conceptual-level study of dewatering alternatives draft...

D R A F T P H A S E - 1 S T U D Y

CONCEPTUAL-LEVEL STUDY OFDEWATERING ALTERNATIVES

Prepared forCotter CorporationP.O. Box 1750Canon City, CO 81215

May 2008

URS Corporation8181 E. Tufts AvenueDenver, CO 80237

Project No: 22240078

DRAFT Table of Contents

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Executive Summary.....................................................................................................................................ES-1

Section 1 ONE Introduction ..........................................................................................................................1-1

1.1 Purpose and Objectives......................................................................... 1-11.2 Background .......................................................................................... 1-11.3 Existing Conditions .............................................................................. 1-1

1.3.1 Subdrain System ....................................................................... 1-21.3.2 Liner System............................................................................. 1-21.3.3 Drain System ............................................................................ 1-21.3.4 Impoundment Tailings .............................................................. 1-2

1.4 Evaluation Criterion ............................................................................. 1-3

Section 2 TWO Evaluation of Phreatic Surface Level................................................................................2-1

2.1 General................................................................................................. 2-12.2 Settlement Criterion and Methodology ................................................. 2-1

2.2.1 Primary Consolidation .............................................................. 2-22.2.2 Secondary Settlement................................................................ 2-2

2.3 Material Properties ............................................................................... 2-32.3.1 Cover Material .......................................................................... 2-42.3.2 Clay Subliner ............................................................................ 2-42.3.3 Tailings..................................................................................... 2-4

2.4 Upper and Lower Bound Settlement Estimates ..................................... 2-62.5 Differential Settlements and Cover Strain ............................................. 2-6

Section 3 TH REE Conceptual Dewatering Alternatives and Time Estimates ............................................3-1

3.1 General................................................................................................. 3-13.2 Alternative 1 – Existing Dewatering Drain System............................... 3-13.3 Alternative 2 – Existing Dewatering Drain System With Wick

Drains................................................................................................... 3-23.4 Alternative 3 – Dewatering Wells ......................................................... 3-33.5 Alternative 4 – Existing Dewatering Drain System With

Dewatering Wells ................................................................................. 3-3

Section 4 FOUR Sensitivity Analyses for Liner Leakage............................................................................4-1

Section 5 FIVE Comparative Evaluation of Conceptual Alternatives .....................................................5-1

5.1 Rating Criteria ...................................................................................... 5-15.2 Conceptual Alternatives........................................................................ 5-25.3 Discussion ............................................................................................ 5-4

Section 6 SIX Conclusions..........................................................................................................................6-1


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Section 7 SE VEN General Information.............................................................................................................7-1

Section 8 EIGHT References............................................................................................................................8-1

List of TablesTable 1 Summary of Material PropertiesTable 2 Summary of Settlement Calculations

Table 3 Calculated Differential Settlements and Horizontal Tensile StrainsTable 4 Calculated Dewatering Time for 5-foot Phreatic Surface Drawdown

Table 5 Sensitivity Analysis for Liner LeakageTable 6 Comparison of Dewatering Alternatives

List of FiguresFigure 1 Cotter Primary Tailings Impoundment, General Plan

Figure 2 Cotter Primary Tailings Impoundment, Cross SectionsFigure 3 Upper Bound Total Settlement Curves for Cover Placement and Dewatering

Figure 4 Lower Bound Total Settlement Curves for Cover Placement and DewateringFigure 5 Total Settlement Profile with No Dewatering Prior to Placing Cover, Upper

Bound EstimateFigure 6 Total Settlement Profile with No Dewatering Prior to Placing Cover, Lower

Bound EstimateFigure 7 Total Settlement Profile with No Dewatering Prior to Placing Cover, Combination

of Upper and Lower Bound EstimatesFigure 8 Total Settlement Profile with Partial Dewatering Prior to Placing Cover,

Combination of Upper and Lower Bound EstimatesFigure 9 Dewatering Alternative 1

Figure 10 Dewatering Alternative 2Figure 11 Dewatering Alternative 3

Figure 12 Leaky Liner Seepage Analyses for Slime TailingsFigure 13 Leaky Liner Seepage Analyses for Sand/Slime Tailings

AttachmentsAttachment A – Material Properties

Attachment B – Dewatering Alternatives


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Acronyms and Abbreviations

ALARA As Low As Reasonably Achievable

cm/s centimeters per secondCotter Cotter Corporation

CPT cone penetration testDOE U.S. Department of Energy

FLAC Fast Lagrangian Analysis of Continuaft foot or feet

ft3 cubic feetgpm gallons per minute

NRC Nuclear Regulatory Commissionpcf pounds per cubic foot

psf pounds per square footPI plasticity index

UMTRCA Uranium Mill Tailings Radiation Control ActUMTRA Uranium Mill Tailings Remedial Action

URS URS Corporation

Executive Summary

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This report presents a conceptual-level alternatives study, called “Phase 1 Study” for dewateringthe existing tailings within the primary impoundment. The Phase I study presented in this reportfocuses on the geometry of the existing primary tailings impoundment with the existingdewatering drain system and liner. A 6-foot thick, multi-layered soil cover will be placed on topof the existing tailings when the tailings impoundment is closed.A soil cover can be an effective barrier for radon gas emission from uranium tailings deposits.However, soil covers can lose integrity and crack if the horizontal tensile strains induced bydifferential settlements are excessive. This report evaluates the effect of tailings dewatering andtotal settlement on the cover integrity and develops a target phreatic surface drawdown level thatshould be achieved in the tailings prior to placing the cover material.

Total settlement of the tailings impoundment results from primary consolidation of the tailingsunder the weight of the cover material and loads induced by dewatering, plus secondarysettlement due to secondary compression and creep. The tailings properties will affect the timefor dewatering and drainage. For the properties used in this study it was calculated that thephreatic surface in the tailings would need to be lowered at least 4 feet below the existing levelprior to placing the cover material in order to limit differential settlement of the tailings to amagnitude that will not result in excessive strains in the soil cover.Four conceptual alternatives were evaluated for lowering the phreatic surface within the tailingsprior to cover placement in order to minimize differential settlement. After the alternatives weredeveloped and analyzed, a comparative evaluation study was performed for all alternatives.Ranking criteria for the various alternatives include: cost, safety of operations, complexity ofoperations, reliability/redundancy, drainage rate, As Low As Reasonably Achievable (ALARA),etc. The alternatives were ranked from the most favorable to the least favorable as follows:Alternative 1 – Existing Dewatering Drain System, which requires pumping from the existingsump and dewatering system;Alternative 4 – Existing Dewatering Drain System with Dewatering Wells, which requires theinstallation of dewatering wells and use of the existing dewatering drain system sump;Alternative 3 – Dewatering Wells only, which requires installation and operation of dewateringwells without the added benefit of the existing dewatering drain system;Alternative 2 – Existing Dewatering Drain System with Wick Drains, which requires installationof wick drains and use of the existing dewatering drain system sump.The last section of the report presents a sensitivity analyses for liner leakage which shows thatthe underlying clay subliner would be effective in reducing leakage through a damaged Hypalonliner.

