i 

## Minnesota Power Boswell Energy Center Unit 3 Dry Ash Placement and Incremental Closure Plan

Table of Contents

1.0 Introduction.................................................................................................................................1 1.1 Purpose.......................................................................................................................... 1 1.2 Unit 3 AQCS Upgrade Project Summary ..................................................................... 2 1.3 Pond Operation Summary............................................................................................. 2 1.4 Plan Organization.......................................................................................................... 4

2.0 Pond 3 Development Plan...........................................................................................................6 2.1 Pond 3 Layout and Operations...................................................................................... 6 2.2 Ash Generation Rates ................................................................................................... 7

3.0 Overall Flyash Fill Procedures................................................................................................. .10 3.1 Flyash Fill Procedures................................................................................................. 10 3.2 On-Site Equipment...................................................................................................... 10 3.3 Geotechnical Considerations....................................................................................... 11

4.0 Closure Schedule ............................................................................................................... .......15

5.0 Closure Design................................................................................................................. .........17 5.1 Design Approach ........................................................................................................ 17 5.2 Cover Design .............................................................................................................. 18 5.3 Variable Grading Design ............................................................................................ 19 5.4 End Use....................................................................................................................... 19 5.5 Maintenance................................................................................................................ 19

6.0 Progress Reporting............................................................................................................. .......21 6.1 Closure Design Submittal ........................................................................................... 21 6.2 Annual Survey and Fill Rate Calculation ................................................................... 21 6.3 Progress Report........................................................................................................... 21ii 

### List of Tables

Table 2-1 Projected Ash Generation Rates Table 2-2 Lower and Higher Annual Ash Generation Rate Projection Table 2-3 25-Year Lower and Highe r Ash Generation Rate Projection Table 3-1 Summary of Sl ope Stability Analysis Table 4-1 Incremental Closure Schedule
List of Figures

Figure 1 – AQCS Solids Ponds Figure 2 – Pond 3 Delta Area Figure 3 – Pond 3 Delta Closure Figure 4 – Lower Ash Genera tion Rate Fill Contours Figure 5 – Higher Ash Generation Rate Fill Contours Figure 6 – West-East Cross Section Figure 7 – North South and Fi nal Cover Cross Sections

List of Attachments

Attachment A – Geotechnical Report Attachment B – Technical Speci fications for Cover Materials1 

# 1.0 Introduction

1.1 Purpose On March 9, 2007, NP DES Permit MN0001007 was reissued to Minnesota Power’s (MP) Boswell Energy Center (BEC). The permit acknowledges that BEC is in the process of upgrading the air emissions quality control equi pment (AQCS) for Unit 3. Construction of the new Unit 3 scrubber and associated facilities (baghouse, selective catalytic reduction, low NOx burners, and absorber tower) began in April 2007 with a proj ected in-service date of fall 2009.
The project includes changing th e method of handling Unit 3 fl yash from “slu rry” to “dry” form. The Permit authorizes placement of dry flyash over the existin g ash delta in Pond 3 (Figure 1) and requires BEC to incrementally c over ash placed above the delta area of Pond 3. This document presents the Incremental Closure Plan and Schedule that provides for both the closure of the dry ash fill over a 25-year period and for th e disposal of all dry flyash generated during this time period.

This document is intended to meet the Permit requireme nt of Chapter 3 Special Requirements, Part 1.2, which states:
Chapter 3 Special Requirements

1.2 The Permittee shall submit a Pond 3 Incremental Closure Plan and Schedule (Plan) for review and approval to the MPCA within six months of permit issuance.
The Plan shall include specifications for each unique layer of the cover, the implementation schedule for the placement of the cover material (which shall not exceed three years), and shall in clude considerations for erosion and the ultimate use for the site.
It should be noted that while this plan ge nerally accommodates management of flyash expected to be generated at the BEC over th e next 25-year timefram e, actual plant life can reasonably be expected to exte nd beyond 25 years. As such, flyash management needs will continue well into the future and this pl an will require periodi c updates (most likely concurrent with the 5-year cycle of NPDES permit renewa l activities but possibly more frequently) to reflect flyash management needs as they evolve over time. This may also2 

### include the option for disposal of dry ash from other MP facilities such as the Rapids and Hibbard Energy Centers at the BEC.

1.2 Unit 3 AQCS Upgrade Project Summary Upgrades of the emission control equipment for BEC Unit 3 are in response to a number of legislative and regulatory mandates, including EPA’s Regional Haze Ru le (RHR), the Clean Air Act Interstate Rule (CAI R), the Clean Air Mercury Rule (CAMR), and the Minnesota Mercury Emissions Reduction Act of 2006. The new equipment will resu lt in significant annual maximum emission reductions of sulfur di oxide, oxides of nitrogen, particulate matter and mercury. MP is installing Best Availabl e Control Technology (BACT) to achieve these air emission reductions. The BACT installations include:

• Fabric filter baghouse for PM , P10 and Mercury control,

• Wet flue gas desulphurization (FGD) system for S0 2 

### control,

• Low NOx burners, over-fire air and selectiv e catalytic reduction (SCR) for nitrogen oxides control, and

• Installation/upgrades of combustion c ontrols for carbon monoxide control.

1.3 Pond Operation Summary The current BEC air quality cont rol system solid waste stream s are managed wet in a pond system that is located northwest of the plant site. Overall pond la yout is shown on Figure 1. The pond system includes three separate pond areas surrounded by a perimeter dike with a crest elevation of 1321 to 13 22. The following section desc ribes current pond operations.

1.3.1 Current Pond Operating Procedures



### • Units 1,2,3,4 Bottom Ash Pond – The bottom ash pond accep ts bottom ash from each of the four BEC generating units . The bottom ash is transpor ted from the plant site to the ponds through pipelines as wet slurr y. Additional discharge to the bottom ash pond occurs at a variable rate from the coal pile sump. Excess wa ter is pumped either to the Central Wastewater Treatment Fac ility (CWWTF) for treatment and discharge under the existing NPDES permit (Outfall SD 004), or back to the Unit 1 through 4 bottom ash system for use in subsequent transport of additional bottom ash to the pond.3 

### • Unit 3 Flyash Pond – The Un it 3 flyash pond (a.k.a. Pond 3) accepts flyash collected from BEC generating Unit 3. The flyash settles out in the pond and excess pond water is discharged, during ice-free months, at an average rate of 1000 gpm to the plant water intake structure located on Nort h Blackwater Lake (Outfall SD 006) in accordance with the existing NPDES permit.

• Unit 1, 2 and 4 Flyash and Un it 4 Scrubber Solids Pond – This pond (a.k.a. Pond 4) accepts flyash collected from BEC genera ting Units 1, 2, and 4 and scrubber solids from BEC generating Unit 4. Th ese solids are transported as slurry through a pipeline to Pond 4. Excess water from P ond 4 is returned to the plant for subsequent use in the Unit 4 scrubber system and for slurry transp ort of additional AQCS solids to Pond 4.
1.3.2 Pond Operating Procedures following Unit 3 AQCS Upgrade


### As noted previously, th e flyash collected from the upgrad ed Unit 3 AQCS w ill be transported dry to the pond area for disposal. The overa ll pond operating procedures that will result are as follows:

• Bottom Ash Pond—The botto m ash pond is generally expected to continue to operate as it does now with the exception that botto m ash will occasionally be transported dry (excavated from bottom as h pond and relocated) for use in the dry ash fill construction in Pond 3. The bottom ash could also be discharged wet as needed into Pond 3 to facilitate th e dry ash fill activities.

• Pond 3 Delta Area—The Pond 3 delta area w ill be converted to dry filling with Units 1, 2 and 3 flyash (and bottom ash as needed to provide truck access or to facilitate other dry ash fill activities).

• Pond 3 Basin Area—In preparation for the Un it 3 AQCS upgrade and for as long after its start-up as possible, excess pond water will be discharged dur ing ice-fr ee months at an average rate of 1000 gpm to the plan t water intake structure located on North Blackwater Lake (Outfall SD 00 6) in accordance with the water quality limits of the existing NPDES permit. The Pond 3 basi n area will begin accepting FGD slurry from Unit 3 after discharging excess water.

• Pond 4—Pond 4 will continue ac cepting slurry transported flyash solids from Units 1, 2, and 4, as well as FGD scrubber solids fr om Unit 4. Pond 4 may also temporarily accept slurry FGD scrubber solids from Unit 3.4 

### As noted above, flyash from Un its 1 and 2 is currently trans ported as a slurry with Unit 4 flyash for deposition in Pond 4. In early 2009, Minnesota Power anticipates that the slurry transport approach for flyash from Units 1 and 2 will be disc ontinued, in which case Unit 1 and 2 flyash would be transported dry for deposition in the Pond 3 delta area.
While the purpose of this docum ent is to present the plan and schedule for the incremental placement and covering of dry flyash from Unit 3 that is placed on the Pond 3 delta, it also discusses potential future upgrades to the Unit 4 AQCS. Currently MP has not formally, in the context of providing for long term ash and flue gas desulfurization (FGD) sludge management at the BEC, decided on a plan to implement a similar upgrade to the Unit 4 AQCS. However, MP believes that it is importa nt to also consider potential future system modifications that could impact solid waste stre am types and volumes to be managed in each ash pond. Potential changes, if they occur to the Unit 4 AQCS, will li kely occur in the 2012 to 2014 timeframe; outside the time-frame of the current NPDES permit. If these changes occur, they would likely have the affect as described below.

