Landfill Stability Demonstration

Landfill Stability Demonstration John Twitty Energy Center Utility Waste Landfill Greene County, Missouri for City Utilities of Springfield, Missouri

October 15, 2018

Landfill Stability Demonstration

John Twitty Energy Center Utility Waste Landfill Greene County, Missouri

for City Utilities of Springfield, Missouri

October 15, 2018

3050 South Delaware Avenue Springfield, Missouri 65804 417.831.9700

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TABLE OF CONTENTS

1.0 EXECUTIVE SUMMARY ........................................................................................................................ 1

2.0 INTRODUCTION .................................................................................................................................... 2

3.0 GEOLOGICAL SETTING ......................................................................................................................... 2

3.1 Stratigraphy ..................................................................................................................................... 2 3.1.1 Regional Geologic Setting ..................................................................................................... 2 3.1.2 Bedrock Stratigraphy ............................................................................................................ 2 3.1.3 Local Surficial Geology .......................................................................................................... 3

3.2 Hydrologic Conditions ..................................................................................................................... 3 3.2.1 Regional Hydrology ................................................................................................................ 3 3.2.2 Bedrock Aquifers and Confining Units ................................................................................. 3 3.2.3 Perched Shallow Groundwater Conditions .......................................................................... 4

3.3 Karst Development ......................................................................................................................... 4

4.0 LANDFILL SITING, EXPLORATION AND DESIGN ................................................................................. 5

5.0 LANDFILL OPERATING HISTORY AND PERFORMANCE .................................................................... 5

6.0 POTENTIAL UNSTABLE AREA MECHANISMS .................................................................................... 6

7.0 SITE INVESTIGATION MEANS AND METHODS ................................................................................... 6

8.0 GEOTECHNICAL INVESTIGATIONS ...................................................................................................... 7

9.0 GEOPHYSICAL INVESTIGTIONS ........................................................................................................... 9

9.1 ERT Data ........................................................................................................................................ 10 9.2 MASW Data .................................................................................................................................... 11

10.0 SUBSURFACE INVESTIGATIONS .................................................................................................... 12

11.0 FINDINGS AND CONCLUSIONS ...................................................................................................... 12

12.0 CERTIFICATION ............................................................................................................................... 14

13.0 REFERENCES .................................................................................................................................. 15

LIST OF FIGURES

Figure 1. Site Location Map Figure 2. Site Diagram Figure 3. Stratigraphic Column Figure 4. Surface Geology Map Figure 5. JTEC Karst Features Figure 6. JTEC Landfill Development Figure 7. Landfill Area Collapse Repair Locations Figure 8. UWL Borehole Locations Figure 9. ERT Traverse and MASW Sounding Locations Figure 10. Ground Surface Elevations Figure 11. Top of Rock Elevations Figure 12. Soil and CCR Thickness Figure 13. ERT Lineament Map

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Figure 14. ERT Bedrock Topography and Lineament Map Figure 15. Borehole Location Map

APPENDICES

Appendix A. Landfill Geotechnical Reports Appendix B. ERT Data Appendix C. MASW Data Appendix D. Drilling/Coring Data

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1.0 EXECUTIVE SUMMARY

A Landfill Stability Demonstration was undertaken at the City Utilities of Springfield, MO John Twitty Energy Center (JTEC) Utility Waste Landfill (UWL) in accordance with the US Environmental Protection Agency’s (USEPA’s) Coal Combustion Residuals (CCR) Rule, published in the Federal Register on April 17, 2015. The specific requirements for Stability Demonstrations (unstable area demonstrations) are provided in 40 CFR 257.64. The JTEC CCR Landfill is subject to these requirements and is located in a karst area. Karst terrain is one of the considerations to be evaluated in determining whether a CCR landfill is stable.

The JTEC landfill site was investigated in early 1980 in conjunction with the Missouri Geological Survey. The site was determined to be suitable for development of a CCR landfill. In November 1980 the Missouri Department of Natural Resources (MDNR) approved the landfill construction plans, specifications and operating procedures previously submitted, and issued Special Solid Waste Disposal Area Permit Number 707702. Construction of the landfill commenced in 1981 and was completed in 1982, at which time the landfill (Phase I area) began accepting CCR. Subsequent permit modifications have been approved by the MDNR. The final cover system for the Phase II Area has been modified to align with current MDNR and CCR regulations.

The landfill is sited at the head of a small valley and has been developed in two phases. The Phase I area encompasses the upper (northern) portion of the site and included a stormwater detention basin and perimeter berm which defined the limit of filling and precluded stormwater run-on. The Phase II stormwater detention pond was also constructed at the same time (1981 to 1982) that the Phase I area was developed. When the working face of the fill approached the Phase I area stormwater detention basin, the Phase II area of the landfill was developed by constructing east and west perimeter berms and moving the access road further south. The JTEC landfill has operated for over 30 years with no evidence of instability. When the landfill approaches completion, a gravel drainage layer will be placed along the final 200 feet of the landfill floor prior to placement of CCR materials in that area

The JTEC landfill stability demonstration involved site reconnaissance to assess karst features; geophysical surveys to determine whether cover collapse sinkholes were forming beneath the landfill; drilling and geotechnical testing within the landfill to determine engineering properties of the fill material; drilling, coring and hydrologic testing around the landfill to characterize stratigraphy, hydrology and karst development; and review of existing stability studies which assessed slope stability, bearing capacity and settlement.

The demonstration found the landfill to be stable. No cover collapse sinkholes were found to be forming within the subsoils beneath the landfill. No free moisture was detected within the fill material which minimizes the potential for sinkhole formation. Minimal solutional voids were found in the underlying bedrock, further indicating the potential for sinkhole formation is low. Landfill slopes were found to be stable. Total settlement which may be experienced at maximum fill heights was considered insignificant to low, and differential settlement which may be experienced along the gravel leachate drainage layer was considered negligible to low. Sections 8.0, 9.0 and 10.0 of this report discuss these findings in greater detail.

