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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........................................................................................................... 21

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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 Higher Ash Generation Rate Projection 

Table 3-1 Summary of Slope 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 Generation Rate Fill Contours 

Figure 5 – Higher Ash Generation Rate Fill Contours 

Figure 6 – West-East Cross Section 

Figure 7 – North South and Final Cover Cross Sections 

List of Attachments 

Attachment A – Geotechnical Report 

Attachment B – Technical Specifications for Cover Materials 

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1.0 Introduction 

1.1 Purpose 

On March 9, 2007, NPDES 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 equipment (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 projected in-service date of fall 

2009. 

The project includes changing the method of handling Unit 3 flyash from “slurry” to “dry” 

form. The Permit authorizes placement of dry flyash over the existing ash delta in Pond 3 

(Figure 1) and requires BEC to incrementally cover 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 the disposal of all dry flyash 

generated during this time period. 

This document is intended to meet the Permit requirement 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 include considerations for erosion and the ultimate use 

for the site. 

It should be noted that while this plan generally accommodates management of flyash 

expected to be generated at the BEC over the next 25-year timeframe, actual plant life can 

reasonably be expected to extend beyond 25 years. As such, flyash management needs will 

continue well into the future and this plan will require periodic updates (most likely 

concurrent with the 5-year cycle of NPDES permit renewal activities but possibly more 

frequently) to reflect flyash management needs as they evolve over time. This may also 

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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 Rule (RHR), the Clean 

Air Act Interstate Rule (CAIR), the Clean Air Mercury Rule (CAMR), and the Minnesota 

Mercury Emissions Reduction Act of 2006. The new equipment will result in significant 

annual maximum emission reductions of sulfur dioxide, oxides of nitrogen, particulate matter 

and mercury. MP is installing Best Available 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 S02 control, 

• Low NOx burners, over-fire air and selective catalytic reduction (SCR) for nitrogen 

oxides control, and 

• Installation/upgrades of combustion controls for carbon monoxide control. 

1.3 Pond Operation Summary 

The current BEC air quality control system solid waste streams are managed wet in a pond 

system that is located northwest of the plant site. Overall pond layout 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 1322. The following section describes current pond operations. 

1.3.1 Current Pond Operating Procedures 

• Units 1,2,3,4 Bottom Ash Pond – The bottom ash pond accepts bottom ash from each 

of the four BEC generating units. The bottom ash is transported from the plant site to 

the ponds through pipelines as wet slurry. Additional discharge to the bottom ash 

pond occurs at a variable rate from the coal pile sump. Excess water is pumped either 

to the Central Wastewater Treatment Facility (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. 

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• Unit 3 Flyash Pond – The Unit 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 North Blackwater Lake (Outfall SD 006) in 

accordance with the existing NPDES permit. 

• Unit 1, 2 and 4 Flyash and Unit 4 Scrubber Solids Pond – This pond (a.k.a. Pond 4) 

accepts flyash collected from BEC generating Units 1, 2, and 4 and scrubber solids 

from BEC generating Unit 4. These solids are transported as slurry through a pipeline 

to Pond 4. Excess water from Pond 4 is returned to the plant for subsequent use in the 

Unit 4 scrubber system and for slurry transport of additional AQCS solids to Pond 4. 

1.3.2 Pond Operating Procedures following Unit 3 AQCS Upgrade 

As noted previously, the flyash collected from the upgraded Unit 3 AQCS will be transported 

dry to the pond area for disposal. The overall pond operating procedures that will result are 

as follows: 

• Bottom Ash Pond—The bottom ash pond is generally expected to continue to operate 

as it does now with the exception that bottom ash will occasionally be transported dry 

(excavated from bottom ash 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 the dry ash fill activities. 

• Pond 3 Delta Area—The Pond 3 delta area will 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 Unit 3 AQCS upgrade and for as long after 

its start-up as possible, excess pond water will be discharged during ice-free months 

at an average rate of 1000 gpm to the plant water intake structure located on North 

Blackwater Lake (Outfall SD 006) in accordance with the water quality limits of the 

existing NPDES permit. The Pond 3 basin area will begin accepting FGD slurry from 

Unit 3 after discharging excess water. 

