revised final closure plan - Salinas Valley Solid Waste Authority

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Revised Final Closure Plan July 2010 Crazy Horse Sanitary Landfill -15- 103-97133 from the site.” The 1993 analysis estimated seismically induced permanent displacement in the base liner and LCRS “on the order of less than one-half of one foot.” In 2004 (Emcon/OWT, Inc. 2004b) both veneer (final cover) and global (through refuse mass) slope stability analyses were performed to evaluate the affects of the vertical expansion of the landfill to the final maximum elevation of 635 feet amsl. This analysis is attached to the RFCP in Appendix C. For the slope stability analysis performed in the 2004 study, the design ground motion was chosen as that having a 10 percent probability of exceedance in 50 years 2 and calculated at 0.53 g. This design ground motion is more stringent than the ground motion associated with the MCE estimated in the 1993 analysis. The 2004 slope stability analysis concluded the following regarding global and veneer stability for the proposed site conditions analyzed in 2004: • For the proposed final landfill slopes, the static factor of safety for global stability is equal to or greater than 1.5, the generally accepted value for static factor of safety. • For the proposed final landfill slopes, a displacement analysis concluded the permanent displacement was less than 1 foot for global stability supportive of the findings in the initial 1993 analysis. • For the proposed final cover on 3H:1V slopes, a veneer stability analysis for a soil vegetative layer determined the seismically induced permanent displacement would be less than 1 foot if the interface shear strengths within the final cover system equaled or exceeded a shear strength envelope defined by a friction angle of 31 degrees. This friction angle would also provide a static factor of safety greater than the required 1.5 value. However, these previous analyses did not consider the existence of localized slopes steeper than 3H:1V at the crest of the landfill. Overall global stability of the landfill is not affected by these slope conditions as the previous critical surfaces were determined to be over the lined areas on the west side of the topdeck. Sideslopes along the lined area will all be 3H:1V or less, therefore the stability assumptions, conditions, and findings of those previous studies over the critical surfaces on the lined area remain unchanged. Both the 1993 and 2004 global stability analyses concluded the existing base liner and LCRS at the CHLF would experience seismically induced permanent displacement of less than the generally accepted limit of 12 inches. 2.13.1.2 Existing Steep Slopes The CHLF will possess a few localized areas of over steepened slope during the postclosure period. In 2009, Golder performed additional veneer stability calculations regarding these slopes which range from 2.3H:1V to 2.2H:1V. Slope measurements on the final slopes are shown on Figure C-1 (Appendix C). The upper slope above a 12-foot wide bench on the eastern face of the topdeck (facing the entrance facilities) currently has an average slope of 2.2H:1V in steepness over an approximate area of 1.0 acres, with a localized area approaching 2H:1V. The slope height of the over steepened section is approximately 24 feet. The SVSWA directed its site operator to stop this steep slope construction and return to typical 3H:1V sideslopes above the steepen area in anticipation of site closure. The 3H:1V slope segment will be approximately 10 feet in height at final closure. Below 2 per the United States Geological Survey (USGS) Probabilistic Seismic Hazard Assessment. Golder Associates
Revised Final Closure Plan July 2010 Crazy Horse Sanitary Landfill -16- 103-97133 the mid-slope bench are sideslopes generally 3H:1V in steepness with some localized areas as steep as 2.6H:1V as shown on Figure C-1. The uppermost slope of the south face is approximately 2.3H:1V in steepness over approximately 1.1 acres as shown on Figure C-1 in Appendix C. The slope height in this area is approximately 40 feet. Slopes to be constructed beyond the main access road and western bench road are to be constructed at 3H:1V slopes per direction given by the SVSWA to its site operator. These slopes were relatively short hence a sliding block analysis was performed versus an infinite slope analysis. The 2009 slope stability analysis concluded the following regarding veneer stability for the proposed final cover system with a vegetative soil layer: • Upper Eastern Slope Face: To limit the estimated seismically induced permanent displacements to less than 12 inches, the vegetative soil required tapering from a thickness of 2 feet at the slope crest to a thickness of 4 feet at the mid-slope bench. This vegetative layer configuration also resulted in an acceptable factor of safety greater than 1.5 for this slope. • Upper Southern Slope Face: The computed factor of safety and estimates of seismically induced permanent displacement were acceptable with a uniform vegetative soil layer thickness of 2 feet. • These analyses were based on interface shear strengths within the final cover system equal or exceeding a shear strength envelope defined by a friction angle of 32.5 degrees. This friction angle was based on direct shear test results of the internal shear strength of the proposed vegetative soil material. Geosynthetic Final Cover Stability. The proposed geosynthetic cover system eliminates the use of a thick vegetative soil layer. This also eliminates the potential for damage to the final cover system due to excessive seismically induced permanent displacements. With elimination of the 2-foot thick vegetative layer, an infinite slope is a more appropriate means of stability analysis for the proposed geosynthetic materials. For the required factor of safety (FS) of 1.5, the minimum shear strength envelope is determined by: tanφ => 1.5 x tanα Where φ is the internal friction angle of the veneer interface being considered and α is the slope angle. For a 2.2H:1V slope and FS = >1.5, the internal friction angle must be greater than or equal to 34.5 degrees (rounding to the nearest half degree). The manufacturer of the proposed geosynthetic mechanically erosion-resistant layer, Closure Turf LLC, had direct shear tests performed on the material to represent both the closed condition and final cover construction conditions at the CHLF: These test results are presented in Appendix O. These results are summarized in the table below and plotted against the minimum shear strength envelope in the accompanying chart. The tests were conducted using a concrete sand infill in the artificial turf component placed against the stud side of the structured geomembrane. The tests were conducted under “soaked” conditions and run at a shear rate of 0.04 inches per minute. Golder Associates
Page 1 and 2: REVISED FINAL CLOSURE PLAN (2010) C
Page 3 and 4: Revised Final Closure Plan July 201
Page 21: Revised Final Closure Plan July 201
Page 41 and 42: TABLES Table 1 - CalRecycles Final
Page 43 and 44: Initial Closure Final Cover Phase I
Page 45 and 46: This figure was originally produced
Page 47 and 48: CRAZY HORSE CANYON ROAD CRAZY HORSE
Page 49 and 50: A - 1 - - TYPICAL ARRAY SECTION LEG
Page 51 and 52: ID Task Name Duration Start Finish
Page 53 and 54: LEGEND NOTES REVISED FINAL CLOSURE
Page 55 and 56: LEGEND A-55 SW-1 NOTES REVISED FINA
Page 57 and 58: LEGEND E A-15 NOTES REVISED FINAL C
Page 59 and 60: NE SW A 4 12 LANDFILL CROSS SECTION
Page 61 and 62: LEGEND NOTES J 14 14 OVERSIDE DRAIN
Page 63 and 64: 1 16 16 2B 16 16 2A 16 16 U 4 16 MA
Page 65: V 18 18 NOTES V 18 18 MODULE 1 FINA

revised final closure plan - Salinas Valley Solid Waste Authority

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