SECTIONONE DRAFT Introduction

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1. Secti on 1 ONE Introducti on

1.1 PURPOSE AND OBJECTIVESCotter Corporation (Cotter) requested URS Corporation (URS) evaluate the dewateringrequirements for the tailings within the primary impoundment at the Cotter Cañon City MillingFacility and develop conceptual-level alternatives to achieve the required dewatering. Werelated the degree of dewatering for the existing tailings to the stress (potential for cracking) inthe future cover soil to define “required dewatering”. In other words, the required dewatering isthe minimum lowering of the phreatic surface prior to cover placement so that the long-termsettlement of the cover would be acceptable. The Phase I study presented in this report focuseson the geometry of the existing primary tailings impoundment, with the existing dewateringdrain system and liner. A 6-foot thick, multi-layered cover will be placed on the existing tailingswhen the associated impoundment is closed. The overall work scope was split into Phase I andPhase II studies. In the Phase I study, reasonable assumptions based on available information,were made for the tailings properties in order to complete the settlement and seepage(dewatering) analyses. These assumptions will be evaluated and refined during future Phase IIwork. The Phase II work will include a tailings characterization study involving field andlaboratory testing.

1.2 BACKGROUNDThe Cotter Cañon City Milling Facility is located in Fremont County, Colorado, approximately 2miles south of Cañon City and approximately 1.5 miles southwest of Lincoln Park in Section 16,Township 19 South, Range 70 West. Cotter started processing ores at the Cañon City MillingFacility in 1958, recovering uranium with an alkaline leach process. Other custom millingprocesses were employed from 1958 to 1979, including acid leaching for recovery of othermetals. The mill tailings from these processes were discharged into a series of ponds east of themill site. This area, called the Old Ponds Area, was used for alkaline tailings disposal untilconstruction of the new tailings impoundments in 1979.The new tailings impoundments, with primary and secondary impoundments, were designed in1978 for acid-leach mill operations. The impoundments were constructed in 1979 and containboth new tailings and solids transferred from the Old Ponds Area. The impoundmentsincorporate a composite liner system consisting of a compacted clay subliner overlain by asynthetic liner (Hypalon) and a protective soil cover (MFG 2005).

The primary impoundment covers approximately 91 acres at its maximum permitted tailingselevation. It was used for disposal of approximately 1.0 million cubic yards of acid leachprocess tailings from 1979 through 1987. Short-term acid mill operations took place in 1990 and1992, stockpiled ore was processed in 1999, and caldacite ore was test processed in 2000 (MFG2005). The mill processed ores from mines along the western slope of Colorado between 2004and 2006.

1.3 EXISTING CONDITIONSExisting topography of the primary impoundment, based on the March 9, 2007 photogrametricsurvey, is provided on Figure 1. Information about construction of the primary impoundmentand the stored tailings was obtained from the 1980 W.A.Wahler & Associates First Stage



Construction Report and the 1999 Shepherd Miller, Inc., Tailings Investigation Report,respectively.

1.3.1 Subdrain SystemAccording to information provided in the W.A.Wahler & Associates 1980 First StageConstruction Report, springs were discovered along the northern half of the impoundment basinduring the excavation and foundation preparation process. A subdrain system drained byprimary and secondary outfall pipes was installed to reduce water pressure buildup under theliner. The discharge from both the embankment and reservoir subdrains is collected in aconcrete sump located outside the limits of the downstream toe of the embankment.

1.3.2 Liner SystemThe primary impoundment has a composite liner system consisting of a clay subliner and asynthetic Hypalon liner. Placement of the Zone 1A clay subliner began on March 12, 1979 andwas completed on October 26, 1979. The 18-inch thick clay subliner was placed and compactedin 6-inch lifts. Based on 61 in-place density tests, the average dry unit weight and water contentwere 115.9 pounds per cubic foot (pcf) and 14.5 percent (0.7 percent above average optimum),respectively. This indicates an average relative compaction of 98.7 percent. A 60-mil Hypalonliner was placed below elevation 5585 and a 45-mil Hypalon liner was placed above elevation5585 over the clay subliner and seamed with a vulcanizing process to fuse the Hypalon sheets.The Hypalon liner was reportedly inspected and repaired as needed prior to being covered. Theextent of the Hypalon liner is shown on Figure 1.A 12-inch protective cover layer of sandy clay was placed over the Hypalon liner. Covermaterial was spread with tracked dozers apparently no larger than a Caterpillar D-8. However,in certain areas, use of a Caterpillar D-9 working above 24-plus inches of protective cover wasallowed. The as-built top of liner protective cover topography is shown on Figure 1.

1.3.3 Drain SystemA dewatering system was installed over the protective cover. The dewatering drain system in theprimary impoundment consists of approximately 15,000 linear feet of finger drains, 6,500 linearfeet of collection pipe drains, and a 24-inch diameter sump. The drain system layout and sectiondetails of the drains are provided on Figures 1 and 2, respectively. The dewatering system wasdesigned to remain idle throughout the operational life of the mill and to be used for dewateringafter the tailings impoundment has been decommissioned. The dewatering system was designedto drain the solution from the tailings by gravity feed to a sump.

1.3.4 Impoundment TailingsThe extent of the existing tailings, existing topography, and pond water surface on March 9,2007 are provided on Figure 1. The primary impoundment holds tailings produced from theSchwartzwalder (near Golden, Colorado) and West Slope (near Nucla, Colorado) ore bodies.Due to discharge and deposition characteristics, tailings consist of horizontally bedded sand,sand/slime, and slime. The majority of the tailings deposit was classified as sand/slime in the



2005 MFG report. Regardless of parent ore type, the tailings located closer to the area ofdischarge (the perimeter of the embankment) are generally coarser-grained than those in theinterior of the impoundment.An interim cover was placed over the exposed tailings along the western edge of theimpoundment to minimize blowing of tailings dust and to decrease radon gas flux from theimpoundment. The interim cover is typically 2 feet thick, with indications of mixed tailings andcover material to depths of 10 feet or more in some areas.

1.4 EVALUATION CRITERIONThe amount of phreatic surface drawdown in the tailings was evaluated based on the loweringrequired to achieve satisfactory performance of the planned soil cover.A soil cover can be an effective barrier for radon gas emission from uranium tailings deposits.However, soil covers can lose integrity and crack if the horizontal tensile strains induced bydifferential settlements are excessive. Generally a limited amount of horizontal tensile strain canbe tolerated by the cover without cracking. The Nuclear Regulatory Commission (NRC) and theU.S. Department of Energy (DOE) have suggested several methodologies for computingallowable cover strain to achieve compliance with the directives of the Uranium Mill TailingsRadiation Control Act (UMTRCA) as modified in Part 18, Appendix A (6 CCR 1007-1). Onesuch methodology, based on the work done by Larson and Keshian (1988), Cadwell and Reith(1993), and Claire, Kuo, and Wanket (1994) relates the plasticity index (PI) of the cover materialto the allowable horizontal tensile strain. This relationship, presented in Equation 1, was adoptedfor this study.

εf = 0.05 + 0.003 PI (1)Where,

εf is the allowable horizontal tensile strain (%) and PI is the plasticity index (%)of the cover liner.

