engineering design report - dnr.mo.gov · 03-05-2017 · atterberg limits % moisture . liquid...

Appendix E

Engineering Design Report

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Appendix E Engineering Design Report

Class 3 RCRA Permit Modification Application for Phase 7 of the Slag Storage Area Landfill and

Interim Measures Plan for SWMUs 2, 4, 29/37, and 32

May 2017

Contents

1.0 Introduction ........................................................................................................................................................................... 1

2.0 Geotechnical Evaluation .................................................................................................................................................... 2

2.1 Grain-Size Analysis and Atterberg Limits .............................................................................................................. 2

2.2 Moisture Density Curves .............................................................................................................................................. 4

2.3 Cone Penetrometer Testing ........................................................................................................................................ 4

2.4 Triaxial Shear Strength .................................................................................................................................................. 5

2.5 Permeability (Clay Borrow Source) .......................................................................................................................... 6

2.6 Geotechnical Data Interpretation ............................................................................................................................. 6

3.0 Facility Development .......................................................................................................................................................... 8

3.1 Site Preparation ............................................................................................................................................................... 8

4.0 Landfill System ...................................................................................................................................................................... 9

4.1 Final Cover System .......................................................................................................................................................10

4.2 Upper Leachate Collection System ........................................................................................................................10

4.3 Primary Liner ...................................................................................................................................................................11

4.4 Leak Detection System ...............................................................................................................................................11

4.5 Lower Composite Liner ...............................................................................................................................................12

5.0 Slope Stability......................................................................................................................................................................13

5.1 Static Slope Stability ....................................................................................................................................................13

5.1.1 Infinite Slope Stability ............................................................................................................................................13

5.1.2 Perimeter Dike Stability.........................................................................................................................................14

5.1.3 Global Stability .........................................................................................................................................................15

5.1.4 Perimeter Dike and Global Stability Modeling Methodology ...............................................................15

5.1.4.1 Geometry ..........................................................................................................................................................15

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5.1.4.2 Groundwater ...................................................................................................................................................16

5.1.4.3 Loading Conditions ......................................................................................................................................16

5.1.4.4 Slope Stability Analysis Input Parameters ...........................................................................................16

5.2 Seismic Analysis .............................................................................................................................................................19

5.2.1 Site Seismicity and Acceleration Time History .............................................................................................19

5.2.2 Seismic Deformation Analysis ............................................................................................................................20

5.2.2.1 Computation of Yield Acceleration ........................................................................................................20

5.2.2.2 Computation of Earthquake-Induced Acceleration .........................................................................20

5.2.2.3 Computation of Seismic-Induced Permanent Deformation.........................................................21

5.2.3 Results of Seismic Deformation Analysis .......................................................................................................22

6.0 Settlement Analysis ...........................................................................................................................................................23

7.0 Facility Performance Evaluation ...................................................................................................................................26

7.1 Hydrologic Evaluation of Landfill Performance (HELP) ..................................................................................26

8.0 Action Leakage Rate (ALR) .............................................................................................................................................29

9.0 Stormwater Management Plan ....................................................................................................................................30

9.1 Stormwater Management System .........................................................................................................................30

9.1.1 Modeling and Methodology ...............................................................................................................................31

10.0 References ............................................................................................................................................................................32

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List of Tables

Table 2-1 Residuum Laboratory Test Results – 2005 Table 2-2 Residuum Laboratory Test Results – 2016 Table 2-3 Clay Borrow Laboratory Test Results – 2016 Table 2-4 Residuum Modified Proctor Results – 2005 Table 2-5 Clay Borrow Modified Proctor Results – 2005 Table 2-6 Consolidated-Undrained (CU) Triaxial Test Results – 2005 Table 2-7 Consolidated-Undrained (CU) Triaxial Test Results – 2016 Table 2-8 Clay Borrow Permeability Test Results – 2005 Table 5-1 Slope Stability Safety Factors Table 5-2 Parameters Used in Stability Analysis Table 5-3 Slope Stability Factors of Safety for Static Conditions Table 5-4 Peak Ground Accelerations (PGA) Table 5-5 Selected Earthquake Records Table 5-6 Stiffness and Damping Parameters Table 5-7 Seismic Deformation Results Table 7-1 Summary of HELP Model Results

List of Figures

Figure E-1 Drainage Area Map

List of Attachments

Attachment E-1 Permit Drawings Attachment E-2 Technical Specifications Attachment E-3 Geotechnical Evaluation Data Attachment E-4 Pipe Strength Computations Attachment E-5 Slope Stability and Settlement Analysis Attachment E-6 Facility Performance Evaluation Attachment E-7 Action Leakage Rate Computations Attachment E-8 Storm Water Runoff Computations

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1.0 Introduction This appendix constitutes the engineering report portion of the permit modification request. It includes descriptions of the proposed facility development and design, supporting design computations, and references other reports that have been prepared as parts of the overall permit modification request. A permit modification drawing set has been prepared to accompany this report and to present the facility development plans. The permit drawings (Attachment E-1) include 16 plan sheets as follows:

G-01 Title Sheet, Sheet Index, and Site Location Map C-08 Site Cross Section

C-01 Site Topography and Boring Locations C-09 Site Cross Sections

C-02 Double Liner Grading Plan (Top of Double Liner) C-10 Liner System Sections

C-03 Cell 7 Leachate Collection System Piping Plan C-11 Side Slope Riser Pipe Sections

C-04 Leachate Collection System and Leak DetectionSystem Header Pipe - Plan and Profile

C-12 Side Slope Riser Pipe Sections andDetails

C-05 Final Cover Grading Plan w/Surface Water RunoffControls

C-13 Sections and Details

C-06 Surface Water Runoff Grading C-14 Sections and Details

C-07 Liner Development and Closure Sequencing Plans C-15 Surface Water Collection SystemSections

Technical specifications have been included (Attachment E-2) to further detail the facility development plans and construction requirements.

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2.0 Geotechnical Evaluation A description of the 2016 site exploration activities is provided in the geologic site investigation (Appendix C) of this permit modification application. These activities were conducted to supplement existing information on the geologic and hydrologic characteristics of the site and to gather information to further the understanding of geotechnical considerations at the site. The site exploration activities performed as part of the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006) were reviewed and utilized in the development of these site exploration activities and in the design of Phase 7.

The proposed Phase 7 will consist of a system of engineered dikes, liners, and piping systems as described in Sections 3.0 and 4.0 of this appendix. Sufficient characterization of geotechnical conditions at the site is required to confirm that foundation soils are of sufficient strength to support the proposed dikes, liners, and piping systems without allowing excessive deformation of these engineered systems.

The following paragraphs briefly characterize the geotechnical testing performed on the materials encountered at the site as background for the subsequent descriptions of facility design. It should be noted that much of the testing performed is for general characterization of the site subgrade soils in compliance with rule requirements. Test results such as grain-size analysis and Atterberg Limits are useful for understanding the general performance of the soils that can be anticipated and for confirming the suitability of the use of these soils in construction if so needed. However, most of the geotechnical analysis relies on the results of the triaxial shear strength tests and on the results of previously performed geotechnical laboratory testing, including 2005 cone penetration testing (CPT).

2.1 Grain-Size Analysis and Atterberg Limits Grain-size analyses (ASTM Method D422) were performed in 2005 on four distinct soil types to characterize materials at the site. Supplemental grain-size analyses were performed in 2016 on soil samples collected in and around the Phase 7 footprint.

Grain-size analyses (ASTM Method D422) were performed on the shallow slag/soil fill mix that has been placed as fill over a large portion of the site, the residuum (weathered bedrock) that underlies this fill, slag, and an offsite clay that may be used for construction of the clay component of the liner system.

Atterberg Limits (ASTM Method D4318) tests were performed on the cohesive soils in 2005 and supplemented with additional testing in 2016 on samples collected in and around the Phase 7 footprint.

The shallow slag/soil fill mix varied considerably in classification and classifications varied from silty sands (SM) and silts (ML) to lean clays (CL) as defined by the Unified Soil Classification System in borings SB-21 through SB-36 (Appendix C and Attachment E-3). Concrete and other construction debris was present in some samples.

The residuum (weathered bedrock) is generally classified as clayey sand with gravel (SC), clayey gravel (GC), or fat clay (CH) by the Unified Soil Classification System. These classifications are on the basis of

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assumed specific gravities ranging from 2.68 to 2.70. The grain-size analyses and Atterberg Limits data for the residuum tested in 2005 are included in Section G of the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006). Atterberg Limits for the residuum tested in 2005 ranged as follows:

Table 2-1 Residuum Laboratory Test Results – 2005 (Phases 1 through 6)

Atterberg Limits % Moisture

Liquid Limit 36.5 to 107.7

Plastic Limit 17.8 to 29.2

Plasticity Index 10.0 to 79.6

Supplemental grain-size analyses and Atterberg Limits data for the residuum tested in 2016 are included in Attachment E-3. Atterberg Limits for the residuum tested in 2016 ranged as follows:

Table 2-2 Residuum Laboratory Test Results – 2016 (Phase 7)





As shown above, the Atterberg Limits of each residuum sample tested in 2016 fell within the result ranges from the 2005 testing.