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2.0 INTRODUCTION

The CCR Rule establishes five (5) location restrictions for CCR landfills. Four (4) of the location restrictions (placement above the uppermost aquifer, wetlands, fault areas, and seismic impact zones) apply only to new CCR landfills. The fifth, unstable areas, applies to both new and existing CCR landfills. Unstable areas are defined as locations that are susceptible to natural or human-induced events or forces capable of impairing the integrity, including structural components of some or all of the CCR unit that are responsible for preventing releases from the unit. Unstable areas can include poor foundation conditions, areas susceptible to mass movement, and karst terrains.

40 CFR 257.64 requires an owner or operator of an existing CCR landfill to demonstrate by October 17, 2018 that recognized and generally accepted good engineering practices have been incorporated into the design of the CCR landfill to ensure that the integrity of the structural components of the CCR landfill will not be disrupted. Section 257.64 further requires the owner or operator to consider all of the following factors, at a minimum, when determining whether an area is unstable: 1) onsite or local soil conditions that may result in significant differential settlement, 2) onsite or local geologic and geomorphological features, and 3) onsite or local human-made features or events (both surface and subsurface).

The JTEC UWL is located in an area of karst terrain. This demonstration details the investigations undertaken to assess the stability of the JTEC UWL with respect to site-specific karst features and forms the basis for certification of the UWL’s stability. The location of JTEC is shown in Figure 1. A diagram of the JTEC UWL is provided as Figure 2.

3.0 GEOLOGICAL SETTING

3.1 Stratigraphy

3.1.1 Regional Geologic Setting

The JTEC site is located in the Springfield Plateau Sub-province of the Ozark Plateau Physiographic Province. The bedrock surface of the Springfield Plateau generally consists of thick Mississippian-age limestones and cherty limestones above Ordovician- and Cambrian-aged strata. Bedrock generally dips gently toward the west with minor folding and faulting. Most of the area faults have less than 50 feet of displacement. The predominantly limestone strata in the area has been extensively weathered, and the irregular bedrock surface is hidden below a mantling of cherty clay residuum with thicknesses that vary from a few feet to over 40 feet.

3.1.2 Bedrock Stratigraphy

The Springfield Plateau is underlain at depth by Precambrian crystalline basement rock that serves as the lower groundwater confining unit. The Precambrian basement is overlain by the Cambrian-aged Lamotte Sandstone consisting of approximately 150 feet of sandstone. This unit is overlain by around 200 feet of dolomite, the Bonneterre Formation, and 150 feet of shale, the Davis Formation. The Derby-Doerun, Potosi and Eminence Dolomites overlie the Davis Formation and are cumulatively around 500 feet thick. The top of the Eminence Formation marks the top of the Cambrian Period, as well.


The Ordovician-aged Gasconade Formation consisting of 350 feet of dolomite and up to 25 feet of sandstone overlies the Eminence Formation. The overlying Roubidoux Formation consists of around 150 feet of dolomite, dolomitic sandstone, and sandstone. Between 250 and 600 feet of dolomite divided into the Jefferson City and Cotter Formations mark the top of the Ordovician Period below the Springfield Plateau.

In the vicinity of JTEC, the Cotter Dolomite is unconformably overlain by around 30 feet of the Mississippian aged Compton Limestone; the Devonian and Silurian Periods being entirely missing from the geologic record. The Compton Limestone is overlain by between 5 and 30 feet of shale and siltstone of the Northview Formation. The Northview Formation is overlain by less than 100 feet of the Pierson Limestone, which is overlain by the Elsey-Reeds Spring Formation, consisting of up to 200 feet of cherty limestone. The Elsey-Reeds Spring Formation is capped by the Burlington-Keokuk Limestone, which forms the bedrock surface across most of the Springfield Plateau and due to the high degree of weathering, may be from 150 to 250 feet thick. A stratigraphic column is provided as Figure 3.

3.1.3 Local Surficial Geology

The native clay residuum in the JTEC area is mapped as cherty clay residuum consisting of clay loam to silty clay loam containing subangular to angular fragments of chert up to one foot in diameter as individual clasts and relict chert layers (Whitfield and others, 1993). Geologic mapping of the Springfield 1-degree-by-2-degrees quadrangle indicates that the JTEC site is located in an area immediately underlain by Mississippian-aged Burlington-Keokuk Limestone (Middendorf and others, 1987). The Burlington-Keokuk is a coarsely crystalline limestone composed of nearly pure calcium carbonate formed from the deposition of the primarily calcareous fragments of shallow marine organisms. Formation thickness is highly variable due to weathering but can reach a maximum of 200 feet. Joints in the limestone influence both surface and subsurface drainage and have caused the bedrock surface to be weathered into cutters and pinnacles, commonly with 10-15 feet of relief or more. The Burlington-Keokuk Limestone is immediately underlain by the Elsey-Reeds Spring Formation at the JTEC site. Karst development at the site is principally concentrated in the upper 40 feet of the Burlington-Keokuk Limestone and in the lower 20 feet of the Burlington-Keokuk where it transitions into the Elsey-Reeds Spring Formation. A surface geology map is included as Figure 4.

3.2 Hydrologic Conditions

3.2.1 Regional Hydrology

The Springfield Plateau is underlain by three bedrock aquifers, the St. Francois Aquifer, the Ozark Aquifer, and the Springfield Aquifer (Emmett et al, 1978). Of these, the Ozark Aquifer is the most important groundwater source in the Springfield area. The Ozark Aquifer has an average thickness of approximately 1,200 feet in the Springfield area, thinning to the north and thickening to the south. The Ozark Aquifer is confined and is artesian where pumping hasn’t produced a large cone of depression in the potentiometric surface.