• Pond 4—Pond 4 will continue accepting slurry transported flyash solids from Units 1, 

2, and 4, as well as FGD scrubber solids from Unit 4. Pond 4 may also temporarily 

accept slurry FGD scrubber solids from Unit 3. 

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As noted above, flyash from Units 1 and 2 is currently transported 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 discontinued, 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 document 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 important to also consider potential future system 

modifications that could impact solid waste stream types and volumes to be managed in each 

ash pond. Potential changes, if they occur to the Unit 4 AQCS, will likely 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 operation 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 solids from Unit 4 and possibly Unit 3. 

1.4 Plan Organization 

The remainder of this document is organized into the following sections: 

Section 2 of this document provides additional background information regarding the current 

Pond 3 operations. This section also provides information on the projected dry ash 

generation rate and volumes following completion of AQCS upgrades. 

Section 3 describes the planned flyash fill procedures and the geotechnical evaluation that 

was performed to confirm feasibility of the approach. 

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Section 4 provides information regarding the schedule for the incremental closure of the 

flyash fill above the Pond 3 delta area. 

Section 5 presents the delta area fill design. 

And finally, Section 6 presents the plan for monitoring and reporting progress on plan 

implementation. 

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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 Figure 2. Total area 

within the Pond 3 perimeter dike is approximately 198 acres. The current Pond 3 delta has an 

area of approximately 95 acres. The Pond 3 delta area will be somewhat larger at the time of 

start-up of the upgraded Unit 3 AQCS. For purposes of this plan 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 flyash will be pneumatically transported to an ash silo 

for temporary holding until it is transferred to a pin-paddle mixer for moisture 

conditioning. After moisture conditioning, the ash will be transported by truck for 

disposal. The moisture conditioning in the pin-paddle mixer will minimize fugitive 

dust emissions during subsequent transport and unloading. Disposal of the ash is 

proposed to occur above the existing ash delta in Pond 3. 

• FGD Sludge – FGD sludge from the Unit 3 AQCS upgrade will be transported as 

slurry and discharged into the Pond 3 water 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 the Unit 3 AQCS upgrade; 

however, as pond capacity is consumed by bottom ash solids the ash pond effectiveness will 

be affected by the reduced volume and will subsequently have 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 the pond to maintain separation between the proposed dry ash 

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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 flyash by truck for deposition above the existing ash delta; 

forming a dry ash management segment within Pond 3. 

• Transportation of Unit 3 FGD sludge by pipeline 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 deposition 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 and McDonnell, estimates flyash generation rates of 15.5 tons 

per hour (tph) for Unit 3 and 26.5 tons per hour for Units 1, 2 and 4 (1.7 tph, 1.4 tph, and 

23.4 tph for Units 1, 2 and 4 respectively) and 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 generation rates are presented in Table 2-1 assuming 

8320 hours of operation per year (8,760 hours per year at 24 hours per day and assuming 5% 

total down time annually) and in-fill ash 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 48,000 

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In order to design for the air-space fill volume that may be needed to accommodate the dry 

ash generated over the next 25 years; projections of ash volume generated 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 bottom ash will be filled 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 106,000 cubic yards per year; 

• At some future date dry flyash will be generated from Unit 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 accommodate the dry ash 

generated for the 25 year period was based on the following assumptions: 

• Unit 3 generates dry flyash during the entire 25 year period, 

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• 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 (or 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 the bottom ash pond. However, the Figures included with this 

report depict filling of the delta area of Pond 3, which provides 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 flyash from Units 1 and 

2 will always be associated with flyash from Unit 4. This is due to the commingling of 

flyash from Units 1, 2 and 4 in the current wet 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 the dry flyash from Unit 3. 

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3.0 Overall Flyash Fill Procedures 

3.1 Flyash Fill Procedures 

Following the start-up of the Unit 3 AQCS upgrades, flyash 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 pond. At the silo the flyash will be moisture 

conditioned and then transported by truck to the active flyash fill area on the ash delta in 

Pond 3. The flyash will be dumped, spread by dozer, and compacted. 

During performance of the geotechnical evaluation subsequently described in Section 3.3, 

ash fill construction considerations were also identified, including: 

• Use of light weight non-vibratory equipment on initial lifts of dry ash fill so as to 

avoid liquefaction in the underlying ash fill. 