The slag samples retrieved during the 2005 site exploration are classified as silty sand (SM) ranging to a sandy lean clay (CL) by the Unified Soil Classification System. The specific gravity of the slag samples tested was 3.79. It should be noted that this characterization is on the basis of two grain-size distribution tests and one specific gravity test, and may not be representative of all slag at the site or the slag that will be placed in the slag storage facility. The grain-size analyses and specific gravity data for the slag are included in Section G of the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006). Slag samples collected in 2005 were considered representative of the slag that will be stored in Phase 7.

An offsite clay borrow source was identified for facility liner construction and soils were obtained for testing prior to Phase 1 construction. The clay from this borrow source is classified as a fat clay (CH) by the Unified Soil Classification System. Percent passing the number 200 sieve ranged from 60 to 84 percent. Percent passing 0.002 mm ranged from 27 to 67 percent. The grain-size analyses and Atterberg Limits data are included in Section G of the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006). Atterberg Limits for the clay ranged as follows:

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Table 2-3 Clay Borrow Laboratory Test Results – 2016





These results are representative of soils obtained from the existing clay borrow source planned for use in Phase 7 construction. Other clay borrow sources may be identified and evaluated prior to initiating development of Phase 7 of the slag storage facility.

2.2 Moisture Density Curves Modified Proctor tests (ASTM Method D1557) were performed on residuum samples collected in 2005. Test results are provided in Section G of the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006). Standard Proctor tests (ASTM Method D698) were performed on samples obtained from the clay borrow source and test results are provided in Section G of the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006).

For the residuum, modified proctor results are summarized as follows:

Table 2-4 Residuum Modified Proctor Results – 2005

Modified Proctor Range Mean Standard Deviation

Maximum Dry Density (pcf) 109.3 to 137.8 118.3 8.3

Optimum Moisture Content (%) 6.3 to 21.7 13.3 3.7

For the clay borrow source, Standard Proctor results are summarized as follows:

Table 2-5 Clay Borrow Modified Proctor Results – 2005

Standard Proctor Range Mean Standard Deviation

Maximum Dry Density (pcf) 76.5 to 112.9 91.4 12.1

Optimum Moisture Content (%) 15.8 to 39.5 29.3 8.0

2.3 Cone Penetrometer Testing CPT was performed at the site by Stratigraphics of Glen Ellyn, Illinois in 2005. The Stratigraphics report is contained in Appendix C-7 of Section G of the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006). Twenty-three CPT soundings with electrical conductivity testing (CPT-EC) and one CPT sounding with shear wave velocity testing (CPT-S) were performed. The CPT soundings were performed to obtain a comprehensive evaluation of the shear strength of the soils that will support the slag storage facility. The primary data output of interest from the CPT soundings include

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the drained friction angle (Ø'), the undrained shear strength (Su), and a general evaluation of the variability (or uniformity) of soil strength with soil type and depth.

2.4 Triaxial Shear Strength Four triaxial shear strength tests (ASTM Method D4767) were performed in 2005 for comparison to the CPT shear strength data and for obtaining additional strength data for use in the slope stability analysis for the facility. The following table presents the results of the triaxial shear strength testing presented in Appendix C-8 in Section G of the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006); namely, the friction angle and cohesion values.

Triaxial shear strength tests are most representative when performed on undisturbed soil samples. In this case, the testing was performed on remolded soil samples.

Table 2-6 Consolidated-Undrained (CU) Triaxial Test Results – 2005 (Phase 1 through 6)

Sample Type and Depth

Triaxial Shear Strength Test Results (see notes)

Drained Friction Angle (Ø') –

Degrees

Drained Cohesion (c') – Pounds Per

Square Foot

Undrained Friction Angle (Ø) –

Degrees

Undrained Cohesion (c) – Pounds Per

Square Foot

Weathered Limestone, Clayey Gravel w/Sand (GC) 10.0–15.0 feet

5.9 (10.2)

3,580 (2,820)

7.5 (9.9)

2,900 (2,140)

Weathered Limestone, Clayey Gravel w/Sand (GC) 15.0–23.0 feet

18.6 (19.6)

1,640 (1,500)

12.9 (13.0)

1,520 (1,400)

Weathered Limestone, Clayey Sand w/Gravel (SC) 40.0–42.4 feet

32.2 (35.6)

1,200 (520)

8.9 (15.0)

5,880 (3,860)

Slag (Existing) Sand, Medium Grained (SP)

32.4 (29.2)

340 (480)

18.2 (18.8)

600 (280)

Note: Values shown are based on maximum deviator stress failure criteria. Values in parenthesis are averages of test results using maximum deviator stress, maximum stress ratio, maximum pore pressure, and 15 percent strain failure criteria.

An additional triaxial shear strength test was performed to supplement the 2005 data. A thin-wall residuum sample was collected from 44-46 feet in SB-24A and subjected to a CU triaxial shear strength test. Results from this test are provided below.

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Table 2-7 Consolidated-Undrained (CU) Triaxial Test Results – 2016 (Phase 7)

Sample Type and Depth

Triaxial Shear Strength Test Results (see notes)

Drained Friction Angle (Ø') –

Degrees

Drained Cohesion (c') – Pounds Per Square

Foot Undrained Friction

Angle (Ø) – Degrees

Undrained Cohesion (c) – Pounds Per Square

Foot

Weathered Limestone, Fat Clay (CH) 44.0 – 46.0 feet

25.7 (24.4)

1,000 (893)

13.1 (12.5)

1,900 (1,893)

Note: Values shown are based on maximum deviator stress failure criteria. Values in parenthesis are averages of test results using maximum deviator stress, maximum stress ratio, and 15 percent strain failure criteria.

2.5 Permeability (Clay Borrow Source) Six falling head permeability tests (ASTM Method D5084) were performed on soil samples retrieved from the clay borrow source in 2005. The samples were tested at percent compaction (of Standard Proctor ASTM D698 Maximum Dry Density) ranging from 93.5 to 99.9 percent. The following table presents the permeability test results.

Table 2-8 Clay Borrow Permeability Test Results – 2005

Sample No. Pail 1 Pail 2 Pail 3 Pail 4 Pail 5 Pail 6

Test No. 1 % Compaction

93.6 94.8 95.2 96.0 93.5 94.2

Test No. 1 Coefficient of Permeability (cm/sec)

3.5 x 10-8 6.2 x 10-8 5.7 x 10-8 4.9 x 10-8 1.6 x 10-7 1.6 x 10-8

Test No. 2 % Compaction

— 99.9 — — 99.5 —

Test No. 2 Coefficient of Permeability (cm/sec)

— 4.5 x 10-9 — — 5.9 x 10-9 —

2.6 Geotechnical Data Interpretation It is apparent from the site exploration activities and the soil testing that the materials that underlie the existing and proposed slag storage facilities encompass a wide range of material types and material strengths. Material types include various combinations of sand, silt, gravel, and clay, which are mostly descendant from the weathering of dolomitic bedrock. These soils with Unified Soil Classification System classifications of SC, GC, and CH have a significant clay fraction. They are expected to perform adequately for foundation support but will require careful moisture conditioning and compaction during construction to the extent that they are excavated and reused.

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Shear strengths of these materials as derived from the CPT are reasonably high (reasonably high in-place internal angles of friction) but show some loss in strength upon remolding and recompaction to in-place density. This is evident from comparison of the CPT shear strength data to the triaxial shear strength data, with the former being lower than the CPT values on a fairly consistent basis. This can be expected when the structure of the soil is disturbed by remolding. Slope stability analyses presented in Section 4.0 of Section G of the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006) and analyses presented in Section 5.1 of this appendix rely on the triaxial shear strength data (remolded strength) for the entire residuum soil strata so are likely a bit conservative.

The near-surface materials at the site contain slag and other various fill materials. Most of these materials appear to have been placed somewhat randomly and possibly in an uncontrolled state, meaning they were not likely heavily compacted during placement. This will necessitate some additional subgrade preparation work prior to liner construction.

One clay borrow source has been evaluated. Based on the testing performed, this borrow source contains clay of the quality required for liner construction. The coefficient of permeability is the most important of the geotechnical characteristics of the clay borrow source. As with all soils, the coefficient of permeability of the clay is dependent on the percent compaction. For this particular borrow source to achieve the 1 x 10-7 cm/sec coefficient of permeability required, it appears that it will be necessary to specify percent compaction of roughly 95 percent or higher of the Standard Proctor maximum dry density. Alternatively, the required coefficient of permeability can easily be achieved by specifying a high percentage of the Modified Proctor maximum dry density. Use of the Standard Proctor is preferable as it is the more common approach.

