GEOMECHANICAL MODELING OF

POTENTIAL MINE DESIGNS FOR WESTERN EXPANSION OF THE AMERICAN ROCK SALT HAMPTON CORNERS MINE, NEW YORK Topical Report RSI-2608 prepared for American Rock Salt 5520 State Route 63 P.O. Box 190 Mount Morris, New York 14510 June 2016

GEOMECHANICAL MODELING OF POTENTIAL MINE DESIGNS FOR WESTERN EXPANSION OF THE AMERICAN ROCK SALT HAMPTON CORNERS MINE, NEW YORK

Topical Report RSI-2608 by Jay R. Nopola Samuel J. Voegeli Leo L. Van Sambeek

RESPEC 3824 Jet Drive Rapid City, South Dakota 57703 prepared for American Rock Salt 5520 State Route 63 P.O. Box 190 Mount Morris, New York 14510 June 2016

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EXECUTIVE SUMMARY OBJECTIVES AND APPROACH American Rock Salt (ARS) is proposing a western expansion at its Hampton Corners Mine near Mount Morris, New York. The objective of this report is to provide geomechanical modeling of potential mine designs that could be considered for the western expansion. A generally conservative approach was taken in the numerical modeling, where no bridging of overburden loads were allowed and each pillar must be fully capable of supporting the earth above the pillar and the adjoining rooms. The numerical modeling was performed in the following sequence: 1. Modeling the current mine design with the intention of reproducing the observed behavior and assessing influential factors. This effort provides confidence in the appropriateness of the modeling method and material models. 2. Evaluating and comparing the impact of expected geological variations, which include a reduced overburden stress in the western expansion. 3. Evaluating and comparing the impact of potential changes to the mine design on in-seam conditions (i.e., floor heave, roof conditions in the salt and shale, and salt conditions in the pillars) and far-field conditions (specifically, subsidence). SUMMARY The overarching findings from this study are:

• Based on available well logs, laboratory testing, underground observations, and typical variations, the stratigraphy near the mining horizon, the material properties of these units, and the thickness of these units are expected to be similar between the existing mine and the western expansion area. Laboratory testing from Well 1303 [Arnold, 2015] found that the dilational strength of the salt is similar to salt from Retsof and stronger than an average of all salts [Van Sambeek, et al., 1993] • Geological conditions west of the existing Hampton Corners Mine are considered suitable for mining and are, in fact, generally expected to be more favorable than conditions of the existing mine. This is because of the decreased overburden stress to the west, which improves in-seam performance metrics (i.e., floor heave, roof conditions, and salt conditions). • Several mine-design options have been identified that are predicted to further improve in-seam performance metrics. The potential mine-design models also predict subsidence and subsidence rates that are equivalent to or less than the current mine design. • Subsidence parameters have been determined by fitting the models to actual measured data from the existing mine. Within the time-period evaluated (50 years), the subsidence-related impacts (e.g., subsidence, subsidence rate, tilt, and strain) above the western expansion are predicted to be similar to or less than the impact predicted over the existing mine.

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TABLE OF CONTENTS

1.0 INTRODUCTION .............................................................................................................................................................. 1 1.1 BACKGROUND INFORMATION ...................................................................................................................... 1 1.2 PROJECT OBJECTIVES ........................................................................................................................................ 2 1.3 REPORT ORGANIZATION ................................................................................................................................. 2 2.0 TECHNICAL APPROACH ............................................................................................................................................. 4 2.1 STRATIGRAPHY .................................................................................................................................................... 4 2.2 MATERIAL PROPERTIES ................................................................................................................................... 6 2.2.1 Elastic Properties and Densities...................................................................................................... 7 2.2.2 Plastic Properties ................................................................................................................................... 7 2 2 2 1 Shear Strength and Tensile Strength ............................................................................ 7 2 2 2 2 Salt Dilation ............................................................................................................................. 12 2.2.3 Constitutive Model for Salt Creep ................................................................................................... 13 2.3 IN SITU STRESS ..................................................................................................................................................... 14 3.0 NUMERICAL-MODEL DESCRIPTION .................................................................................................................... 16 3.1 FINITE DIFFERENCE PROGRAM ................................................................................................................... 16 3.2 THREE-DIMENSIONAL CURRENT MINE-DESIGN MODEL ................................................................. 16 3.3 MODEL VARIATIONS .......................................................................................................................................... 19 3.4 MODELING APPROACH AND PERFORMANCE METRICS .................................................................... 20 4.0 MODELING RESULTS .................................................................................................................................................... 22 4.1 BASELINE-MODELING RESULTS ................................................................................................................... 22 4.1.1 Elastic and Creep Behavior ................................................................................................................ 22 4.1.2 Brittle-Plastic Behavior ....................................................................................................................... 24 4.1.3 Abutment Influence .............................................................................................................................. 24 4.2 INFLUENCE OF GEOLOGICAL FACTORS .................................................................................................... 27 4.2.1 Thickness of Floor Dolomite and Floor Salt ............................................................................... 29 4.2.2 Overburden Depth ................................................................................................................................ 31 4.2.3 Maximum Horizontal-Stress Direction ......................................................................................... 31 4.3 INFLUENCE OF POTENTIAL MINE DESIGN .............................................................................................. 31 4.3.1 Floor Heave .............................................................................................................................................. 34 4.3.2 Salt and Roof-Shale Conditions ........................................................................................................ 35 5.0 SUBSIDENCE ..................................................................................................................................................................... 42 5.1 SUBSIDENCE-MODEL DESCRIPTION .......................................................................................................... 42 5.2 SUBSIDENCE COMPONENTS ........................................................................................................................... 44

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TABLE OF CONTENTS (continued)

5.3 SUBSIDENCE COMPARISON ............................................................................................................................ 45 5.3.1 Change in Elevation .............................................................................................................................. 45 5.3.2 Horizontal Strains.................................................................................................................................. 48 5.3.3 Tilt ................................................................................................................................................................ 48 6.0 SUMMARY AND DESIGN OPTIONS ........................................................................................................................ 53 7.0 REFERENCES .................................................................................................................................................................... 54

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LIST OF TABLES

TABLE PAGE 2-1 Depths and Thicknesses of the Modeled Stratigraphy at the Hampton Corners Mine Based on Well 9441 .............................................................................................................................................................. 6 2-2 Overburden Thickness and Composition ...................................................................................................... 6 2-3 Elastic Properties and Densities for the Modeled Geologic Units at the Hampton Corners Mine ............................................................................................................................................................................... 8 2-4 Shear and Tensile Strength Properties ........................................................................................................... 11 2-5 RESPEC Dilation Criterion Properties for Dilational Strength of Salt for the Hampton Corners Mine ............................................................................................................................................................. 13 2-6 Two-Component Power Law Parameters Used to Simulate the Creep Behavior of the Salt Units at the Hampton Corners Mine ................................................................................................................ 14 3-1 Overview of the Parameter Variations Evaluated in the Investigation of the Geological Factors .......................................................................................................................................................................... 20 3-2 Overview of the Parameter Variations Evaluated in the Investigation of the Mine-Design Factors .......................................................................................................................................................................... 21