Equation 1 was used in the DOE 2006 calculation package for the Crescent Junction DisposalSite for the Moab UMTRA Project.The induced horizontal tensile strain in the cover is a function of the differential settlements andcan be calculated with Equation 2:

(%)10022

⋅−+

=x

xsxfε (2)

Where,εf is the horizontal tensile strain in the cover material, %s is the differential settlement between locations of calculated settlement, feetx is the horizontal distance between locations of calculated settlement, feet

For the Phase I analyses, due to limited information available on the proposed cover material, aPI value of 0 was conservatively assumed for the soil cover. Accordingly, 0.05 percent wasselected as the maximum allowable horizontal tensile strain in the cover for this study. Thiscriterion will be adjusted for the Phase II study after evaluating the site-specific planned covermaterial properties.

SECTIONTWO DRAFT Evaluation of Phreatic Surface Level


2. Secti on 2 TWO Evalua ti on of P hreati c Surf ace Le vel

2.1 GENERALTotal settlement of the Cotter Primary Tailings Impoundment will result from primaryconsolidation settlement of the tailings under the weight of the cover material and loads inducedby dewatering, plus secondary settlement due to secondary compression and creep. Tailingsproperties will affect the time for dewatering and drainage. The amount of settlement that occursafter the cover is placed may impact its integrity. Differential settlements can result in tensilestress in the cover. If these stresses exceed the cover material’s tensile capacity, the cover maycrack and have its integrity compromised. This section discusses how much dewatering isneeded to meet the evaluation criterion given in Section 1.4.

2.2 SETTLEMENT CRITERION AND METHODOLOGYConsolidation settlement analyses were performed to evaluate the minimum drawdown requiredwithin the tailings prior to cover placement resulting in acceptable long-term differentialsettlements of the cover. For these analyses, a 6-foot thick, multi-layered cover was placed ontop of the existing tailings. The tailings will settle under the weight of this cover. Themagnitude of post-cover placement settlement will depend on the location of the initial phreaticsurface at the time of cover placement. Largest settlements will occur if, at the time of coverplacement, the initial phreatic surface is at the top of the tailings. The smallest settlements willoccur if the initial phreatic surface is at the bottom of the tailings. Between these two dewateringextremes, there is a phreatic surface level (“target” phreatic surface level) that should beachieved before cover placement so that the integrity of the cover due to settlement can bemaintained. The step-by-step approach used for estimating the settlement corresponding to thetarget phreatic surface level is as follows:

• A study section was located to pass through the maximum tailings depth area of the pond.

• On the study section, five tailings columns were located with different tailingsthicknesses. Settlements for each tailing column were calculated for various sequences ofcover placement and dewatering alternatives.

• Total settlements for the less compressible sand/slime and more compressible slimetailings columns were estimated, assuming the initial phreatic surface was (i) at the tailingssurface at the time a 6-foot thick cover is placed, (ii) at the bottom of the tailings column, and(iii) at intermediate depths between the top and bottom. Total settlements calculated in thisstudy are the summation of primary settlement of tailings under the weight of the cover andthe weight of the tailings itself after dewatering plus secondary settlement of the tailings in1,000 years. Total settlement values were plotted along the study section to represent thelower and upper bound settlement profiles for sand/slime and slime tailings, respectivelycorresponding to complete dewatering (lowering the water table to the base of the tailings).

Case studies reported by Wels et al. (2000); Matyas, Welch, and Reades (1984); DOE (2006),and a study by Gates (1982) indicate consolidation theory predictions provide a good fit to thesettlement response of uranium tailings. Consolidation, or time-delayed compression, is thereduction in volume associated with a reduction in pore water pressure under load, and it occursin all soils. Consolidation occurs quickly in coarse-grained soils such as sands and gravels and isnot distinguishable from elastic deformation. Consolidation of fine-grained soils such as clays



and organic materials can be significant and take considerable time to complete. Consolidationsettlements consist of primary consolidation and secondary settlement. The methods forcalculating primary consolidation and secondary settlement of the tailings in the primaryimpoundment are described below.

Earthquake-induced settlement calculations were beyond the scope of the Phase I analysis.

2.2.1 Primary ConsolidationPrimary consolidation settlement occurs during dissipation of excess pore fluid pressure, and iscontrolled by the granular expulsion of fluid from voids in the soil, leading to associatedcompression of the soil skeleton. Primary consolidation settlement of a soil column is calculatedby Equation 3:

1

2log1 σ

σ⋅

+⋅=

oc e

HCs (3)

Where,

s is the primary consolidation settlement, feetσ1 is the initial effective stress at the midpoint of the tailings column, psfσ2 is the stress after cover placement and dewatering, psfCc is the compression index, dimensionlessH is the height of the tailings column, feeteo is the initial void ratio of the tailings, dimensionless

The computer program Fast Lagrangian Analysis of Continua (FLAC) version 5.0 (Itasca 2005)was used to perform the primary consolidation settlement analysis for the Cotter primaryimpoundment. FLAC is a two-dimensional, explicit, finite difference computer program thatwas developed for geotechnical applications. FLAC analyses were performed using theadvanced constitutive model Modified Cam Clay, which is applicable for modeling the responseof soft soils such as normally consolidated mine tailings. Upper and lower bound settlementresponse model parameters were selected to match the reported consolidation test data. Usingcalibrated material properties allow for a close match between the calculated material behaviorand the actual laboratory-measured soil response. Using the calibrated material properties, it waspossible to calculate the settlement response and void ratio change due to the added cover loadand removal of the buoyant pressure via dewatering. Each loading or water removal sequencehad zero pore water pressure (100 percent consolidation) representing the end of primaryconsolidation. FLAC settlement analyses were also checked by hand for some of the sections.

2.2.2 Secondary SettlementSecondary settlement consists of secondary compression and creep, which are associated withthe compression and distortion at constant water content of compressible soils such as clays,silts, organic materials, and peat. They are time-dependent deformations that occur at essentiallyconstant effective stress, with negligible change in pore water pressure. Secondary compressionand creep may be a dispersion process in the soil structure causing particle movement and maybe associated with electrochemical reactions and flocculation. Although creep is caused by thesame mechanism as secondary compression, they differ in the geometry of confinement. Creep



is associated with deformation without volume and pore water changes in soil subject to shear,whereas secondary compression is associated with volume reduction without significant waterpressure changes.Secondary settlement is calculated by Equation 4.

1

2log1 t

te

HCso

⋅+

⋅= α (4)

Where,s is the secondary settlement, feett1 is the time of primary consolidation, yearst2 is the design life of the facility (1,000 years, in this case), yearsCα is the secondary compression index, dimensionlessH is the height of the tailings column, feeteo is the initial void ratio of the tailings, dimensionless

Time of primary consolidation in Equation 4 can be calculated with Equation 5:

vCHTt

2

1⋅

= (5)

Where,t1 is the time of primary consolidation, yearsT is the time factor, which is a function of consolidation, dimensionlessH is the layer height, feetCv is the coefficient of consolidation, cm2/sec

We calculated the time for primary consolidation t1 corresponding to 90-percent consolidationfor multiple tailings column heights with single drainage. For a 20-foot tailings column, 1 yearis a reasonable amount of time to reach 90 percent of the primary consolidation under the weightof the cover material. Similar consolidation time estimates were provided in the MFG (2005)report (0.27 to 10.6 years). For secondary settlement calculations, we assumed t1=1 year for alltailings columns, regardless of tailing type and column height. Secondary settlement valueswere calculated for each tailings column developed in the primary settlement calculations.