1.3.3 Pond Operating Procedures if Unit 4 AQCS Upgrades are Implemented



### • Bottom Ash Pond—Continued ope ration as described for the Unit 3 AQCS upgrade.

• Pond 3 Delta Area—The Pond 3 delta area will accept flyash from Units 1 through 4 and bottom ash from Units 1 through 4 as described for the Unit 3 AQCS upgrade.

• Pond 3 Basin Area—The Pond 3 Basin Area will accept FGD scrubber solids from Unit 3 and/or Unit 4.

• Pond 4—Pond 4 will accept FGD scrubber soli ds from Unit 4 an d possibly Unit 3.

1.4 Plan Organization
The remainder of this document is or ganized into the following sections:
Section 2 of this document provides additiona l background informatio n regarding the current Pond 3 operations. This section also prov ides information on the projected dry ash generation rate and volumes follo wing completion of AQCS upgrades.
Section 3 describes the planned flyash fill procedures and th e geotechnical evaluation that was performed to confirm f easibility of the approach.5 

### Section 4 provides information regarding the schedule for the increm ental closure of the flyash fill above the Pond 3 delta area.
Section 5 presents the delta area fill design.
And finally, Section 6 presen ts the plan for monitoring and reporting progress on plan implementation.6 

# 2.0 Pond 3 Development Plan

2.1 Pond 3 Layout and Operations Overall Pond 3 layout and the delta area within Pond 3 are shown on Fi gure 2. Total area within the Pond 3 perimeter dike is approximate ly 198 acres. The current Pond 3 delta has an area of approximately 95 acres. The Pond 3 delta area w ill be somewhat larger at the time of start-up of the upgraded Unit 3 AQCS. For purposes of this pl an it is assumed that the Pond 3 delta will total approximately 110 acres by year-end 2009. Actual delta area will be somewhat lower or higher.
Following start-up of the Unit 3 AQCS upgrade, the ash transportation and disposal system in Pond 3 is to be operated as follows:

• Flyash – Transported dry in trucks and placed dry above the existing ash delta area in Pond 3.
From the new baghouse, the fl yash will be pneumatically tr ansported to an ash silo for temporary holding until it is transferre d to a pin-paddle mixer for moisture conditioning. After moisture conditioning, the ash will be transported by truck for disposal. The moisture c onditioning in the pin-paddle mixer will mini mize fugitive dust emissions during subsequent transport and unloading. Disposal of the ash is proposed to occur above the exis ting ash delta in Pond 3.

• FGD Sludge – FGD sludge from the Unit 3 AQC S upgrade will be transported as slurry and discharged into the Pond 3 wate r basin area in the south half of the pond.

• Bottom Ash – The AQCS upgrade on Unit 3 is not anticipated to result in significant changes to the overall bottom ash system.
The bottom ash system is expected to be largely unaffected by th e Unit 3 AQCS upgrade; however, as pond capacity is co nsumed by bottom ash solids the ash pond effectiveness will be affected by the reduced vo lume and will subsequently ha ve shortened water retention time. To resolve this issue and to also facilitate vehicle traffic on the existing ash delta in Pond 3, bottom ash will be used in Pond 3 as a foundation layer to separate existing wet ash deposits from the proposed overlying dry ash deposits within the pond. Furthermore, bottom ash may be used to segment th e pond to maintain separation between the proposed dry ash7 

### deposit and the proposed FGD slurry discharge into Pond 3. MP will excavate bottom ash from the bottom ash pond on an as-needed basis for use in Pond 3 and to maintain the operational effectiveness needed in the bottom ash pond for continued bottom ash deposition.
In summary the ash management approach includes:

• Transport of Unit 3 AQCS flya sh by truck for deposition a bove the existing ash delta; forming a dry ash manageme nt segment within Pond 3.

• Transportation of Unit 3 FGD sludge by pipe line for deposition in a wet segment of Pond 3.

• Transfer of bottom ash from the bottom ash pond to Pond 3 on an as -needed basis to maintain separation of new dry ash deposits from underlying wet ash deposits to aid trafficability, and to maintain ash depositio n capacity within the bottom ash pond.

2.2 Ash Generation Rates For the upgraded Unit 3 AQCS and assuming the Unit 4 AQCS is upgraded in the future, MP’s design engineer, Burns an d McDonnell, estimates flyash generation rates of 15.5 tons per hour (tph) for Unit 3 and 26.5 tons per h our for Units 1, 2 and 4 (1.7 tph, 1.4 tph, and 23.4 tph for Units 1, 2 and 4 respectively) an d an annual bottom ash generation rate of approximately 58,000 tons per year. The Pond 3 Development Plan is based on these ash generation rates. The BEC annual ash genera tion rates are presented in Table 2-1 assuming 8320 hours of operation per year (8,760 hours per year at 24 ho urs per day and assuming 5% total down time annually) and in-fill as h density of 90 pounds per cubic foot.

Table 2-1 Projected Ash Generation Rates Source Unit 3 Flyash Units 1, 2 and 4 Flyash Bottom Ash Average Generation, Tons Per Hour 15.5 26.5 N/A
Average Generation, Tons Per Year 129,000 220,000 58,000
Average In-Place Volume, Cubic Yards Per Year 106,000 181,000 48,0008 

### In order to design for the air-s pace fill volume that may be n eeded to accommo date the dry ash generated over the next 25 years; projec tions of ash volume ge nerated are made for a lower rate case and a higher rate case. The lower ash generation rate case is based on the assumptions that:

• Dry flyash will be generated only from Unit 3 at a rate of 106, 000 cubic yards per year; and

• 8,000 cubic yards of bot tom ash will be fille d with the flyash.
The higher ash generation rate case is based on the assumptions that:

• Dry flyash will be generated from Unit 3 at a rate of 1 06,000 cubic yard s per year;

• At some future date dry flya sh will be generated from Un it 1, 2 and 4 at a rate of 181,000 cubic yards per year; and

• All of the 48,000 cubic yards of bottom ash generated per year will be incorporated with the flyash fill.
The projections for total annual ash fill volume (cubic yards/year) for the lower and upper rate cases are presented in Table 2-2.

Table 2-2 Lower and Higher Annual Ash Generation Rate Projection Source Lower Rate Case Higher Rate Case Unit 3 Flyash, CY/yr 106,000 106,000
Bottom Ash, CY/yr 8,000 48,000
Units 1, 2, and 4 Flyash, CY/yr 0 181,000
Total Annual Generation, CY/yr 114,000 335,000



### This plan provides for the filling of the Pond 3 delta area over at least a 25 year period (depending on actual compared to projected ash generation rate and in-place ash density). Design for the potential range in air-space volume needed to acco mmodate the dry ash generated for the 25 year period was based on the follo wing assumptions:

• Unit 3 generates dry flyash duri ng the entire 25 year period,9 

### • Units 1, 2 and 4 begin generating dry flyash in year 6 of the period for the higher rate case only,

• 8,000 cubic yards of bottom ash used annually for the lower rate case, and

• 48,000 cubic yards of bottom ash used annually for the higher rate case.
The 25-year projected fill volumes for the lower rate fill case and upper rate fill case are presented in Table 2-3.

Table 2-3 25-Year Lower and Higher Rate Ash Generation Rate Projection Time Period Lower Rate Case Higher Rate Case
Years 1 to 5, CY 570,000 770,000
Years 6 to 25, CY 2,280,000 6,700,000
Total 25 Year Period, CY 2,850,000 7,470,000



### Thus the fill plan for the delta area will need to be adjusted as needed to provide for an estimated ash fill volume ranging from 2.9 to 7.5 million cubic yards (o r more depending on actual plant life). In the future , this may include extension of the ash fill area onto the ash delta area of Pond 4 or onto th e bottom ash pond. However, th e Figures included with this report depict filling of the delta area of Pond 3, which provid es for nearly 6,000,000 cubic yards of ash disposal; much nearer the high rate case than the low rate case fill projection.

In the ash generation rate analysis presented above it is assumed that fl yash from Units 1 and 2 will always be associ ated with flyash from Unit 4. Th is is due to the commingling of flyash from Units 1, 2 and 4 in the current we t ash transportation and management approach at BEC. It is possible that at a future date flyash from Units 1 and 2 will be managed dry while flyash from Unit 4 continues to be managed wet. In this event, dry flyash from Units 1 and 2 will be placed with th e dry flyash from Unit 3.10 

# 3.0 Overall Flyash Fill Procedures

3.1 Flyash Fill Procedures Following the start-up of the Unit 3 AQCS upgrades, fl yash will be transported pneumatically in a dry condition by pipeline from the plant site to an ash silo located near the southeast corner of the existing bottom ash pon d. At the silo the fl yash will be moisture conditioned and then transported by truck to the active flyash fill ar ea on the ash delta in Pond 3. The flyash will be dum ped, spread by do zer, and compacted.