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3.0 Facility Development The approved July 2006 Class 3 RCRA Permit Modification Application for the Slag Storage Area (Landfill Unit) Report (Barr, 2006) proposed a phased construction of six cells of the slag storage area (SSA) with a total storage volume of 519,000 cubic yards. Due to changes in the smelter operations, the original anticipated life of the SSA has decreased, and the addition of SSA Phase 7 is necessary. The estimated acreage to be developed for the Phase 7 expansion is 4.5 acres. The estimated storage capacity is approximately 140,000 cubic yards.

The permit application drawings and specifications (Attachment E-1 and E-2) include significant detail regarding overall facility development. More detailed specifications and drawings will be prepared for bidding and construction of Phase 7. Development procedures and facility features and characteristics are described in the following paragraphs.

3.1 Site Preparation The foundation of Phase 7 will be prepared to provide support to the liners and resistance to pressure gradients above and below the liners to prevent failure of the liner due to settlement, compression, or uplift. Preparation of the foundation for Phase 7 of the SSA will consist of clearing, grubbing, and grading in preparation for liner construction. The foundation will be sloped from southeast to northwest at generally an approximate 5 percent slope to minimize overall earthwork requirements and to take advantage of existing slopes. The slope perpendicular to the primary slope will be approximately 2 percent between the leachate collection pipes.

To accommodate construction above the existing slag and other uncontrolled fill at the site, dynamic compaction will be used. Dynamic compaction consists of repeatedly dropping a large weight from significant height until no other subgrade consolidation occurs from subsequent drops. This approach to subgrade treatment is effective where the water table is low as it is at this site and where the uncontrolled fill materials are regular in nature.

Perimeter dikes shown on the permit drawings will be constructed to direct surface water from outside the dikes into Impoundment E for sediment control, where existing grades allow, and to define the area where the double composite liner (liner) will be constructed. The facility will be graded to accommodate the liner grades shown on the permit drawings. Construction drawings and technical specifications will be prepared prior to construction of Phase 7. Requirements for dynamic compaction are included in the technical specifications included in Attachment E-2 of this permit modification request.

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4.0 Landfill System Prior to installation of the double composite liner system at Phase 7, the site subgrade soils will be prepared as described in Section 3.0 of this appendix and as described in the technical specifications contained in Attachment E-2. This preparation will result in a foundation capable of providing support to the liner and resistance to pressure gradients above and below the liner to prevent failure of the liner due to settlement, compression, or uplift.

The double composite liner system will meet the requirements of 40 CFR 270.21 as incorporated by 10 CSR 25-7.264 and will consist of two composite liners with a leachate collection and removal system above and between the liners. The profile of the landfill system is described as follows, listed from top to bottom (see Engineering Drawings included in Attachment E-1):

• Final Cover – The final cover will include a 40-mil low-density polyethylene (LDPE) geomembraneoverlain by a granular drainage layer, a rooting soil layer, and a topsoil layer planted with grasses.

• Upper Leachate Collection System – A 12-inch-thick granular drainage layer will serve as theleachate removal system to be located above the primary liner.

• Primary Liner – The primary liner will consist of a geosynthetic clay liner (GCL) overlain by a 60-milhigh-density polyethylene (HDPE) geomembrane.

• Leak Detection System – The lower leak detection system will consist of a geocomposite drainagelayer (geonet sandwiched between geotextile filter fabrics) that drains to leachate collection pipesconnected to a sump. This leak detection system will also function as a leachate removal system.

• Bottom Liner – A 60-mil HDPE geomembrane will function as the upper component of thecomposite bottom liner. A 3-foot-thick compacted clay layer will be placed on the subgradewithin the perimeter dike to function as the lower composite bottom liner.

The liner system includes the primary liner and the lower composite liner. As discussed in this appendix, the landfill system is designed and will be constructed and installed to prevent any migration of waste out of the landfill to the adjacent subsurface soil or groundwater or surface water at any time during the active life (including the closure period) of the landfill. The liner system will be constructed of materials that prevent wastes from passing into the liner during the active life of the facility and will cover all surrounding earth likely to be in contact with the waste or leachate.

The leachate collection and removal system consist of the upper leachate collection system and the leak detection system. This system will be capable of detecting, collecting, and removing leaks of hazardous constituents at the earliest practicable time through all areas of the primary liner likely to be exposed to waste or leachate during the active life and post-closure care period.

The landfill system will have the appropriate chemical properties and sufficient strength and thickness to prevent failure due to pressure gradients (including static head and external hydrogeologic forces),

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physical contact with the waste or leachate to which they are exposed, climatic conditions, the stress of installation, and the stress of daily operation. These properties are outlined in the technical specifications (Attachment E-2).

4.1 Final Cover System The final cover will consist of a 40-mil LDPE geomembrane overlain by a granular drainage layer, a rooting soil layer, and a topsoil layer, as shown on the permit drawings (Attachment E-1). To promote drainage and minimize erosion or abrasion of the cover, the final cover will be planted with a variety of grasses intended to be tolerant of the dry subsoil conditions that the final cover system yields. The final cover will effectively reject surface water impacting the site, will provide long-term minimization of the migration of liquids through the landfill, and will function with minimum maintenance.

The final cover system is designed to block surface water infiltration into the underlying slag. As described in Section 7.0 of this appendix, the performance of the final cover system has been shown to be equivalent to that of the landfill base liner system in preventing migration of liquids through the system and meets the regulatory requirements for the final cover system. As discussed in Section 6.0, the final cover, sloped at approximately 3.5 horizontal to 1.0 vertical, will accommodate settling and subsidence so that the cover's integrity is maintained.

Stormwater runoff control is a critical function of the final cover system. Surface water runoff will be collected by a toe-of-slope surface water collection system consisting of a drainage ditch around the perimeter and discharging to Impoundment E. The design of the stormwater runoff control system is described in Section 10.0.

4.2 Upper Leachate Collection System As specified in the federal regulations, a leachate collection and removal system will be installed overlying the primary liner. The leachate collection system will consist of an approximate 12-inch-thick granular drainage layer that will drain to leachate collection pipes as shown on the permit drawings. Coarse filter material will be placed around the leachate collection pipes as shown on the permit drawings to prevent migration of fine particles into the leachate collection system. The coarse filter material will be separated from the geomembrane by a geotextile to prevent puncture of the geomembrane. During periods of high leachate flow, flow is expected to occur in both the leachate collection pipe and the underlying coarse filter material. During periods of low flow, leachate conveyance to the sump will be via the coarse filter layer.

Leachate will be conveyed to sumps where it will be pumped via sidewall riser pipes to a header pipe. The header pipe will discharge to a lift station for subsequent pumping via forcemain to the water treatment plant. No components of the leachate collection system protrude through the landfill liner; rather a sump and sidewall riser system is used to remove leachate.

The header pipe that both the leachate collection system and leak detection system sumps will discharge into will be constructed incrementally. The header will drain by gravity to the lift station that was

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constructed in conjunction with the Phase 1 liner construction. The forcemain from the lift station to the wastewater treatment plant consists of a small diameter polyethylene pipe. An insulating soil cover or other pipe insulation approach will be used as necessary. The forcemain location may be moved during the construction of Phase 7 if necessary.

Cleanout access will be provided on each end of the leachate collection pipes. The maximum length of the collection pipes is approximately 450 feet, allowing a maximum reach of approximately 225 feet from each end to clean the entire pipe length. Pipe cleaning equipment is available that is capable of providing well in excess of these cleaning lengths. Pipe strength design computations are included in Attachment E-4.

The granular materials and leachate collection pipes that were selected will minimize clogging during the active life and post-closure care period. All stormwater collected within the active portion of Phase 7 will be pumped to the onsite wastewater treatment plant, or other locations approved by the Missouri Department of Natural Resources (MDNR) in an expeditious manner, but in no case exceeding seven calendar days after the last storm event and in no case shall the head on the bottom liner exceed one foot.

4.3 Primary Liner The primary liner will consist of a 60-mil HDPE geomembrane overlain by a GCL. A composite primary liner was selected to reduce the odds of collecting leachate in the leak detection system. This composite primary liner design exceeds the regulatory requirements for a single geomembrane for the primary liner above the leak detection layer.

4.4 Leak Detection System The leak detection system will consist of a geocomposite (geonet and filter fabric in combination) drainage layer overlying the lower composite liner, draining to a sump as shown on the permit drawings. As shown on the permit drawings, the geocomposite drainage layer will be constructed with a bottom slope of approximately one percent or greater and will have a transmissivity of 3×10−5 m2 /sec or more. The geocomposite drainage layer is designed to minimize clogging during the active life and post-closure care period and will be above the seasonal high water table.

