Advantages of Dry Stack TSF Over Conventional Slurry Tailings§ Tailings are placed under unsaturated conditions resulting in minimal seepage§ Dry Stack TSF not susceptible to breaching because there is no reclaim pond§ Significant water conservation minimizes water usage and consumption requirements§ Facilitates concurrent reclamation and revegetation during operation§ Minimizes disturbance area§ Minimizes visual impact from surrounding areas
Dry Stack TSF Design Criterian Production rate = 75,000 tpd (tons per day) or 27 MT per yearn Storage capacity estimated at 596 MT and mine life estimated at approximately 21 yearsn Average tailings in-place dry density = 109 pcf (pounds per cubic foot)n Compliance with all applicable regulations including the Arizona Best Available Demonstrated Control Technology (BADCT) standardsn Rockfill Buttresses are placed around the perimeter of the facility in 50-foot high lifts with 3H:1V side slopes and 25 foot benchesn 3.5H:1V overall side slopen Dry Stack TSF will be constructed in two phases (Phases I and II)n Implement dust control suppression measures throughout the production periodn Concurrent reclamation and revegetation during operations
Phase I Dry Stack TSF Characteristicsn Contains Approximately 12 years of productionn Maximum Buttress elevation = 5250 feetn Maximum Tailings surface elevation = 5237.5 feetn Total capacity = 343 million tons (MT)n Total footprint of 706 Acresn Footprint outside of McCleary Canyon
Phase I Dry Stack TSF Characteristicsn Evaporation ponds will be incorporated into tailings lifts to capture stormwater runoff from the tailings surfacen Temporary perimeter ditches will be constructed where necessary to route stormwater runoff to evaporation pondsn A temporary diversion channel will be constructed at start-up to capture stormwater runoff upstream of the phase I Dry Stack TSF through production year 4n A permanent Diversion Channel sized for the PMF and armored for the 200 year storm will be constructed at start-up and will divert stormwater upgradient of the plant site into McLeary canyon north of Phase I
Phase II Dry Stack TSF Characteristicsn Contains approximately 8 years of productionn Maximum Buttress elevation = 5250 feetn Maximum Tailings surface elevation = 5237.5 feetn Total capacity = 253 million tons (MT)n Total footprint 400 Acres
Phase II Dry Stack TSF Characteristicsn Evaporation ponds will be incorporated into tailings lifts to capture runoff from the tailings surfacen Temporary perimeter ditches will be constructed where necessary to divert stormwater to evaporation pondsn An additional permanent diversion channel will be constructed in year 12 to divert stormwater upstream of phase II as well as from the diversion channel upgradient of the Plant siten Dry Detention Basins will be constructed as part of Permanent Diversion Channel system and will greatly reduce peak runoff produced by storm events
Climaten Tetra Tech conducted the meteorological analysis as part of their Feb 2009 design process Month Precipitation Pan Evaporation Projected Pan Evaporation January 1.10 3.59 4.13 February 0.85 4.46 4.28 March 0.90 7.01 7.11 April 0.39 9.35 8.50 May 0.22 11.91 10.38 June 0.47 13.31 10.75 July 4.34 10.00 4.93 August 4.13 8.28 2.89 September 1.55 8.06 4.40 October 1.33 7.17 6.15 November 0.66 4.49 4.11 December 1.43 3.57 3.89 Total 17.37 91.20 71.52 Event 1-Hour 3-Hour 6-Hour 24-Hour 2-yr 1.42 1.60 1.83 2.21 5-yr 1.85 2.03 2.30 2.75 10-yr 2.16 2.38 2.68 3.18 25-yr 2.57 2.86 3.22 3.77 50-yr 2.87 3.24 3.66 4.23 100-yr 3.17 3.63 4.12 4.75 500-yr 3.84 4.59 5.24 6.00 1000-yr 4.14 5.03 5.76 6.57
Site Geology Summaryn Project specific geology is discussed in the Tetra Tech report entitled “Geologic Hazards Assessment” dated June 2007n The geologic units underlying the Dry Stack TSF include • Gila Conglomerate • Mount Fagan Rhyolite • Apache Canyon Formation • Willow Creek Formation • Alluvial materials
Seismic Hazard Analysis Summaryn The Maximum Credible Earthquake (MCE) based on a deterministic analysis was used for the design of the TSFn The deterministic analysis included: • Identifying the largest potentially active fault close to the site • Determining earthquake magnitude that the fault is capable of producing • Determining the Peak Ground Acceleration (PGA) that will be produced at the site from this eventn The Santa Rita fault zone determined to be the controlling of 27 contributing fault sources within a 200 kilometer radius of the project site with a distance from site of 11.2 kilometers and a length of approximately 52 kilometers.n The Santa Rita fault zone capable of producing a PGA of 0.33g and a magnitude 7.1 event
Geotechnical Investigationn Geotechnical field investigation were carried out in two phases by Tetra Tech, between November 2006 and March 2007 and between May and July of 2008. The objective of the investigations included the following: • To define general subsurface conditions for use in evaluation of the Dry Stack TSF stability • To identify suspect zones that could affect the performance of the Dry Stack TSF • To quantify engineering characteristics of the materials incorporated into the Dry Stack TSFn A total of 10 test pits and 38 geotechnical borings in the vicinity of the Dry Stack TSF allowed subsurface conditions to be definedn A total of approximately 13,000 feet of seismic refraction survey was also completed near the vicinity of the Dry Stack TSF footprint