2.3 MATERIAL PROPERTIESA field investigation was performed by Shepherd Miller, Inc. (1999) to characterize the primaryimpoundment tailings. The subliner clay was characterized by W.A.Wahler & Associates (1978)during design and construction.

Material properties calculated from field and laboratory tests (Shepherd Miller, Inc. 1999 andW.A.Wahler & Associates 1978), values from literature (Bardet 1997, Keshian and Rager 1988),and values selected for Phase I analysis are summarized in Table 1 and discussed below.



2.3.1 Cover MaterialTotal Unit Weight: Based on the designs provided in the 2005 MFG report, the cover will be a 6-foot thick compacted, layered medium consisting of clayey soils. We estimated the average totalunit weight of the cover materials to be approximately 130 pcf. The corresponding stressincrease for the tailings under the cover weight will be 780 pounds per square foot (psf) (130pcf×6 = 780 psf).Plasticity Index: The PI of the proposed cover soil is not available. A value of zero wasconservatively assumed for horizontal tensile strain calculations of the cover.

2.3.2 Clay SublinerPermeability: Permeability tests performed for W.A.Wahler & Associates (1978) on sublinerclay at different void ratios provided a permeability range of 2×10-7 to 2×10-8 centimeters persecond (cm/s). Adverse chemical reaction or deterioration of subliner clay due to leakage is notexpected based on compatibility test results in the W.A.Wahler & Associates report (1978). Weused 1×10-8 and 1×10-7 cm/s for the vertical permeability of the clay subliner in the leaky linersensitivity analysis presented in Section 4.

2.3.3 TailingsTwenty-four uranium tailings piles from different processing operations across the country wereinvestigated by Keshian and Rager (1988) for the DOE’s UMTRA program. They concludedthat although tailings underwent different grinding processes and came from different regions,the material properties fit into a relatively narrow band. The results of this study were used,along with site-specific data and URS experience with other tailing facilities. The figures inAttachment A provide a comparison of our selected values with the UMTRA data. The Cotterprimary impoundment tailings are divided into coarser-grained, less compressible, and morepervious sand/slime for the lower bound settlement estimates and finer-grained, morecompressible, less pervious slime tailings for the upper bound settlement estimates.

Specific Gravity: Specific gravity values for the primary impoundment tailings from the 2005MFG report are presented on Figure A-1 in Attachment A. Values are on the higher end of therange that is expected for natural soils and observed for the UMTRA samples. However, highervalues are possible for some pure minerals. Based on the average value of 2.93; a specificgravity of 2.9 was selected for the Cotter tailings.Gradation: The primary impoundment tailings tested in the Shepherd Miller report consisted oftailings from the Schwartzwalder and West Slope ore bodies. The tailings were located along thewestern and northern perimeter of the impoundment and had fines content (percent passing theNo. 200 sieve) ranging from 12 to 99 percent. Uranium tailings were classified based on grainsize according to the Uranium Mill Tailings Remedial Action (UMTRA) program (Keshian andRager, 1988) as defined below:

• Sand: contains less than 30 percent passing the No. 200 sieve

• Sand/slime: contains between 30 percent and 70 percent passing the No. 200 sieve

• Slime: contains greater than 70 percent passing the No. 200 sieve



Water Content and Unit Weight: The 2005 MFG report provided the water content and unitweight values for the tailings. Dry unit weights ranged from 66.9 pcf to 101.0 pcf with anaverage of 80.0 pcf. Water contents ranged from 28.6 to 64.5 percent with an average of 48percent. Total unit weights ranged from 109 pcf to 130 pcf with an average of 117 pcf. Basedon in situ properties from the consolidation tests in the MFG report, we used water contents of 40and 60 percent and total unit weights of 115 and 110 pcf for the sand/slime and slime tailings,respectively. This results in dry unit weights of approximately 82 and 69 pcf for the sand/slimeand slime tailings, respectively.

Void Ratio: Void ratios of the primary impoundment tailings from the 2005 MFG report arepresented on Figure A-2 in Attachment A. The void ratio values were calculated fromconsolidation and specific gravity test results. Based on the consolidation test data, a void ratioof 1.2 was used for sand/slime, and a void ratio of 1.8 was used for slime tailings. As shown onFigure A-2, these values are within the range reported for UMTRA samples.Compression Index, Secondary Compression Index, Coefficient of Consolidation: Consolidationtest results from the 2005 MFG report are presented on Figure A-3 and A-4 in Attachment A asstress-strain and stress-void ratio graphs. We calculated compression index values in the rangeof 0.15 to 0.71, compared to 0.12 to 0.67 calculated by Advanced Terra Testing, Inc. in the MFGreport. The K-C14615 and A-C14682 sample test results were used as the less compressible andmore compressible tailings, respectively. Values of Cc = 0.12 for sand/slime and Cc=0.33 forslime tailings were selected. These values are in the midrange of data from the UMTRA samples(Figure A-5 in Attachment A).Generally, the secondary consolidation coefficient for soils range from about 2.5 percent to 10percent of the primary compression index (Bardet 1997). The expected averages for silt and clayare approximately 5 percent based on EM 1110-1-1904. This average value was used tocalculate the secondary compression index: Cα = 0.05×Cc. Thus, secondary compression indexvalues were calculated to be 0.12×0.05 = 0.006 and 0.33×0.05 = 0.017 for sand/slime and slimetailings, respectively. These calculated values were adjusted based on the values presented byKeshian and Rager (1988). For sand/slime tailings, 0.004 was selected based on the averagevalue for slime tailings from the UMTRA samples. For slime tailings, 0.02 was selected basedon the values presented for UMTRA samples (Figure A-6 in Attachment A).

The average coefficient of consolidation from the 2005 MFG report ranged from 0.005 to 0.019,for the expected stress range (400 to 3,200 psf). These values were adjusted based on the valuespresented by Keshian and Rager (1988). For sand/slime and slime tailings respectively, 0.02 and0.005 cm2/s were selected. These values are in the midrange of data from the UMTRA samples(Figure A-7 in Attachment A).Horizontal and Vertical Hydraulic Conductivity: Horizontal and vertical hydraulic conductivityvalues for UMTRA (Keshian and Rager 1988) and Elliot Lake (Matyas, Welch, and Reades1984) tailings were found to vary over several orders of magnitude. Hydraulic conductivity is afunction of void ratio, dry unit weight, and grain size of the uranium tailings. Horizontal andvertical hydraulic conductivity values were calculated from the cone penetration test (CPT) andconsolidation test results presented in the 2005 MFG report, respectively. The anisotropy ratiofor hydraulic conductivity (kh/kv) was approximately 5. Based on ranges presented in the 2005MFG report, 5×10-5 and 1×10-6 cm/s were selected for horizontal hydraulic conductivities of



sand/slime and slime tailings, respectively. The selected values are also within the UMTRAsamples range of values (Figure A-8 in Attachment A).