Phase 7 will drain to its own sump where collected leachate will be extracted via a sump pump and sidewall riser pipe system and will discharge to the header pipe discussed above. The header pipe will discharge to the lift station for subsequent pumping to the wastewater treatment plant. The sump and pumps will be of sufficient size to collect and remove liquids from the sump and prevent liquids from backing up into the drainage layer. The sump will be capable of measuring and recording the volume of liquids present in the sump and of liquids removed.

As with the upper leachate collection system, a sidewall riser pumping system will be utilized to lift collected leachate over the perimeter dike and into the header pipe for subsequent discharge to the water treatment plant. The sidewall riser approach to leachate removal accomplishes several objectives:

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• It avoids penetrations in the liner system.

• It facilitates monitoring of flows from individual phases of the facility for both the leachatecollection system and leak detection system.

• It yields a flexible system—if initial pump sizing is found to be inadequate, then smaller or largerpumps can be installed.

• It yields a redundant system—there are multiple points of leachate extraction from the facilityrather than a single point.

4.5 Lower Composite Liner Per the federal regulations, a landfill must include a composite bottom liner consisting of at least an upper and a lower component to minimize the migration of hazardous constituents if a breach in the upper component were to occur. To fulfill this requirements of 40 CFR 264.301(c)(1)(i)(B), a 60-mil HDPE geomembrane will be used as the upper component of the composite bottom liner. The lower component of the lower composite liner is designed to minimize the migration of hazardous constituents if a breach in the upper component were to occur and will, at a minimum, consist of at least 3 feet of compacted soil material with a hydraulic conductivity of no more than 1x10-7 cm/sec.

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5.0 Slope Stability The landfill liner and landfill cover systems are multi-layer systems—layers of differing types of materials are stacked atop one another to achieve finished systems. With multi-layer systems, there is the potential for one layer to slide or slip relative to the other, leading to a slope failure. Infinite slope stability analyses were performed to confirm stability of these systems and to aid in the final specification of the geosynthetic material components of the liner and cover systems. Stability of perimeter dikes and stability of the overall landfill system (global stability) were evaluated using limit equilibrium analysis of randomly generated failure planes to determine the perimeter slope and global stability safety factors.

Landfill liner and cover systems, perimeter dikes, and global stability for the completed slag storage facility were evaluated relative to the generally accepted slope stability safety factors of 1.3 for the condition immediately after construction and 1.5 for long-term conditions. The safety factors were computed by dividing the sum of forces resisting sliding to the sum of forces causing sliding. Slopes having safety factors above 1.0 should (in theory) remain stable. Use of safety factors of 1.3 and 1.5 accommodate the variability in soil strengths, water conditions, slope geometry, vegetation conditions, and other factors that are difficult to predict with complete certainty.

5.1 Static Slope Stability Infinite slope stability computations, perimeter dike modeling, and global stability modeling were performed to analyze the static stability of Phase 7. Static stability analysis methods and results are described below.

5.1.1 Infinite Slope Stability The stability of the landfill liner system on the 3.5 horizontal to 1.0 vertical side slopes and the stability of the landfill cover system on the approximate 3.5 horizontal to 1.0 vertical cover slopes were evaluated using techniques for evaluation of infinite slopes. More specifically, the resistance to sliding of each layer relative to the underlying layer of the cover and liner systems was evaluated. The basic equation for analysis of stability of infinite slopes is as follows:

F.S. = (N tan δ) / (W sin β); where

F.S. = Factor of Safety against Sliding

N = Normal Force of Overlying Materials

δ = Interface Friction Angle between Adjacent Materials

W = Weight of Overlying Materials

β = Slope Angle

Since N = W cos β; F.S. = (tan δ) / (tan β)

The results of the stability of infinite slope computations for the facility liner and cover system are presented in Section G of the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006).

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The stability of the final cover system is adequate with a safety factor of 1.86 as compared to the minimum acceptable values of 1.3 (after construction) and 1.5 (long term). Likewise, the stability of the landfill liner system is adequate with safety factors ranging from 1.5 to greater than 2.0 (depending on the layers considered) as compared to the generally accepted minimum values. Therefore, stability of infinite slopes (liner and cover) is adequate.

Table 5-1 Slope Stability Safety Factors – Final Cover and Landfill Liner

Final Cover System

Upper Layer Lower Layer Interface Friction Angle, δ (typical)

Stability Safety Factor, F.S.

Granular Drainage Layer 40 mil. Textured LDPE 28° 1.86

40 mil. Textured LDPE Granular Drainage Layer 28° 1.86

Landfill Liner System

Upper Layer Lower Layer Interface Friction Angle, δ (typical)

Stability Safety Factor, F.S.

Granular Drainage Layer 60 mil. Textured HDPE 28° 1.86

60 mil. Textured HDPE GCL 25° 1.64

GCL GCL 23° 1.50

Geocomposite Drainage Layer 60 mil. Textured HDPE 25° 1.64

60 mil. Textured HDPE Clay Liner 32° > 2.00

Note: The GCL/geocomposite drainage layer interface friction angle is assumed. Confirm all interface friction angles and stability safety factors for specific geosynthetics selected at time of construction.

In addition to the interface shear strength analysis, the internal shear strength of the GCL must be considered. For this application, a reinforced GCL is required on the side slopes of the perimeter berm. Reinforced GCLs such as those by Bentofix have internal shear strengths in the range of 27 to 34 degrees, yielding infinite slope stability safety factors above the 1.3 and 1.5 minimums acceptable.

5.1.2 Perimeter Dike Stability Stability of the perimeter dike was evaluated using SLOPE/W computer software that systematically generates assumed failure planes through the perimeter dike to search for the failure plane that yields the lowest combination of forces resisting sliding to forces causing sliding—the lowest slope stability safety factor. The results of the perimeter dike stability analysis are presented graphically in Attachment E-5 on Figure E-5-1. As indicated by the graphical output from the stability analysis, the lowest factor of safety generated for perimeter dike stability by the SLOPE/W program is 1.78, which is greater than the generally accepted minimum values of 1.3 after construction and 1.5 long term. Section 5.1.4 provides a more detailed description of the perimeter dike stability analysis.

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5.1.3 Global Stability As with the perimeter dike stability analysis, global stability of the completed Phase 7 was evaluated using SLOPE/W computer software to search for the failure plane yielding the lowest combination of forces resisting sliding to forces causing sliding—the lowest slope stability safety factor. The results of the global stability analysis are presented graphically in Attachment E-5 on Figure E-5-2. As indicated by the graphical output from peak strength stability analysis, the lowest factor of safety generated for global stability by the SLOPE/W program is 1.89, which is greater than the generally accepted minimum values of 1.3 and 1.5. Therefore, global stability is adequate.

The global stability referenced above was performed using the peak shear strength of the residuum underlying the site. This is the soil strength that must be overcome before approaching an unstable condition. Global slope stability analyses typically are based on the peak shear strength of the foundation soil. For this site, the analysis was taken one additional step to confirm that even if the peak shear strength of the foundation soils were overcome the slope would remain stable (slope stability safety factor of 1.0 or greater) for a case where the soils exhibit their residual shear strength. The results of this analysis are presented graphically in Attachment E-5 on Figure E-5-3. As indicated by the graphical output from the residual strength stability analysis, the lowest factor of safety generated for global stability by the SLOPE/W program is 1.13, which is greater than the minimum value of 1.0. This analysis represents a very conservative evaluation of stability and indicates that even under an extreme condition where only the residual shear strength of the foundation soils is mobilized, the facility remains stable. Section 5.1.4 provides a more detailed description of the global stability analysis.

5.1.4 Perimeter Dike and Global Stability Modeling Methodology A limit equilibrium slope stability modeling software called Slope/W within the GeoStudio suite of stability analysis applications was used to analyze the most critical cross section at the proposed SSA. The software, produced by GEO-SLOPE International, Ltd. of Calgary, Alberta, Canada, was utilized to estimate the factor of safety for the proposed SSA under various loading conditions.

5.1.4.1 Geometry The cross section analyzed for perimeter dike and global stability was developed based on exploratory hollow-stem auger borings and well logs, with the critical section (the section expected to yield the lowest factor of safety) passing nearly through borings SB-22, SB-23, SB-25, and SB-35. These boring locations are shown on Figure C-2. Stability at this section was deemed to be most critical due to the height of the perimeter berm at this location and its length of slope, and due to the proximity of Impoundment E to the toe of slope.

As shown in Figure E-5-1 in Attachment E-5, the geometry analyzed for perimeter dike stability consists of the existing slag and fill material overlying a gravelly clay residuum. The contact between the residuum and the parent dolomitic bedrock ranges from an approximate elevation of 1,380 to 1,345 feet in the area of interest. A pond, referred to as Impoundment E, is located at the toe of the existing slope to the northwest with a surface elevation of about 1,345. As shown on Figures E-5-2 and E-5-3 in Attachment E-5, the geometry analyzed for global stability comprises a dike constructed of mostly

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granular materials, an approximate 3-foot-thick clay liner, geosynthetic liner materials (not shown), an approximate 1-foot-thick drainage layer, and slag material that will be stored above the clay layer.