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LIST OF FIGURES

FIGURE PAGE 1-1 Hampton Corners Mine Layout as of July 2015 With Typical Pillar and Room Dimensions Indicated ...................................................................................................................................................................... 3 2-1 Simplified Stratigraphic Model Based on Well 9441 ................................................................................ 5 2-2 Comparison of Laboratory-Derived Young’s Modulus Versus Depth and Lithology in Well 1303 ............................................................................................................................................................................... 9 2-3 Schematic Illustrating the Ductile and Brittle Postpeak Behavior Used to Model the Floor Shale .............................................................................................................................................................................. 11 3-1 Location and Extent of the Three-Dimensional Current Design Model Within the Typical Hampton Corners Mine Pattern ........................................................................................................................ 17 3-2 Three-Dimensional Current Design Model Used in the Floor-Heave Investigation .................... 18 4-1 Model-Predicted Salt-Creep Closure Rates (Without Floor Heave) and Measured Closure Rates From the Hampton Corners Mine ........................................................................................................ 23 4-2 Predicted Factors of Safety for Units Near the Mine Horizon in the Current Design Model at 5 Years After Excavation.................................................................................................................................. 25 4-3 Overview of Brittle-Plastic Behavior and Floor Heave Predicted in the Current Design Model ............................................................................................................................................................................ 26 4-4 Vertical-Stress Profiles in the Current Design Model and Abutment Model at 5 Years ............. 28 4-5 Predicted Percent of Failed Floor Shale Below the Beltway for Varying Floor-Dolomite Thicknesses ................................................................................................................................................................ 30 4-6 Predicted Percent of Failed Floor Shale Below the Beltway for Varying Floor-Salt Thicknesses ................................................................................................................................................................ 30 4-7 Predicted Percent of Failed Floor Shale Below the Beltway for Different Overburden Weights ........................................................................................................................................................................ 32 4-8 Predicted Percent of Failed Floor Shale Below the Beltway for Varying Horizontal In Situ Stress Directions ...................................................................................................................................................... 32 4-9 Modeled Dimensions for the Five Mine Designs Selected for Further Analysis............................ 33 4-10 Schematic of a Larger Array of the Chain-Pillar Design .......................................................................... 34 4-11 Predicted Percent of Failed Floor Shale Below the Beltway for the Selected Mine Designs ... 35 4-12 Predicted Factors of Safety in the Roof Shale at 2 Months for the Selected Mine Designs ....... 36 4-13 Predicted Factors of Safety in the Roof Shale at 5 Years for the Selected Mine Designs ........... 37 4-14 Predicted Salt Dilation Factors of Safety in the B6 Salt at 2 Months for the Selected Mine Designs ......................................................................................................................................................................... 39 4-15 Predicted Salt Dilation Factors of Safety in the B6 Salt at 5 Years for the Selected Mine Designs ......................................................................................................................................................................... 40 4-16 Comparison of Salt-Creep Closure Rates Versus Time for the Selected Mine Designs .............. 41 5-1 Components of Ground Movement as a Result of Subsidence .............................................................. 44

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LIST OF FIGURES (continued)

FIGURE PAGE 5-2 Estimated Vertical Subsidence (Feet) Over the Hampton Corners Western Expansion at 5 Years .......................................................................................................................................................................... 46 5-3 Estimated Vertical Subsidence (Feet) Over the Hampton Corners Western Expansion at 10 Years ....................................................................................................................................................................... 47 5-4 Estimated Vertical Subsidence (Feet) Over the Hampton Corners Western Expansion at 50 Years ....................................................................................................................................................................... 49 5-5 Estimated Tensile Strains (Feet/Feet) Over the Hampton Corners Western Expansion at 50 Years ....................................................................................................................................................................... 50 5-6 Estimated Compressive Strains (Feet/Feet) Over the Hampton Corners Western Expansion at 50 Years............................................................................................................................................ 51 5-7 Estimated Tilt (Feet/Feet) Over the Hampton Corners Western Expansion at 50 Years ......... 52

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1.0 INTRODUCTION

American Rock Salt (ARS) is proposing a western expansion at its Hampton Corners Mine near Mount Morris, New York. RESPEC has recently been working with ARS to investigate the specific conditions at the Hampton Corners Mine, which included numerical modeling, instrumentation, and laboratory testing [Van Sambeek and Groff, 2014; Arnold, 2015; Nopola et al., 2016]. The objective of this report is to provide geomechanical modeling of potential mine designs that could be considered in the western expansion. This report presents the technical approach, results, and design options from the investigation. 1.1 BACKGROUND INFORMATION The Hampton Corners Mine is located in western New York and is situated within the glaciated Allegheny Plateau physiographic province. The target ore zone is an approximately 14- to 26-foot-thick salt bed denoted as the B6 Salt of the Vernon Formation. In general, the stratigraphy above and immediately below the B6 Salt is dominated by alternating beds of shale, limestone, dolomite, and salt that are of Devonian and Silurian age. Immediately to the west of the mine shafts, the Genesee River creates a bedrock low where lacustrine deposits have infilled a glacial-scoured valley. The western extent of the mineral rights is located within this valley, where the overburden thickness above the mine is at a minimum of approximately 1,200 feet (ft). As the mineral rights extend to the east, the overburden thickness increases to a maximum of approximately 1,800 ft. Currently, the active mine faces are located near the center of the mineral rights, where the overburden thickness is approximately 1,500 ft. The Hampton Corners Mine uses a large-pillar design that consists of nominally 95-ft square pillars separated by 55-ft-wide crosscuts and rooms. The main travelway/beltway rooms are approximately 40 ft wide and bordered by a staggered series of 95-ft-wide by 177.5-ft-long rectangular pillars. The term extraction ratio (ER) is used to define the amount of ore that can be recovered by mining in relation to the total area of ore. The ER can be defined as follows: ( )( )ER 1 wl

w R l C

= − + +

(1-1) where:

=

=

=

=

pillar widthpillar lengthroom widthcrosscut widthw

l

R

C

Based on the existing mining dimensions, the ER is nominally 60 percent; this refers to the local ER around individual pillars or production areas. The global ER is slightly less because of the presence of narrower beltway rooms (40 ft) and the staggered rectangular pillars. A “chevron” mining front is used to advance the active mining faces and results in “yard” widths of approximately 2,000 ft. The mine height is nominally 13 ft with varying thicknesses of roof and floor salt. A plan view of the existing Hampton

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Corners Mine is illustrated in Figure 1-1 (depicted on the right side [east] of the figure). The proposed western mine expansion is also illustrated in Figure 1-1 (bounded by the green outline of mineral rights with a year-by-year mining plan of the mains and yards). Several previous geotechnical evaluations and laboratory-testing reports were available for review [Van Sambeek and Groff, 2014; Arnold, 2015; Nopola et al., 2016; Agapito Associates, Inc., 1995, 2006; SRK Consulting, Inc., 2013a, 2013b; Frayne, 2000] and provided background information on the stratigraphy, material properties, and geotechnical considerations at the mine. Rock-mechanics measurements, well logs, and mine maps were also provided by ARS for evaluation. 1.2 PROJECT OBJECTIVES The primary objective of this modeling effort is evaluating potential mine-design options that could be used in the western expansion and evaluating the impact of these designs. The goal for changing the design is to maintain or improve conditions in the mine, while also maintaining or improving far-field conditions (specifically, subsidence). 1.3 REPORT ORGANIZATION This report contains seven chapters, including this introduction. Chapter 2.0 presents the technical approach used in the geomechanical investigation. A description of the numerical-modeling software and modeling approach is provided in Chapter 3.0, and the modeling results are presented and discussed in Chapter 4.0. The approach to modeling subsidence and the results are provided in Chapter 5.0. The conclusions and design options from the study are provided in Chapter 6.0, and references are listed in Chapter 7.0.

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Figure 1-1. Hampton Corners Mine Layout as of July 2015 With Typical Pillar and Room Dimensions Indicated.