During performance of the geotechnical evalua tion subsequently descri bed in Section 3.3,
ash fill construction considerations were also identified, including:

• Use of light weight non-vibr atory equipment on initial lift s of dry ash fill so as to avoid liquefaction in th e underlying ash fill.

• Placement of the initial lift of ash to a th ickness of several feet to accommodate traffic on the softer existing ash.

• Use of grade control so that desired fill slopes are achieved without overfilling.

The primary form of dust control will be the moisture conditioning that takes place in the pin-paddle mixers at the ash s ilo. However, provisions will be made for spraying of the ash
fill surface with water obtained from Pond 3 or Pond 4. Once ar eas are filled with flyash to final grade, these areas will r eceive intermediate cover consisting of either granular soil, bottom ash, or geosynthetic cover. These interm ediate covers will be placed as needed to
control wind-blown dust.

3.2 On-Site Equipment As noted previously, ash will be hauled by truc k from the silo to the ac tive fill area. Current plans are to subcontract ash hauling activities; most likely to an earthwork contractor who
will use on-road trucks for the ash hauling. However, off-road construction trucks (such as mining or articulated dump trucks) may be used in the future. Truck type will likely evolve11 

### over time to maintain efficient operations, pa rticularly if the Unit 4 AQCS upgrades are implemented, resulting in substantially higher ash transport requirements. The effect of higher axle-weight vehicles on the existing compacted ash road system will be considered before implementation of heavier truck types.

In addition to the ash hauling vehicles, a dozer will be stati oned on site to spread the ash after dumping. Here again, dozer size and type will likely vary over time to maintain the overall efficiency of the operation. At the low ash delivery rates (from the Unit 3 AQCS only) a relatively small dozer should suffice, wh ereas at higher ash delivery rates (from the Unit 3 and 4 AQCS possibly in the future) a relatively larger dozer may be needed. At this phase of project development, use of a rubber- tire dozer is anticipated. Use of a rubber tire dozer will aid in compacting the ash while it is being spread. Howeve r, early in the fill development some experimentation may also oc cur with a self-propelled compactor to weigh the relative gains in ash fill density. Finally, wa ter trucks will also be stationed on site for use in watering haul roads as needed and for use in watering the active fill area for dust control that is not achieved with the on-site irrigation system.

This equipment is proposed to be housed in an on-site maintenance building to be
constructed near the southwes t corner of the bottom ash pon d. The maintenance building will likely consist of a pre-engineering insula ted steel building with a small tool area for routine equipment maintenance, a small office area for use by facility operations personnel, and for some records storage. The approximate location of the structure is shown on Figure
2.

3.3 Geotechnical Considerations The planned flyash fill procedures depend in part on the ability of the existing ash delta in Pond 3 to support the load imposed by the overlying ash fill activitie s. A geotechnical exploration and analysis were performed, the ma in objectives of which were to determine the geotechnical properties of the existing ash in Po nd 3 and Pond 4, and to evaluate the effects of the proposed vertical development on the stability of the existing ash deposits.12 3.2.1 Ash Characterization 

### The existing ash fill in the delta s was classified as silt and sandy silt and ranged in color from black to gray to tan. Based on field and laboratory testing, the ash has a ‘very loose’ relative density, but also occasionally exhibits cementation properties. The ash behaved in a low to moderate compressive behavior and is estimated to have a hydraulic conductivity of approximately 10 -4 

### cm/s. Under slow rates of loading intended to simulate dry ash placement above the delta, the ash displayed a relatively high friction angle, typically ranging from 35 to 43 degrees. Under rapid loading, the as h had a low undrained strength ratio typically ranging from 0.37 to 1.0.

3.2.2 Slope Stability Considerations 

### The proposed dry ash fill plan for Pond 3 will create dry ash fill embankments and dry ash filling adjacent the existing interior and exteri or pond dikes. Slope stability analyses were conducted to compute a factor of safety for the proposed ash fill slopes and existing dike slopes. Long term (drained) soil strength proper ties were used in the analysis to reflect the relatively slow rate of ash fill placement that will occur. Use of short term (undrained) soil strength properties would be appropriate to a ve ry rapid filling with dr y ash over a relatively large area. Due to the low rate of ash generatio n relative to the area of the existing ash delta in the pond, a rapid rate of filling will not occur in Pond 3.

The analyses were based on a review of previo us investigations at the site, and on a site exploration program including soil borings, co ne penetration testing and in-laboratory material testing. Stability analyses were cond ucted at three cross sections that represent typical subsurface profiles, ma ximum dike heights, and multip le configurations of dry ash fill placement within the confines of the existing dikes. These include:

• Interior Dike – Stability analys is of dry ash fill ad jacent sections of interior dike that
do not have ash fill on the opposite side of the dike.

• Perimeter Dike with Offset Fill – Stability analysis of dry ash fill above the existing ash delta; not adjacent existing dikes.13 

### • Perimeter Dike with Adjacent Fill – Stability analysis of the dry ash fill adjacent
sections of exterior dike.

The stability analysis are based on overall ash fill slopes not ex ceeding 20 percent (5H :1V) and a 100 foot separation distan ce of the ash fill area from futu re high water marks in the pond. Note that future high water location in the pond will likely be controlled by dike construction within the pond; not by location and elevation of the existing ash delta.

The stability analyses yield greater factors of safety than th e minimum recomme nded safety factors. The stability analysis re sults are summarized in Table 3-1.

Table 3-1 Summary of Slope Stability Analysis Long-Term Condition Drained (ESSA) End of Construction Undrained (USSA)
Section Suggested F.S. = 1.5 Suggested F.S. = 1.3 1. Interior Dike 2.75 1.75 2. Perimeter Dike with Offset Fill 2.99 1.47 3. Perimeter Dike with Adjacent Fill 2.45 1.41

3.2.3 Settlement Considerations 

### Deformation/consolidation analysis was conducted at the most cr itical ash fill cross section. The analysis results indicate that the maximum vertical displacement of the ash fill resulting from consolidation of the existing ash delta is e xpected to be on the order of three feet. This displacement is due to the larg e amount of fill placed and the relatively loose nature of the existing ash. Deformation is not a critical desi gn issue as the pond liner system is located below the existing ash delta and will be relativel y unaffected by the overlying ash fill. In addition, the majority of this settlement will occu r during dry ash fill placement; before it could have any detrimental affect on final cover.

The results of the geotechnical exploration and analysis indicate that the existing ash ponds can be converted into a dry as h landfill. The provided recomm endations for ash fill design14 

### and construction are incorporated into this plan. The complete Geotechnical Evaluation Report is provided as Attachment A.15 

# 4.0 Closure Schedule

This plan provides for the incr emental filling of the Pond 3 de lta area. The plan contains sufficient flexibility in final f ill heights and grades to accommod ate 20 to 25 years of dry ash filling. The Pond 3 delta area (1 10 acres) will be incrementally c overed as filling progresses. The preliminary closure schedule is presented in Table 4-1. An aggressive schedule is shown in the event that:

• In-place ash densities are lower than antic ipated and hence cap acity consumption rates higher than anticipated and/or,

• Ash generation rates are higher than anticipated and hence capacity consumption rates higher than anticipated.

Actual area to be closed will de pend strictly on the area that is filled to grade and available for final cover placement at each 3-year increment of closure.

Table 4-1
Incremental Closure Schedule Year after Unit 3 AQCS Upgrade Area Closed, Acres Cumulative Area Closed, Acres

Year 4 15 15

Year 7 15 30

Year 10 15 45

Year 13 14 59

Year 16 15 74

Year 19 12 86

Year 22 12 98

Year 25 12 110



### Assuming that the Unit 3 AQCS Upgrade is placed in service in late 2009 and that year 1 is therefore 2010, the construction of the first increment of closure would occur in 2013. Additional increments of closure construc tion would occur on a three year basis.16 

### The anticipated incremental closure areas are show n on Figure 3. It must be noted that the incremental closure area is based on assumptio ns of fill rates which depend on ash generation rates and in-place fill density. The location and areal extent of individual closure elements will change to accommodate effi cient and effective closure, dependent on the actual ash generation rates and in-place fill density.
Implicit in the closure plan is that the dry as h from Units 1, 2, 3 a nd possibly 4 (under a Unit 4 AQCS upgrade scenario) will be placed as needed to ultimately facilitate closure of the entire ash delta in Pond 3. This is under th e assumption that the ash delta in Pond 3 ultimately will require final cove r independent of the availability of dry flyash fr om the BEC. If dry flyash is not available, then imported soils would be required for grading the existing ash delta in preparation for closur e. However, since dry flyash will be available, the fill plan is intended to make the best possible use of dry ash as the material used to grade the existing ash delta and to prepare it for final closure.17 

# 5.0 Closure Design

5.1 Design Approach The goals/objectives of the dry ash f ill area closure desi gn are as follows:

• Achieve incremental dry ash fill closure,

• Minimize erosion from the cover,

• Provide sedimentation for surface water runoff,

• Provide for surface discharge of non-contact storm water, and

• Accommodate the filling of all dry ash ge nerated under the higher ash fill rate scenario.
In order to provide for the direct discharge of stormwater from fille d and closed areas to surface water, the closure design:

• Separates runoff from the clos ed areas and the active ash fill areas by placing a barrier layer over ash fill materials and by using surface water runoff control berms and ditches.