3.2.2 Bedrock Aquifers and Confining Units

The Springfield Aquifer in the Springfield area varies between 100 to more than 300 feet thick and encompasses the Burlington-Keokuk Limestone, Elsey-Reeds Spring Formation and Pierson Formation. Groundwater flow primarily occurs along fractures, bedding planes, and voids causing flow velocities to be widely variable across the Springfield Plateau. The Northview Formation and the Compton Limestone form an effective aquitard, restricting migration of groundwater into the underlying Ozark Aquifer. The overall

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hydraulic conductivity of the aquifer is estimated to be 2.5x10-4 feet per second and the transmissivity ranges from about 1.0x10-2 to 5.0x10-2 square feet per second. The Springfield Aquifer is not used as a drinking water source in Greene County (designated Sensitive Area C) in accordance with Missouri Well Construction Code regulations as outlined in 10 CSR 23-3. The Upper portion of the aquifer consists of the Cotter and Jefferson City Dolomites and is capable of yielding 30 to 70 gallons per minute. The lower portion of the aquifer includes the Roubidoux Formation, Gasconade Dolomite, Eminence Dolomite, and Potosi Dolomite and yields from 100 to 200 gallons of water per minute. Estimates of transmissivity of the Ozark Aquifer in Springfield, Missouri vary from 860 to 4,320 square feet per day and estimates of the horizontal hydraulic conductivity vary from 4.3 to 43 feet per day (Imes, 1989). The Bonneterre Formation and Lamotte Sandstone form the St. Francois deep aquifer with the Davis Formation acting as the confining unit.

3.2.3 Perched Shallow Groundwater Conditions

The Springfield Aquifer is unconfined, allowing surface water to percolate through clay residuum into the bedrock below. The percolation of groundwater is aided by increased secondary porosity of the jointed bedrock below, and solutional widening of those joints. Transient, perched groundwater is generally found at the bedrock surface across the Springfield Plateau, resulting from the difference in hydraulic conductivities of the native clay residuum and underlying bedrock. The iron-rich clay residuum that forms from the underlying cherty limestone bedrock often retains relict beds of chert in much the same orientation as they were found in the parent bedrock. The high iron content of the clay residuum causes the clay to flocculate and form blocky aggregates, adding to the secondary porosity of the soil. The combination of flocculated clay structure and high chert content results in a higher permeability than would be expected from a more uniform clay soil. The permeability of the native clays has been shown to be reduced substantially by reworking and recompaction of the materials.

3.3 Karst Development

Karst features in southwest Missouri include springs, caves, solution-widened joints, cutters and pinnacles, solutional sinkholes, and collapse sinkholes. Solution-widened joints form when slightly acidic groundwater percolates through the existing joints in soluble bedrock, slowly dissolving and widening the joint. Cutters and pinnacles form where tightly-spaced and roughly perpendicular intersecting joint sets are widened by solutioning, leaving spires of bedrock separated by joints that narrow with depth. A solutional sinkhole is bounded by a definable rim and is generally a circular or oval-shaped depression. Some solutional sinkholes contain an “eye” where groundwater quickly enters subsurface drainage galleries formed by intersecting solution-widened joints. Solutional sinkholes form as the bedrock surface is dissolved by solution, causing the overlying soil to subside and form a surface depression. Collapse sinkholes form when a void in the subsurface soil propagates toward the surface until the roof of the soil cavity collapses, forming a relatively steep-sided roughly cylindrical to cone-shaped cavity from the surface to the bedrock below. Collapse sinkholes occur along bedrock joints and are typically triggered by an increase in soil moisture content. Residual soils in the JTEC area tend to be stiff and strong when dry, but strength decreases, and unit weight increases when wet. Extensive site reconnaissance and geological mapping has been conducted at the JTEC site. Karst features mapped during reconnaissance of the JTEC site are shown in Figure 5.


4.0 LANDFILL SITING, EXPLORATION AND DESIGN

The existing JTEC UWL was developed in the early 1980s and was sited at the head of a small valley to the east of the electric generating unit (EGU) in an area devoid of sinkholes. Based on my recollection and personal experience as CU engineer of record, the site was drilled extensively to determine depth to bedrock, and a number of soil trenches were excavated to the bedrock surface in order to characterize soil structure and the nature of the bedrock surface. Trenches which intercepted deep cutters (indicative of solution-widened joints) were left open for a period of time and periodically filled with water to assess the permeability of the bedrock and to induce sinkhole collapse. Site exploration was conducted by Gary Pendergrass, PE, RG, the undersigned, in conjunction with J. Hadley Williams, PhD, Director of the Missouri Geological Survey. The site was determined to be suitable for development of a CCR landfill and was permitted and constructed in the mid-1980s. The landfill was constructed in accordance with the MDNR requirements which existed at the time. An initial layer of very impermeable fly ash/scrubber sludge mixture was placed incrementally on the floor of the landfill prior to placement of operational CCR.

5.0 LANDFILL OPERATING HISTORY AND PERFORMANCE

The 42-acre UWL was developed and operated in two phases, as shown in Figure 6. The initial phase consisted of the northernmost 20 acres and incorporated a stormwater detention basin at the downstream (southern) end and a perimeter berm. The perimeter berm serves two functions; to direct stormwater runoff from the interior of the landfill toward the detention basin and to prevent stormwater run-on into the landfill footprint/working area (run-on/runoff control). Placement of CCR fill progressed from north to south. When the working face of the fill approached the original stormwater detention basin, Phase II of the landfill was developed. The Phase II stormwater detention pond was constructed at the same time (1981 to 1982) that the Phase I area was developed. A gravel leachate drainage layer with integral perforated PVC pipe will be placed along the final 200 feet of the landfill floor prior to placement of CCR materials in that area. The Phase II area liner protective layer requirements were approved and incorporated into the existing UWL permit through a modification dated April 30, 2001. The permit indicates that the liner consists of a scarified, wetted (if needed) and recompacted top six inches of soils/clay materials. The subsoil liner is protected with fly ash/scrubber sludge matrix in the Phase I area and a layer of Type C fly ash/bottom ash in the Phase II area; placed in 30- to 50-foot wide strips prior to moving into each area. The Phase I area requirements (fly ash/scrubber sludge) were included in the landfill development plans and incorporated by reference to the original November 1980 landfill permit approved by the MDNR.