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2.0 TECHNICAL APPROACH

To achieve the project objectives, the technical approach used in this investigation initially involved gathering and analyzing the necessary information to define the stratigraphy, material properties, and in situ stress of the geologic units at the Hampton Corners Mine. These topics are described in the following sections and comprise the analysis procedure used in this study. Whenever uncertainties were present, conservatism was used throughout the technical approach. 2.1 STRATIGRAPHY The stratigraphy used in the numerical modeling was primarily based on Well 9441, which is located approximately 3,500 ft north-northeast of the shafts, and Well 1302, which is located in the proposed western expansion. The influence of stratigraphy is most important near the mine horizon, where stress concentrations are greater, whereas the far-field stratigraphy has less of an impact. Therefore, special attention was given to the geologic units that are immediately above and below the B6 Salt because these units are expected to have the greatest impact on the mine’s geomechanical behavior. Figure 2-1 provides the simplified stratigraphic model used in the geomechanical investigation. The depths and thicknesses of the geologic units are also listed in Table 2-1. To further simplify the stratigraphic model, the Devonian shales and glacial till above the Onondaga Limestone were not explicitly included in the model but, instead, were represented solely by their overburden weight acting on top of the Onondaga. This is considered a conservative simplification because the overlying shales and till are unable to provide any geomechanical support (e.g., “bridging”). Because glacial scour has eroded part or all of the Devonian shales, the overburden thickness and composition above the Onondaga Limestone varies across the mineral rights of the Hampton Corners Mine. At the western extent of the mineral rights, glaciation has eroded all of the Devonian shales and the upper portion of the Onondaga Limestone and has replaced them with a thick sequence of glacial sediments. At the eastern extent of the mineral rights, the Devonian shales are more preserved with only a relatively thin overlying layer of deposited tills. To account for these variations in overburden thickness and composition, the overburden associated with the current mining area and the western regions of the mineral rights were estimated based on Well 9441 and Well 1302, respectively. Table 2-2 summarizes the overburden thickness and composition for each region as well as the calculated lithostatic stress in the ore zone for the current mine and the western expansion, which indicates a reduction in the lithostatic stress of nearly 10 percent in the western expansion compared to the existing mine. An important observation from a review of well logs and material properties (Section 2.2) is that the relatively stiffer dolomite layer located immediately below the B6 Salt was expected to have an influence on local conditions in the mine. The thickness of this floor-dolomite layer is expected to vary across the mineral rights. Therefore, well-log information from around the mine was used to approximate minimum, intermediate, and maximum floor-dolomite thicknesses. A range of floor-salt thicknesses was also considered in the models based on variations in the overall thickness of the B6 Salt. The specific floor-dolomite and floor-salt thicknesses incorporated into each model are indicated in Section 3.3.

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Figure 2-1. Simplified Stratigraphic Model Based on Well 9441.

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Table 2-1. Depths and Thicknesses of the Modeled Stratigraphy at the Hampton Corners Mine Based on Well 9441

Geologic Unit Lithology Depth at Top

(ft) Depth at Bottom

(ft) Thickness

(ft)

Glacial Till Sediments 0 45 45

Devonian Shales Shale 45 661 616

Onondaga Limestone 661 794 133

Bertie Dolomite 794 872 78

Camillus Shale 872 943 71

Syracuse Dolomite 943 1,152 209

Vernon

Shale 1,152 1,268 116

Dolomite 1,268 1,278 10

Shale 1,278 1,318 40

B6 Salt 1,318 1,340 22

Dolomite 1,340 1,343 3

Shale 1,343 1,376 33

B5 Salt 1,376 1,383 7

Shale 1,383 1,390 7

B1-B4 Salt 1,390 1,452 62

Shale 1,452 1,627 175

Lockport Dolomite 1,627 3,000(a) 1,373

(a) The bottom of the modeled stratigraphic column was simplified by extending the bottom of the Lockport geologic unit to a significant depth.

Table 2-2. Overburden Thickness and Composition

Location Glacial-Till Thickness

(ft)

Devonian-Shale Thickness

(ft)

Total Thickness Above Onondaga

(ft)

Calculated Lithostatic

Stress in the B6 Salt

(psi)

Current 45 616 661 1,481

West 643 0 643 1,361

2.2 MATERIAL PROPERTIES Appropriate material models and properties were defined for the geologic units of the Hampton Corners Mine to allow accurate modeling of the geomechanical behavior. Deformation of the geologic units can be broadly separated into elastic, plastic, and creep behaviors. Elastic behavior results in deformations that are reversible (i.e., the material was not permanently deformed). Plastic behavior occurs when geologic units experience stress conditions that exceed the strength of the rock. Salt creep (sometimes defined as

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viscoplastic behavior) occurs whenever the salt is subjected to a stress difference. Plastic and viscoplastic deformations are not reversible, and the original condition of the rock is permanently altered. All of the geologic units have an elastic component, but only geologic units that were considered subject to failure were modeled with a plastic component. In particular, the large displacements associated with floor heave seen in the mine suggest that the floor dolomite and floor shale are experiencing plastic failure. Salt deformation is typically dominated by time-dependent viscoplastic (creep) behavior, which was also included in the modeling. RESPEC has performed laboratory testing on core from the B6 Salt as well as the rock immediately above and below the B6 Salt obtained from Well 1303 [Arnold, 2015]. The laboratory testing results were used as the foundation for defining the properties of those geologic units. Information summarized in previous geomechanical studies of the Hampton Corners Mine [Agapito Associates, Inc., 2006] were used to define the material properties of the remaining geologic units. The following sections describe the properties and material models used in the current investigation. 2.2.1 Elastic Properties and Densities The elastic properties of the geologic units were defined in terms of Young’s modulus and Poisson’s ratio. Table 2-3 lists the elastic properties, densities, and sources for each of the geologic units included in the model. Of particular interest are the contrasting stiffnesses of the floor dolomite and floor shale. Figure 2-2 displays a graphical representation of Young’s modulus versus depth based on laboratory testing of core from Well 1303. This laboratory testing indicated that the Young’s modulus of the floor dolomite (and the salt) is nearly four times greater than the Young’s modulus of the floor shale, which suggests that the floor dolomite (and salt) may form a relatively stiff beam that separates the mine excavations from the relatively soft floor shale. In addition, the contrasting stiffness between the roof shale and floor shale indicates that the roof shale is also significantly stiffer than the floor shale. This contrast may help explain the greater propensity of floor heaves when compared to roof falls. 2.2.2 Plastic Properties As previously mentioned, failure of the geologic units results in plastic behavior and permanent deformations. This failure is generally evaluated in terms of the material’s shear strength and tensile strength. Because plastic behavior is only anticipated near the mine horizon, only the roof and floor rock were evaluated for plastic failure. The salt beds were evaluated in terms of fracturing and dilation. The plastic models and properties are discussed further in the following sections.

Shear Strength and Tensile Strength Shear failure and tensile failure of the B6 Salt and the roof/floor rock were evaluated based on a strain-softening Mohr-Coulomb (M-C) model. This material model describes the onset of failure based on a peak strength that is followed by a postfailure softening (weakening) of the material to a residual strength. The initiation of shear failure is defined by a peak cohesion and a peak friction angle, while the initiation of tensile failure is defined by the tensile strength. After failure is initiated, the material will begin to plastically (irreversibly) strain in response to any additional stress that exceeds its strength.