Capillary Moisture Retention: Capillary moisture retention was measured for the Cotter tailingsand repoted in the 2005 MFG report (Figure A-9 in Attachment A). One representative curve(C14517R,A,8-0-10.0’) was conservatively selected to develop a volumetric water contentfunction and partially saturated conductivity function based on the van Genuchten functionestimation method in the Seep/W computer program (Krahn, 2004).Strength: No strength tests were performed on the samples. However, based on the CPT dataand the strength values presented for UMTRA samples, effective friction angles for the materialsare expected to be on the order of 28 degrees, 32 degrees, and 35 degrees for slime, sand/slime,and sand tailings, respectively. (Note that strength parameters were not required for thecalculations in this Phase I study)

2.4 UPPER AND LOWER BOUND SETTLEMENT ESTIMATESForty-eight FLAC analysis runs were performed to calculate the primary consolidation responseof four sand/slime and slime tailings columns (10, 20, 30, and 40-foot column height) under sixloading conditions. Secondary settlement values for a 1,000-year design life were analyticallycalculated for the four sand/slime and slime tailings columns. Settlement of the 25-foot columnwas also estimated based on interpolation of the 20-foot column and 30-foot column settlements.Total settlement is the sum of the primary and secondary settlements for each column. Totalsettlement values are provided on Figures 3 and 4 for upper and lower bound predictions.Results are also summarized in Table 2.

Figures 5, 6, 7, and 8 show total calculated settlements for the tailings columns analyzed atSection A. Figure 5 and 6 show calculated total settlements for slime and sand/slime tailings,respectively if the cover is placed with the current phreatic surface level and then dewatered.Based on Shepherd Miller, Inc. (1999) tailings sampling in the primary impoundment, thetailings located along the western and northern perimeters of the impoundment were mostlysand/slime. Tailings from the interior of the impoundment are expected to have more finescontent (slime tailings) due to the depositional nature of the material. Figure 7 combines theslime settlement estimates from Figure 5 and Figure 6 in order to model slime tailings locatedadjacent to sand/slime tailings at Section A. This results in a relatively high differentialsettlement condition between the 40-foot sand/slime column and the 30-foot slime column.Figure 8 shows calculated settlements for a case with the piezometric level in the tailingslowered approximately 5 feet before the cover is placed.

2.5 DIFFERENTIAL SETTLEMENTS AND COVER STRAINFigures 5 and 6 show calculated settlements if the tailings in Section A were entirely sand/slimeor slime, respectively. These figures have differential settlements and horizontal tensile strainsless than the maximum allowable criterion provided in Section 1.4. However, it is likely that thetailings deposit will be non uniform as depicted on Figure 7. As shown, for this case, thedifferential settlement between the 40-foot sand/slime column and the 30-foot slime column is1.6 feet. For 45 feet of separation, horizontal tensile strains in the cover soil were calculated to



be 0.063 percent (Table 3), which is slightly higher than the maximum allowable maximumallowable criterion provided in Section 1.4.

Partial dewatering before cover liner placement can reduce the differential settlements. Asshown on Figure 8 and in Table 3, 4 feet of dewatering reduces the calculated differentialsettlement to 1.1 feet and reduce the horizontal tensile strain to 0.030 percent. This is less thanthe maximum allowable horizontal tensile strain requirement (0.05 percent). Lowering thephreatic surface level by 4 feet prior to cover placement would also result in a drier workingsurface. This would be beneficial during cover placement and could provide a higher strengthcrust with the necessary bearing capacity to support the construction equipment during coverplacement (Wels, Robertson, and Jakubick 1999).

SECTIONTHREE DRAFTConceptual Dewatering Alternatives and Time Estimates


3. Secti on 3 THREE Conc eptual Dew ater i ng A lternati ves and Time Est imate s

3.1 GENERALFour conceptual dewatering alternatives were developed and analyzed to evaluate the targetphreatic surface level identified in Section 2.5, i.e., 4 feet of dewatering required to reduce thehorizontal tensile strain in the cover to less than 0.05 percent. However, 5 feet of dewateringwas taken as the targeted drawdown for the seepage calculations in order to account forfluctuations in the pond water level. The alternatives include the existing dewatering drainsystem, the existing dewatering drain system with wick drains, dewatering wells, and acombination of the existing dewatering drain system and dewatering wells. Each alternative wasmodeled for slime tailings and for sand/slime tailings to capture the likely variation in tailingsconductivity. Table 4 presents dewatering time estimates for each conceptual dewateringalternative that is discussed below.

Two-dimensional transient seepage analyses using the Seep/W (GeoStudio 2004, Version 6.19)computer program were performed for each alternative to estimate the time required to achievethe target phreatic surface. Seep/W is a two-dimensional, finite element analysis softwaredeveloped by Geo-Slope International. Seep/W can numerically model a real physical problem,such as dewatering, under transient conditions. The program can be used to model drainage of asaturated medium or a partially saturated medium that has a volumetric water content and apartially saturated hydraulic conductivity.

3.2 ALTERNATIVE 1 – EXISTING DEWATERING DRAIN SYSTEMThe first alternative we evaluated was use of the existing dewatering drain system on top of theliner protective cover to dewater the tailings. The dewatering drain system is described inSection 1.3.3. The flow capacity of the sand, gravel, and pipes in the drain system wascalculated based on the geometry and slopes shown on Figures 1 and 2 and on permeabilitycorrelated from the grain size of the materials. The following flow capacities were estimated.

• Finger drains: 138 cubic feet (ft3)/day or 0.718 gallons per minute (gpm)

• Collection pipe drains: 24,500 ft3/day or 127 gpmA two-dimensional Seep/W model was developed for slime and sand/slime tailings to simulatedrainage into the existing dewatering drain system for a 25-foot thick saturated tailings section.For the initial run, nodes at the drain were given a zero pressure boundary condition, indicating afreely draining system with no flow restrictions. The amount of flow at the drain exceeded thefinger drain capacity for sand/slime tailings; therefore, flow quantity was specified for the nodesso that drain capacity would not be exceeded. Based on a transient seepage analysis for 25 feetof slime and 25 feet of slime/sand tailings, 5 feet of dewatering was calculated to takeapproximately 50 years and 1.8 years, respectively (Figure 9). Dewatering times will also beinfluenced by the actual effectiveness of the drain system, i.e. whether it is or becomes cloggedand the site specific permeability of the tailings which were considered to be homogenous in themodel.



3.3 ALTERNATIVE 2 – EXISTING DEWATERING DRAIN SYSTEM WITH WICKDRAINS

We evaluated the installation of wick drains to supplement the existing dewatering system Wickdrains generally shorten the drainage path thereby accelerating the drainage time (DOT, 1992).Wick drains can improve the vertical hydraulic conductivity of the tailings. A hydraulicconductivity anisotropy ratio of 5 (kh/kv=5) was used for the primary impoundment tailings asdiscussed in Section 2. We have assumed that with the installation of wick drains the verticalhydraulic conductivity values will rise as high as the horizontal hydraulic conductivities,effectively creating equivalent isotropy for the tailings. A 5-foot thick undisturbed zone oftailings was left at the bottom of the tailings during wick drain installation so the Hypalon linerwould not be punctured and it was assumed no wick drains will be installed in areas where thetailing thickness is less than 5 feet.