Once the Phase II area was prepared, the Phase I stormwater detention pond was drained and filled as the working face progressed.

The CCR landfill has operated continuously since development with no indication of instability. One small cover collapse sinkhole formed along the interior toe of the perimeter berm in (June 2007). A second small cover collapse sinkhole formed along the exterior toe of the perimeter berm in (August 2017). Both of these features formed in locations that were not within areas where CCR is currently placed and managed, and both formed in areas which periodically receive storm water runoff. The locations of these features are shown in Figure 7. Two additional cover collapse sinkholes were induced during the original site exploration. Both were excavated and successfully stabilized by construction of engineered graded filters and clay seals prior to landfill development in those areas. Cover collapse sinkholes typically form where fluctuations in soil moisture content occur, such as beneath storm water runoff channels. These cover collapse sinkholes

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are not an indication of instability beneath the CCR landfill, since soil moisture content beneath the fill tends to be relatively stable.

The JTEC CCR landfill was designed to promote stormwater runoff and minimize changes in moisture content within the foundation materials. These design elements constitute recognized and generally accepted good engineering practices to ensure that the integrity of the structural components of the CCR landfill would not be disrupted.

6.0 POTENTIAL UNSTABLE AREA MECHANISMS

The one type of karst feature which has potential to compromise the integrity of the UWL is the cover collapse sinkhole. Cover collapse sinkholes originate at the soil-bedrock interface when soil is eroded into underlying bedrock solution cavities. The resulting soil void can propagate upward until it manifests as a surface collapse or, in the case of CCR landfills, until it undermines the structural support of landfill components such as liners and leachate collection systems. The mechanics of cover collapse sinkhole formation are well understood. The formation of a cover collapse sinkhole is typically initiated by changes in soil moisture content. As soil moisture content increases, the unit weight of the soil increases and soil strength decreases. Cover collapse sinkholes can form in areas with no surface expression of karst but do require the existence of a solution-widened bedrock joint which is filled with residual soil that supports the overlying soil and a solution cavity beneath the residual soil to receive the eroded material.

7.0 SITE INVESTIGATION MEANS AND METHODS

A Landfill Stability Demonstration in a karst setting must assess whether a cover collapse sinkhole has formed, or is forming, beneath the landfill. A cover collapse sinkhole would be manifest as a vertical void extending from solution cavities within the underlying limestone bedrock upward toward the landfill material.

GeoEngineers has extensive experience investigating cover collapse sinkholes in karst areas and has found that Electrical Resistivity Tomography (ERT) coupled with Multichannel Analysis of Surface Waves (MASW) is a very effective means to identify and characterize these features. ERT is a non-intrusive imaging method designed to measure spatial variations (2-D or 3-D) in the electrical resistivity of soil and rock. The ERT tool also measures spatial variations in the electrical resistivity of CCR. Cover collapse sinkholes would be manifest as vertical zones of very high electrical resistivity (air-filled voids) extending upward from the bedrock surface. MASW is a non-intrusive acoustic imaging method designed to measure spatial variations (1-D) in the average shear wave velocity of soil and rock. It also measures spatial variations (1-D) in the shear wave velocity of CCR. ERT and MASW surveys were completed at the JTEC landfill.

The geophysical data were complimented by borehole, core hole, geotechnical data compiled at the site since 2007. The information from these boreholes was reviewed and compared to corresponding ERT data in order to refine and support the data interpretations.


8.0 GEOTECHNICAL INVESTIGATIONS

The potential for water to percolate through the UWL fill material and reach the foundation materials was investigated by advancing three (3) exploratory boreholes into the capped portion of the UWL. Drilling and associated geotechnical testing was completed by Palmerton & Parrish, Inc. (PPI). PPI’s drilling logs and geotechnical data are provided in Appendix A. Figure 8 shows the landfill boring locations. The three boreholes were advanced along the landfill crest, which generally corresponds with the pre-landfill valley centerline beneath the fill. Borehole CCRB-1 was completed at the north end of the landfill crest, at the head of the pre-landfill valley. CCRB-3 was completed at the south end of the landfill crest, within the pre-landfill valley. Both CCRB-1 and CCRB-3 were terminated approximately ten feet above the landfill base. CCRB-2, which is located midway along the landfill crest, was drilled approximately three feet into the landfill foundation material and completed as a temporary piezometer. All three boreholes confirmed the absence of free moisture, both during drilling and subsequent 6 months of water level measurements. CCRB-2 was maintained as a piezometer for approximately six months and consistently showed no free moisture. These data are also consistent with ERT data, which indicate high resistivity/low moisture content within the landfill fill material. Correspondingly, the results of these three boreholes indicate that free water is not percolating through the UWL fill material within the capped portion of the UWL. The piezometer and two boreholes have since been properly abandoned and plugged.