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SI-2608

Table 2-3. Elastic Properties and Densities for the Modeled Geologic Units at the Hampton Corners Mine

Geologic Unit Lithology Depth at Top

(ft)

Young’s Modulus (106 psi)

Poisson’s Ratio

(-)

Density (lbf/ft3) Source

Glacial Till Sediments 0 Modeled as overburden load 148.0 Agapito Associates, Inc. [2006]

Devonian Shales Shale 45 Modeled as overburden load 156.6 Agapito Associates, Inc. [2006]

Onondaga Limestone 661 7.96 0.26 167.4 Agapito Associates, Inc. [2006]

Bertie Dolomite 794 5.60 0.25 176.1 Agapito Associates, Inc. [2006]

Camillus Shale 872 6.93 0.20 174.0 Agapito Associates, Inc. [2006]

Syracuse Dolomite 943 6.75 0.26 169.0 Agapito Associates, Inc. [2006]

Vernon

Shale 1,152 4.88 0.17 158.0 Agapito Associates, Inc. [2006]

Dolomite 1,268 5.88 0.22 171.1 Arnold [2015]

Shale 1,278 4.02 0.15 157.5 Arnold [2015]

B6 Salt 1,318 3.92 0.22 134.8 Arnold [2015]

Dolomite 1,340 4.26 0.17 156.0 Arnold [2015]

Shale 1,343 1.18 0.14 148.1 Arnold [2015]

B5 Salt 1,376 3.92 0.22 134.8 Assumed same as B6 Salt

Shale 1,383 1.18 0.14 148.1 Assumed same as floor shale

B1-B4 Salt 1,390 3.92 0.22 134.8 Assumed same as B6 Salt

Shale 1,452 4.88 0.17 158.0 Assumed same as upper Vernon Shale

Lockport Dolomite 1,627 6.75 0.26 169.0 Assumed same as Syracuse

8

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Figure 2-2. Comparison of Laboratory-Derived Young’s Modulus Versus Depth and Lithology in Well 1303 [Arnold, 2015].

1,210

1,215

1,220

1,225

1,230

1,235

1,240

1,245

1,250

1,255

1,2600 1 2 3 4 5 6

Dep

th (

ft)

Young's Modulus (10^6 psi)

Roof Shale

Roof Dolomite

Salt

Floor Dolomite

Floor Shale

Shale

Dolomite

Salt

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As the plastic strain accumulates, the cohesion, friction angle, and tensile strength are adjusted (softened) until they achieve the final residual values. This softening behavior accounts for the progressive fracturing and weakening of the rock. Eventually, enough plastic strain occurs that the rock is reduced to consistent residual values of cohesion, friction angle, and tensile strength. In general, the softening behavior can be classified as either ductile or brittle. Very ductile materials (e.g., steel or clay) may exhibit residual strength values that are equivalent to peak strength values (i.e., there is no softening). Materials with moderate ductile softening behavior can exhibit relatively large amounts of postfailure plastic strain before the residual strength is approached. In this situation, ductile failure is a relatively gradual weakening process. In contrast, materials with brittle softening behavior can only withstand a small amount of postfailure plastic strain before the residual strength is abruptly approached (e.g., glass or concrete). Based on laboratory testing, mine observations, and engineering judgment, the floor salt and floor dolomite were conservatively assumed to have perfectly brittle behavior. This means that when the peak strength is exceeded, the strength is immediately softened to residual strength values. The relatively soft floor shale was further evaluated by using both brittle and ductile behavior, depending on the modeling approach (Section 3.4). For the floor shale, brittle behavior is considered a more conservative approach, while the ductile behavior is considered more realistic. Table 2-4 provides the peak and residual shear and tensile strengths for the various geological units near the mine horizon. Peak strength values for the roof shale, floor dolomite, and floor shale are based on a least-squared-fitting procedure of the laboratory testing [Arnold, 2015]. Using M-C strength criteria to evaluate failure of salt is typically useful only at very low confining stress and rapid strain rates. M-C strength properties for the salt were based on a fit to the Hoek-Brown criteria reported in Agapito Associates, Inc. [2006] for intact rock properties. The tensile strength used for all of the units was 171 psi, which is the average value for salt reported by Arnold [2015]. This approach is considered slightly conservative because the salt had the lowest tensile strength of all units tested but is also considered reasonable because of the salt-filled syneresis structures that are observed in the nonsalt geological units near the mine. Residual strength properties were only available on the direct-shear samples of the salt-dolomite interface reported by Arnold [2015], and these residual strength properties were used for all of the units. The floor-rock softening was defined as a linear relationship between peak and residual strengths. In addition, the plastic-strain thresholds that describe the ductile or brittle behavior were defined as 5.0 millistrain and 0.1 millistrain, respectively. These two relationships are illustrated in Figure 2-3. Factor-of-safety (FS) values were used to quantify the resistance of the geological units against shear failure by using the M-C Criterion. The M-C factor of safety ( )MCFS is defined as the ratio between the strength of the material ( )fS and the developed internal stress ( )S and may be written as: 1 30MC 2

cos sin2FS cosf SSS J

σ + σ φ + φ = =

ψ (2-1)

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Table 2-4. Shear and Tensile Strength Properties

Geologic Unit

Peak Strength Properties Residual Strength Properties

Cohesion (psi)

Friction Angle

(°)

Tensile Strength

(psi)

Cohesion (psi)

Friction Angle

(°)

Tensile Strength

(psi)

Roof Shale 794 47.3 171

0 32.0 0 B6 Salt 754 45.7 171

Floor Dolomite 559 57.9 171

Floor Shale 528 27.9 171

Figure 2-3. Schematic Illustrating the Ductile and Brittle Postpeak Behavior Used to Model the Floor Shale.

0 2 4 6 8

Incr

easi

ng

Str

ess

Dif

fere

nce

→

Millistrain

Ductile

Brittle

Peak Strength

Residual Strength

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where: ( ) ( ) ( )

( )

2 2 22 1 2 2 3 3 11 1 2 31 3

1 2 3 1 2 30

16 2tan (Lode angle)3, , principal stress components ( , compression positive)cohesionangle of internal friction.

J

S

−

= σ − σ + σ − σ + σ − σ

σ − σ + σψ =

σ − σ

σ σ σ = σ ≥ σ ≥ σ

=

ϕ =

In this equation, compression is positive and tension is negative. Strength increases as the principal stresses (i.e., increasing confinement) increase. The Lode angle describes the relative magnitudes of the principal stresses.

Salt Dilation In addition to brittle failure, some evaporites (e.g., salt) tend to progressively lose strength as microfractures form, grow, and coalesce within the crystalline structure; this process is referred to as “damage.” As damage progresses in salt, the initiation and growth of microfractures cause the salt to dilate (expand) and the strength to progressively weaken. Salt dilation is a term used to describe the transition in the deformation behavior from isochoric (constant volume) creep to a combination of creep and the volume-increasing dilation process. The onset of dilation was estimated by using the RESPEC Dilation (RD) criterion and fit laboratory tests reported by Arnold [2015]. The dilation limit of the RD criterion, 2,dilJ , is defined as:

( )1 1 0 02,dil 2/3cos sinnD I T

JD

σ +=

ψ − ψ (2-2)

where: ( ) ( ) ( )2 2 22 1 2 1 3 2 3

1 1 2 31 2 3

1 20

16, , most-compressive, intermediate, and least-compressive principal stresses , , coefficients of the RD criterion determined for each unit unconfine

J

I

D D n

T

= σ − σ + σ − σ + σ − σ

= σ + σ + σ

σ σ σ =

=

=

0d tensile strengthdimensional constant equal to 145 psiLode angle.σ =

ψ =

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When 2J is less than 2,dilJ , no dilation is expected. Dilation is expected to occur when 2J exceeds 2,dilJ , and its intensity will increase with increasing values of 2J . Factors of safety against dilation (FS) can be used to quantify the potential for dilation in the salt as follows:

2,dil2FS J

J= (2-3)

An FS of 1.0 is the limit stress state when the onset of dilation is expected to occur. The likelihood (and intensity) of dilation increases when FS values decrease. Values used to define the RD criterion at the Hampton Corners Mine are provided in Table 2-5. Table 2-5. RESPEC Dilation Criterion Properties

for Dilational Strength of Salt for the Hampton Corners Mine

1D

(psi) 2D

(–) N

(–) 0T

(psi)

135.9 0.61(a) 0.69 171.0

(a) Assumed based on typical triaxial extension relationship.

