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• north-south and is overlain by steep topography. Typical <br />overburden thickness is approximately 1,250 feet, so that <br />monuments are spaced at about 0.1 times the average <br />overburden depth. The network extends approximately 850 feet <br />beyond the limit of mining along the panel and 550 feet <br />beyond the limit of mining across the panel. The location of <br />this monitoring network should be reviewed if the mine <br />layout in the vicinity of 5 north panel is altered from that <br />which was proposed when the monitoring network was <br />originally designed. W'e understand that, subseouent to our <br />initial involvement in this project, additonal detailed <br />monitoring grids have been installed, and more may be <br />required. <br />Survey monuments for the close-spaced network consist <br />of 3/9-inch rebar four feet long which are driven 3 1/2 feet <br />into the ground, leaving 6 inches above ground. The network <br />was installed and initially surveyed in November, 1981. No <br />mining has been conducted as yet in the area of the survey <br />network. <br />Mining of the 5 north panel is scheduled to commence in <br />late 1983. Surveying will be undertaken annually until that <br />time, which will result in least 12 months of monitoring <br />baseline data prior to mining. Subsequent to that time, <br />monitoring will be undertaken twice yearly until subsidence <br />• due to the mining of 5 north panel stops. It is considered <br />that this survey network and proposed monitoring program <br />should provide definitive data on angle of draw applicable <br />to the Orchard Valley Mine. More frequent monitoring is not <br />practical due to snow cover in winter and inaccessible roads <br />in spring and fall. <br />A quarterly (or as weather permits) visual inspection <br />for signs of surface subsidence is also conducted above <br />active mining arras. To date, this has located subsidence <br />cracks in the vicinity of first and second west panels. The <br />locations of major cracks are shown on Plates 2, 10 and 17. <br />Development of these panels was initiated around May, 1979 <br />with pillar recovery being completed with sealing of the <br />panel in November, 1980. Subsidence cracks were first <br />detected in October, 1980 during the first walkover <br />inspection. Overburden cover above the panels <br />where the subsidence cracks have been observed ranges <br />between approximately 520 and 690 feet. Geologic conditions <br />in the area are considered to be typical with the exception <br />that the south side of the panel is defined by the burn <br />line. Visual observations will be continued to determine <br />whether subsidence cracks develop north or west of *_hose <br />mapped to data. <br />• - 25 - <br />