5.1.4.2 Groundwater Although pockets of saturated material were present in the residuum during the geotechnical investigation, readings from deep wells indicate that the continuous groundwater table is located in the dolomite bedrock. These conditions are not expected to change significantly over time. The groundwater table in the stability models was modeled at the interface between bedrock and overlying residuum.

5.1.4.3 Loading Conditions Two scenarios were modeled in GeoStudio. First, the end-of-construction case (perimeter dike) was analyzed using a total stress analysis in which undrained shear strength was used for the residuum. This type of analysis is also referred to as an undrained strength stability analysis (USSA). The geometry analyzed consisted of the perimeter dike, the clay liner, and the drainage layer. The purpose of modeling this geometry was to evaluate stability of the perimeter dike just after construction—the point in time when the soil pore water pressures are highest, soil strengths are lowest and in turn, slope stability safety factors should be at their lowest. The proposed slag material was not included because the loading from the slag is anticipated to take place over time. The presence of the slag is reflected in the second scenario consisting of a long-term, drained analysis (global stability); also referred to as an effective stress stability analysis (ESSA). The cross section for this scenario included the perimeter dike and liner system and the proposed slag. The shear strengths used for the residuum were the drained shear strength envelopes for peak and residual shear strength.

5.1.4.4 Slope Stability Analysis Input Parameters Slope stability analysis input parameters were selected based on the results of the geotechnical site investigation and geotechnical laboratory testing described in Section 2 of this appendix and previously performed site investigation activities detailed in the Class 3 RCRA Permit Modification Application For Slag Storage Area (Landfill Unit) Report (Barr, 2006).

The following table summarizes the parameters used in the slope stability analysis. The strength parameters are given by the friction and cohesion according to the Mohr-Coulomb model. Unit weights for all materials were assumed based on typical values, but were selected to fall within the range of soil densities derived from material testing performed in conjunction with this permit application and the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006).

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Table 5-2 Parameters Used in Stability Analysis

Material Unit Weight

[pcf]

Undrained (USSA) Drained (ESSA)

c [psf] φ [deg] c’ [psf] φ’ [deg]

Dolomite Bedrock (1) (1) (1) (1) (1)

Residuum 120 2,000 10 (2) (2)

Existing Slag/Fill 120 500 18 400 30

Dike 120 0 35 0 35

Clay Liner 120 2,000 0 0 (3)

Drainage Layer 120 0 35 0 35

Future Slag Fill 120 0 36 0 36

Notes: (1) Assumed impenetrable, thus unit weight and shear strength are irrelevant in the limit equilibrium analysis.(2) Both fully softened peak and residual shear strength envelopes were used (ref. Figure 5).(3) φ’=25º (peak), φ’=15º.

The parameters shown above include the undrained parameters used in the USSA and the drained parameters used in the ESSA.

Estimation of undrained shear strength su (also referred to as c) for the residuum from 2005, CPT data is made using the following equation:

kt

votu N

qs

)( s−=

Where Nkt is an empirical cone factor, svo is the total in-situ vertical stress, and qt is the corrected cone tip resistance. The Nkt factor varied as a function of tip resistance and total overburden stress according to Stratigraphics (2005). Figure 2 in the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006) shows the undrained shear strength determined from the CPT data versus depth. It can be seen from this figure that the undrained shear strength is highly variable, as expected in a residual soil. Undrained shear strength data from triaxial shear tests were reported in Attachment E-3 of this appendix.

The peak and residual drained strength envelopes for the residuum were estimated based on Atterberg Limit tests performed on samples obtained in the residuum and on the basis of the previously reported triaxial shear tests. The peak shear strength was used for the standard/typical analysis of long-term stability to confirm that slope stability safety factors are above 1.5 long term. The residual shear strength was used for a conservative analysis of long-term stability to confirm that in any case slope stability safety factors are above 1.0. Although, it is unlikely that the residuum would exhibit its residual shear strength, research by Mesri and Shahien (2003) has shown that it is possible for the residual strength of a clay to be mobilized for a first-time slope failure, especially for stiff, fissured clays. Therefore, both the peak and

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residual shear strength envelopes were used for the residuum, and an analysis was performed for each case.

The peak and residual drained strength envelopes for the residuum used in stability analyses of Phase 1 through Phase 6 were selected for use in Phase 7 stability analysis because Atterberg Limit tests performed on residuum samples in the Phase 7 footprint fell within the same range as those in samples collected in the Phase 1 through Phase 6 footprint.

The undrained shear strength for the clay liner was taken as a typical value for a compacted cohesive material. Peak and residual friction angles were estimated for the clay liner as 25 and 15 degrees, respectively. No stress-dependent envelope was attempted for the clay liner because the small thickness of this unit has very little influence on stability.

Because the existing slag/fill is expected to be of lower strength than the proposed slag material, the 2005 CPT data was used to obtain the friction angle for the existing slag/fill and applied to both materials to yield a conservative analysis of stability. Figure 6 of the Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006) shows friction angle as a function of overburden stress and tip resistance. This correlation was proposed by Robertson and Campanella (1983) based on empirical calibration chamber data. It can be seen in the figure that at CPT sounding SB-15, the lowest friction angle was about 36 degrees in the slag material. This lower bound value was selected for the entire existing slag/fill layer and the proposed slag material. SB-15 was chosen because it is the only sounding where the existing slag layer was relatively thick and an adjacent rotasonic boring confirmed that the material was indeed slag.

The friction angles for the dike material and drainage layer were based on estimated typical values. The values used in the analysis were 35 degrees and zero cohesion.

The results from the GeoStudio model are summarized in the following table.

Table 5-3 Slope Stability Factors of Safety for Static Conditions

Case Factor of Safety Minimum Acceptable

Factor of Safety Attachment E-5

Figure No.

End of Construction (USSA) 1.78 > 1.3 E-5-1

Long-Term (ESSA), Peak Strength for Residuum

1.89 > 1.5 E-5-2

Long-Term (ESSA), Residual Strength for Residuum

1.13 > 1.0 E-5-3

As indicated by the results summarized above, the facility design yields slope stability factors above the minimum acceptable values.

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5.2 Seismic Analysis In addition to the static analyses described above, earthquake-induced deformation was analyzed to determine whether or not excessive deformation of Phase 7 would take place under earthquake loading. The following describes the methodology used to evaluate the earthquake-induced deformation.

5.2.1 Site Seismicity and Acceleration Time History The earthquake acceleration data used in the design of Phase 7 is summarized in the following table. This data was computed using a ground motion database website developed by the University of Berkley Pacific Earthquake Engineering Research (PEER) Center that was referenced by the United States Geological Survey (USGS) Earthquake Hazards Program website (USGS, 2014). Earthquake acceleration was based on the unified hazard spectrum for 2,475-year return period (or 2 percent probability of exceedance in 50 years) and based on an input latitude of 37º38’08” North and longitude of 91º07’55” West. The maximum induced acceleration at the base of Phase 7 is considered to be the same as the maximum bedrock peak ground acceleration (PGA).

Table 5-4 Peak Ground Accelerations (PGA)1 – The Buick Resource Recycling Facility, LLC, Boss, Missouri

Probability of Exceedance in 50 Years [%] PGA [x g]

10 0.0908

2 0.2295

1 From USGS (2002).

The uniform hazard spectrum was used for acceleration time histories determination. The uniform hazard spectrum was determined using EZ-Frisk™ software Version 7.50 created by Risk Engineering, Inc. based on the site location. Uniform hazard spectrum was obtained for both near-field and far-field events. These spectra then were used in PEER and the target acceleration time histories for each event was selected based on the result’s minimum squared error from the site hazard spectrum. For each event, the acceleration time histories along north and east directions were applied.

Using the PEER website, the near-field acceleration time history was selected from 30 obtained records. The far-field acceleration time history was chosen from 27 obtained records. The selected records are shown below.

Table 5-5 Selected Earthquake Records

Event

Mean Squared

Error Scale Factor Earthquake Name Year Station Name Magnitude

Near-Field 0.0922 15.09 Sparks_2011-11-06 2011 Luther Middle School 5.68

Far-Field 0.3935 2.85 Mineral_2011-08-23 2011 Palisades_ NY 5.74

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5.2.2 Seismic Deformation Analysis Evaluation of the facility earthquake-induced deformation was completed using the Newmark (1965) method. In this method, the permanent deformation of the facility in response to earthquake loading is estimated. The procedure consists of computing the yield acceleration (e.g., the acceleration required to cause movement in a slope – factor of safety equal to 1.0) and then comparing the yield acceleration with the earthquake-induced acceleration. The following describes the three basic steps of the method.