• Provides for sediment control and erosi on control BMP’s for the closed areas.
Sedimentation is provided by the sedimentatio n basin adjacent to the northwest corner of Pond 3 and the perimeter ditch which co nveys runoff from the entire closed area perimeter to the sedimentation basin (Figure 4). Long term erosion control is provided by vegetative cover and a series of side slope intersection dike/ditches and conveyance pipes. These features will help prevent soil erosion along closed slopes by intercepting and conveying ru noff from the closed slopes to the perimeter drainage ditch. Interception dike/ditches are to be located at a maximum slope length of 200 feet for 20% slopes and at a maximum of 500 feet for slop es of 5% or less. Best Management Practices (BMPs) to be in place during closure construction will be detailed in the construction Stormwater Po llution Prevention Plan (SWPPP) prepared as part of each cover cons truction project bid package.18 

### 5.2 Cover Design The closure system will generally consist of a geomembrane barrier layer overlain by a granular drainage layer or geonet drainage laye r, followed by rooting soil and topsoil. This is illustrated in Figure 7. For the granular dr ainage layer method of cover the profile would likely be:

• Geomembrane Foundation Layer/Interim Co ver (flyash, botto m ash, or sand)

• 40 mil Low Density Polyethylene Geomembran e (or 24 inch clay as alternative)

• 12 inch Granular Drainage Layer

• 12 inch Rooting Soil Layer

• 6 inch Vegetated Topsoil Layer
If a geonet drainage layer is utilized, it would replace the 12 inch granular drainage layer and be supplemented with an additional 6 inches of rooting soil. Template Technical Specifications for the cover materials are in Appendix B. The template technical specifications will be fine-tuned as needed to reflect the sp ecific requirements of each incremental closure construction event.
Grading plans for the lower rate ash fill case and the higher rate ash fill case are shown in Figure 4 and Figure 5 respectively. These plans are shown in cross-section on Figures 6 and 7. Each of these grading plans incorporates sediment and erosion control for a similar direction of drainage for surface runoff.
The lower rate case grading plan provides for:

• Fill volume of 2,900,000 cubic yards,

• Top slope elevations reaching approximately EL 1360

• Side slope grades from 5H :1V,

• Top slope grades from 2% to 3% slope.
The higher rate case grading plan provides for:

• Fill volume of 7,500,000 cubic yards,19 

### • Top slope elevations reaching approximately EL 1380, and

• Side slope grades of 5H : 1V,

• Top slope grades from 2% to 5% slope.

5.3 Variable Grading Design The grading plans shown in Figur es 4 and 5 illustrate the varia tion in top slope elevation and top slope grade necessary to provide for the rang e in filling rate from the lower rate case of 114,000 cubic yards per year to the higher rate case of 335,000 cubic yards per year.

BEC proposes to annually meas ure the ash fill rate and ad just the fill grading plan accordingly to accommodate the known and potential future ash volumes requiring placement at the facility.

5.4 End Use Currently BEC’s planned end use of the closed delta area is open space. The proposed closure design provides for low ma intenance long term control of sediment transport and soil erosion. BEC will seek agency approval for alternate end us es, should alternate uses be contemplated.

5.5 Maintenance Routine maintenance, such as mowing vegetati on to prevent tree growth and to maintain drainageway flow capacity, will be performed as needed. A ll features including, but not limited to, site security, monito ring wells, and surface water runo ff control structures will be properly maintained.

The surface cover will require periodic maintena nce. Supplemental cover soil will be placed to repair the effects of severe erosion or settlement. Seeding, fer tilizing, and mulching of bare soil will be performed as required to restore the area and establish a stable vegetative cover. These areas will be insp ected periodically to ensure th at vegetative growth has been
re-established.20 

### Surface water runoff control sy stems will be maintained on a routine basis. Drainage ditches, terraces, and spillways will be inspect ed for condition and proper function. For systems that are found to be in poor condition and/ or non-functioning, routine maintenance will be performed.21 

# 6.0 Progress Reporting

6.1 Closure Design Submittal Final design plans and specifications will be submitted to the MPCA 30 days prior to construction of each increment of closure. De signs will incorporate pr ovisions for long term sediment and erosion control BMPs. Construction sediment and erosio n control BMPs will be presented in each Construction SWPPP.

6.2 Annual Survey a nd Fill Rate Calculation A topographic survey of the active f ill area will be obtained on a periodic basis. The rate of ash filling will be calculate d based on these surveys. Filling plans for the next year and next three years will be updated as necessary to provide fill grades that both provide for the ash volume generated and for the 3-year cycle of closure construction.

6.3 Progress Report Topographic survey data, fill vol ume computations and the annu al and three year projected filling plan will be provided to the MPCA with the facility Annual Groundwater Report currently submitted each year by July 1st.

## Figures

## Attachment A – Geotechnical Report

# Geotechnical Exploration Report Ash Pond Incremental Closure

Boswell Energy Center Cohasset, Minnesota



## Prepared for Minnesota Power

August 2007i Table of Contents

Executive Summary .............................................................................................................. ........................1
1.0 Introduction.........................................................................................................................................2 1.1 Project Description....................................................................................................................2 1.2 Scope and Objectives ........................................................................................................... .....2
2.0 General Considerations ......................................................................................................... ..............3 2.1 Site Description ............................................................................................................... ..........3 2.2 Site Geologic Setting.......................................................................................................... .......3 2.3 Previous Investigations..............................................................................................................3
3.0 Geotechnical Exploration....................................................................................................... .............4 3.1 Soil Borings...............................................................................................................................4 3.2 Cone Penetration Testing ....................................................................................................... ...5 3.3 Laboratory Testing ............................................................................................................. .......5
4.0 Exploration Results ............................................................................................................ .................8 4.1 General Site Stratigraphy ..........................................................................................................8 4.1.1 Native Materials............................................................................................................. 8 4.1.2 Dike Fill Materials ......................................................................................................... 8 4.1.3 Ash Fill .......................................................................................................................... 8 4.2 Groundwater Conditions ......................................................................................................... ..9 4.3 Cone Penetration Testing ....................................................................................................... ...9 4.3.1 Soil Behavior Type ........................................................................................................ 9 4.3.2 Shear Strength................................................................................................................ 9 4.3.3 Permeability ................................................................................................................. 10 4.4 Field and Laboratory Testing ..................................................................................................1 0 4.4.1 Standard Penetration Tests........................................................................................... 10 4.4.2 Index Tests................................................................................................................... 1 0 4.4.3 Unit Weight.................................................................................................................. 10 4.4.4 Shear Strength.............................................................................................................. 10 4.4.5 Compressibility............................................................................................................ 11
5.0 Analysis....................................................................................................................... ......................12 5.1 Seepage Analysis............................................................................................................... ......12 5.1.1 Seepage Model............................................................................................................. 12 5.1.2 Material Properties....................................................................................................... 13 5.1.3 Boundary Conditions and Model Assumptions ........................................................... 13 5.1.4 Results........................................................................................................................ .. 13 5.2 Slope Stability ................................................................................................................ .........13 5.2.1 Stability Model............................................................................................................. 13 5.2.2 Material Properties....................................................................................................... 14 5.2.3 Results........................................................................................................................ .. 14ii 5.3 Deformation/Consolidation .....................................................................................................1 5 5.3.1 Deformation Simulation............................................................................................... 15 5.3.2 Material Properties....................................................................................................... 15 5.3.3 Results........................................................................................................................ .. 16
6.0 Construction Considerations .................................................................................................... .........17 6.1 Design......................................................................................................................... .............17 6.2 Construction ................................................................................................................... .........17
Refere nces..................................................................................................................... ..............................18

List of Tables

Table 1 Boring Summary............................................................................................................... 4 Table 2 CPT Sounding Summary................................................................................................... 5 Table 3 Laboratory Testing Summary............................................................................................ 7 Table 4 Permeability Values Used in Seepage Analysis .............................................................. 13 Table 5 Shear Strength Parameters Us ed in Stability Analyses.................................................... 14 Table 6 Summary of Slope Stability Analysis.............................................................................. 15 Table 7 Modified CAM-CLAY Soil Para meters used for Ash ..................................................... 16 Table 8 Soil Deformation Parameters of Other Materials ............................................................ 16

List of Figures Figure 1 Geotechnical Inve stigation Location Map Figure 2 CPT Tip Resistance vs. Elevation Figure 3 CPT Pore Pressure vs. Elevation Figure 4 CPT Friction Angle vs. Elevation Figure 5 CPT Undrained Shear Strength vs. Elevation Figure 6 CPT Undrained Shear Strength Ratio vs. Elevation Figure 7 CU Triaxial Shear Streng th Mohr-Coulomb Failure Envelope



### Appendices Appendix A Ebasco Services, Inc. Reference Materials Appendix B Braun Intertec Geot echnical Investigation Report Appendix C Results of Analysis

Certification I hereby certify that this report was prepared by me or under my direct supervision and that I am a duly licensed Professional Engineer under the laws of the State of Minnesota.