• Placement of the initial lift of ash to a thickness 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 silo. However, provisions will be made for spraying of the ash 

fill surface with water obtained from Pond 3 or Pond 4. Once areas are filled with flyash to 

final grade, these areas will receive intermediate cover consisting of either granular soil, 

bottom ash, or geosynthetic cover. These intermediate covers will be placed as needed to 

control wind-blown dust. 

3.2 On-Site Equipment 

As noted previously, ash will be hauled by truck from the silo to the active 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 evolve 

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over time to maintain efficient operations, particularly 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 stationed 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, whereas 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. However, early in the fill 

development some experimentation may also occur with a self-propelled compactor to weigh 

the relative gains in ash fill density. Finally, water 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 southwest corner of the bottom ash pond. The maintenance building 

will likely consist of a pre-engineering insulated 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 activities. A geotechnical 

exploration and analysis were performed, the main objectives of which were to determine the 

geotechnical properties of the existing ash in Pond 3 and Pond 4, and to evaluate the effects 

of the proposed vertical development on the stability of the existing ash deposits. 

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3.2.1 Ash Characterization 

The existing ash fill in the deltas 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 ash 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 exterior 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 properties 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 very rapid filling with dry ash over a relatively 

large area. Due to the low rate of ash generation 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 previous investigations at the site, and on a site 

exploration program including soil borings, cone penetration testing and in-laboratory 

material testing. Stability analyses were conducted at three cross sections that represent 

typical subsurface profiles, maximum dike heights, and multiple configurations of dry ash 

fill placement within the confines of the existing dikes. These include: 

• Interior Dike – Stability analysis of dry ash fill adjacent 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. 

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• 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 exceeding 20 percent (5H :1V) 

and a 100 foot separation distance of the ash fill area from future 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 the minimum recommended safety 

factors. The stability analysis results 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 critical 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 expected to be on the order of three feet. This 

displacement is due to the large amount of fill placed and the relatively loose nature of the 

existing ash. Deformation is not a critical design issue as the pond liner system is located 

below the existing ash delta and will be relatively unaffected by the overlying ash fill. In 

addition, the majority of this settlement will occur 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 ash landfill. The provided recommendations for ash fill design 

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and construction are incorporated into this plan. The complete Geotechnical Evaluation 

Report is provided as Attachment A. 

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4.0 Closure Schedule 

This plan provides for the incremental filling of the Pond 3 delta area. The plan contains 

sufficient flexibility in final fill heights and grades to accommodate 20 to 25 years of dry ash 

filling. The Pond 3 delta area (110 acres) will be incrementally covered 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 anticipated and hence capacity 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 depend 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 construction would occur on a three year basis. 

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The anticipated incremental closure areas are shown on Figure 3. It must be noted that the 

incremental closure area is based on assumptions 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 efficient and effective closure, dependent on the actual ash 

generation rates and in-place fill density. 

Implicit in the closure plan is that the dry ash from Units 1, 2, 3 and 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 the assumption that the ash delta in Pond 3 

ultimately will require final cover independent of the availability of dry flyash from the BEC. 

If dry flyash is not available, then imported soils would be required for grading the existing 

ash delta in preparation for closure. 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. 

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5.0 Closure Design 

5.1 Design Approach 

The goals/objectives of the dry ash fill area closure design 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 generated under the higher ash fill rate 

scenario. 

In order to provide for the direct discharge of stormwater from filled and closed areas to 

surface water, the closure design: 

• Separates runoff from the closed 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 erosion control BMP’s for the closed areas. 

Sedimentation is provided by the sedimentation basin adjacent to the northwest corner 

of Pond 3 and the perimeter ditch which conveys 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 runoff 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 slopes of 5% or less. Best 

Management Practices (BMPs) to be in place during closure construction will be 

detailed in the construction Stormwater Pollution Prevention Plan (SWPPP) prepared 

as part of each cover construction project bid package. 

