2007-01-11_PERMIT FILE - C1982057

Home Browse Search

March 2004 Seneca Coal Company * Seneca 11 W Mine Sedimentation Po 016A As -built Stability Analysis ♦ 1 1.0 INTRODUCTION • 1.1 PURPOSE The purpose of.this stability analysis is to calculate the factor of safety (FOS) under static conditions for the as -built conditions of Sedimentation Pond 0 1 6A embankment in order to document design compliance with applicable regulatory standards under Rules 4.05.9 (6) and 4.05.9 (8) (b). 1.2 METHODOLOGY The static. FOS for Sedimentation Pond 016A was calculated using a basic, two - dimensional equilibrium analysis program called SLOPE/W (Version 5.1). The calculated FOS is.the ratio of those resisting forces that would prevent embankment movement or failure to the corresponding driving forces. Resisting forces include the internal strength parameters for both the soil materials used in the construction of the embankment and the underlying base materials on which the facility was constructed. Potential driving forces include gravitational forces on the embankment and foundation materials and the gravitational and lateral forces associated with the water volume that could be impounded behind the embankment. For this analysis, a circular failure mode was assumed, and the FOS was calculated using the Bishop's method. 1.3 PROCEDURE The design embankment cross - section used to evaluate embankment stability was determined from the digitized topography shown on Drawing 13 -9C, Pond 16A As -built Drawing for the Seneca II W Permit. A water surface elevation behind the embankment of 7,525.5 feet above mean sea level (amsl) was used for the analysis. This water surface elevation is equivalent to the surveyed elevation of the invert to the principal spillway. The embankment geometry analyzed is based on the as -built topography cut perpendicular the downstream • topography. The potential phreatic surface for the stability analysis is based on a water surface elevation of 7,525.5 ft -amsl. The phreatic surface through the embankment was estimated based on Darcy's law and observation of similar homogenous pond embankments at the mine site. At these locations seepage does not occur above the embankment toe. The resulting surface is shown on the SLOPE/W computer printouts contained in Appendix B. An undisturbed. sample was collected from the foundation area of Pond 016. The foundation soil sample was obtained from a Shelby tube sample taken from a test pit at the location of the embankment. Triaxial (with pore pressure), grain size and Atterberg limits tests were run on the sample and the results can be found in Appendix A, and are summarized on Table 1. The results of these tests were used to approximate the properties of the Pond 016A foundation conditions due to the close proximity of the two embankments. During construction of Pond 016A, embankment fill material was sampled and subjected to sieve analysis and Atterberg limit testing. The strength properties for the embankment fill were obtained from tests on a sample from Pond 010. The Pond 010 embankment strength results were selected to represent the strength properties of the Pond 016A embankment fill. This was judged appropriate due to the similarity in index and sieve properties between the sample from the Pond 016A embankment fill and the pond 10 material (see Table 1). The Pond 010 embankment shear strength test was performed on a remolded sample at 95% of-the-maximum-dry density-from-the-standard Proctor test (ASTM D698). The results can be found in Appendix A, and are summarized on Table 1. The parameters described in Table I were input into the SLOPE/W program to evaluate static stability of the Pond 016A structure and are shown in the attached program printouts with associated results. `• MWH, 1801 California Street, Suite 2900 * Denver, Colorado 80202 * (303) 291 -2222 JA 2470108SenecaCoal \ Ponds Arbll \W*ord\POND 16A ASbuillSTABII_I7Y.doc 03102104 dim