Date: August 30, 2007 License No.: 209511 

# Executive Summary

This report summarizes the geotechnical exploration and analys is completed in support of development of an incremental closure plan of th e Boswell Energy Center flyash ponds. The main objectives of the geotechnical e xploration were to determine th e geotechnical properties of the existing ash in Pond 3 and Pond 4, and to evaluate the effects of the proposed incremental closure on the stability of the existing ash ponds.
The existing ash fill was classified as silt and sandy silt and ranged in color from black to gray to tan. Based on field and laboratory testing, the ash has a ‘very loose’ relative density, but also occasionally exhibits cemen tation properties. The ash behaved in a low to moderate compressive behavior and is estimated to have a permeability of 10 -4 cm/s. Under drained conditions, the ash displayed a relatively high friction angle, typical ly ranging from 35 to 43 degrees. Under undrained conditions, the ash had a low undrained strengt h ratio typically ranging from 0.37 to 1.0.
The engineering analyses were based on a review of previous investigations at the site, and a field exploration program including soil borings, cone penetration testing and laboratory testing.
Seepage and stability analyses were conducted at three cross sections that represent typical subsurface profiles, maximum dike heights and mu ltiple configurations of dry ash fill placement within the existing dikes. The stability analyses yield greater factors of safety than the minimum recommended safety factors for dr ained and undrained conditions.
Deformation analysis was conducted at the most critical cro ss section. The analys is results indicate that the maximum vertical displacement occurring below the dry ash fill is expected to be on the order of three to four feet. This displacement is due to the relatively loose nature of the pond ash. Deformation is not expected to be a critical design issue, rather a check for excess pore pressures and undrained strength conditions.
The results of the geotechnical e xploration and analysis indicate th at the ash ponds can be closed incrementally. The provided reco mmendations for fill design and co nstruction should be followed to maintain stable pond embankments.2 

# 1.0 Introduction

1.1 Project Description The incremental closure project i nvolves converting portions of the Unit 3 Flyash Pond (Pond 3) at the BEC from a wet ash pond system to a dry ash landfill to accommodate dry ash that will be collected from the Unit 3 air quality control system equipment upgrades and placed to incrementally close the pond. It is possible that Unit 4 will undergo similar upgrades in the future. 

## 1.2 Scope and Objectives The main objectives of the geotechnical exploration we re to determine the geotechnical properties of the existing ash in Pond 3 and Pond 4, and to evaluate the effects of the proposed dry ash fill on the stability of the existing ash ponds. In order to meet these objectives, the following exploration scope was developed:

• Perform a subsurface exploration to collect undisturbed samples of the ash.

• Perform in-situ and laboratory testing to eval uate the geotechnical properties of the ash.

• Research previous studies done at the site.

• Conduct analyses regarding seepage, stabilit y and settlement of the proposed fill.

• Summarize the findings in a report for use in designing the dry ash filling plan.
This report describes the field exploration, testing and analysis performed and describes the exploration results.3 

# 2.0 General Considerations

2.1 Site Description The Boswell Energy Center is located about three mi les west of Cohasset, Mi nnesota in southwestern Itasca County, on the northern bank of the Mississippi River. The BEC coal ash management system includes three adjacent ponds located northwest of the plant: Pond 3, Pond 4 and a small bottom ash pond. The ponds were constructed in the late 1970’s and cover a total area of approximately 645 acres. Earthen dikes made of recompacted, rando m sandy borrow material with a slope of 3:1 (H:V) make up the interior dikes. The pond perimeter dike s are of similar construc tion with a clay barrier on the upstream slope. The ponds were constructe d using in-situ clay as a natural liner. 

## 2.2 Site Geologic Setting The description of the site geology is based on a lit erature review. The site is located within the Superior Upland portion of the Canadian Shield phy siographic province. The region is characterized by low hills and rocky ridges formed by exposed Precambrian metamorphi c and igneous rocks and thick, recent glacial deposits.
The site is situated on the Aitken Lacustrine Plain, comprised mostly of fine sands, silts and clays, deposited by glacial meltwater about 10,000 years ago. The lacustrine deposits of the region can be more than 200 feet thick. Bedrock at the site is the Lower Precambrian Giants Range Granite and was encountered at a depth of 265 feet during a site geotechnical exploration in 1976 (Ebasco, 1977). 

## 2.3 Previous Investigations A dike and foundation study of the ash disposal ponds was performed in 1977 by Ebasco Services Incorporated of Norcross, Georgia. The study included a field exploration, in-laboratory material testing, stability and seepage analysis, and ash pon d design. Geotechnical parameters developed for in-situ materials during the Ebasco exploration were re-evaluated by Barr and used in the current analysis. The Ebasco exploration report, accompanying tables and laboratory test results are included in Appendix A.4 

# 3.0 Geotechnical Exploration

The geotechnical exploration for the ash pond incr emental closure consiste d of soil borings, cone penetration tests and field and laboratory testing of so il samples. The site exploration was conducted by Braun Intertec of Hibbing, Minnesota, and coordinated by a Barr Engineering geotechnical engineer. The exploration was conducted from Marc h 6 to March 9, 2007. Figure 1 shows the boring and sounding locations completed for the project. 

## 3.1 Soil Borings Eleven soil borings were completed for the exploratio n and are summarized in Table 1. Eight of the soil borings were performed in Pond 3, and the remaining th ree soil borings were performed in Pond 4. Each boring was advanced with a 31⁄4 inch hollow stem auger, using a CME 750 balloon-tired, all-terrain drill rig. Nine borings were advanced to depths of native materials, ranging from 111⁄2 feet in B-P3-3 to 441⁄2 feet in B-P4-5. One boring, B-P3-5, was advanced to a depth of 6 feet when auger refusal occurred in what was apparently cemented ash. Boring P4-7 was terminated once sufficient samples and test results had been obtained in Pond 4. Generally, Standard Penetration Tests and split spoon samples were collected at 21⁄2 foot intervals. Four representa tive thin walled piston samples were collected. The borings were abandoned using one bag of quick grout to seal the bottom of the boring, then backfilled with auger cuttings. Braun Intertec provided Barr with SPT calibration charts for the rig so the SPT hammer efficiency could be evaluated. Boring logs depicting the subsurface materials are included in the Braun Intertec exploration report located Appendix B.
Table 1 Boring Summary Coordinates* Boring

ID Easting Northing Elevation (feet) Total Depth (feet) Notes

B-P3-1 3657.6 -7769.3 1314.2 11.5
B-P3-2 3809.8 -747 7.6 1315.9 19.0
B-P3-3 3508.1 -7501.5 1315.0 13. 5 Piston sample at 4.5 feet
B-P3-4 2649.6 -559 3.7 1314.2 21.5
B-P3-5 2867.9 -5538.5 131 5.8 6.0 Auger Refusal
B-P3-6 2695.5 -540 1.4 1314.2 25.5
B-P3-7 Boring not performed
B-P3-8 2927.4 -5165.0 1316.2 23. 0 Piston sample at 4.5 feet
B-P3-9 2762.3 -502 1.0 1315.5 20.5
B-P4-5 4461.3 -6234.0 1311.3 44. 5 Piston sample at 7 feet
B-P4-7 3921.7 -6050.3 131 1.4 11.5 Terminated in ash
B-P4-9 3566.7 -5929.5 1311.3 39. 5 Piston sample at 9.5 feet *Locations surveyed by SEH and are in local coordinate system5 

## 3.2 Cone Penetration Testing Ten cone penetration test soundings were performed at locations adjacent to the corresponding soil borings. Penetration test depths and locations are summarized in Table 2. Seven soundings were performed in Pond 3 and three in Pond 4. The s ounding locations were pr e-drilled through the frost depth, to approximately 3.5 to 4 feet. The sounding s were advanced to depths of native materials, interpreted from the completed borings.
The soundings were performed using a Hogentogler digital cone pushed by the CME 750 drill rig. For the CPT test, a cylindrical cone is pushed vertically into the ground at a constant penetration rate of 20 mm/sec. The cone used in the exploration has a 10 cm 2 base area and an unequal area end ratio a of 0.80. During penetration, measurements are made of the cone tip resistance q c , the side friction f s on the cylindrical shaft just above the tip, and pore water pressure u 2 generated by cone pe netration. Pore pressure dissipation (PPD) tests were performed in the ash at representative locations.
Braun Intertec provided BARR with complete records of tip resistance, sleeve friction, pore pressure, and friction ratio of all CPT soundings. These records in cluded a hard copy showing the graphical variations of all readings with depth, including an interprete d soil behavior type classification. Sounding logs and PPD test results are located in Appendix B.
Table 2 CPT Sounding Summary Coordinates* Sounding ID Northing Easting Elevation (feet) Total Depth (feet) Notes

CPT-P3-1 3657.6 -7769.3 1314.2 12.63 PPD test at 9.0 feet
CPT-P3-2 3809.8 -7477.6 1315.9 17.55 PPD test at 14.6 feet
CPT-P3-3 3508.1 -7501.5 1315.0 14.44 PPD test at 11.0 feet
CPT-P3-4 2649.6 -5593.7 1314.2 6.40 Refusal in cemented ash
CPT-P3-6 2695.5 -5401.4 1314.2 27.56 PPD test at 14.6 feet
CPT-P3-8 2927.4 -5165.0 1316.2 12.30 Refusal in cemented ash
CPT-P3-9 2762.3 -5021.0 1315.5 22.31 PPD test at 11.0 feet
CPT-P4-5 4461.3 -6234.0 1311.3 48. 06 PPD tests at 12.8 and 26.6 feet
CPT-P4-7 3921.7 -6050.3 1311.4 29.20 PPD tests at 9.8, 19.8 and 29.2 feet
CPT-P4-9 3566.7 -5929. 5 1311.3 30.18 PPD test at 16.2 and 29.9 feet *Locations surveyed by SEH and are in local coordinate system 

## 3.3 Laboratory Testing The following tests were perform ed on collected soil samples:

• Sieve analyses in accordance with ASTM D 422, “Standard Test Method for Particle-Size Analysis of Soils.”