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5.2 Cover Design 

The closure system will generally consist of a geomembrane barrier layer overlain by a 

granular drainage layer or geonet drainage layer, followed by rooting soil and topsoil. This 

is illustrated in Figure 7. For the granular drainage layer method of cover the profile would 

likely be: 

• Geomembrane Foundation Layer/Interim Cover (flyash, bottom ash, or sand) 

• 40 mil Low Density Polyethylene Geomembrane (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 specific 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, 

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• 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 Figures 4 and 5 illustrate the variation in top slope elevation and 

top slope grade necessary to provide for the range 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 measure the ash fill rate and adjust 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 maintenance long term control of sediment transport and soil 

erosion. BEC will seek agency approval for alternate end uses, should alternate uses be 

contemplated. 

5.5 Maintenance 

Routine maintenance, such as mowing vegetation to prevent tree growth and to maintain 

drainageway flow capacity, will be performed as needed. All features including, but not 

limited to, site security, monitoring wells, and surface water runoff control structures will be 

properly maintained. 

The surface cover will require periodic maintenance. Supplemental cover soil will be placed 

to repair the effects of severe erosion or settlement. Seeding, fertilizing, and mulching of 

bare soil will be performed as required to restore the area and establish a stable vegetative 

cover. These areas will be inspected periodically to ensure that vegetative growth has been 

re-established. 

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Surface water runoff control systems will be maintained on a routine basis. Drainage 

ditches, terraces, and spillways will be inspected for condition and proper function. For 

systems that are found to be in poor condition and/or non-functioning, routine maintenance 

will be performed. 

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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. Designs will incorporate provisions for long term 

sediment and erosion control BMPs. Construction sediment and erosion control BMPs will 

be presented in each Construction SWPPP. 

6.2 Annual Survey and Fill Rate Calculation 

A topographic survey of the active fill area will be obtained on a periodic basis. The rate of 

ash filling will be calculated 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 volume computations and the annual 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. 

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Attachment A – Geotechnical Report 

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Geotechnical Exploration Report 

Ash Pond Incremental Closure 

Boswell Energy Center 

Cohasset, Minnesota 

Prepared for 

Minnesota Power 

August 2007

 

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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 ..................................................................................................10

4.4.1 Standard Penetration Tests........................................................................................... 10

4.4.2 Index Tests................................................................................................................... 10

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.......................................................................................................................... 14

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5.3 Deformation/Consolidation .....................................................................................................15

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

References...................................................................................................................................................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 Used in Stability Analyses.................................................... 14

Table 6 Summary of Slope Stability Analysis.............................................................................. 15

Table 7 Modified CAM-CLAY Soil Parameters used for Ash ..................................................... 16

Table 8 Soil Deformation Parameters of Other Materials ............................................................ 16

List of Figures 

Figure 1 Geotechnical Investigation 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 Strength Mohr-Coulomb Failure Envelope 

Appendices 

Appendix A Ebasco Services, Inc. Reference Materials 

Appendix B Braun Intertec Geotechnical 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.: 20951

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Executive Summary 

This report summarizes the geotechnical exploration and analysis completed in support of 

development of an incremental closure plan of the Boswell Energy Center flyash ponds. The main 

objectives of the geotechnical exploration were to determine the 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 cementation 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, typically ranging from 35 to 43 degrees. Under undrained 

conditions, the ash had a low undrained strength 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 multiple 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 drained and undrained conditions. 

Deformation analysis was conducted at the most critical cross section. The analysis 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 exploration and analysis indicate that the ash ponds can be closed 

incrementally. The provided recommendations for fill design and construction should be followed to 

maintain stable pond embankments. 

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1.0 Introduction 

1.1 Project Description 

The incremental closure project involves 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 were 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 evaluate the geotechnical properties of the ash. 

• Research previous studies done at the site. 

• Conduct analyses regarding seepage, stability 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. 

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2.0 General Considerations 

2.1 Site Description 

The Boswell Energy Center is located about three miles west of Cohasset, Minnesota 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, random sandy borrow material with a slope of 3:1 (H:V) 

make up the interior dikes. The pond perimeter dikes are of similar construction with a clay barrier 

on the upstream slope. The ponds were constructed using in-situ clay as a natural liner. 