The geotechnical data show a wide range in grain size distribution, as would be expected with landfill materials ranging from slag and bottom ash to fly ash. Grain size distributions were run on four discrete samples, collected at a 5-foot depth in CCRB-1, at 5-foot and 65-foot depths in CCRB-2, and at a 5-foot depth in CCRB-3. The three shallow samples provided an assessment of lateral grain size distribution across the landfill. The deep sample provided an assessment of grain size distribution near the base of the fill. Hydraulic conductivity was also run on the deep sample. The shallow sample from CCRB-1 contained 10.1 percent gravel-sized particles, 29.2 percent sand-sized particles, and 60.7 percent silt/clay-sized particles, suggesting it is composed primarily of fly ash. The shallow sample from CCRB-2 contained 21.1 percent gravel-sized particles, 52.8 percent sand-sized particles, and 26.1 percent silt/clay-sized particles, suggesting it is composed primarily of bottom ash. The shallow sample from CCRB-3 contained 19.7 percent gravel-sized particles, 43.9 percent sand-sized particles, and 36.4 percent silt/clay-sized particles, suggesting it is composed of a mixture of bottom ash and fly ash. This spatial variation in grain size distribution is consistent with the manner in which materials are placed and compacted within the utility waste landfill. The deep sample from CCRB-2 contained 4.4 percent gravel-sized particles, 18.5 percent sand-sized particles, and 77.1 percent silt/clay-sized particles, suggesting it is composed primarily of fly ash. Hydraulic conductivity was measured at 1.25 x 10-5 cm/sec in the deep sample from CCRB-2, which is generally equivalent to the MDNR requirement for hydraulic conductivity of landfill cap material (10 CSR 80-11.010(14)(C)3.).

Standard Penetration Test (SPT) blow counts were recorded at 5-foot intervals in each borehole to provide a measure of the bulk density of landfill materials. Blow counts below 10 indicate a low bulk density, blow counts between 10 and 30 indicate a medium bulk density, and blow counts above 30 indicate a high to very high bulk density. Blow counts in CCRB-1 ranged from 9 to 65, with an average of 28, indicating a material with some variability, but generally with a medium to high bulk density. Blow counts in CCRB-2 ranged from 4 to 51, with an average of 20, indicating a material with some variability, but generally with a medium bulk density. (Note: the final blow count in CCRB-2 was recorded in the foundation materials and was not included in calculation of average CCR blow counts.) Blow counts in CCRB-3 ranged from 3 to 53,

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with an average of 28, indicating a material with some variability, but generally with a medium to high bulk density. The variation in blow count is likely due to a combination of factors, including a) the variation in material type, b) the natural consolidation that is taking place due to landfill surcharge, and c) the natural cementing process that occurs in pozzolanic materials such as fly ash.

Samples were collected from CCRB-1, CCRB-2 and CCRB-3 at 5-foot intervals and analyzed in the laboratory to determine moisture content. Moisture content is determined by weighing the as received (wet) sample, drying the sample in an oven, and weighing the dry sample. Moisture content is the ratio of the weight of water present in the material to the dry weight of the material. Moisture content in CCRB-1 ranged from 26.4 percent to 50.7 percent, with an average of 43.3 percent. Moisture content in CCRB-2 ranged from 32.7 percent to 65.7 percent, with an average of 45.8 percent. (Note: the final moisture content in CCRB- 2 was recorded in the foundation materials and was not included in calculation of average CCR moisture content.) Moisture content in CCRB-3 ranged from 24.8 percent to 52.1 percent, with an average of 34.1 percent. The variation in moisture content is to be expected given the grain size distribution in the fill material. Moisture in fine-grained materials is more readily retained, while moisture in coarse grained materials can more readily drain.

Anderson Engineering, Inc. (AE) conducted earlier geotechnical investigations within the JTEC UWL in 2007 and 2011 for the purpose of analyzing slope stability and assessing the effects of karst features on settlement and bearing capacity (Anderson Engineering, 2007 and 2011). These investigations provide useful data regarding properties of the fill material. The 2007 investigation advanced three boreholes to approximately 37 feet below ground surface (BGS), and the 2011 investigation advanced two boreholes to approximately 50 feet BGS. All of the boreholes were completed in fly ash, bottom ash and scrubber sludge, and show some stratification of materials. All of the boreholes were terminated above the base of the fill material. Boreholes B-2-07, B-3-07, and B-1 had an average water content of 31 percent, 32 percent and 38 percent, respectfully. B-1-07 had an average water content of 40 percent from 0-12 feet BGS, and an average water content of 22 percent from 12 to 37 feet BGS. Borehole B-3 had an average water content of 41 percent from 0 to 25 feet BGS, and an average water content of 29 percent from 25 to 50 feet BGS. In general, the water content was shown to decrease slightly with depth in these particular borings, which may indicate the borings were completed following a period of rainfall. AE’s geotechnical report also notes that saturated materials were not encountered in any of the boreholes. AE’s stability analysis found the JTEC landfill to meet generally accepted standards for slope stability. AE also concluded that the likelihood of encountering shallow karst features under the landfill was low and, if encountered, the effects on settlement and bearing capacity would not be significant. Moreover, AE concluded that the settlement which may be experienced at maximum fill heights is considered insignificant to low, and that the differential settlement which may be experienced along the gravel drainage layer is considered negligible to low. AE borehole locations are also shown on Figure 8. AE reports are included in Appendix A.

On a volumetric basis, the landfill fill material is comprised of air, water and solid material. Only when all of the available void space is filled with water is the sample considered to be saturated; overcoming surface tension and allowing water to move freely. Since no free water was observed in the boreholes, the landfill material appears to be well below saturation levels and minimal water would be expected to flow through the fill material into the foundation materials. Consequently, changes in moisture content within the foundation’s materials, which could contribute to cover collapse sinkhole formation beneath the landfill, are not expected.


9.0 GEOPHYSICAL INVESTIGATIONS

ERT and MASW data were acquired across and in proximity to the JTEC UWL as shown in Figure 9. In total, 2-D ERT data were acquired along 103 separate traverses on the JTEC site. MASW data were acquired at a total of 37 discrete locations.