5.2.2.1 Computation of Yield Acceleration The yield acceleration is defined as the average acceleration producing a horizontal inertial force on a potential sliding mass so as to produce a factor of safety equal to unity and thus cause the sliding mass to experience permanent displacement. The yield acceleration is computed by applying an inertial force equal to the sliding mass multiplied by the seismic coefficient. The seismic coefficient at which the full strength of the sliding mass is mobilized (i.e., factor of safety equal to 1) is the yield acceleration. Values of yield acceleration are a function of the slope geometry, the strength of the soil mass, and the location of the potential sliding mass. The computer program SLOPE/W was used to compute the yield acceleration. The strength properties of each material type used in the limit-equilibrium stability analysis are shown in Section 5.1.4.4. A yield acceleration of 0.265g was computed for Phase 7.

5.2.2.2 Computation of Earthquake-Induced Acceleration Earthquake-induced accelerations in the slope are computed using the dynamic response analysis. Finite elements or finite difference procedures utilizing strain-dependent soil properties are used to calculate the time histories of acceleration. FLAC 8.0 is an explicit finite difference modeling software and was used for fully dynamic analysis simulation. It uses strain-dependent soil properties and strain-dependent modulus and damping to compute acceleration time histories within the sliding mass.

In the analysis presented herein, the input ground motions at bedrock level included in Table 5-5 were used in FLAC model. FLAC uses an explicit finite difference scheme to solve full equations of motion. It simulates fully nonlinear dynamic response of a system to excitation from an external or internal source as opposed to the equivalent linear model, which is commonly applied in earthquake engineering analyses. The shear modulus and damping reduction curves proposed by Sun et al. (1989) were used to compute the time history of acceleration along the slip surface. The program uses the hysteretic damping approach and allows variable damping to be used in different elements. The shear (G) and bulk (K) moduli as well as the corresponding shear and hysteretic damping reduction constants based on the default model for each material type in the model are shown in Table 5-6. To remove high frequency noise and avoid low-level oscillation a small amount (0.2 percent) of stiffness-proportional Rayleigh damping was added to the model.

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Table 5-6 Stiffness and Damping Parameters

Material Density

(slug/ft3) Shear Velocity

(ft/s) Shear Modulus

(psf) Bulk Modulus

(psf)

Hysteretic Damping Parameters

L1 L2

Slag 3.73 550 1.12824E+06 1.88040E+06 -3.325* 0.823*

Clay Liner 3.73 570 1.21179E+06 2.01964E+06 -3.156* 1.904*

Existing Slag/Fill 3.73 750 2.09797E+06 3.49661E+06 -3.325* 0.823*

Compacted Clay Dike 3.73 800 2.38702E+06 3.97837E+06 -3.156* 1.904*

Residuum 3.73 1000 3.72972E+06 6.21620E+06 -3.156* 1.904*

* Corresponds to shear modulus reduction curve and damping ratio curve for sand (upper range), after Seed et al. (1989).** Corresponds to shear modulus reduction curve and damping ratio curve for cohesive soil (upper range), after Seed et al.

(1989).

FLAC allows calculation of the time history of each node or element. The time history of acceleration along the slip surface was computed as the average of the time history of 12 equally spaced points along the slip surface. The results of the FLAC analysis in terms of the average acceleration-time histories and yield acceleration are included in Figures E-5-6, E-5-7, E-5-8, and E-5-9 in Attachment E-5. The computed average time history is then used to calculate the permanent displacement.

The input acceleration time histories obtained from PEER are from bedrock; therefore, the dolomite bedrock was not considered in the modeling effort. The modeling procedure is to introduce the acceleration to the bottom of the model where the bedrock and residuum interface is located. The resultant acceleration at each point along slip surface is then monitored and recorded during the earthquake event and an average of acceleration along the slip surface is calculated. The resultant average acceleration is then compared to the yield acceleration to see if any plastic deformation can be expected.

5.2.2.3 Computation of Seismic-Induced Permanent Deformation Once the yield acceleration and the average acceleration-time history of the potential sliding mass are determined, the permanent deformation can be readily calculated. Earthquake-induced accelerations in the slope are computed using the dynamic response analysis. When the earthquake-induced acceleration of the sliding mass exceeds the yield acceleration, permanent movements are assumed to occur along the direction of the failure plane and the magnitude of the displacement is evaluated by a double integration procedure.

A computer code to perform the double integration was developed by Barr when the yield acceleration is exceeded by the earthquake-induced acceleration. The code was written in MATLAB and was developed as an executable file. When the average acceleration exceeds the yield acceleration, this code will be applied to compute the permanent deformation.

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5.2.3 Results of Seismic Deformation Analysis The results of the seismic deformation analysis are summarized in Table 5-7 in terms of the yield acceleration and computed permanent deformation. In general, permanent deformations less than 60 cm (2 feet) are considered tolerable. These deformations can result in some cracking, which can be repaired after earthquake shaking. It can be seen from Table 5-7 that earthquake-induced permanent deformation is equal to 0 inches for both near-field and far-field event. This is concluded based on the resultant average acceleration for each event being below the yield acceleration; therefore, causing no plastic (permanent) deformation.

Table 5-7 Seismic Deformation Results

Yield Acceleration ky [g]

Near-Field Displacement [inch]

Far-Field Displacement [inch]

0.265 0.0 0.0

The results summarized in Table 5-7 indicate that Phase 7 is expected to experience minimal permanent deformation during the design earthquake events. These results are similar to those presented in Section G of Class 3 RCRA Permit Modification Application for Slag Storage Area (Landfill Unit) Report (Barr, 2006) for the initial phases of the SSA.

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6.0 Settlement Analysis Development of the slag storage facility produces stress in the foundation soils, thereby causing consolidation (settlement). Settlement was modeled using the SIGMA/W program within the GeoStudio software suite. This program uses finite elements to model the interaction of stress and strain as a result of various types of loading. The same geometry that was used for the slope stability model was used for the settlement computations.

Initial stresses must be computed before the effect of loading can be modeled. Thus, only the bedrock, residuum, and existing slag/fill were modeled initially, and the resulting stresses were imported into another model where the dike, clay liner, drainage layer, and slag were placed above the underlying materials. The added load from these materials was simulated as occurring instantaneously, even though the placement of the slag will take place over many years. This should make no difference with respect to estimates of total displacements.

Elastic properties of the foundation soils (existing slag/fill and residuum) were computed based on results from the triaxial shear tests that were performed on samples of residuum and slag. The particular tests were CU tests in which cylindrical samples are subjected to equal all-around pressure under drained conditions. Because the sample is consolidated in this manner, the change in volume due to the applied pressure can be correlated to a drained bulk modulus according to:

∆p = K’εv

where

∆p = change in pressure

K’ = drained bulk modulus

εv = volumetric strain

If a drained Poisson’s ratio is assumed, the drained Young’s modulus for the soil can be calculated by:

Ε’ = 3K’(1-2ν’)

where

Ε’ = drained Young’s modulus

ν’ = drained Poisson’s ratio

An assumed drained Poisson’s ratio of 0.25 and the resulting Young’s moduli for the residuum and the existing slag/fill were used in the model. Young’s modulus computations are included in Attachment E-3.

The differential loading produced by development of the slag storage facility produces vertical settlement and horizontal displacement. This is apparent from the settlement analysis results presented in Attachment E-5. From Attachment E-5, it can be determined that the estimated total vertical settlement

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from facility loading is 1.0 foot in the completed facility and 0.9 foot at the location of the facility liner. The estimated maximum differential settlement is 0.6 foot occurring over a distance of approximately 65 feet.

It can be estimated that the maximum strain that the liner system will experience is 0.92 percent (equal to 0.6 foot divided by 65 feet)—well below strain levels that the facility liner system can tolerate. The slope of the leachate collection system, which is 5 percent without consideration of settlement, would be reduced to 4 percent if the predicted settlement does occur. This slope remains above the preferred minimum slope of 2.0 percent in the leachate collection system.

The settlement analysis indicates that the perimeter dikes will settle as much as 0.7 foot. Some differential settlement will occur within the dikes as well. Most of this settlement will occur during construction of the perimeter dikes. Impacts on the alignment and grades of the surface water runoff control system and leachate transport system constructed within the dikes should be minimal, as some of the settlement will have occurred by the time these systems are installed.

Vertical settlement of the final cover system will also occur. The majority of the settlement will occur during filling of the facility with slag. Therefore, impacts on the final cover system will be minimal. Furthermore, no detrimental effects on the final cover are expected from the settlement given the approximate 3.5H:1V slope of the cover system and the lack of infrastructure placed on the cover system.

Total and differential horizontal displacements resulting from facility settlement are lower than the vertical displacements. All of the infrastructure to be constructed at the facility can be expected to tolerate the predicted displacements of 0.25 foot total and 0.35 foot differential, resulting in an estimated maximum strain of 0.23 percent (equal to 0.35 foot divided by 150 feet).