2.2.3 Constitutive Model for Salt Creep The salt at the Hampton Corners Mine was modeled as a viscoplastic material and described by a two-component power law that is similar to the Norton power law [Norton, 1929]. This two-component power law is defined as: ε = ε vp de

ij eqij

∂σ∂σ

(2-4) where:

( ) ( )1 2

22

1 2

1 2

viscoplastic strain-rate tensorε stress tensor 3 (effective stress)12 (deviatoric stress tensor)1 (mean stress)3Kronecker delta,

vpij

deq

ij

e

ij ji

ij ij ij m

m k

n ne e

k

ij

J

J

A A

s

s

A A

s

σ + σ

ε =

=

σ =

σ =

=

= σ − δ σ

σ = σ

δ =

1 2, material propert es, i .n n =

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The creep properties ( )1 2 1 2, , ,A A n n were based on laboratory creep testing of the B6 Salt from both the Hampton Corners Mine and the Retsof Mine. In total, three creep tests from the Retsof Mine and five creep tests from the Hampton Corners Mine (including two tests performed in 2015) were used to define the creep behavior of the B6 Salt. A least-squares-fitting procedure was applied to the creep test data, and the resulting creep properties were reported by Arnold [2015]. Table 2-6 lists the creep properties that were used in the current investigation. Table 2-6. Two-Component Power Law Parameters

Used to Simulate the Creep Behavior of the Salt Units at the Hampton Corners Mine

Parameter Unit Value

1A 1 1psi yrn− − 5.64 10–23

2A 1 1psi yrn− − 3.47 10–19

1n ― 5.5

2n ― 5.0

2.3 IN SITU STRESS The geomechanical behavior of the rock surrounding an underground excavation depends not only on the material properties but also on the in situ state of stress that exists in the rock before excavation. When an opening is excavated, the preexisting stresses redistribute around the opening and must be supported by the remaining rock. This redistribution of stress results in the formation of deviatoric stresses (i.e., a stress difference) in the rock surrounding the excavation. In nonsalt rocks that surround the excavation, these deviatoric stresses can potentially lead to failure if the shear strength and/or tensile strength is exceeded. When in situ stress differences are present, the deviatoric stress formed by excavations is enhanced. Because the deviatoric stresses are greater when an in situ stress difference is present, failure is more likely in this scenario (compared to an isotropic in situ state of stress where all in situ stresses are equal). Therefore, in situ stress conditions representative of the preexisting stress distributions at the Hampton Corners Mine need to be specified to reliably model the geomechanical behavior of the mined excavations. The in situ stress state is often defined with a vertical component and two orthogonal, horizontal components that are all predominately a function of the overburden weight. Typically, the in situ vertical stress is lithostatic and determined by the densities and thicknesses of the overlying rocks. The individual horizontal in situ stresses can often be either less, equal to, or greater than the vertical stress, depending on both local and regional tectonic influences. The in situ stress state at the Hampton Corners Mine has been measured by Kim [1995] based on hydraulic-fracturing tests performed in Well 9463. Ten tests were conducted from the Devonian shales down into the upper Vernon shales; however, the three test intervals within the Camillus, Syracuse,

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and Vernon Formations were unsuccessful. Based on the Kim [1995] results, Agapito Associates, Inc. [2006] defined the anisotropic in situ stress ratios of the nonsalt units as: 1.6H

h

h v

σ=

σ

σ = σ

(2-5) where: maximum horizontal in situ stressminimum horizontal in situ stressvertical in situ stress.

H

h

v

σ =

σ =

σ =

The two orientations of the maximum horizontal in situ stress measured by Kim [1995] were (1) approximately aligned in the east-west direction and (2) oriented N80°E. In a regional study of the Appalachian Basin [Evans, 1989], the maximum stress direction for western New York was reported to range between an orientation of N59°E and N77°E. Over geologic ages, salt creep typically relieves any differences between the horizontal and vertical in situ stress components. Therefore, the salt was assumed to have an isotropic stress state, which indicates that both the horizontal-stress components are equal to the vertical-stress component: H h vσ = σ = σ (2-6) The stratigraphy provided in Table 2-1 and the densities listed in Table 2-3 were used to calculate the overburden weight and, subsequently, the lithostatic in situ vertical stress. The in situ horizontal stresses were then calculated by using Equation 2-5 for all nonsalt units and Equation 2-6 for salt. As discussed in Section 2.1, this study evaluated two different overburden thicknesses and compositions that are listed in Table 2-2. The minimum overburden weight occurs at the western extent of the mineral rights, and the maximum overburden weight occurs at the eastern extent. To account for the overburden variations, the vertical stress applied to the top of the Onondaga Limestone was adjusted, based on the densities and thicknesses of the Devonian shales and till. Well 1302 was used to estimate the minimum overburden weight at the western extent. Stress differences applied perpendicular to the model boundaries were used to include in situ horizontal-stress differences in the numerical models (which are discussed further in Section 3.3). In the models, the maximum horizontal stress was oriented parallel to the direction of the beltway for comparison purposes, unless otherwise noted. As was indicated above, some literature suggests that the maximum horizontal-stress direction in this region may actually be oriented in a somewhat northeast-to-southwest direction and is, therefore, oblique to the mined excavations.

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3.0 NUMERICAL-MODEL DESCRIPTION

Numerical-modeling software was used to evaluate the geomechanical response of the geologic units. This chapter describes the finite difference program that was used to simulate the mine behavior and defines the numerical models and modeling approach that were developed to represent the Hampton Corners Mine. 3.1 FINITE DIFFERENCE PROGRAM The explicit finite difference code, FLAC3D Version 5.01 [Itasca Consulting Group, Inc., 2014], was used to investigate the floor-heave behavior at the Hampton Corners Mine. FLAC3D is designed to simulate the behavior of materials in response to applied stresses and/or displacements. Materials are represented within the model by polyhedral elements that form a three-dimensional (3D) grid, which can be adjusted to fit the shape of the object being modeled. Each element behaves according to a prescribed linear or nonlinear stress-strain law (constitutive model) in response to the applied forces or boundary restraints. If the stresses (or stress gradients) are large enough to cause the material to yield or flow, the grid can actually deform and move with the material represented in the model. FLAC3D is based on a “Lagrangian” calculation scheme that is well suited for modeling large distortions. Because FLAC3D was developed primarily for geotechnical applications, it embodies special features to accurately represent the mechanical behavior of geologic materials. FLAC3D has several built-in material models that range from the “null” model, which represents excavations in the grid, to the shear- and volumetric-yielding models, which include strain-hardening and -softening behavior, nonlinear shear failure, and compaction. FLAC3D also has several built-in constitutive models that permit the behavior of highly nonlinear materials such as salt. The features and capabilities of FLAC3D that were required for simulating the 3D model include the following:

• 3D geometries • Kinematic and traction boundary conditions • Arbitrary specification of in situ stresses as a function of depth • Multiple layers of materials with different properties and material behaviors • A two-component power law constitutive model to represent creep behavior • A strain-softening M-C constitutive model to represent plastic failure followed by material weakening.

