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Appendix a Settlement and Slope Stability Analyses

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Appendix a Settlement and Slope Stability Analyses Appendix A Settlement and Slope Stability Analyses SETTLEMENT & SLOPE STABILITY ANALYSES Saugus RESCO Landfill Saugus, Massachusetts Prepared for: June, 2008 Prepared by: 11 Northeastern Boulevard Salem, NH 03079-1953 Project 119504 Table of Contents________________________________________________ List of Tables ..................................................................................................................................................ii List of Attachments .........................................................................................................................................ii 1.0 Introduction.....................................................................................................................................1-1 2.0 General Subsurface Conditions ......................................................................................................2-1 3.0 Settlement Analysis ........................................................................................................................3-1 3.1 General.................................................................................................................................3-1 3.2 Settlement Model..................................................................................................................3-1 3.3 Calculations..........................................................................................................................3-2 4.0 Static and Seismic Slope Stability Analyses ...................................................................................4-1 4.1 General.................................................................................................................................4-1 4.2 Method of Analyses..............................................................................................................4-1 4.2.1 Slope Stability Analyses ..........................................................................................4-1 4.2.2 Permanent Displacement Analysis for Seismic Conditions .....................................4-2 4.3 Cross-Section Across Phase III............................................................................................4-2 4.3.1 Critical Slope Stability Cross-Section ......................................................................4-2 4.3.2 Subsurface Conditions for Analysis.........................................................................4-2 4.3.3 Shear Strength of Boston Blue Clay........................................................................4-3 4.3.4 Results of Slope Stability Analyses .........................................................................4-4 4.3.4.1 Static Slope Stability Assuming No Consolidation of BBC ........................4-4 4.3.4.2 Static Slope Stability Assuming Consolidation of BBC..............................4-4 4.3.4.3 Slope Stability under Seismic Conditions..................................................4-4 4.4 Cross-Section Across Phase IV............................................................................................4-6 4.4.1 Critical Slope Stability Cross-Section ......................................................................4-6 4.4.2 Soil Parameters.......................................................................................................4-6 4.4.3 Results of Static Slope Stability Analysis.................................................................4-6 4.4.4 Results of Seismic Slope Stability Analysis.............................................................4-7 5.0 Conclusions and Recommendations...............................................................................................5-1 5.1 Settlement ............................................................................................................................5-1 5.2 Slope Stability.......................................................................................................................5-1 6.0 Bibliography....................................................................................................................................6-1 N:\projects\119504\2008\VF FEP Docs\Global Stability i List of Tables ___________________________________________________ Table 1 Boston Blue Clay Consolidation Calculations Table 2 Undrained Shear Strength of Boston Blue Clay Under Partial Consolidation Table 3 Undrained Shear Strength of Boston Blue Clay Under 100% Consolidation Table 4 Summary of Slope Stability Analyses Results – Cross Section Across Phase III Table 5 Summary of Slope Stability Analyses Results – Cross Section Across Phase IV List of Attachments ______________________________________________ Attachment A Settlement Analysis Attachment B Results of Static and Seismic Slope Stability Analyses Cross-Section Across Phase III B-1 Location of Slope Stability Cross-Section and Test Boring B-2 Results of Static Slope Stability Analysis B-3 Probabilistic Seismic Hazard Based on USGS B-4 Pseudo-Static Slope Stability Analysis to Determine Yield Acceleration B-5 Permanent Displacement Calculations Attachment C Results of Static and Seismic Slope Stability Analyses Cross-Section Across Phase IV C-1 Results of Static Slope Stability Analysis C-2 Results of Seismic Slope Stability Analysis N:\projects\119504\2008\VF FEP Docs\Global Stability ii 1.0 Introduction Refuse Energy Systems Company (RESCO), now known as Wheelabrator Saugus, J.V. (Wheelabrator), owns and operates a waste-to-energy plant and the adjacent ash residue disposal site, known as the Saugus RESCO Landfill (Landfill), in Saugus, Massachusetts. The Landfill accepts municipal solid waste (MSW) ash residue from the waste-to-energy plant located adjacent to the Landfill. The currently approved final grading plan for the Landfill consists of a ridge and valley configuration with a maximum elevation of 50 feet mean sea level (MSL) along the ridges. In all, there are five valley lines and six ridge lines. In addition, the Landfill consists of five phases, I through V. Phases I and II have been filled to the currently approved final grading, and Phases III, IV, and V are currently uncapped and either active or dormant covered with a layer of intermediated cover. In accordance with the 2008 Revisions to the Final Engineering Plan (2008 FEP), additional ash residue will be placed within the area between ridges by flattening the cross-slope (this is referred to as the “valley fill”). The minimum cross-slope will be decreased from a five percent to a two percent slope, with a side slope of three horizontal to one vertical immediately adjacent to the valley line and in the perimeter areas. The slope along the length of the valley line will be maintained at one percent. The maximum Landfill elevation will remain at 50 feet MSL. This report presents the results of analyses performed by Shaw Environmental, Inc. (Shaw) to determine the impact of the valley fill on landfill settlement and global slope stability of the Landfill Phases III, IV, and V. The settlement analysis was performed using an idealized cross section taken at a critical cross section located within Phases III and IV. The slope stability was analyzed using two cross sections. The first slope stability cross section was taken perpendicular to the western perimeter of Phase III. The second slope stability cross section was taken perpendicular to the northwestern perimeter of Phase IV. N:\projects\119504\2008\VF FEP Docs\Global Stability 1-1 2.0 General Subsurface Conditions Boring logs of existing test borings and monitoring wells were used to interpret the general subsurface conditions at the Landfill. No field investigation or laboratory tests were performed as part of this study. A review of these logs shows that the Landfill is underlain by the following strata, starting from the top: • Existing municipal solid waste (MSW) ash residue • Municipal solid waste • A layer of peat (not encountered at all locations) • A layer of sand • Boston Blue Clay (BBC) • Glacial till In general, ash residue deposits vary from 0 to 40 feet in thickness to a maximum elevation of 50 feet MSL overlying the MSW. The bottom of MSW is at an approximate elevation of 0 feet MSL with a maximum thickness of approximately 25 to 30 feet. The thickness of peat varies from 0 to 7 feet. The thickness of sand is between 5 to 10 feet. The thickness of the BBC varies from 20 to 80 feet. The BBC consists of an over-consolidated upper crust which has a medium stiff consistency. The lower part of the BBC is soft, and it is normally consolidated. N:\projects\119504\2008\VF FEP Docs\Global Stability 2-1 3.0 Settlement Analysis 3.1 General The purpose of the settlement analysis performed was to determine whether the proposed two percent cross-slope will be adequate to accommodate any differential settlements in the Landfill that may occur in the future. If excessive differential settlement were to occur, surface water runoff to the valleys would be impeded which could result in ponding of surface water. The settlement analysis was performed for an idealized cross-section taken between a critical ridge and valley identified based on the subsurface conditions. 3.2 Settlement Model The MSW ash residue is relatively incompressible due to its cementing properties. Similarly, the settlement of the sand layer can be assumed to be very small because it is dense and its thickness is small. Therefore, the only strata that would contribute to landfill settlement include the following:
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