sylvester Gulch (800 to I,O50 feet), the alluvium in the South of Divide mining area contains more Wasatch clays than in Deep Creek and Sylvester Gulch. Therefore, any effects on the alluvium due to mining are even less likely to occur. However, in order to compare field results with predicted results in this important stream valley environment (as described in Section 11.3), detailed subsidence monitoring is recommended above the western limits of longwall panels E2 and E3-an area of minimum overburden and maximum predicted subsidence effects for the South of Divide mining area. 4. Mining impacts on rockfalls were not observed during annual subsidence observations in the Apache Rocks and Box Canyon mining areas over the last six years (1999 to 2004 inclusive), in areas where the estimated rockfall potential was moderate to high. The highest estimated rockfall potential is classified as moderate to high in the South of Divide mining area, and many of these are located in areas where no mining is currently planned. Consequently, the natural rockfall process is not expected to be significantly accelerated by longwall mining. Evidence of naturally occurring rockfalls, such as remnant boulders neaz roads near the base of steep slopes, should be documented prior to mining. It is suggested that signs stating, "Watch for Falling Rock" (similar to signs • posted along major highways in high rockfall potential areas), be posted on the more well-traveled roads, such as the Dry Fork and Main Fork of Minnesota Creek roads, when (or before) mining begins in these areas- 5. As discussed in Sections 5.32, 11.0, and 15.3, no cracks are predicted to occur in the alluvium in the Dry Fork of Minnesota Creek and its tributazies, or in Lick Creek and its tributaries. This prediction is based on annual subsidence observations that no cracks occurred in alluvium during longwall mining in the Apache Rocks area. Therefore, water flow in these streams, and tributaries, and diversion ditches would is not expected to be impacted. However, it is suggested that stream flow be monitored before, and during, and after mining within the area of mining influence and also outside of the area of mining influence in the Dry Fork and Lick Creek drainages, in order to verify this prediction. 6. Based on ten years of annual observations in the Jumbo Mountain, Apache Rocks, and Box Canyon mining areas, the author believes that the landslides that are located north and south of Minnesota Reservoir and above the northern part of longwall panel E9, will not be noticeable impacted by subsidence or seismic activity caused by longwall mining. Based on field observations during the past ten years, the major fording is that landslide movement occurs in response to moisture and ground saturation, and is not noticeably affected by subsidence or any mine-related seismic activity caused by (ongwatl mining beneath or neaz the landslides. However as discussed in Section 11.1, it is recommended that this area be periodically monitored prior to mining, during mining, and after mining to verify predictions, based on the historical evidence. 831-032.690 Wright Water Engineers Inc.