Koerner (1998) reports that tensile failure strains for geotextile-based GCLs are in the range of 10 percent to 19 percent, depending upon the specific GCL manufacturer selected. Overlapped seams in GCL installations are compromised at somewhat higher strains ranging from 12 percent to 81 percent, depending upon the overburden stress on the GCL at the time of deformation, and depending upon the amount of overlap at the seam. Koerner (1998) and others report tensile failure strains for HDPE geomembranes of 12 percent at yield and 100 percent at break.

The GCL of the upper liner has a lower strain tolerance than does the geomembrane of the upper liner. However, it is apparent from comparing the 10- to 19-percent strain tolerance of the GCL to the computed strains from settlement that the liner system can be expected to perform well, even under greater than estimated differential settlements. Even if the GCL undergoes significant strain, it will continue to perform as intended up until a point at which, if strains exceed approximately 10 percent, performance will start to decline, but the geomembrane will remain intact and capable of performing as intended.

The tolerance of a natural clay liner to strain is highly dependent on the plasticity of the clay and the confining stress on the clay. It is not uncommon for clay specimens to tolerate 10 to 15 percent strains or higher without showing significant signs of cracking that would compromise their containment

25

capabilities (overall coefficient of permeability). Clays of very low plasticity may show signs of cracking and their overall coefficient of permeability may be compromised at much lower strains (3 to 5 percent). The soil contained in the clay borrow source is of relatively high plasticity. As such, it is expected that the clay will easily tolerate the small strains expected to occur at this site due to settlement.

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7.0 Facility Performance Evaluation 7.1 Hydrologic Evaluation of Landfill Performance (HELP) The HELP computer model was used to evaluate overall facility performance, to estimate leachate generation rates, to estimate leachate quantities in the leak detection system, and to estimate base liner and final cover performance. The HELP computer program is a quasi-two-dimensional hydrologic model of water movement across, into, through and out of landfills. The model accepts weather, soil and design data, and uses solution techniques that account for the effects of surface storage, snowmelt, runoff, infiltration, evapotranspiration, vegetative growth, soil moisture storage, lateral subsurface drainage, leachate recirculation, unsaturated vertical drainage, and leakage through soil, geomembrane, and composite liners. The model facilitates rapid estimation of the amounts of runoff, evapotranspiration, drainage, leachate collection, and liner leakage that may be expected to result from the operation of a landfill facility. The primary purpose of the model is to assist in the comparison of design alternatives as judged by their water balance. As such, the results generated by the model are only estimates of the performance that can be expected, and actual results can vary significantly depending upon the quality of construction of the facility, actual climatic conditions, and other factors.

The HELP model that was developed for the approved July 2006 Class 3 RCRA Permit Modification Application for the Slag Storage Area (Landfill Unit) Report (Barr, 2006) was reviewed and, since the design for Phase 7 is the same as for the previous phases, the HELP model was found to still be applicable. The model is described below.

The landfill was modeled for three open conditions (no slag, half-filled, and full) and for two closed conditions (half-filled and full). A summary of the model results is provided on the following page. Detailed results are provided in Attachment E-6. The scenarios modeled were:

• Scenario 1 – Operating facility with slag depth over base liner of 5 feet (final cover not yetconstructed).



• Scenario 4 – Closed facility with slag depth over base liner of 30 feet (final cover constructed).

• Scenario 5 – Closed facility with slag depth over base liner of 60 feet (final cover constructed).

As indicated on the data summary table, the proposed base liner design is very efficient. The HELP model predicts that no leakage through the base liner will occur. This is under the assumption of a good installation, which can be achieved by the implementation of the proposed construction quality control program.

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Table 7-1 Summary of HELP Model Results

Landfill Condition

Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5

Open: Initial Filling (Slag

Depth of 5 ft)


Depth of 30 ft)


Depth of 60 ft)

Closed: (Slag Depth

of 30 ft)

Closed: (Slag Depth

of 60 ft)

Average annual rainfall, inches 34.27 34.27 34.27 34.27 34.27

Evapotranspiration, inches 20.03 20.03 20.03 27.34 27.34

Runoff, inches (includes surface water runoff and includes drainage layer lateral drainage in Scenarios 4 and 5)

11.38 11.38 11.38 6.97 6.97

Leachate, inches 2.87 2.48 1.97 0.00 0.00

Leakage through final cover geomembrane, inches

N/A N/A N/A 0.00 0.00

Leakage through upper composite base liner, inches

0.00 0.00 0.00 0.00 0.00

Leakage to leak detection system, inches

0.00 0.00 0.00 0.00 0.00

Leakage through lower composite base liner, inches

0.00 0.00 0.00 0.00 0.00

Maximum head of final cover geomembrane, inches

N/A N/A N/A 11.22 11.22

Maximum head on base liner, inches

9.43 6.10 5.11 0.00 0.00

Liner efficiency (%) 100 100 100 100 100

Cover efficiency (%) N/A N/A N/A 100 100

Note: Reference scenario descriptions on previous page.

Of the average annual rainfall of 34 inches, the HELP model predicts that roughly one-third will be lost to evapotranspiration, whether the facility is in an open or closed condition. In the open condition, the remaining third of the average annual rainfall is collected as runoff and leachate. In the closed condition, the balance of the average annual rainfall that is not lost to evapotranspiration is collected as runoff from the surface of the landfill and from the cover system drainage layer.

The HELP model results indicate that the upper composite liner on the base of the landfill, consisting of the geomembrane overlying the GCL, should be effective at blocking leachate migration into the leak detection system and underlying composite liner system. The underlying composite liner system (geomembrane over approximately 3 feet of clay) appears largely redundant and will function as a backup liner system in the event of an unforeseen leak in the primary liner that may not be identified and repaired at the time of construction.

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A comparison of the performance of alternative final cover system designs in preventing infiltration of precipitation into the slag was completed utilizing the HELP model, the results of which are included in Attachment E-6. Scenario 1 is an evaluation of a design suggested by the MDNR, Hazardous Waste Program (HWP); Scenario 2 is an evaluation of the design proposed by Barr Engineering; and Scenario 3 is an evaluation of the design specified for sanitary landfills in the Missouri Solid Waste Management Regulations. All of these scenarios would meet the hazardous waste regulatory requirements for a final cover design, but the evaluations show that Scenarios 2 and 3 utilizing a drainage layer are essentially equivalent in their performance and are more efficient in preventing infiltration of precipitation into the landfill than is Scenario 1. The annual average volume of water allowed to infiltrate the slag in Scenario 1 is two to three orders of magnitude higher than Scenarios 2 and 3. This is largely the result of the greater head allowed on the geomembrane component of the final cover in Scenario 1 and the corresponding retardation of runoff that results from placement of the vegetative and frost protection layer in Scenario 1 in place of the granular drainage layer that is utilized in Scenarios 2 and 3.

As with the liner system, the HELP model results indicate that the proposed cover system will be effective at blocking surface water infiltration into the underlying material. While the landfill base liner system achieves its performance by combining a geomembrane and GCL to achieve high runoff values through the leachate collection system, the final cover system achieves its performance by allowing high evapotranspiration rates and relatively high runoff values. In either case, the base liner or final cover systems reject 100 percent of the precipitation impacting these systems such that their performance is at least equivalent.

As noted previously, the primary value of the HELP model is for comparison of the average performance of one design to another design. Operating conditions at the site will vary significantly from year to year and precipitation, evapotranspiration, runoff, and leachate generation rates will vary as well.

29

8.0 Action Leakage Rate (ALR) Per MDNR and U.S. Environmental Protection Agency (USEPA) guidance Action Leakage Rates for Leak Detection Systems (USEPA, 1992), the ALR is equal to 10 percent of the computed flow rate capacity of the leak detection system after a safety factor of 2.0 has been applied to the computed flow rate capacity. An evaluation to attempt to identify the cause of the leakage and to identify a potential remedy is triggered if the flow rate in the leak detection system meets or exceeds the ALR. The ALR that was developed for the approved July 2006 Class 3 RCRA Permit Modification Application for the Slag Storage Area (Landfill Unit) Report (Barr, 2006) was reviewed and, since the design for Phase 7 is the same as for the previous phases, the ALR was found to still be applicable. The ALR is described below.

ALR computations for the facility are included in Attachment E-7.

As depicted in the ALR computations, the following input data have been utilized for computation of the ALR for a GSE Fabrinet geocomposite:

• A maximum head in the leak detection system of 1.0 foot.

• A geocomposite layer thickness of 5.0 millimeters and hydraulic conductivity of 4.0 feet/minute.

• An average leak detection system slope of 5.3 percent (the vector of the 5.0 percent and 2.0percent slopes of the leak detection system).