3.2 THREE-DIMENSIONAL CURRENT MINE-DESIGN MODEL A 3D model that represents the typical mine layout at the Hampton Corners Mine was first developed to investigate the conditions experienced in the mining horizon. Figure 3-1 illustrates the location and extent of the 3D model within the mine. The particular location and extent of the model was selected because it includes the pillars and excavations that immediately surround the beltway, which is of primary interest in the current study. Based on this location and extent, the 3D model illustrated in Figure 3-2 represents a repeating array of pillars and rooms and provides a reasonable approximation of the complete pattern used at the Hampton Corners Mine. The model was developed with enough refinement to provide an adequate representation of the stress gradients near the excavations.

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Figure 3-1. Location and Extent of the Three-Dimensional Current Design Model Within the Typical Hampton Corners Mine Pattern.

17

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Figure 3-2. Three-Dimensional Current Design Model Used in the Floor-Heave Investigation.

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The model extends vertically from the top of the Onondaga Limestone to a depth of 3,000 ft below the ground surface. This lower-boundary location was selected to isolate the response of the mine from the influence of the bottom boundary. Kinematic constraints were specified to restrict normal displacements along the bottom boundary and vertical boundaries of the model. These boundaries and constraints represent planes of symmetry. The upper surface of the model (top of the Onondaga) was free to move in response to the mined excavation below it. A vertical pressure was applied to the upper surface of the model to account for the weight of the rocks overlying the Onondaga. As discussed in Section 2.1, the Devonian shales and glacial sediments above the Onondaga were omitted from the model because (1) it is unlikely that they significantly influence the floor-heave behavior; (2) it allows for simple adjustments to account for changes in overburden thickness and composition; and (3) the elimination of zones allows the model to compute more quickly, which is a benefit when numerous model iterations are investigated. This is considered a somewhat conservative approach as it does not allow for any bridging of stress above the Onondaga. All of the overburden weight (above each pillar and halfway into each adjacent room) must be supported by each pillar. Several modifications were made to this current design model. These modifications are discussed briefly in Section 3.3. 3.3 MODEL VARIATIONS Dozens of model variations were considered as part of this study and, for the sake of brevity, not all of the models considered are detailed in this report. The primary variations considered in the modeling effort can be simplified as follows:

• Baseline Modeling – Current design model – Current design model including an abutment pillar

• Geological Variations (natural factors that cannot be controlled) – Changes in overburden depth – Variations in floor-dolomite thickness – Variations in floor-salt thickness – Different maximum horizontal-stress directions – Variations in the material properties of floor shale and dolomite

• Mine-Design Variations (factors that can be controlled) – Room width – Intersection type – Pillar dimensions – Other considerations. More specific details regarding each of these model variations are provided in the individual results section. Table 3-1 (comprising the baseline model and geological factors) and Table 3-2 (mine-design changes) provide an overview of the primary model variations included in this report and a list of the input changes for each model. All of the mine-design variations considered in this study are based on tributary loading conditions, where each pillar is capable of supporting the full weight of the overburden above it and above adjacent rooms. These designs are in contrast to a yield pillar design, which is based on a stress arch of overburden above the panel that must be supported by adjacent abutments.

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Table 3-1. Overview of the Parameter Variations Evaluated in the Investigation of the Geological Factors (Pink Shading Highlights Deviations From the Current Design Model)

Model Name(a)

Input Variables

Overburden Depth

Maximum In Situ Stress Direction

Floor-Dolomite Thickness

(ft)

Floor-Salt Thickness

(ft)

Current Design Current Mine Parallel to beltway 3.5 4.5

No Floor Dolomite Current Mine Parallel to beltway 0 4.5

Thick Floor Dolomite Current Mine Parallel to beltway 7 4.5

Thin Floor Salt Current Mine Parallel to beltway 3.5 1

Thick Floor Salt Current Mine Parallel to beltway 3.5 7

Perpendicular Horizontal Stress Current Mine Perpendicular to beltway 3.5 4.5

Western Mine Western Mine Parallel to beltway 3.5 4.5

(a) The current mine design was used for each model.

3.4 MODELING APPROACH AND PERFORMANCE METRICS The primary means in evaluating the relative impact of the geological variables and mine-design changes was a comparative approach. Results from the modeled variations were compared to those of the current design to assess whether the performance measures were better or worse. The modeling approach was developed to allow for a quantitative analysis of the relative impact of the model variations. In this approach, the model simulated 20 years of creep without allowing any of the rock to fail. Therefore, only elastic- and creep-driven deformations were initially simulated to obtain the time-dependent stress distribution that results from salt creep. At discrete times selected for further analysis, the shale was then allowed to fail with brittle postpeak behavior (a conservative approach). The dolomite and salt, however, were not allowed to fail so that all of the models were capable of reaching a stable and consistent solution for comparison purposes. This decoupled approach provides a much faster simulation runtime than simultaneous calculations of creep and plastic failure. In addition, the decoupled approach also accentuates the impact that creep has on redistributing stresses and the subsequent impact on floor-shale failure and other considerations. Using the aforementioned modeling approach, the modeling process and performance measures were generally conducted in the following sequence for each of the model variations analyzed: • Floor shale—quantitative calculation of the potential for failed shale in the beltway. The results are normalized and presented as percentage of failed shale in the beltway. • Salt—evaluate the potential for dilation and the creep-driven closure rate • Roof shale—evaluate the potential for failure • Subsidence—evaluate subsidence impacts.

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Table 3-2. Overview of the Parameter Variations Evaluated in the Investigation of the Mine-Design

Factors (Pink Shading Highlights Major Deviations From the Current Design Model)

Model Name(a)

Input Variables

Beltway Room Width

(ft)

Crosscut Room Width

(ft)

Beltway- Crosscut

Intersection

Pillar Dimensions

(ft)

Local Extraction

Ratio (%)

Model Extraction

Ratio(b) (%)

Current Design 40 55 4-Way 95 × 95 60 57

70x70 Pillars 40 40 4-Way 70 × 70 60 57

40w Rooms 40 40 4-Way 95 × 95 50 48

Large Pillars 40 40 4-Way 200 × 400 24 24

Chain Pillars 40 40 3-Way 48 × 150 57 57 (a) The baseline-geological setting was used for each model. (b) Although the nominal local ER of the current design is for example, 60%, incorporating the narrower beltway room width and rectangular pillars

may reduce the model ER slightly.