2.2 Site Geologic Setting 

The description of the site geology is based on a literature review. The site is located within the 

Superior Upland portion of the Canadian Shield physiographic province. The region is characterized 

by low hills and rocky ridges formed by exposed Precambrian metamorphic 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 pond 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. 

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3.0 Geotechnical Exploration 

The geotechnical exploration for the ash pond incremental closure consisted of soil borings, cone 

penetration tests and field and laboratory testing of soil 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 March 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 exploration and are summarized in Table 1. Eight of the soil 

borings were performed in Pond 3, and the remaining three 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 representative 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 -7477.6 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 -5593.7 1314.2 21.5 

B-P3-5 2867.9 -5538.5 1315.8 6.0 Auger Refusal 

B-P3-6 2695.5 -5401.4 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 -5021.0 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 1311.4 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 system 

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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 sounding locations were pre-drilled through the frost 

depth, to approximately 3.5 to 4 feet. The soundings 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 cm2

 base area and an unequal area end ratio a of 

0.80. During penetration, measurements are made of the cone tip resistance qc, the side friction fs on the 

cylindrical shaft just above the tip, and pore water pressure u2 generated by cone penetration. 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 included a hard copy showing the graphical variations 

of all readings with depth, including an interpreted 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 performed 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 ASTM D 2216, “Standard Test Method for Laboratory 

Determination of Water (Moisture) Content of Soil and Rock by Mass. 

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• 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 field and laboratory test results are summarized in 

Table 3 and presented in full in Appendix B. 

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Table 3 Laboratory Testing Summary 

Atterberg Limits Gradation 

Boring 

Sample 

Depth 

(feet) 

Material 

Classification 

Water 

Content 

(%) 

Specific 

Gravity 

Dry 

Density, 

pcf LL PL PI 

% 

Gravel 

% 

Sand 

% 

Silt 

% 

Clay 

B-P3-1 9.5-11.5 

Sandy Silt 

(MLS) 39.2 0 45.9 31.9 22.2 

B-P3-3 4.5-6.5 Silt (ML) 

72.6 70.9 93.4 71.0 2.45 

53.9 53.9 44.6 53.2 NP NP NP 0.0 13.7 80.5 5.8 

B-P3-6 6.5-8.5 Silt (ML) 41.3 0.0 5.2 60.5 34.4 B-P3-8 4.5-6.5 

Sandy Silt 

(ML) 

27.7 33.6 50.7 31.6 3.24 

97.7 90.8 65.0 95.0 NP NP NP 0.0 36.0 61.2 2.8 

B-P4-5 7-9 Silt (ML) 

37.4 69.2 57.6 51.0 2.84 

82.7 55.4 62.0 68.1 29.5 26.8 2.8 0.0 12.1 85.1 2.8 

B-P4-9 9.5-11.5 Silt (ML) 32.7 0.0 10.1 79.8 10.1 
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4.0 Exploration Results 

A description of the field exploration and laboratory testing 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 located 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 silt with some sand. This layer is generally 10 

feet thick across the site 

• Clay – A layer of clay is located below the sand and silt, ranging in thickness from 0 to 40 

feet. The clay is thickest in the center of the pond, tapering in the east and west directions. 

Clay was not encountered 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 bedrock 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 borrowed 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. 

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4.2 Groundwater Conditions 

There are two aquifers generally recognized in the area 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 direction 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 water 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 qt, friction ratio Rf, and pore-water 

pressure parameter Bq, The corrected tip resistance qt 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 undrained 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 partially-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 from 30 to 40 degrees, shown in Figure 4. 

CPT data were also used to estimate the undrained shear 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 are 

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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, Kh, was calculated based on results from the pore pressure dissipation 

tests and laboratory consolidation tests (Baligh and Levadoux, 1980). Estimations of permeability based 

on soil behavior (Lunne, Robertson and Powell, 1997) were coupled with the Kh 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 exploration, standard penetration tests (SPT) were conducted at each soil boring 

location. The SPT results can be used to infer the approximate soil strength profile at the soil boring 

location. The SPT values of the ash in terms of N60 (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 classified 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 samples to accurately classify 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 collected 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 determine 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/ft3

 with an average of 103.3 lb/ft3. 

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 undrained 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. 