A two-dimensional model was developed to simulate drainage into the existing dewatering drainsystem. Initially, the 25-foot thick tailings section was saturated. At t = 0, nodes at the drainwere given a zero pressure boundary condition, indicating an unrestricted and freely drainingsystem. The amount of flow was checked with flux sections. The flow was above the fingerdrain capacity for sand/slime tailings; therefore, flow quantity was specified for the nodes so thatdrain capacity would not be exceeded. Wick spacing was parametrically modeled in Seep/W. Awick drain spacing of 5 feet resulted in a 10-percent increase in vertical drainage and equivalentisotropic conditions. Based on the transient seepage analysis of 25 feet of saturated slime andslime/sand tailings, 5 feet of dewatering will take approximately 45 years and 1.6 years,respectively (Figure 10). The general layout of Alternative 2 and typical wick drain details areprovided on Figure B-1 in Attachment B.Wick drains with surcharging have been used effectively in uranium tailings for consolidatingtailings and reducing structure induced settlements (Miller and Range, 1989; Matyas et al.,1984). Wick drains and surcharging typically consists of a drainage layer at the top and bottomof the loaded soft soil. This allows excess pore water pressure to reach the wick drains and draineffectively to the top or bottom drainage layer. Wick drains with surcharging typically involvesinstalling wick drains within the tailings and placing a surcharge loaded above the tailings. Theloading generates pore pressures that are relieved by the wick drains that are in contact withdrainage layers at the top and bottom of the loaded tailings.Although surcharging is an effective tool to promote consolidation and settlement of the tailings,wick drains with surcharging is not a feasible option for lowering the piezometric level in thecurrent primary impoundment for the following reasons.

1) The underlying Hypalon liner may be punctured during wick-drain installation. Asafe, undisturbed zone should be left between the Hypalon liner and the bottom of thewick drains. This undisturbed zone can impede the flow towards the drain system.2) The liner cover protective material is described as clayey sand in the constructionreport (W.A.Wahler & Associates, 1980) and is not expected to provide an effectivebottom drainage layer.

3) The voids in the tailings eventually decrease because of the surcharge inducedsettlement of the tailings. This causes the tailings to become less permeable thus slowingthe dewatering process.



3.4 ALTERNATIVE 3 – DEWATERING WELLSDewatering wells were evaluated as another alternative dewatering method. Wells can beeffective in dewatering layered tailings because they penetrate the individual layers. A two-dimensional axisymmetric Seep/W model was developed to simulate three-dimensional drainageinto a well. The axisymmetric model rotates a two-dimensional model around a vertical axis ofsymmetry. Wells were modeled with a zero pressure boundary condition at the axis ofsymmetry. A no-flow boundary condition was defined at half the distance between the wells.Drainage into the well from the initial phreatic surface level was simulated with transientanalysis for sand/slime and slime tailings. A 5-foot thick undisturbed zone of tailings was left atthe bottom of tailings during dewatering well installation to protect the Hypalon liner from beingpunctured and it was assumed no dewatering wells will be installed in areas where the tailingthickness is less than 5 feet.Based on the transient seepage analysis, 25 feet of saturated slime tailings was calculated to drain5 feet in 350 years with 180-foot well spacings and drain 5 feet in 80 years with 90-foot wellspacings. The 25-foot saturated sand/slime tailings was calculated to drain 5 feet in 7 years and5 feet in 1.6 years, with 180-foot and 90-foot well spacings, respectively. Results of the transientanalysis are provided on Figure 11. The general layout of Alternative 3 for sand/slime tailings,with 180-foot well spacing and typical dewatering well details, is provided on Figure B-2 inAttachment B. In areas of slime tailings the number of wells would need to be increased.

3.5 ALTERNATIVE 4 – EXISTING DEWATERING DRAIN SYSTEM WITHDEWATERING WELLS

Combining Alternative 1 with Alternative 3 can provide an efficient dewatering alternative.Numerical analysis results from alternatives 1 and 3 were combined analytically to calculate thedewatering times as described below and summarized in Table 4.The approximate time for dewatering 25 feet of saturated slime tailings by 5 feet with 180-footdewatering well spacing combined with the existing drain system is approximately(1/50+1/350)-1=44 years. The approximate time for dewatering with the well spacing reduced to90 feet is (1/80+1/50)-1=31 years.The approximate time for dewatering 25 feet of saturated sand/slime tailings by 5 feet with 180-foot dewatering well spacing combined with the existing drain system is approximately(1/1.8+1/7)-1=1.4 years. The approximate time for dewatering with the well spacing reduced to90 feet is (1/1.8+1/1.6)-1=0.8 years.The well spacing and layout pattern were not optimized in this analysis. The actual layout of thewells and dewatering times will depend on additional analyses which are beyond the scope ofthis study.

SECTIONFOUR DRAFT Sensitivity Analyses for Liner Leakage


4. Secti on 4 FOUR Sensi tivi ty A naly ses for Liner Le aka ge

There may be concerns that the integrity of the existing liner may have been punctured and/ortorn since it was installed, resulting in some leakage into the underlying soils. The existingsystem consists of up to 40 feet of tailings, 1 foot of protective soil cover, a 60-mil Hypalonliner, and 1.5 feet of clay subliner. Sensitivity analyses were performed to evaluate seepagethrough a potentially leaky liner by varying the hydraulic conductivity of the clay subliner.Select SEEP/W files, developed in Section 3, were re-run with steady state analysis by removingthe no-flow boundary from the bottom of the model, applying a constant head at the surface, andvarying the permeability of the clay subliner. The seepage analyses results were assigned tovarying percentages of the impoundment bottom area to account for varying degrees ofcompromised Hypalon liner integrity.

Results of the SEEP/W steady state analyses are provided on Figures 12 and 13 for the slime andsand/slime tailings, respectively. These seepage analyses results were hypothetically applied to1%, 5%, and 10% of the approximately 55 acre bottom area of the impoundment to quantify theleakage that would occur if varying portions of the Hypalon liner were torn or punctured. Table5 presents the results of the sensitivity analysis. The hypothetical leakage ranges from 3.7×10-2

gpm beneath slime tailings with 1% of the liner area affected and a clay subliner permeability of1×10-8 cm/s to 5.56 gpm for sand/slime tailings with 10% of the liner area affected and a claysubliner permeability of 1×10-7 cm/s. Note that even with 10% of the liner torn or punctured, thecalculated leakage is less than 6 gpm.

SECTIONFIVE DRAFTComparative Evaluation of Conceptual Alternatives


5. Secti on 5 FIVE Comparat ive Ev al uati on o f Conce pt ual Alter nati ves

5.1 RATING CRITERIAA comparative evaluation study of the conceptual alternatives was performed after theconceptual alternatives were developed and analyzed. The format of this study was very similarto the Cotter comparative study on depositional methods that was presented to CDPHE inDecember 2007. The dewatering alternatives were evaluated using the decision matrix presentedin Table 6. Ranking criteria for the various alternatives include: cost, safety of operations,complexity of operations, reliability/redundancy, drainage rate, As Low As ReasonablyAchievable (ALARA), etc. Each alternative was given a weight and rating. The weight given toeach criteria is a relative number based on judgment and experience. Ratings were based onquantitative and qualitative comparisons. The relative score is calculated by dividing the initialscore by the score of the highest scoring alternative. A discussion of the rating criteria ispresented below.Safety of Installation: Safety of installation is a qualitative measure of safety during constructionwhich is related to the total number of personnel required during installation of the alternative,presence of rotating machinery and other physical hazards, the duration of exposure to thehazard, and the likelihood, and duration of chemical exposure. Risk of puncturing the Hypalonliner during construction is also included in safety of installation.