21

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4.0 MODELING RESULTS As described in the previous chapter, the modeling approach was to first evaluate the current design to understand the mechanics in the mine horizon. After the mechanics were understood, various geological variations were evaluated to understand the range of conditions that can be encountered just from natural conditions. Lastly, mine-design variations were evaluated according to the approach described in Section 3.4. This chapter presents the modeling results in a similar manner: (1) baseline-design results; (2) the impact of geological factors; and (3) the impact of changes to mine design, which is divided into a preliminary analysis and a further analysis of the selected designs. 4.1 BASELINE-MODELING RESULTS The objective of the baseline modeling was to develop a modeling method that is able to provide a reasonable representation of the observed conditions and to understand the mechanics driving this behavior. Three initial simulations of the model were performed to obtain a preliminary understanding of the mine behavior. First, the model was simulated without allowing any rock to fail in order to isolate the impact of creep. Second, the model was simulated while allowing all of the floor rock (shale, dolomite, and salt) to fail with brittle postpeak behavior to attempt to simulate the floor-heave phenomenon. Third, the model boundaries were extended to evaluate the influence of an abutment. The results of these initial simulations aided in defining the approach that was used to investigate floor heave and are discussed in the following sections. 4.1.1 Elastic and Creep Behavior For this preliminary task, the model simulated 20 years without allowing any of the rock to fail; therefore, only elastic- and creep-driven deformations were simulated. The goal of this simulation was to evaluate the mine behavior in the absence of any plastic failure and assess the relative impact of creep on room closure. This simulation was analyzed in terms of the room-closure and closure rate as well as the predicted condition of the rock near the mined excavations. In general, measurements from closure stations within the Hampton Corners Mine have indicated relatively high closure rates. Based on RESPEC’s experience, the measured closure rates are greater than would be expected, given the ER, creep properties, and mine depth at the Hampton Corners Mine. Figure 4-1 compares the model-predicted closure rates (located between pillars within the beltway) to an overview of all measured closure rates in the mine that were normalized to time since the initial excavation. Note that closure rate is presented on a log scale in Figure 4-1. Figure 4-1 also depicts model-predicted closure rates based on generic creep properties that represent very fast-creeping salt. The measured closure rates are nearly all greater than the model predictions for salt-creep closure (in the absence of any plastic failure); therefore, the comparison of closure rates in Figure 4-1 suggests that nearly all of the measured closure rates are dominated by floor heave caused by floor-rock failure, and that some degree of floor heave appears to be occurring in the majority of the mine. Salt creep is only a secondary influence on room closure and is often predicted to contribute only approximately 1/10th of the total room closure.

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Figure 4-1. Model-Predicted Salt-Creep Closure Rates (Without Floor Heave) and Measured Closure Rates From the Hampton Corners Mine.

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The current design model was also evaluated for potential failure in the units near the mine horizon. Figure 4-2 shows the model-predicted factors of safety in the roof shale, B6 Salt, floor dolomite, and floor shale after 5 years of simulation time. Based on the model-predicted factors of safety, the following important observations can be made: • Most of the potential floor failure is predicted to occur in the floor shale, particularly in the intersections • The floor salt and floor dolomite have relatively high factors of safety, except for limited areas of potential failure under the pillar ribs • The floor salt and floor shale appear to have a much greater potential for failure in comparison to the roof units. These observations provide a preliminary understanding of the mechanisms that are likely responsible for the floor heave. The floor-shale failure appears to be the driving mechanism that influences floor heave, based on the significant volume of potentially failed shale; however, the floor beam (comprised of the floor salt and dolomite) must also fail for sizable floor heave to occur. The overarching message from a review of these driving mechanisms is that providing confinement to the floor shale and limiting shear stress in the floor beam will overall reduce the potential for floor failure. This observation was considered in the potential mine designs investigated in Section 4.3.

4.1.2 Brittle-Plastic Behavior A reproduction of the observed floor heave was considered necessary to validate the modeling approach. To accomplish this, the floor rock (i.e., salt, dolomite, and shale) was simulated using brittle-plastic properties that allow floor heave to occur in response to failure of the floor rock. Using this approach, Figure 4-3 provides an example of the model-predicted floor heave at 5 years of simulation time. The results clearly indicate that the floor has heaved into the mined excavations. A contour plot of the predicted plastic strain is also depicted in Figure 4-3. The plastic-strain contours indicate that distinct failure planes have developed in the floor, as denoted by the white dashed lines. These failure planes allow for differential movement of discrete “blocks” in the floor that eventually cause the floor rock to be squeezed upward into the rooms, as illustrated by the white arrows. The contours of the predicted vertical displacement shown in Figure 4-3 indicate that over 7 ft of floor heave has occurred. Also note that the vertical-displacement contours show that the pillars have sunk over 1 ft into the floor. The brittle-plastic behavior simulated in this model resulted in an unstable solution. The model indicated that the floor will continue to heave and the pillars will continue to sink, and a final stable solution will not be attained until the rooms have completely filled with rock from the floor heave. This makes it difficult to quantitatively evaluate floor heave based on the conservatively assumed brittle-failure behavior. 4.1.3 Abutment Influence Another preliminary simulation was performed to investigate the influence of unmined rock adjacent to a series of pillars and excavations. Specifically, this simulation was intended to estimate how much the proximity of active (and inactive) mine faces can influence the geomechanical behavior. To accomplish this, the previously discussed current design model was extended to represent a complete half-yard and an adjacent 1,000-ft-wide area of unmined rock (abutment). Another model was created using a 200-ft

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Figure 4-2. Predicted Factors of Safety for Units Near the Mine Horizon in the Current Design Model at 5 Years After Excavation.

25

Low Factors of Safety in the Dolomite and Salt

Low Factors of Safety in the Shale

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Figure 4-3. Overview of Brittle-Plastic Behavior and Floor Heave Predicted in the Current Design Model.

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abutment and produced very similar results as the 1,000-ft abutment model. A plan view of this extended model is shown at the right side of Figure 4-4. The support provided by the unmined rock is anticipated to reduce the vertical load that the adjacent pillars must carry. Consequently, the geomechanical behavior and floor-heave potential may improve as the proximity to unmined rock decreases. Conversely, heightened stress concentrations in the abutment may create difficult mining conditions for adjacent yards that mine through the abutment. Figure 4-4 compares the predicted profiles of vertical stress along the midheight and midwidth of the pillars after 5 years of simulation time. This simulation was performed with only elastic and creep deformations (i.e., floor failure and heave was not allowed). Simulations that allow floor failure were also performed and delivered very similar results. Based on the vertical-stress profiles illustrated in Figure 4-4, the following observations can be made: • The extended model suggests that the unmined rock has a limited influence on the geomechanical behavior of the pillars. Specifically, the support provided by the unmined rock provides a slight reduction in the vertical load (stress) carried by only the closest two pillars. The pillar closest to the abutment has a vertical stress of approximately 75 percent of the tributary load, and the second pillar from the abutment supports close to 90 percent of the tributary load. Pillars near the beltway are essentially uninfluenced by the unmined rock (tributary loading conditions). • The current design model and the extended model show nearly identical vertical-stress results, which is indicated by the vertical-stress profiles of 0 to 300 ft from the beltway centerline. Based on this observation, the full tributary loading in the current design model also appears to be represented in the extended model. This suggests that the limited extent of the current design model sufficiently represents the full tributary loading behavior that is expected near the beltways. • The area of increased vertical stress in the abutment is primarily located within approximately 100 ft of the mined area. Within this distance, increased floor heave is possible for adjacent yards that are mined back toward previously mined yards. Overall, the results of this abutment model indicate that unmined rock only reduces the pillar load (within a proximity of approximately two pillars), and that the pillar load is still approximately 75 percent or more of the tributary load. Therefore, the third pillar behind the active faces has likely reached full tributary loading as mining advances. In addition, if unmined rock flanks the length of a yard, the same full tributary behavior can also be expected to occur on approximately the third pillar away from the unmined rock. Note that a conservative approach was taken in the model by applying the units that overlie the Onondaga Limestone as a traction instead of explicitly including them in the model. Because the units above the Onondaga Limestone are applied as a force in the model, they are unable to transfer load to the abutment. This is considered a reasonable approximation for areas where unconsolidated sediments or heavily fractured rocks overlie the Onondaga. However, this modeling approach may be overly conservative in areas where stiff materials overlie the Onondaga and are capable of transferring more of the overburden load onto the abutment.