The ERT survey was conducted with the goal of imaging the subsurface to a depth of approximately 100 feet and providing insight into the structural stability of the JTEC UWL. The ERT data were acquired using an AGI SuperSting system coupled to a dipole-dipole array consisting of 168 electrodes spaced at 5-foot intervals. The 5-foot electrode spacing was employed to ensure relatively high-resolution (vertical and lateral) ERT data were recorded. A non-standard array of 168 electrodes was employed so that the subsurface could be imaged to depths in excess of 120 ft., as the depth of investigation is approximately equal to 20 percent of the electrode array length (167 x 5 feet x 0.2). The west-east oriented ERT traverses were spaced at 20- foot intervals (north-south) so they could be processed as a 3-D data set. Interpreted ERT profiles are presented in Appendix B.

The MASW data were acquired using a 24-channel Seistronix engineering seismograph and 4.5 Hz geophones spaced at a 5-foot interval and, in some instances, a 2.5-foot interval, as well with the intent of imaging the subsurface to depths on the order of 100 feet, as the depth of investigation is typically equal, more or less, to the length of the geophone array (23 x 5 feet). In areas where interpretable MASW data could not be acquired using a 5-foot geophone spacing (generally because of the irregular depth to top of rock along the length of the geophone array), a geophone spacing of 2.5 feet was also employed. The MASW was conducted to determine the engineering properties of the materials below and in proximity to the JTEC UWL to depth of approximately 100 feet. The MASW data are provided in Appendix C.

The fifty-six W-E oriented ERT traverses labeled 600 to 655, inclusive, in Figure 9 are spaced at 20 foot-intervals (south to north). The ERT data acquired along each of these fifty-six traverses were initially processed as a 2-D data set, with the initial output of processing being a suite of fifty-six 2-D ERT profiles. The entire suite of fifty-six 2-D ERT profiles was subsequently reprocessed as a single 3-D data set. This 3- D ERT data set is included in Appendix B as a suite of fifty-five ERT profiles (labeled 600-601 to 654- 655), each of which was extracted from the processed 3-D ERT data set for visualization and interpretation purposes.

The twenty-nine W-E oriented ERT traverses labeled 659-687, inclusive, in Figure 9 are spaced at 20 foot-intervals. The ERT data acquired along each of these twenty-nine traverses were initially processed as a 2- D data set; with the initial output of processing being a suite of twenty-nine 2-D ERT profiles. The entire suite of twenty-nine 2-D ERT profiles was subsequently reprocessed as a single 3-D data set. This 3-D ERT data set is included in Appendix B as a suite of twenty-eight ERT profiles (labeled 659-660 to 686-687), each of which was extracted from the processed 3-D ERT data set for visualization and interpretation purposes.

The eight N-S oriented ERT traverses labeled 688-695, inclusive, in Figure 9 are spaced at 20 foot-intervals. The ERT data acquired along each of these eight traverses were initially processed as a 2-D data set, with the initial output of processing being a suite of eight 2-D ERT profiles. The four N-S oriented ERT traverses labeled 696-699, inclusive, in Figure 9 are also spaced at 20 foot-intervals. The ERT data acquired along each of these four traverses was processed as 2-D data only; the output of processing being a suite of four 2-D ERT profiles labeled 696-699 and included in Appendix B).

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The ten ERT traverses labeled 30945 NW-SE, 30950 W-E, 30950 N-S, 30951 NW-SE, 30952 SW-NE, 30953 N-S, 31000 NW-SE, 31001 N-S, 31172 SW-NE and 31184 N-S were acquired along the perimeter of the JTEC landfill as shown in Figure 9). The ERT data acquired along each of these ten traverses were processed as 2-D data only; the output being a suite of ten 2-D ERT profiles labeled 30945 NW-SE, 30950 W-E, 30950 N-S, 30951 NW-SE, 30952 SW-NE, 30953 N-S, 31000 NW-SE, 31001 N-S, 31172 SW-NE and 31184 N-S and included in Appendix B).

MASW data were acquired at specific locations along west-east oriented ERT traverses and (mostly) at 200- foot intervals, as shown in Figure 9). Where necessary, MASW data acquisition locations were shifted because of access issues (surface topography, roadways, etc.).

9.1 ERT Data

Three contoured maps were generated for the JTEC site based on the interpretation of the acquired 3-D ERT control: 1) JTEC Ground Surface Elevation, 2) JTEC Top of Rock Elevation, and 3) JTEC Combined Soil and CCR Thickness. These three contoured maps are presented as Figures 10, 11 and 12, respectively.

As shown on the JTEC Ground Surface Elevation map (Figure 10), the surface of the CCR landfill is elevated relative to the ground surface in areas adjacent to the landfill. Although some storm water would seep through the grass-covered clay cap, it is anticipated that most storm water would flow (as runoff) down the flanks of the landfill. Some of the run-off from the landfill would seep into the soil and underlying rock (mostly near the toe of the landfill). The rest would flow away from the landfill, as surface runoff, along natural and/or man-made surface drainage pathways. Some seepage (into the underlying soil and rock) along the run-off routes would be expected, especially where flow was concentrated (in drainage ditches, for example).

In Figure 11, the JTEC Top of Rock Elevation map is presented. This map is consistent with ERT control (where top of rock can be confidently identified on the ERT profiles), MASW control, and general topographic trends. The JTEC Top of Rock map confirms the landfill was constructed in a north-south trending topographic valley.

In Figure 12, the JTEC Combined Soil and CCR Thickness map is presented. This map is consistent with ERT control (where top of rock can be confidently identified on the ERT profiles), MASW control, and general topographic trends.