Geotechnical Investigation Summaryn Depth of Bedrock varied across the footprint from 0 to 100 feetn Average depth to bedrock approximately 40 feetn Soils included 1 to 3 feet of topsoil underlain by alluvial materialn Groundwater elevations vary across the footprint from elevations 4,650 to 4,850 feet
Geotechnical Investigation Summaryn The foundation consists primarily of relatively shallow, dense to very dense granular soils.n Foundation preparation will require stripping loose surficial soils providing a uniformly dense founding surface for the tailings.n A Laboratory testing program was completed on select disturbed samples and bench scale tailing samples obtained from field investigations and pilot plant studies.n Two bench scale tailings samples, Colina and MSRD-1 were tested. Both samples were determined to be low-plastic silt (ML) with a plasticity index of 1.n Colina maximum dry density of 115.8 at 14.9%n MSRD-1 maximum dry denstiy of 118.9 at 14.8%
Geologic Hazard Summaryn Landslides or rockfall hazard potential will be minimal within the Dry Stack TSF project area.n Collapsible soils are not considered to be an issue within the footprint.n Historic mining activity will require further field reconnaissance to determine the extent of workings for remediation purposes.n Earthquake induced ground failure (liquefaction) is not anticipated to occur within either the foundation or the Dry Stack TSF.
Geochemical Test Resultsn Tailings generally contain less than 0.01 percent sulfide-sulfurn Tailings possess high capacity for acid neutralizationn Tailings produce very low metal concentrations in the resulting leachaten Total-sulfur concentrations less than 0.3 percent and a neutralization potential ratio greater than 3n Testing indicate the tailings meet ADEQ criteria as inert
Engineering Properties of Tailingsn Laboratory gradations of the tailings indicate an average of approximately 72.6 percent by weight passing the No. 200 sieven Atterberg limit testing indicates the tailings have: • PI of 1 • PL of 20 • LL of 21n The tailings classify as a low-plastic silt (ML), as defined by the USCSn Average effective shear strength approximately 36.5 degrees
Engineering Properties of Alluvium/Foundationn Average of approximately 26.8 percent by weight passing the No. 200 sieven Atterberg limits ranging between non-plastic and 26n Average effective shear strengths ranging between 33 and 41 degrees with cohesions ranging between 1,600 and 2,500 psf
Dry Stack TSF Designn The Dry Stack TSF consists of two separate areas referred to as Phase I and Phase IIn Phase I is located between the McCleary Canyon wash and the Waste Rock Storage Area (12 years, 343 MT)n Phase II is an extension of the phase I facility and will be constructed north of Phase I within McCleary Canyon (years 12-21, 253 MT)n Tailings properties were determined through testing of bench scale tailings samples.n The specified moisture range of placed tailings is 15% (by weight) plus or minus 3%.
Dry Stack TSF Designn Foundation preparation will include clearing and grubbing, tree removal, access road construction and topsoil salvaging and stockpiling.n In the TSF footprint, most of the existing natural drainages will be filled with inert rock and function as flow-through drains.n An initial starter buttress will be constructed in the lower Barrel Canyon drainage to accommodate three months of tailings storage.n Rockfill Buttresses will advance ahead of tailings in 50-foot high lifts using upstream construction methods.n Buttresses will have 150-foot top widths to accommodate two-way haul traffic and outer slopes of 3H:1V.
Dry Stack TSF Designn Dry tailings will be delivered from the filter plant by conveyor and placed in 25-foot lifts using a radial stacker upgradient of the Rock Buttress.n Tailings will be spread with a dozer and compacted with a vibratory smooth drum roller to provide compaction for trafficability of the conveyor and to minimize dust.n The outer perimeter of the tailings beneath the Rock Buttress will be placed in 5-foot lifts and compacted 90% of standard proctor density.n A bypass conveyor will be provided to allow temporary disposal of tailings during primary conveyor movement, maintenance or upset conditions.
Dry Stack TSF Designn Flow-through drains will be constructed of 12-inch minus rockfill and separated from the tailings above by a layer of 10 oz/yd2 geotextile.n Seepage is anticipated to peak at year 18 at a rate of 8.4 gpm.n Natural seepage and springs will be captured with collection drains consisting of shallow trenches filled with rockfill wrapped in 10 oz/yd2 non-woven geotextile.n Existing water wells within the Dry Stack TSF footprint will be abandoned according to ADWR regulations.