• Moisture content tests in accordance with A STM D 2216, “Standard Test Method for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass.6 • Atterberg limit tests in accordance with ASTM D 4318, “Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils.”

• Specific gravity tests in accordance with ASTM D-854, “Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer”

• Compressive strength tests in accordance with ASTM D 4767-95, “Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils”

• Compressibility tests in accordance with ASTM D 2435-96, “Standard Test Method for OneDimensional Consolidation Properties of Soils” Soil laboratory tests were conducted by Braun Intertec in their Hibbing, Minnesota laboratory, and by Soil Engineering Testing in Bloomington, MN. The fi eld and laboratory test results are summarized in Table 3 and presented in full in Appendix B.7Table 3 Laboratory Testing SummaryAtterberg LimitsGradationConsolidationBoringSampleDepth(feet)MaterialClassificationWaterContent(%)SpecificGravityDryDensity,pcfLL PL PI%Gravel%Sand%Silt%ClayCcPc,tsfVoidRatioB-P3-1 9.5-11.5Sandy Silt(MLS) 39.2 0 45.9 31.9 22.2B-P3-3 4.5-6.5 Silt (ML)72.670.993.471.02.4553.953.944.653.2NP NP NP 0.0 13.7 80.5 5.8 0.5 2.41.841.842.431.88B-P3-6 6.5-8.5 Silt (ML) 41.30.0 5.2 60.5 34.4B-P3-8 4.5-6.5Sandy Silt(ML)27.733.650.731.63.2497.790.865.095.0NP NP NP 0.0 36.0 61.2 2.8 0.2 2.11.071.191.991.06B-P4-5 7-9 Silt (ML)37.469.257.651.02.8482.755.462.068.129.5 26.8 2.8 0.0 12.1 85.1 2.8 0.2 3.01.152.21.861.6B-P4-9 9.5-11.5 Silt (ML) 32.70.0 10.1 79.8 10.18 

# 4.0 Exploration Results

A description of the field exploration and laboratory t esting procedures has been provided in Section 3. Section 4 presents the data from the exploration and testing and provides further analysis of these results. 

## 4.1 General Site Stratigraphy The geotechnical exploration focused on the ash fill within the ponds. The descriptions of the native dike fill materials are taken from the Ebasco exploration and are included to provide a complete site description.

4.1.1 Native Materials

• Peat – Peat was encountered in borings P3-1, P3-6, P3-8 and P3-9. The 1976 exploration indicates that peat was locate d in a drainage swale extending from the north-west of the pond to the center of the east dike.

• Upper Sand and Silt – Below a layer of topsoil or the localized peat is strata comprised of material ranging from sand with some silt to si lt with some sand. This layer is generally 10 feet thick across the site

• Clay – A layer of clay is located below the sa nd and silt, ranging in thickness from 0 to 40 feet. The clay is thickest in the center of th e pond, tapering in the east and west directions. Clay was not encount ered in the southwest corner of the pond. The clay is medium stiff to stiff, gray silty clay with very slight evidence of varves of fine silt.

• Lower Sand – Extending from below the clay layer to be drock is a dense to very dense sand. This layer is more coarsely grained than the upper sand and silt layer and includes pockets of gravel. This strata can be more than 200 feet thick and extends to bedrock.

4.1.2 Dike Fill Materials

• Central Zone – The interior and perimeter dams were constructed of compacted random material consisting of sand and silty sand bo rrowed from an area west of the pond.

• Clay Liner – Clay borrowed from the site was used for the clay blanket on the upstream side of the interior dikes and a clay liner where the native clay layer was discontinuous in the southwest corner of the ash disposal area.

4.1.3 Ash Fill The ash fill was classified as silt and sandy silt and ranged in color from black to gray to tan. The ash appeared to be more coarse grained in Pond 4. The thickness of the ash deposit at the boring locations ranges from 11.5 to 23.5 feet in Pond 3 to up to 44.5 feet in Pond 4. The borings performed in Pond 4 were located in the clay borrow area used during construction; the deepest part of Pond 4. Ash in the remaining area of Pond 4 is expected to be approximately 20 feet thick.9 

## 4.2 Groundwater Conditions There are two aquifers generally recognized in the ar ea of the ponds; a deep aquifer associated with the river elevation, and a shallow aquifer perched on top of the clay stratum. Groundwater movement in the area of the ponds is generally in a southeasterly dir ection from the higher elevation toward the Mississippi River. The permanent groundwater surface slopes from 1300 feet MSL in the high ground area to 1274 feet MSL at the river (Ebasco 1981).
While drilling, water was encountered at a depth of about six feet in Pond 3 and eight feet in Pond 4. These observations represent pond wate r conditions at the time of the field exploration and may not be indicative of other times or seasons. 

## 4.3 Cone Penetration Testing The results of the CPT program were compiled to obtain an adequate understanding of the ash fill within the ponds. The CPT results were used to interpret soil behavior type, shear strength, and permeability. The field data from the CPT program are included in Appendix B. The following describes the procedures used to interpret the CPT data and the interpretation results.
4.3.1 Soil Behavior Type According to the soil behavior type classification proposed by Robertson (Lunne, Robertson and Powell, 1997), the ash behaved as a variety of materials, including clays, silt mixtures, sand mixtures and sands. Occasionally, the ash was classified as a gravely sand to sand, likely due to a higher tip resistance in areas of ash cementation. Generally the soil behavior classifications correlated well with the soil type encountered while drilling.
The classification system is based on the corrected tip resistance q t , friction ratio R f , and pore-water pressure parameter B q , The corrected tip resistance q t ranged from 2 to 472 tsf, but most values were in the 3 to 20 tsf range. Figure 2 shows corrected tip resistance vs. elevation for all soundings. Pore pressures vs. elevation are shown in Figure 3.
4.3.2 Shear Strength
CPT data were used to estimate the drained and undr ained shear strength of the ash. During much of the testing, high excess pore pressures were not generated at the normal rate of CPT penetration. In other words, the ash often behaved in a par tially-drained manner during penetration.
Drained strength was evaluated using the method by Kulaway and Mayne (Kulaway and Mayne, 1990). The drained friction angle typically ranged fro m 30 to 40 degrees, shown in Figure 4.

CPT data were also used to estimate the undrained sh ear strength of the fine grained material (cohesive material). The undrained shear strength values ranged from 185 to 2170 psf, with most values less than 1000 psf. The undrained shear strength ratio (USR) is a parameter used in design and is defined as the undrained shear strength divided by effective vertical stress. The USR resulting from the CPT soundings performed in Pond 3 typically ranged from 0.37 to 1.0. The undrained shear strength and USR results are10 shown in Figures 5 and 6, respectively. The CPT strength results correspond well to the laboratory test results which are discussed in a following section.
The USR calculated from the CPT sounding P4-5 had a lower bound of around 0.10 at depths from about 41 to 45 feet. This zone of lower strength appears to be localized in the former clay borrow area, below a zone of higher strength, possibly cemented ash. Further investigation of this area should be done if ash is to be place in this portion of Pond 4.
4.3.3 Permeability The coefficient of permeability, K h , was calculated based on results from the pore pressure dissipation tests and laboratory consolidation tests (Baligh a nd Levadoux, 1980). Estimations of permeability based on soil behavior (Lunne, Robertson and Powell, 1997) were coupled with the K h calculation to confirm an average ash permeability of 10 -4 cm/s. 

## 4.4 Field and Laboratory Testing

4.4.1 Standard Penetration Tests As part of the subsurface exploratio n, standard penetration tests (SPT) were conducted at each soil boring location. The SPT results can be used to infer th e approximate soil strength profile at the soil boring location. The SPT values of the ash in terms of N 60 (i.e. SPT value correct to account for 60 percent hammer efficiency) were generally in the range from ‘weight of hammer’, or zero blows, to three blows per foot. The relative density of this material is cla ssified as ‘very loose’. Higher values were obtained in borings where the ash had been cemented, and in the upper few feet of each boring due to frost.