As indicated on the suite of ERT profiles presented in Appendix B, the interpreted top of rock correlates well with the 125 ohm-m contour interval, except in areas where shallow rock is anomalously moist. The ERT profiles are consistent with available MASW estimated depths to top of rock.

Visual examination of the ERT data indicates the moisture content of CCR materials and foundation materials increases with depth. However, in places, pod-shaped zones of higher electrical resistivity are imaged on the ERT profiles. These are interpreted as zones comprised of more porous and permeable CCR. With respect to structural stability of the landfill, there is no evidence on any of the 3-D ERT profiles that air-filled voids have developed within the CCR material or the foundation materials.

As indicated on the suite of ERT profiles presented in Appendix B, shallower soils are generally drier and characterized by electrical resistivity values higher than 125 ohm-m. Soils at depth are generally more moist and are characterized by electrical resistivity values lower than 125 ohm-m. The soil/rock interface is


readily mapped on the ERT profiles where the soil is relatively moist, and the rock is relatively dry. In places where the soil and rock are either both moist or both dry, the soil/rock contact is less well-defined on the ERT profiles. With respect to structural stability of the landfill, there is no evidence on any of the 3-D ERT profiles that significant air-filled voids have developed within the soils either beneath or immediately adjacent to the landfill. As explained previously, air-filled voids would be imaged as areas of extremely high electrical resistivity. The ERT data does indicate elevated moisture in areas near the toe of the landfill where runoff is impeded or allowed to soak into the subsurface naturally. Because these areas are outside of the UWL footprint the threat of cover collapse sinkhole formation to the UWL is minimal. Even if there were a minor collapse, it might only impact the toe of a slope which would be readily visible and easily repaired.

Bedrock below the UWL is characterized by electrical resistivity values above 900 ohm-m except in areas where the shallow, more intensely weathered rock is anomalously moist and in proximity to prominent, mostly near-vertical, seepage pathways. Inasmuch as moisture has been seeping into the subsurface along identified seepage pathways beneath the landfill only as long as the landfill has been operative, it is inconceivable that seepage along this pathway could cause significant solution-widening of the existing fractures within bedrock. There is no visual evidence on any of the 2-D or 3-D ERT profiles that either pre-existing air-filled voids or pre-existing or newly infilled clay-filled voids are present within the rock underlying the landfill.

A visual examination of 3-D ERT profiles acquired across the landfill indicates that the electrical resistivity of the landfill material (near the structural top of the landfill) typically decreases with depth. This indicates that the shallower CCR material is relatively dry and that the moisture content of the material generally increases with depth. If there is any seepage thru the landfill, most of it would tend to flow vertically to the underlying bedrock surface and then laterally (down dip) at or near the bedrock surface towards near-vertical fractures. There is no evidence on any of the 3-D ERT profiles that air-filled voids have developed within the landfill, or that there are any cover collapse sinkholes in progress within the UWL area.

Significant karst features, such as active or inactive sinkholes or prominent solution-widened joints, are not identified on any of the acquired ERT data. Significant clay-filled karst features, such as sinkholes and prominent solution-widened joints, are generally characterized by anomalously low electrical resistivity values (<125 ohm-m; attributable to high moisture content and presence of conductive clays). Even though the exact interface between the fill materials and the foundation materials is not discernable due to the consistent moisture content and consistent ERT signature, vertical air-filled voids associated with cover collapse sinkholes have a very distinct signature (anomalously high electrical resistivity) which would be readily discernable in either material. All of the low electrical resistivity zones on the JTEC ERT data set (Appendix B) can be confidently attributed to the seepage of moisture along mostly near-vertical pathways through the soil and into the underlying pervasively fractured rock.

9.2 MASW Data

The MASW data were acquired at the JTEC site indicate that the shallow vehicle-compacted soil (depths < 5 feet) are characterized by shear wave velocities averaging 1500 feet/s and that the underlying less compacted soils are characterized by velocities averaging about 1000 feet/s. The bedrock surface at the site is very irregular due to solutional weathering and development of cutters and pinnacles, with the top of rock ranging from roughly 20 to 40 feet in depth, as characterized by shear wave velocities averaging about 3000 ft/s. Rock at depths greater than 40 feet is characterized by velocities averaging more than 4800 feet/s.

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The entire suite of interpretable MASW data (including field records, dispersion curves and 1-D shear-wave velocity profiles) are presented in Appendix C. Overall, the interpretations of the ERT and MASW data are comparable.

The contact between CCR fill and the underlying soil cannot be mapped on either the ERT data or the 1-D shear wave velocity profile, since the shear wave velocities of the materials are very similar. However, an assessment of the 1-D MASW shear wave velocity profiles indicates that the velocity of the CCR fill and underlying soil increases with depth from about 400 feet/s to more than 3000 feet/s, which suggests that consolidation of the materials is taking place as a result of landfill surcharge.

The shear wave velocity of rock generally increases significantly with depth (from 1500 feet/s to in excess of 5000 feet/s), indicating that shallower rock is more weathered than rock at greater depth. This is consistent with the interpretations that moisture could flow laterally (down dip) at or near the top of the pervasively fractured bedrock.

10.0 SUBSURFACE INVESTIGATIONS

A total of 29 boreholes were drilled and cored in the vicinity of the JTEC landfill site in conjunction with design, installation and testing of the groundwater monitoring system network. Locations of boreholes are shown in Figure 15. Drilling logs and core photographs are provided in Appendix D. The drilling program found residual clay soils to overlay an irregular bedrock surface exhibiting well-developed cutters and pinnacles and clay-filled joints. Depth to bedrock ranges from a few feet to more than 40 feet. Inspection of rock core indicates there are relatively few solution cavities and solution-widened bedding planes in the vicinity of the landfill, which lessen the possibility of cover collapse sinkhole formation. The solutional features observed in the rock core tend to be small (a few inches in dimension) and tend to diminish with depth. Based on our investigations, it appears stormwater moves through the residual soils rather rapidly to the bedrock surface, then migrates primarily downward through vertical solution-widened bedrock joints. Lateral movement of water through solution-widened bedding planes appears to be a lesser component. Very little perched water was observed above the bedrock surface. Given the rapid drainage of water through the residual soils and bedrock, it does not appear that residual soils remain saturated for an extended period of time, which also further lessens the possibility of cover collapse sinkhole formation.