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The results of the triaxial compression tests indicate 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 tested based on the maximum deviator stress failure 

criterion was φcu = 17o to 24° and cu = 400 to 500 psf. The drained strength properties of the ash are 

generally higher than typical silt-sized material, which 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 characteristics 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 deformation of the ash due to loading. 

The consolidation test results indicate that the compression ratio for the three ash samples ranges from 

0.075 to 0.190. The recompression ratio lies between 0.010 and 0.028, and the 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. 

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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 and 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 ash 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 analysis 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 influence 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 located 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 existing dikes and proposed dry ash filling plan. The 

seepage simulations presented in this report model groundwater flow under steady-state conditions. 

Existing seepage profiles were established 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 the cross sections was conducted 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. 

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5.1.2 Material Properties 

The main material parameter relevant to the seepage 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 types. 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 review and pressure heads, among other 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 reported in recent years. 

Therefore, the seepage analyses are assumed to be conservative. The seepage conditions are 

represented on the cross-sections 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 embankment. The slope stability analyses were 

conducted using SLOPE/W. SLOPE/W uses the 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 during the seepage analysis were imported into 

SLOPE/W to compute effective stress. 

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Spencer’s method was used as the search technique to determine the factor of safety of the dikes and 

dry ash fill. This method is considered the most adequate because it satisfies 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 materials 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 

Material Mohr-Coulomb Strength Ratio Mohr-Coulomb 

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 critical 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 the suggested factor of safety under each condition 

analyzed. 

It can be seen from Table 6 that the long-term condition 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 1.3 

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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 deformation/consolidation analysis was to develop an understanding of the 

deformations, pore water pressures and stresses that would be generated 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 conditions 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 with 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 soil 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 normal 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 test, the triaxial consolidation test, and in-situ testing. 

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Table 7 CAM-CLAY Soil Parameters 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 properties are given by the Young’s modulus (E) and Poisson’s Ratio, n. 

The strength parameters are given by the friction 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 issue, 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 vertical displacements and 

pore pressures after construction are shown in Appendix C. 

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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 ash deposits which could compromise the pond 

stability. 

• The ash fill should not be constructed near ponded water where 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 taken 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 accommodate workability on 

the softer, existing ash. 

• Subsequent ash lifts should 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 the 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. 

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References 

Baligh, M.M. and Levadoux, J.N. 1980. Pore Pressure Dissipation after Cone Penetration. 

Massachusetts Institute 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 Disposal Pond, Dike and Foundation Studies, Engineering Report. 

Ebasco Services Inc. 1981. Minnesota Power Clay Boswell SES 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., and 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. 

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Figure 2. Tip Resistance vs. Elevation

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1,315

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Ele

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CPT-P3-2

CPT-P3-3

CPT-P3-6

CPT-P3-8

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Figure 3. Pore Pressure vs. Elevation

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1,305

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1,315

-20 30 80 130 180

Pore Pressure (Pressure Head), u2 [ft]

Ele

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atio

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CPT-P3-2

CPT-P3-3

CPT-P3-6

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Figure 4. Friction Angle vs. Elevation (Kulhawy & Mayne, 1990)

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Ele

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CPT-P3-2

CPT-P3-3

CPT-P3-6

CPT-P3-8

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Figure 5. Undrained Shear Strength from CPT vs. Elevation 

(Nkt=16)

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Undrained Shear Strength, su [psf]

Ele

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CPT-P3-2

CPT-P3-3

CPT-P3-6

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

(Nkt=16)

1,265

1,270

1,275

1,280

1,285

1,290

1,295

1,300

1,305

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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, su/V’vo

Ele

v

atio

n [ft]

CPT-P3-1

CPT-P3-2

CPT-P3-3

CPT-P3-6

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Confined-Undrained Triaxial Test

0

2000400060008000

10000

0 2000 4000 6000 8000 10000 1

Normal Stress (psf)

Shear Stress (psf)

Ash (Silt), DrainedAsh (Silt), UndrainedAsh (Sandy Silt), DrainedAsh (Sandy Silt), UndrainedDrained Upper BoundDrained Lower BoundUndrained Upper BoundUndrained Lower Bound

Drained Friction 

Undrained FricUndrained Stre
Appendix A

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