Dry Stack TSF Surface Water Managementn Water management will be addressed in the Water Management Plan to be submitted in July 2009. General water management concepts specific to the Dry Stack TSF are listed below:n Perimeter ditches and evaporation ponds will collect stormwater runoff from the tailings surfacen Flow-through drains will allow stormwater that does not come into contact with tailings to be routed beneath the Dry Stack TSFn Diversion channels will be constructed in two phases concurrent with the Dry Stack TSF phases. They will be sized to pass the PMF and armored to protect against the 200 year/24 hour storm.n A Temporary diversion channel will be constructed upstream of the initial lifts of phase I and will function through year 4
Dry Stack TSF Seepage Analysisn Seepage analysis conducted using the finite element method based computer program SVFlux Version 2.0.13n Tailings modeled at average moisture content of 18% (or less) by weightn One-dimensional tailings column models were incrementally evaluated using 50 foot lifts to the full height of 550 feetn Developed isopach maps representing average depths of tailings for each lift and phasen Each successive model incorporated the pore water distributions from the previous model
Dry Stack TSF Seepage Analysisn Included climatic flux comprised of environmental factors including precipitation, pan evaporation, relative humidity and temperature.n The greatest average annual precipitation of 22.2 inches was used, and the lowest average annual pan evaporation of 71.5 inches was used.n The dry stack tailings are considered to be relatively homogeneous in nature.n Laboratory testing was performed to determine hydraulic conductivity at various depths.n Hydraulic conductivity ranges between 4 x 10-3 cm/sec near the top of the Dry Stack TSF and 6 x 10-7 cm/sec at depths of 50 feet or greater.
Dry Stack TSF Seepage Analysisn A series of moisture retention laboratory tests were completed on the tailings samples .n These tests were used to develop a soil water characteristic curve (SWCC) for the tailings materials.n The SWCC defines the soil’s ability to store and release moisture.
Soil Water Characteristic CurveINSERT SWCC Curves
Dry Stack TSF Seepage Analysis Resultsn As the Dry Stack TSF expands over time, the estimated seepage rate increases to a peak value of approximately 8.4 gpm, at production year 18.n The upper 8 feet of the tailings performs as a storage-release unit, where moisture lost to evaporation is replenished by precipitation.n Based on the model, the seepage is due solely to drainage of pore water.n Meteoric influences will have a small recharging effect on the top several feet of tailings, but due to the large evaporation rate there will be an overall negative flux at the surface.n A two-dimensional model of the ultimate Dry Stack TSF was also developed to verify the results.
Moisture Content with Depth Over Time Note: The data represents a typical 100-foot column. The initial moisture content was modeled at 18% by weight
Dry Stack TSF Seepage Analysis Resultsn The estimated maximum seepage from the Dry Stack TSF is expected to be 0.007 gpm/acre. For comparison, the following tailings disposal methods and associated expected seepage rates are as follows: n Slurry Tailings (no liner) 6.4 gpm/acre n Slurry Tailings (with liner) 0.06 gpm/acre n Paste and Thickened tailings 0.4 gpm/acre
Dry Stack TSF Stability Analysisn Establishment of stability design criteria for static and seismic loading conditions based upon laboratory testing, field investigation, and seismic hazard analysisn Development of representative cross sections.n Completion of static and seismic stability analyses utilizing limit equilibrium methods.n Slope stability was evaluated using Spencer’s method.
Dry Stack TSF Stability Analysis Methodologyn The minimum factors of safety used in accordance with the BADCT Guidance Manual guidelines are 1.3 and 1.0 for static and seismic analyses, respectively with appropriate laboratory and field testing.n The stability of the Dry Stack TSF under earthquake loading was evaluated using the pseudostatic approach.n The cross sections were developed at the maximum sections of the facilityn For conservatism, the tailings 1,100 feet from the crest of the buttress were modeled as having no strength.
Dry Stack TSF Stability Analysis Resultsn For tailing impoundment facilities the minimum factors of safety, as required by the BADCT Guidance Manual, are 1.3 and 1.0 for static and seismic analyses. Static Pseudostatic Cross Section Analysis Modeled Factor of Safety Factor of Safety Effective 2.3 1.2 Phase I Total 1.9 1.0 Effective 2.3 1.2 Phase II Total 1.9 1.0
Dry Stack TSF Stability Analysis - Liquefactionn Liquefaction can be generally defined as the loss of shear strength in loose, saturated, and cohesionless soils due to the generation of excess pore pressures as a result of large shear strains induced by undrained cyclic loading.n The dry stack tailings will be unsaturated and will be under large confining pressures producing a uniformly dense fill, hence the propensity for liquefaction will be very low and is not anticipated to occurn The majority of native foundation soils were very dense or hard for granular and fine grained material and are not susceptible to liquefaction.
Dry Stack TSF Closure Conceptn The primary goal of closure/post-closure plan is to eliminate any reasonable probability of further discharge from the Dry Stack TSF.n Concurrent with operations, portions of the Dry Stack TSF will be reclaimed to reduce erosion due to wind and water.n The top of the Dry Stack TSF will be graded inward to create an evapotranspiration pond capable of containing the PMP.n The top of the Dry Stack TSF will be revegetated with native seed mixes designed to maximize evapotranspiration.