This paper outlines the analysis methods and results used in a geotechnical evaluation performed on the stability of two declines at the Skyline Mine. The Lower O'Connor-A (LOA) seam has been displaced by a fault within the study area at the mine. The declines have been driven in relatively thin interburden and connect the existing mains in the LOA seam to the mains in the same coal seam in the proposed underlying mine level. The primary objective of the study was assess the roof and rib stability of both the declines and the connecting crosscut, while establishing a viable brow thickness above the decline entrance. The impact of driving the declines on the stability of the mains pillars in the proximity of the decline in the upper and lower levels were analyzed, while concurrently evaluating the effect of multi-seam mining in the lower level mains pillars. The study was performed using a combination of numerical modeling tools, LAMODEL and FLAC3D. The study results indicated likely rib and roof stability within the declines and crosscut, as well as the pillars in the vicinity of declines in both the mining levels.


The Skyline Mine (Skyline) is a deep underground coal in Central Utah located southeast of Salt Lake City. A geotechnical evaluation was performed to assess the stability implications of driving two declines in the relatively thin interburden at the Skyline Mine. This study was performed prior to the work to evaluate the stability of two declines connecting the existing LOA seam mains in Mine-2 (mid-level) at Skyline to the LOA mains in the underlying SWR (Southwest Reserves, bottom level)), as the seam has been displaced by a fault. The proposed mains were developed after ramping down two declines from the existing mains in the up-thrust side of the LOA Seam to the down-thrust side. The newly developed mains are directly overlain by existing mains in the Lower O'Connor-B (LOB) Seam in Mine-2 (Figure 1) and sealed workings in Mine-1 (upper level) in the Upper O'Conner (UO) seam. Information from drill holes near the decline bottom indicated the SWR mains are separated from the Mine-2 mains by an interburden thickness of approximately 26 feet (ft). The SWR mains in the LOA seam will service several proposed longwall panels in the lowermost level. These mains are anticipated to be in service for upwards of 10 years.

Since the completion of the study, the mains have been developed by ramping down two declines from the existing mains in the up-thrust side of the LOA Seam to the down-thrust side. The interburden was excavated utilizing a continuous mining machine for the initial floor. A roadheader was then utilized to establish the brow, and was used for the remainder of the project. The declines were excavated at an angle of approximately 8 degrees from horizontal. The declines are spaced approximately 59-feet apart (rib-to-rib) and have their centerlines coincident with the mains entries in Mine-2 (mid-level) and SWR workings (lower level).

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