• Darcy’s Law for flow rate computation; q = kiA, where:

o q = the leak detection system flow capacity per 1.0 foot width of geocomposite

o k = the hydraulic conductivity of the geocomposite

o i = the hydraulic gradient; the length of geocomposite over which 1.0 foot of head isdissipated, computed as 0.054 for the proposed slag storage facility

o A = the cross-sectional area of a 1.0-foot-wide section of geocomposite

Based on the input data summarized above for the GSE Fabrinet geocomposite leak detection layer, the ALR is computed as 408 gallons/acre/day. Using a Tenax TDP 1000, geocomposite increases the computed ALR to approximately 17,900 gallons/acre/day. The ALR is variable depending upon the actual geocomposite used in construction. The approved geocomposite material properties are described in the technical specifications (Attachment E-2). However, as illustrated in the ALR calculations, the transmissivity rate for the actual geocomposite utilized is expected to still result in a high ALR.

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9.0 Stormwater Management Plan Stormwater runoff will be managed in several ways, depending upon the source of the stormwater and the timing relative to overall development of the facility. During the early stages of facility development, stormwater collected on the site perimeter and stormwater draining onto undeveloped portions of the site from tributary areas will be diverted to Impoundment E. This stormwater will be pumped to the wastewater treatment plant for treatment and discharge. The stormwater that enters the active SSA will be collected by the leachate collection or leak detection system and then discharged to the wastewater treatment plant for treatment. As areas of the facility are filled to final grade and covered, stormwater impacting covered areas will continue to be discharged to Impoundment E, or alternatively will be managed as clean surface water runoff and discharged to the environment.

9.1 Stormwater Management System As detailed in Section 4.0, a leak detection system will be installed between the primary liner and the lower composite liner to collect any moisture that drains into Phase 7 until the area can be covered. During facility operations, runoff within the perimeter dikes cannot be separated from water that may have contacted material, so it will be collected with the leachate and handled as such. The leachate collection system will remain active throughout the life of the facility.

The permeability of the material will be somewhat variable such that during and after heavy rain events some temporary ponding of surface water on flatter portions of the material surface can be expected. There is some potential that surface water ponding within the active area of the landfill will persist during extended wet periods of the year or after heavy rainfall events. This situation could result from the somewhat low permeability that is expected of the material to be placed in the SSA. Since the permeability of the underlying granular drainage layer is expected to be higher than that of the overlying material, a ponding situation will not be reflective of leachate head on the liner system. Therefore, surface water ponding will likely be rectified only to the extent needed to maintain routine operations of the facility. This may be through occasional use of temporary pumps and piping to transfer ponded liquid to the lift station, or some occasional spraying of leachate on the material surface to suppress dust if dust generation becomes problematic during facility operations.

Once the final cover is constructed, surface water drainage from the final cover and leachate from the upper leachate collection system will be collected by a toe-of-slope surface water collection system. This system consists of a drainage ditch around the perimeter and discharging to Impoundment E. The perimeter ditches will direct stormwater runoff to one of two separate storm drain systems—one that drains the northeastern portion of the site and one that drains the southwestern portion of the site. The two storm drain systems will be directed to Impoundment E via an outlet pipe that was installed for the final cover perimeter storm drain for SSA Phases 1 through 6. The outlet pipe collects water from the slag storage facility stormwater drainage system upon closure of each phase and conveys the drainage down the steep slope to Impoundment E. Due to the steep slopes of the outlet pipe, energy dissipation measures are used to reduce the erosive potential of the water prior to discharging into the basin. Upon

31

closure, Phase 7 drainage will be connected to the existing outlet pipe to convey stormwater to Impoundment E.

The storm drainage ditch was sized to convey the 25-year, 24-hour storm event. The drainage ditches will be installed beyond the extent of the material and will be installed on native or fill soil. This will reduce the likelihood of differential settlement interfering with the planned drainage patterns. The final cover drainage ditch associated with run-on and runoff control systems will be emptied or otherwise managed expeditiously after storms to maintain design capacity of the system.

Stormwater runoff collected by the surface water drainage system is expected to be clean. As discussed in Appendix H, periodic evaluation of the system in the form of water quality monitoring will be made to determine what portion of the collected runoff can bypass Impoundment E and be discharged directly to the environment.

9.1.1 Modeling and Methodology The stormwater flow rates and volumes were determined by entering specific rainfall events into the HydroCAD version 10.00 software stormwater model, which is based on the Soil Conservation Service (SCS) TR-20 method. The storm events are evaluated based on the rainfall depths for Iron County, Missouri listed in the Rainfall Frequency Atlas for the Midwest and the SCS Type II distribution. The stormwater system for the final cover is designed based on the 25-year, 24-hour storm event. The time-of-concentration method used is based on the SCS Lag Method/Curve Number methodology. The HydroCAD model that was developed for the approved July 2006 Class 3 RCRA Permit Modification Application for the Slag Storage Area (Landfill Unit) Report (Barr, 2006) was reviewed and Phase 7 area was added into the model. The model was also modified to reflect the amendment to the final cover plan to remove the catch basins and closed pipe design along with open channels to be replaced by only open channel conveying the stormwater into Impoundment E. The updated HydroCAD-generated report for the 25-year, 24-hour event has been included in Attachment E-8.

Figure E-1 shows the drainage area map used in the updated hydrologic model. The final, capped and vegetated slag storage facility site will generate approximately 20 cubic feet per second (cfs) of runoff during the 25-year, 24-hour storm event and approximately 35 cfs will run off the site during the 100-year, 24-hour event. During the 25-year design event, approximately 10 cfs will drain through thenortheast storm drainage system and approximately 10 cfs will drain through the southwest stormdrainage system.

32

10.0 References Barr Engineering Company, 2006. Class 3 RCRA Permit Modification Application for Slag Storage Area

(Landfill Unit) Report, Prepared for The Doe Run Company, Buick Resource Recovery Facility. November 2005, revised July 2006.

Huff, Floyd A. and James R. Angel, 1992. Rainfall Frequency Atlas Of The Midwest. MCC Climate Analysis Center, National Weather Service, NOAA. Illinois State Water Survey, Champaign, IL. Bulletin 71, 1992. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs141p2_024033.pdf. Accessed April 12, 2017.

Idriss, I. M., 1985. Evaluating Seismic Risk in Engineering Practice, Proceedings. 11th International Conference on Soil Mechanics and Foundation Engineering, Vol. 1., Balkema Publishers, the Netherlands, pp. 255-320. 1985.

Join, A. and J. N. Mandal, 2005. Computer Aided Design and Analysis of Geosynthetic Landfills. Proceedings of the 20th International Conference on Solid Waste Technology and Management, Philadelphia, PA. April 2005.

Koerner, R. M., G. R. Koerner and M. A. Eberle, 1996. Out-of-Plane Tensile Behavior of Geosynthetic Clay Liners. Geosynthetic International, Vol. 3, No. 2, pp. 277-296. 1996.

Makdisi, F. I., and H. B. Seed, 1978. Simplified Procedure for Estimating Dam and Embankment Earthquake-Induced Deformations. J. Geotech. Engineering Division, ASCE, 104 (7), pp. 849-867. 1978.

Mesri, G. and M. Shahien, 2003. Residual Shear Strength Mobilized in First-Time Slope Failures. Journal of Geotechnical and Geoenvironmental Engineering, Volume 129, Issue 1, pp. 12-31. January 2003.

NAUE GmbH & Co. KG, 2008. Slope Design with Bentofix GCLS. NAUE GmbH & Co. KG, Gewerbestraße 2, 32339 Espelkamp-Fiestel, Germany. February 2008. http://www.cirtex.co.nz/wp-content/uploads/2016/01/BentoFix-GCL-Slope-Design.pdf. Accessed April 12, 2017.

Newmark, N. M., 1965. Effects of Earthquakes on Dams and Embankments. Geotechnique 15, (2), pp. 139-159. 1965.

Stark, T., H. Choi and S. McCone, 2005. Drained Shear Strength Parameters for Analysis of Landslides. Journal of Geotechnical and Geoenvironmental Engineering, Volume 131, Issue 5, pp. 575-588. May 2005.

Stratigraphics, 2005. Cone Penetration Testing with Soil Electrical Conductivity and Seismic Shear Wave Velocity Measurements, Doe Run Buick Facility, Boss, Missouri. June 2005.

https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs141p2_024033.pdf

http://www.cirtex.co.nz/wp-content/uploads/2016/01/BentoFix-GCL-Slope-Design.pdf

http://www.cirtex.co.nz/wp-content/uploads/2016/01/BentoFix-GCL-Slope-Design.pdf

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United States Environmental Protection Agency, Office of Solid Waste, 1992. Action Leakage Rates for Leak Detection Systems. EPA 530-R-92-004. January 1992.

United States Geological Survey, 2017. USGS Earthquake Hazards Program, Seismic Hazard Maps and Site-Specific Data, United States Lower 48. https://earthquake.usgs.gov/hazards/hazmaps/ Accessed April 12, 2017.

Yazdoni, G., 1996. Interface Friction for Geosynthetics. Poly-Flex, Inc. Geomembrane Lining Systems. 1996.

https://earthquake.usgs.gov/hazards/hazmaps/

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