4.4.2 Index Tests A series of general index tests were conducted on selected sample s to accurately cl assify the ash. Fifteen moisture content tests on six ash samples ranged from 28% to 93% with an average of 52% and standard deviation of 19%. Based on Atterberg Limit tests, all but one of the samples tested were non-plastic. The sample coll ected from B-P4-5 from seven to nine feet had a liquid limit of 29.5 and a plasticity index of 2.8. Grain-size distribution was evaluated in six ash samples from across the site. Two samples classified as sandy silt; the remaining four samples were silt.
4.4.3 Unit Weight Twelve selected soil samples were evaluated to determ ine their dry unit weight. Moisture content tests conducted on the same samples were used to determine the in situ (moist) unit weight. The moist unit weights ranged from 86.3 to 125.0 lb/ft 3 with an average of 103.3 lb/ft 3

. 4.4.4 Shear Strength Laboratory tests were conducted to evaluate the shear strength of the soil encountered in the soil borings. Selected samples were subjected to a consolidated -undrained (CU) triaxial compression test with porewater pressure measurements so that both the drained and undrai ned failure envelopes could be estimated. A range of confining pressures was used in the tests in order to simulate the existing stress in the soil, as well as the anticipated stress increase due to anticipated loading.11 The results of the triaxial compression tests indicat e that the drained failure envelope for the samples tested based on the maximum deviator stress failure criterion was φ ’ =38o to 43 ° and c’ =0 psf.
The undrained failure envelope for the samples test ed based on the maximum deviator stress failure criterion was φ cu = 17o to 24 ° and c u = 400 to 500 psf. The drained strength properties of the ash are generally higher than typical silt-sized material, whic h may be due to cementation properties of the ash.
The USR resulting from the CU triaxial compression tests has a lower bound of 0.32 and an upper bound of 0.45. The drained and undrained laboratory shear strength results are presented in Figure 7.
4.4.5 Compressibility In order to estimate the compressibility characteris tics of the ash, three one-dimensional consolidation tests were conducted. The tests were performed at a range of loads in order to determine the parameters necessary to evaluate the deformati on of the ash due to loading.

The consolidation test results indicate that the co mpression ratio for the three ash samples ranges from 0.075 to 0.190. The recompression ratio lies between 0.010 and 0.028, and th e preconsolidation pressure values ranged from 2.1 to 3.0 tsf. The magnitude of these values indicates that the ash material exhibits low to moderate compressibility.12 

# 5.0 Analysis

This section describes the engineering analyses conducted to evaluate seepage, slope stability and deformation for the proposed ash fill construction. The material properties used, analysis methods and results are described. GeoStudio modeling software developed by GEO-Slope International (Geo-Slope, 2004), comprised of SEEP/W, SLOPE/W an d SIGMA/W, was chosen for this project. The software was chosen because of its capabilities of comprehensive modeling and product integration.
Three cross sections were developed for analysis based on borehole data from the Ebasco exploration and based on the anticipated dry as h filling plan. The three sections represent typical subsurface profiles and maximum dike heights. The sections represent multiple configurations of dry ash fill placement within the existing dikes. The analys is sections are shown on Figure 1 and described below.

1. Interior Dike Cross Section – represents the impact that dry ash fill will have on the interior dike between Pond 3 and Pond 4.
2. Perimeter Dike Cross Section with Offset Fill – represents the impact that dry ash fill will have on existing ash away from the in fluence of the perimeter dikes.
3. Perimeter Dike Cross Section with Adjacent Fill – represents the impact that dry ash fill will have on the perimeter dikes of Pond 3 and Pond 4.
A low area with in-situ soft clay was identified by Ebasco and is located along the eastern perimeter dike of Pond 4. This area is loca ted in the area of the water in Pond 4. This area is not expected to be impacted by the dry ash fill and therefore was not represented in the models. 

## 5.1 Seepage Analysis 5.1.1 Seepage Model The main objective of the seepage analysis was to develop an understanding of the groundwater regime and how it relates to stability of the ex isting dikes and proposed dry ash filling plan. The seepage simulations presented in this report mo del groundwater flow under steady-state conditions. Existing seepage profiles were est ablished based on pond elevations provided by Minnesota Power. Water levels used in the analysis were 1319 feet for Pond 3 and Pond 4. These levels represent the maximum expected high pond levels.
Modeling of the seepage profiles for each of th e cross sections was c onducted using SEEP/W. SEEP/W uses the finite-element technique to model the movement of fluid and pore-water-pressure distribution within porous materials such as soils.13 5.1.2 Material Properties The main material parameter relevant to the seep age analysis is the hydraulic conductivity, or permeability of the materials considered in the analysis. The permeability of the ash was evaluated from consolidation tests and CPT soil behavior type s. The remaining materials were re-evaluated from the Ebasco exploration based on laboratory and field permeability tests. Table 4 summarizes the permeability values used in the seepage analysis.
Table 4 Permeability Values Used in Seepage Analysis

Material Permeability (cm/s) Upper Sand and Silt 10 -3 cm/s Clay 10 -6 cm/s Lower Sand 10 -3 cm/s Central Zone 10 -3 cm/s Clay Liner 10 -6 cm/s Ash 10 -4 cm/s 5.1.3 Boundary Conditions and Model Assumptions The use of boundary conditions in the seepage model is required to simulate actual field conditions and allow the input of seepage revi ew and pressure heads, among othe r variables. For this study, the boundary conditions applied and assumptions in the seepage analyses included:

• A fixed-head boundary condition was used to simulate the pond levels

• The downstream slope of the dikes from the pond beach to the crest contained a boundary condition that allowed seepage to flow from the face.

• All of the materials modeled were assumed to be isotropic, that is, to have the same permeability values in all directions

5.1.4 Results The pond levels used in the analyses were equal to the highest levels repor ted in recent years. Therefore, the seepage analyses are assumed to be conservative. The seepage conditions are represented on the cross-se ctions in Appendix C. 

## 5.2 Slope Stability 5.2.1 Stability Model The main objective of the slope stability analysis was to evaluate the safety of the interior and perimeter dikes and the proposed dry ash fill em bankment. The slope stability analyses were conducted using SLOPE/W. SLOPE/W uses th e limit equilibrium theory to compute a factor of safety on earth and rock slopes. It uses a variety of methods to compute the factor of safety of a slope while analyzing complex geometry, stratigraphy and loading conditions. For each of the sections, the groundwater head files generated dur ing the seepage analysis were imported into SLOPE/W to compute effective stress.14 Spencer’s method was used as the sear ch technique to determine the factor of safety of the dikes and dry ash fill. This method is considered the most ad equate because it satisfie s all conditions of static equilibrium and provides a factor of safety based on both force and moment equilibrium.
The slope stability analysis performed were for end-of-construction (EOC) and long-term conditions (LTC). For the EOC condition, the undrained shear strength parameters were used. Subsequently the factor of safety was computed for the LTC when the excess pore water pressure has been dissipated. The drained shear strength parameters are used for the LTC simulation.
5.2.2 Material Properties The shear strength data for the ash material was interpreted from triaxial compression testing and cone penetration testing. Parameters developed for in-situ ma terials during the Ebasco exploration were re-evaluated by Barr and used in the current evaluation. Table 5 summarizes the unit weight and shear strength parameters used in the slope stability analysis.
Table 5 Shear Strength Parameters Used in Stability Analyses Drained Strength Undrained Strength

Mohr-Coulomb Strength Ratio Mohr-Coulomb Material Unit Weight

(pcf) c (psf) Φ (degrees) Su/s’ v c (psf) Φ (degrees) Lower Sand 126 0 40 0 40 Clay 135 500 20 1200 0 Upper Silty Sand 126 0 40 0 40 Central Zone 122 500 30 500 30 Clay Liner 123 500 20 1000 0
Pond Ash 100 0 35 0.25 Dry Ash Fill 105 0 35 0 35

5.2.3 Results The results of the slope stability analysis, including the output with the crit ical failure surfaces at each section, can be found in Appendix C. Table 6 summarizes the results of the slope stability in terms of the factor of safety and includes th e suggested factor of safety under each condition analyzed.
It can be seen from Table 6 that the long-term c ondition factors of safety are above the suggested minimum of 1.5. The computed factor of safety for end of construction decreases using undrained conditions, although the factors of safety are all above the suggested minimum of 1.315 Table 6 Summary of Slope Stability Analysis
Long-Term Condition Drained (ESSA) End of Construction Undrained (USSA)

Section Suggested F.S. = 1.5 Suggested F.S. = 1.3 1. Interior Dike 2.70 1.82 2. Perimeter Dike with Offset Fill 3.66 1.47 3. Perimeter Dike with Adjacent Fill 2.44 1.40


## 5.3 Deformation/Consolidation

5.3.1 Deformation Simulation The main objective of the deformat ion/consolidation analysis was to develop an understanding of the deformations, pore water pressures an d stresses that would be generate d by placement of dry ash fill above the existing ash ponds. The deformation analyses were conducted using SIGMA/W. SIGMA/W uses the finite element technique to simulate stress/deformation of earth structures.
SIGMA/W can simulate consolidation analysis by coupling with SEEP/W. SIGMA/W computes displacements and stresses while SEEP/W computes changes in pore-water pressure with time. The initial pore-water pressure condi tions were obtained from SEEP/W and the steady-state analysis under existing conditions. Then SIGMA/W was used with the body load given by the unit weight and initial pore-water pressure to establish the initial state of stresses. The coupled analysis was used to establish the initial in-situ stresses.
Deformation analysis was conducted at the section which represents the most fill placement; Profile #3, Perimeter Dike Cross Section wi th Adjacent Fill. The fill placement was modeled in eight stages within a time frame of nearly five years to represent a practical construction sequence. The fill stages are shown on the model results.
5.3.2 Material Properties The material most susceptible to deformation and pore-water pressure generation and thus of most concern from the deformation/consolidation point of view is the existing ash. The material model selected to simulate the ash was Modified CAM-CLAY. The CAM-CLAY model introduced by Roscoe and Schofield (1963), allows simulation of so il behavior in the plastic range of deformations.
Table 7 summarizes the parameters required by the modified CAM-CLAY model (MCC). Lambda, λ , represents the slope of the nor mal consolidation line. Kappa, κ , represents the slope of the swelling line. OCR represents the over consolidation ratio. Gamma, Γ , represents the specific volume of the critical state line at a vertical line p = 1.0 psf. M represents the slope of critical state line. The parameters shown in Table 7 were obtained from the compressibility information of the ash collected in the consolidation te st, the triaxial consolidation test, and in-situ testing.16 Table 7 CAM-CLAY Soil Paramete rs used for Existing Ash MCC Parameter Value λ .1244

κ .0188 Pc 2.5
Γ 1.164

υ 2.838 M 1.418
Table 8 summarizes the parameters used in the deformation/consolidation analysis for other materials. In the deformation analysis, the deformability and strength properties of the soils are required. The deformability prope rties are given by the Young’s modulus (E) and Poisson’s Ratio, n

. The strength parameters are given by the fricti on and cohesion according to the Mohr-Coulomb model.