Safety of Operation: Safety of operation represents the safety of operators and maintenancepersonnel required for the dewatering operation. The presence of splashes and spills of solution,and other physical hazards, the duration of exposure to the hazard, the likelihood and duration ofchemical exposure and ease and frequency of maintenance activities were all part of theevaluation.ALARA: The ALARA guiding principle of minimizing any dose that does not have ananticipated benefit was used in evaluating the alternatives. U.S. Nuclear RegulatoryCommission guidance on the ALARA principle was utilized in assigning this relative rating,including number of personnel exposed and quantity of exposure.Operational and Capital Cost: Preliminary construction and operating cost estimates for eachconceptual dewatering alternative were developed. The costs were then given a relative numericrating based on the range of the highest and lowest costs.

Complexity of Operations: Potential operational issues will require monitoring and maintenanceof the dewatering system. Disconnected pipes, broken pumps, loose wires in the electricalsystem present some of the possible complexities associated with operating the dewateringsystems. A relative numeric rating was assigned to each alternative based on engineeringjudgment for similar systems.Reliability/Redundancy: Drainage systems may clog and lose efficiency. It may not be possibleto flush and clean the drainage system for all the alternatives. A relative numeric rating wasassigned to each alternative based on engineering judgment with similar systems.

Drainage Rate: A relative numeric rating was assigned to each alternative based on the seepageanalyses provided in Section 4.



5.2 CONCEPTUAL ALTERNATIVESA brief discussion of the pros and cons of each conceptual alternative and basis for rating eachcriterion is presented below.

Alternative 1 – Existing Dewatering Drain System:• Safety of Installation: Safest alternative, only pump and collection pipe system

installation. Existing drain system has already been installed.• Safety of Operation: Safest alternative, with activities at the sump area and collection

piping areas only.• ALARA: Radon gas can be contained within the layered tailings and under the cap,

resulting in minimum exposure. Limited installation activity will also result in minimumexposure.

• Operational Cost: Operating costs are limited to operation of a single pump system.• Capital Cost: Only pump and collection pipe system installation cost.• Complexity of Operations: Simplest alternative.• Reliability/Redundancy: Drainage system may clog and lose efficiency, no repair method

or backup system available.• Drainage Rate: Based on a transient seepage analysis, 5 feet of dewatering will take

approximately 50 years and 2 years for 25 feet of saturated slime and 25 feet ofslime/sand tailings, respectively, as discussed in Section 3.

Overall, the existing dewatering drain system is ranked as the most favorable dewateringalternative based on the decision matrix in Table 6.

Alternative 2 – Existing Dewatering Drain System with Wick Drains:• Safety of Installation: Wick drains must be terminated within tailings 5 feet above the

Hypalon liner in order to avoid puncturing the Hypalon liner. There is significant risk ifinstallation equipment over penetrates and punctures the liner. Safety concerns potentialhazards during installation of wicks, pump, and collection pipe system.

• Safety of Operation: Operation requires minimum maintenance and operations personnel.There will be work around the sump area and collection piping areas only.

• ALARA: Wick drains that intercept the layered tailings may create potential flow pathsto enhance the release of radon gas during installation.

• Operational Cost: Operation costs would be similar to that of Alternative 1, with a singlepump operations cost.

• Capital Cost: Alternative 2 would have higher installation costs compared to those of theother alternatives. Capital costs are estimated to be on the order of 6 million dollarsincluding installation of wicks at 5 feet spacing, pumps and collection piping.

• Complexity of Operations: Simple operations similar to that of Alternative 1.• Reliability/Redundancy: Performance of the wick drains will entirely depend on the

operation of the existing drain system that has a chance of clogging and losing efficiencyas in Alternative 1.

• Drainage Rate: The effectiveness of wick drains is reduced because the wick drains mustbe terminated within tailings 5 feet above the Hypalon liner in order to reduce the risk ofpuncturing the liner. Based on the transient seepage analysis, 5 feet of dewatering will



take approximately 45 years and 2 years for 25 feet of saturated slime and slime/sandtailings, respectively, as discussed in Section 3.

Overall, the existing dewatering drain system with wick drains is ranked as the least favorabledewatering alternative based on the decision matrix in Table 6.

Alternative 3 – Dewatering Wells:• Safety of Installation: Wells must be terminated within tailings 5 feet above the Hypalon

liner to avoid puncturing the Hypalon liner. There is a risk associated with over-drillingand puncturing the liner. Safety concerns are related to potential hazards duringinstallation of the wells, well pumps, and collection pipe system.

• Safety of Operation: Will require operation, monitoring, inspection, and maintenance ofeach well, pump, and collection system.

• ALARA: Potential exposure during installations and inspections. The wells will interceptthe layered tailings and may create potential flow paths to enhance the release of radongas.

• Operational Cost: Operation costs of Alternative 3, with multiple low capacity pumps, areexpected to be slightly more than that of Alternative 1.

• Capital Cost: Capital costs associated with installation of alternative 3 are expected to beon the order of 1 million to 2 million dollars for 180-foot and 90-foot well spacings,respectively.

• Complexity of Operations: The multiple well pumps and collection piping system willrequire significant maintenance, operations and monitoring. Disconnected pipes, brokenpumps, electrical system failures present some of the complexity of operations.

• Reliability/Redundancy: Dewatering wells may effectively dewater the layered tailingsbecause they penetrate individual layers and the water is removed by pumping. Wellscreens can clog and lose efficiency, but it is possible to flush and clean them.

• Drainage Rate: Dewatering time will be a function of well spacing, however, it isexpected to be longer than that of Alternative 1 as presented in Table 4.

Overall, dewatering wells are ranked more favorable than Alternative 2 and less favorable thanAlternative 1.

Alternative 4 – Existing Dewatering Drain System with Dewatering Wells:• Safety of Installation: Similar to Alternative 3, there is a risk of puncturing the Hypalon

liner during well installation. Safety concerns are related to potential hazards duringinstallation of wells, well pumps, collection pipe system, and the sump pump.

• Safety of Operation: Will require operation, monitoring, inspection, and maintenancesimilar to that of Alternative 3.

• ALARA: Potential exposure during installations and inspections. The wells will interceptthe layered tailings and may create potential flow paths to enhance the release of radongas.

• Operational Cost: Operation costs of Alternative 4 combines that of Alternative 3, withmultiple low capacity pumps, and that of Alternative 1 with a pump at the sump.

• Capital Cost: Alternative 4 will have slightly higher capital costs than that of Alternative3 to account for a pump and piping associated with the existing dewatering drain system.



• Complexity of Operations: The complexity of operation of the pumps and piping systemwill be similar to that of Alternative 3.

• Reliability/Redundancy: Dewatering wells may effectively dewater the layered tailingsbecause they penetrate individual layers and the water is removed by pumping. Wellscreens can clog and lose efficiency, but it is possible to flush and clean them. Thesystem will continue to operate if the wells fail and the existing dewatering drain systemoperates or vice versa.

• Drainage Rate: Dewatering time will be a function of well spacing, however, it isexpected to be shorter than that of Alternative 1 as presented in Table 4.

Overall, the existing dewatering drain system with dewatering wells is an effective alternative, asit also provides the shortest dewatering time.