11.0 FINDINGS AND CONCLUSIONS

Following are findings and conclusions resulting from the landfill stability demonstration:

1. Recognized and generally accepted good engineering practices were incorporated into the design of the CCR landfill to promote stormwater runoff and minimize changes in moisture content. Since water can be a catalyst for karst development, this helps ensure that the integrity of the structural components of the CCR landfill will not be disrupted.

2. No free moisture was found within the landfill fill material, which reduces the possibility of cover collapse sinkhole formation beneath the landfill.

3. Geotechnical analysis performed by Anderson Engineering, Inc. found the JTEC landfill to meet generally accepted standards for slope stability. Anderson Engineering also concluded that the


likelihood of encountering shallow karst features under the landfill was low and, if encountered, the effects on settlement and bearing capacity would not be significant.

4. Geotechnical analysis performed by Anderson Engineering, Inc. concluded that the settlement which may be experienced at maximum fill heights is considered insignificant to low, and that the differential settlement which may be experienced along the gravel drainage layer is considered negligible to low.

5. The extensive 3D ERT survey conducted at the site found the landfill foundation and fill material to be intact with no indication of cover collapse sinkhole formation.

6. The extensive subsurface investigation undertaken for the groundwater monitoring system network showed the limestone bedrock to be intact with relatively few solutional voids or solutional bedding planes encountered, lessening the possibility of cover collapse sinkhole formation.

7. Stormwater at the site appears to drain rapidly through residual soils into vertical fractures and solution-widened joints. Residual soils do not appear to remain saturated for extended periods of time, which also lessens the possibility of cover collapse sinkhole formation.


13.0 REFERENCES

Anderson Engineering, Inc. Geotechnical Investigation Report for Ash Landfill, Southwest Power Station, Springfield, Missouri, September 19, 2007.

Anderson Engineering, Inc. Geotechnical Investigation Report, John Twitty Energy Center Utility Waste Landfill, December 9, 2011.

Salvati, R. & Sasowsky, I.DD, 2002. Development of Collapse sinkholes in areas of groundwater discharge. Journal of Hydrology, 264. 1-11.

Tharp, T.M., 1999. Mechanics of upward propagation of cover collapse sinkholes. Engineering Geology 52, 13–33.

Waltham, T., Bell, F. G., & Culshaw, M. G. (2005). Sinkholes and subsidence: Karst and Cavernous Rocks in Engineering and Construction.

White, W.B., White, E.L., 1995. Thresholds for soil transport and the long-term stability of sinkholes. In: Beck, B.F., Pearson, F.M. (Eds.), Karst geohazards: engineering and environmental problems in karst terrane, Balkema, Rotterdam, pp. 73–78.

FIGURES

µVicinity Map

Figure 1

City Utilities of Springfield - John Twitty Energy Center Greene County, Missouri

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LegendA Well Locations! Collapse Repair Inside Perimeter Berm

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@A

MW-SA-2A

MW-SA-3

MW-SA-4

MW-PZ-01D

MW-PZ-09D

MW-PZ-08D

MW-PZ-10D

MW-PZ-03D

MW-PZ-11D*

MW-PZ-12D

MW-PZ-13D

MW-PZ-14D

1180

1170

1190

1170

1200

1190

1210

1170

1170

MW-SA-5

MW-SA-6

MW-SA-7

MW-SA-8

MW-SA-9

1

μ@A Monitoring Well

Top of Bedrock (Interpreted from 3D ERT)

Interpreted Lineaments

Landfill_Boundary

* Well Not Currently Equipped With Transducer

JACKSON SPRING(ELE 1145 FT)

Scale 1:3,000

MW-SA-5MW-SA-4MW-SA-3MW-PZ-10DMW-SA-6MW-PZ-11DMW-SA-7MW-PZ-03D

1160.9 ft1161.1 ft1163.9 ft1177.9 ft1183.3 ft1189.6 ft1198.5 ft1214.9 ft

Monitoring Well Bedrock Elevations(ft. NAVD 83)

Figures - Page 4 of 37

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_

_

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MW-SA-5

MW-SA-6

MW-SA-7

MW-SA-8

MW-SA-9

B-1-07

B-2-07

B-3-07

B-3

B-2

B-1

BH CCRB3

BH CCRB1

BH CCRB2

MW-SA-2A

MW-SE-1

MW-SE-2MW-SE-3MW-SA-3

MW-SA-4

MW-PZ-01D

MW-PZ-09D

MW-PZ-08D

MW-PZ-10D

MW-PZ-03D

MW-PZ-11D*

MW-PZ-12D

MW-PZ-13D

MW-PZ-14D

MW-PZ-01

MW-PZ-09

MW-PZ-08

MW-PZ-04

MW-PZ-07

MW-PZ-03

MW-PZ-06

MW-PZ-02

MW-PZ-05

JTEC UWL Borings (2007-Present)

City Utilities of Springfield - John Twitty Energy CenterGreene County, Missouri

Figure 15

µ400 0 400

Feet

Legend_ Boring Locations

Landfill Boundary



P:\15\15723009\GIS\JTEC Stability Report\F15 - JTEC Borings 2007-Present.mxd Date Exported: 10/03/18 by jbrown


Scale 1:3,000

Documents

Landfill Stability Demonstration