Table 8 Soil Deformation Parameters of Other Materials

Material c (psf) Φ , (degrees) Young’s Modulus E, (psf) Poisson’s Ratio

Lower Sand 0 40 700,000 0.35 Clay 500 20 100,000 0.40 Upper Silty Sand 0 40 700,000 0.35 Central Zone 500 30 700,000 0.35 Clay Liner 500 20 100,000 0.40 Dry Ash Fill 0 35 50,000 0.40

5.3.3 Results The results of the deformation analysis are interpreted in terms of the settlement and pore-water pressure. The maximum displacement located below the ash fill is expected to be on the order of three to four feet. This displacement is due to the large amount of fill placed and the relatively loose nature of the pond ash. Deformation is not expected to be a critical design i ssue, rather a check for excess pore pressures and undrained strength conditions if the dry ash were to settle into the existing wet ash.
Zones of high pore pressures are located within the existing ash and clay. It will be important to monitor the existing pore pressures within the clay existing piezometers at the site. Monitoring is further discussed in the following section. Cross- sections showing total ve rtical displacements and pore pressures after constructi on are shown in Appendix C.17 

# 6.0 Construction Considerations

Based on the engineering analysis performed, the ash ponds at BEC can be incrementally closed by placement of dry ash. The following list includes items to consider when designing and constructing the dry ash fill. 

## 6.1 Design

• The overall slope of the deposited ash should not be greater than 20 percent, or 5:1 (H:V). Greater slopes result in higher/thicker as h deposits which could compromise the pond stability.

• The ash fill should not be cons tructed near ponded water wher e there is the potential for water elevation to rise causing the existing ash fill material to lose strength. It is recommended that the new dry ash fill be constructed no closer than 100 feet from the location of the anticipated high water mark in each pond. Alternatively, new dikes can be constructed within the pond to separate dry ash fill areas from future pond areas.


## 6.2 Construction

• When placing the ash, caution should be take n so that only light, non-vibratory equipment is used to avoid liquefaction in the underlying ash fill.

• The first ash lift may be placed in a thickness of a few feet to acc ommodate workability on the softer, existing ash.

• Subsequent ash lifts shoul d be placed in 6-inch lifts and compacted with purpose-built compaction equipment.

• A grade control system should be established so the 5:1 fill slopes are achieved without overfilling.

• Existing piezometers around the perimeter of th e basin should be read on a regular basis and monitored for any increase in piezometric levels within the underlying clay and sand. If piezometric levels increase, the stability of the dry ash fill and existing dikes should be reevaluated.

• The pond water levels should continue to be monitored regularly. If pond levels are significantly greater (on the order of 5 feet) than those which were included in the stability model, the stability of the dry ash fill and existing dikes should be re-evaluated.18 

# References

Baligh, M.M. and Levadoux, J.N. 1980. Pore Pressure Dissipati on after Cone Penetration. Massachusetts Inst itute of Technology. Department of Civil Engineering. Cambridge, Mass. Report R80-11.

Ebasco Services Inc. 1977. Minnesota Power and Light Company Clay Boswell Steam Electric Station Unit No. 4, Ash Dispos al Pond, Dike and Foundation Studies , Engineering Report.

Ebasco Services Inc. 1981. Minnesota Power Clay Boswell SE S Pond Closure and Sealing Plan for the Unit #3 Fly and Bottom Ash Ponds

. GEO-SLOPE International, Ldt. 2004. GeoStudio . Version 6.16.
Kulhawy, F.H. and Mayne, P.H. 1990. Manual on Estimating soil Properties for Foundation Design

. Electric Power Research Institue, EPRI. August, 1990
Lunne, T., Robertson, P.K., a nd Powell, J.J.M. 1997. Cone Penetration Testing in Geotechnical Practice. Routledge. New York
Roscoe, K.H. and A.N. Schofield. 1963. Mechanical Behavior of an Idealized ‘Wet’ Clay . Proc. European Conf. on Soil Mechanics and Foundation Engineering, Wiesbaden (Essen: Deutsche Gesellschaft für Erd-und Grundbau e.V.), Vol. 1, pp. 47-54.

# FiguresC:\Documents and Settings\mlk2\Desktop\030 Data Analysis\CPT\CPTSummary.xls 

## Figure 2. Tip Resistance vs. Elevation 1,265 1,270 1,275 1,280 1,285 1,290 1,295 1,300 1,305 1,310 1,315 0 10 20 30 40 50 60 70 80 90 100 Tip Resistance, q t 

### [tsf]Elevation[ft] CPT-P3-1 CPT-P3-2 CPT-P3-3 CPT-P3-6 CPT-P3-8 CPT-P3-9 CPT-P4-5 CPT-P4-7 CPT-P4-9 CPT-P3-4 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/AC:\Documents and Settings\mlk2\Desktop\030 Data Analysis\CPT\CPTSummary.xls 

## Figure 3. Pore Pressure vs. Elevation 1,265 1,270 1,275 1,280 1,285 1,290 1,295 1,300 1,305 1,310 1,315 -20 30 80 130 180 Pore Pressure (Pressure Head), u 2 

### [ft]Elevation[ft] CPT-P3-1 CPT-P3-2 CPT-P3-3 CPT-P3-6 CPT-P3-8 CPT-P3-9 CPT-P4-5 CPT-P4-7 CPT-P4-9 CPT-P3-4 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/AC:\Documents and Settings\mlk2\Desktop\030 Data Analysis\CPT\CPTSummary.xls



## Figure 4. Friction Angle vs. Elevation (Kulhawy & Mayne, 1990) 1,265 1,270 1,275 1,280 1,285 1,290 1,295 1,300 1,305 1,310 1,315 10 15 20 25 30 35 40 45 50 55 60 Friction Angle, I ’ [degrees]Elevation[ft] CPT-P3-1 CPT-P3-2 CPT-P3-3 CPT-P3-6 CPT-P3-8 CPT-P3-9 CPT-P4-5 CPT-P4-7 CPT-P4-9 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/AC:\Documents and Settings\mlk2\Desktop\030 Data Analysis\CPT\CPTSummary.xls



## Figure 5. Undrained Shear Strength from CPT vs. Elevation 

## (N kt 

## =16) 1,265 1,270 1,275 1,280 1,285 1,290 1,295 1,300 1,305 1,310 1,315 0 200 400 600 800 1,000 1,200 1,400 Undrained Shear Strength, s u 

### [psf]Elevation[ft] CPT-P3-1 CPT-P3-2 CPT-P3-3 CPT-P3-6 CPT-P3-8 CPT-P3-9 CPT-P4-5 CPT-P4-7 CPT-P4-9 CPT-P3-4 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/AC:\Documents and Settings\mlk2\Desktop\030 Data Analysis\CPT\CPTSummary.xls



## Figure 6. Undrained Shear Strength Ratio from CPT vs. Elevation 

## (N kt 

## =16) 1,265 1,270 1,275 1,280 1,285 1,290 1,295 1,300 1,305 1,310 1,315 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Undrained Shear Strength Ratio, s u 

### / V ’ voElevation[ft] CPT-P3-1 CPT-P3-2 CPT-P3-3 CPT-P3-6 CPT-P3-8 CPT-P3-9 CPT-P4-5 CPT-P4-7 CPT-P4-9 CPT-P3-4 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/AFigure 7. Ash Mohr-Coulomb Shear Strength Failure EnvelopeConfined-Undrained Triaxial Test020004000600080001000002000400060008000100001200014000Normal Stress (psf)

Shear Stress (psf)Ash (Silt), DrainedAsh (Silt), UndrainedAsh (Sandy Silt), DrainedAsh (Sandy Silt), UndrainedDrained Upper BoundDrained Lower BoundUndrained Upper BoundUndrained Lower BoundDrained Friction Angle 38-43oUndrained Friction Angle 17-24oUndrained Strength Ratio 0.32-0.45

## Appendix A