5.3 DISCUSSIONThe relative scores for the conceptual alternatives are provided in Table 6. The narrow spread inthe relative scores does not indicate a clear favorite because the margin of error could be largerthan the difference in relative scores. The margin of error was influenced by factors such as theweights that were assigned to each criterion based on judgment and experience, and the ratingsthat were based on quantitative and qualitative comparisons. Therefore, the relative scores couldbe significantly affected if weights or ratings are changed. For example, the capital costs andoperations costs were assigned a weight of 1 in the decisions matrix. If costs were consideredmore important and were given a weight of 2, the order of the relative scores would change. Inorder to make a more educated decision on the preferred alternative, further analysis and costestimates need to be done to refine the ratings..

SECTIONSIX DRAFT Conclusions


6. Secti on 6 SIX Conc lus ions

Cotter requested URS prepare a conceptual-level alternative study for dewatering the existingtailings within the primary impoundment. The Phase I study presented in this report focuses onthe geometry of the existing primary tailings impoundment with the existing dewatering drainsystem and liner. A 6-foot thick, multi-layered soil cover will be placed over the existing tailingsif the existing tailings impoundment is closed.Total settlement of the Cotter Primary Tailings Impoundment results from primary consolidationof the tailings under the weight of the cover material and loads induced by dewatering, plussecondary settlement due to secondary compression and creep. The tailings properties affect thetailings dewatering and drainage period. It was calculated that the phreatic surface level in thetailings should be lowered at least 4 feet below the existing level prior to placing the covermaterial. This is expected to limit total differential settlement of the tailings to a magnitude thatresults in tolerable horizontal tensile strain in the cover material.

Four conceptual alternatives were evaluated for lowering the phreatic surface 5 feet below thecover material prior to cover placement. The results show that use of the existing dewateringsystem may be the most favorable alternative.The sensitivity analyses for liner leakage showed the underlying clay in the subliner is effectivein reducing leakage through the Hypalon liner if the liner is damaged.

SECTIONSEVEN DRAFT General Information


7. Secti on 7 SEVEN Genera l In formati on

The conceptual alternatives and recommendations presented in this report are based on URS’scurrent knowledge of the site, subsurface conditions identified in the limited previous field andlaboratory investigations discussed in Section 2, and previous experience with similar facilities.No additional field and laboratory investigations have been performed for this study in support ofthe design parameters selection. These recommendations are subject to revision based uponfurther field and laboratory tests during detailed design engineering and field monitoring andinspection during dewatering and construction of the cover liner. For example, according toMFG (2005), tailings were sampled where access was possible. Most likely, this resulted insampling a proportionately larger percentage of relatively coarse tailings. Additional studiesmay indicate, tailings in the primary impoundment are finer and more compressible than thesampled tailings which could effect differential settlements and drainage times presented in thisreport.

The opinions and conclusions expressed are limited by the information available and the inherentlimitations of subsurface investigations used to evaluate the tailings facility. URS does notguarantee the performance of the project in any respect; only that our engineering work andjudgments rendered meet the standard of care of the profession. URS represents that our servicesare performed within the limits prescribed by the client, in a manner consistent with the level ofcare and skill ordinarily exercised by other professional consultants under similar circumstances.No other representation to the client, expressed or implied, and no warranty or guarantee areincluded or intended.

SECTIONEIGHT DRAFT References


8. Secti on 8 EIGHT Refere nces

Bardet, J-P. 1997. Experimental Soil Mechanics. Prentice Hall.Cadwell, J. and C. Reit. 1993. Principles and Practice of Waste Encapsulation. Lexis

Publishers.Claire, R.F., J.C. Kuo, and D.R. Wanket. 1994. “Evaluation of the Cover Cracking Potential

Due to Ground Subsidence at UMTRA Project Disposal Cells.” Available:http://www.osti.gov/bridge/servlets/purl/10134903-vRS1LC/native/10134903.pdf.

Gates, T.E. 1982. “Consolidation Theory and Its Applicability to the Dewatering and Coveringof Uranium Mill Tailings.” NUREG/CR-2894 PNL-4402, NTIS.

Itasca Consulting Group. 2004. FLAC Version 5.0 – Fast Lagrangian Analysis of Continua,User’s Guide.

Keshian B. and R.E. Rager. 1988. “Geotechnical Properties of Hydraulically Placed UraniumTailings.” Hydraulic Fill Structures, ASCE Geotechnical Special Publication No. 21, pp.227-254.

Krahn, J. 2004. Seepage Modeling with SEEP/W, GeoSlope International.

Larson, N.B. and B. Keshian. 1988. “Prediction of Strains in Earthen Covers.” Hydraulic FillStructures, ASCE Geotechnical Special Publication No. 21, pp. 367-388.

MFG Consulting Scientists and Engineers (MFG). 2005. 2005 Update of the MillDecommissioning and Tailings Reclamation Plan for the Cotter Corporation Canon CityMilling Facility.

Matyas, E.L,, D.E. Welch, and D.W. Reades. 1984. “Geotechnical Parameters and Behaviour ofUranium Tailings.” Canadian Geotechnical Journal, No. 21, pp. 489-504.

Miller, L. and Range, D., 1989, “The Use of Vertical Band Drains (Wicks) to AccelerateConsolidation of Uranium Tailings”, Geospec, Geotechnical News, Sept. 1989, pp. 27-31.

Shepherd Miller, Inc. 1999. “Canon City Mill Tailings Management Alternative Evaluation.”Prepared for Cotter Corporation, December 22, 1999.

Shepherd Miller, Inc. 2000. “Tailings Investigation Report for the Canon City Milling Facility,Canon City, Colorado.” Prepared for Cotter Corporation in draft form, December 22,1999; completed for Cotter Corporation November 30, 2000.

U.S. Army Corp of Engineers (USACE). 1990. Settlement Analysis, EM 1110-1-1904.U.S. Department of Energy (DOE). 2006. Crescent Junction Disposal Site, Disposal Cell

Design, Settlement, Cracking, and Liquefaction Analysis, Moab UMTRA Project, Calc.No. MOA-02-0502006-03-16-00.

U.S. Department of Transportation, (DOT) Federal Highway Administration, 1992,“Prefabricated Vertical Drains”, Pile Buck, Inc.

Vick, S.G. 1990. Planning, Design, and Analysis of Tailings Dams. Bi Tech Publishers Ltd.,Vancouver, BC, Canada.

W.A. Wahler & Associates. 1978. Site and Laboratory Investigation and Definitive DesignReport. Cotter Corporation Uranium-Varadium Tailings Impoundment.

http://www.osti.gov/bridge/servlets/purl/10134903-vRS1LC/native/10134903.pdf.

SECTIONEIGHT DRAFT References


W.A. Wahler & Associates. 1980. First Stage Construction Report. Cotter CorporationUranium-Varadium Tailings Impoundment.

Wels, C., A.M. Robertson, and A.T. Jakubick. 1999. “Cover Placement on Extremely Weak,Compressible Tailings.” Soft Tailings Stabilization Workshop. Edmonton, Alberta.Available: http://www.robertsongeoconsultants.com/papers/wismut99.pdf. May.

Wels, C., U. Barnekow, M. Haase, M. Exner, and A.T. Jakubick. 2000. “A Case Study on Self-Weight Consolidation of Uranium Tailings.” Available: http://www.robertsongeoconsultants.com/papers/uranium2000.pdf.

http://www.robertsongeoconsultants.com/papers/wismut99.pdf.

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