4.2 INFLUENCE OF GEOLOGICAL FACTORS The next task of the study was to evaluate the relative importance of the geological factors on mine response. Based on the preliminary modeling results discussed in the previous sections, floor-shale failure is considered the primary factor that drives room closure and local ground conditions in the mine (more so than salt creep). Therefore, the potential for increased room-closure rates is likely a function of

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Figure 4-4. Vertical-Stress Profiles in the Current Design Model and Abutment Model at 5 Years.

28

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the amount of failed floor shale and the ability of the floor beam to contain that failure. An increase in the amount of floor-shale failure or a deterioration of the floor beam will likely cause an increase in room closure. The following geological factors that potentially influence overall room closure at the Hampton Corners Mine were identified: • Overburden—thickness and depth of the overlying geologic units • Stratigraphy—thickness and depth of near-mine geological units • In situ stress—overburden weight and horizontal-stress direction/magnitude • Material properties—stiffness, strength, and brittleness. Each of these factors was evaluated by varying a parameter (e.g., rock thickness, overburden weight, and stiffness) to determine its influence on the amount of floor-shale failure. Because the beltway is the primary concern in this study, the geological factors were assessed in terms of the percent of floor-shale failure underneath the beltway.

4.2.1 Thickness of Floor Dolomite and Floor Salt The thickness of the floor dolomite and floor salt is expected to vary throughout the mine. The floor dolomite is stiffer and stronger than the floor shale and acts as a protective beam that separates the floor shale from the excavated rooms. As the floor-dolomite thickness increases, less floor shale fails because the thicker floor-dolomite beam offers more protection. In contrast, when the floor dolomite is absent, a significant amount of the floor shale is predicted to fail. Figure 4-5 depicts the predicted percent of failed floor shale below the beltway for varying thicknesses of the floor dolomite (all of which include a 4.5-ft thickness of floor salt). The percent of floor-shale failure is plotted as a function of time after the rooms were excavated. The results indicate that the thickness of the floor dolomite has a significant influence on the amount of predicted floor-shale failure. Slightly more floor-shale failure is predicted with increasing time. As vertical stress is transferred farther into the pillar as salt creeps, the reduction in vertical confining stress near the pillar rib increases the potential for failure in the shale beneath the rib. As mentioned previously, the thickness of the B6 Salt can vary from 26 ft to only 14 ft. These variations, in addition to mining practices, may cause the floor-salt thickness below the excavated rooms to vary from as little as 1 ft to as much as 8 ft. Figure 4-6 illustrates the predicted percent of failed floor shale below the beltway for varying thicknesses of the floor salt (with a 3.5-ft thickness of dolomite in all cases). Similar to results of the floor-dolomite thickness comparison, Figure 4-6 indicates that increasing the floor-salt thickness causes a decrease in the predicted amount of floor-shale failure. Although the floor salt may not provide a significant contribution to the structural behavior of the floor beam, it likely acts as a “buffer” between the excavation and the floor shale. Increasing the floor-salt thickness likely increases this buffer effect and reduces the floor shale subject to failure. Figure 4-6 also indicates that increasing the floor-salt thickness from 1 foot to 4.5 ft has a much greater impact on the floor shale than when the floor-salt thickness is increased from 4.5 ft to 8 ft. In summary, the thicknesses of the floor salt and floor dolomite that separate the mined room from the floor shale has a major influence on the potential for floor-shale failure. Variations in the combined floor-beam thickness across the mine could clearly influence where floor heave initiates and where movement can arrest.

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Figure 4-5. Predicted Percent of Failed Floor Shale Below the Beltway for Varying Floor-Dolomite Thicknesses.

Figure 4-6. Predicted Percent of Failed Floor Shale Below the Beltway for Varying Floor-Salt Thicknesses.

0%

10%

20%

30%

40%

50%

60%

70%

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Per

cen

t o

f F

aile

d F

loo

r S

hal

e B

elo

w B

eltw

ay (

%)

Time After Excavation (yrs)

8-ft-Thick Floor Salt

4.5-ft-Thick Floor Salt

1-ft-Thick Floor Salt

0%

10%

20%

30%

40%

50%

60%

70%

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Per

cen

t o

f F

aile

d F

loo

r S

hal

e B

elo

w B

eltw

ay (

%)

Time After Excavation (yrs)

No Dolomite (0-ft)

Medium Dolomite Thickness (3.5-ft)

Thick Dolomite (7.5-ft)

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4.2.2 Overburden Depth The overburden depth varies across the current mine reserves. Figure 4-7 depicts the predicted percent of failed floor shale below the beltway for varying overburden weight. The results suggest that as the overburden weight decreases to the west, the amount of floor-shale failure is predicted to decrease. Generally speaking, less floor shale failure is expected to the west compared to the current mine area. 4.2.3 Maximum Horizontal-Stress Direction The influence of the maximum in situ horizontal-stress direction was evaluated by aligning the beltway either parallel or perpendicular to the maximum horizontal stress. Figure 4-8 shows the predicted percent of failed floor shale below the beltway for the different maximum horizontal-stress directions. The results indicate that orienting the beltway in the same direction as the maximum horizontal in situ stress can significantly reduce the amount of failure predicted in the floor shale. This is expected because the stress difference that initiates failure would increase if the beltway is aligned perpendicular to the maximum horizontal in situ stress. As noted earlier, there are some indications that the actual maximum horizontal-stress direction at the Hampton Corners Mine aligns in somewhat of a northeast-to-southwest direction. With the maximum horizontal stress oriented oblique to the mining direction, floor heave is likely to be equally common in both the beltway and the crosscuts. Additionally, if the actual horizontal-stress magnitudes are greater than what was modeled, more failure in the shale would be expected. Furthermore, if the difference between the horizontal stresses increases (e.g., the maximum horizontal is much greater than the minimum horizontal), the benefit of a beltway aligned with the maximum horizontal-stress direction is accentuated. 4.3 INFLUENCE OF POTENTIAL MINE DESIGN For this study, the mining-induced factors are defined as those factors that can be directly attributed to mining operations. Numerous mine designs were investigated to evaluate potential benefits. This report contains only five selected designs that were predicted to provide some benefit. The potential mine designs were evaluated based on the performance metrics of floor heave, closure rate, and roof conditions. The overburden depth used for the comparison is that of the existing mine area, as this represents the maximum overburden depth experienced in the western expansion. As the overburden stress decreases to the west, the performance metrics will improve. Figure 4-9 illustrates the five potential mine designs that were developed and analyzed. All of these mine designs either maintain or reduce the current ER. A description for each design is provided below:

• Current design model • 70×70 pillars model: a “shrunken” version of the existing design with 70- × 70-ft pillars and 40-ft-wide beltway and crosscuts • 40w room model: the current design of 95- × 95-ft pillars with all rooms reduced to 40 ft wide, which decreases the ER to 48 percent

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Figure 4-7. Predicted Percent of Failed Floor Shale Below the Beltway for Different Overburden Weights.

Figure 4-8. Predicted Percent of Failed Floor Shale Below the Beltway for Varying Horizontal In Situ Stress Directions.

0%

10%

20%

30%

40%

50%

60%

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Per

cen

t o

f F

aile

d F

loo

r S

hal

e B

elo

wB

eltw

ay (

%)

Time After Excavation (yrs)

Max Horizontal Stress Aligned withBeltway

Max Horizontal Stress Aligned withCrosscut

0%

10%

20%

30%

40%

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Per

cen

t o

f F

aile

d F

loo

r S

hal

e B

elo

w B

eltw

ay (

%)

Time After Excavation (yrs)

Minimum Overburden Weight - West

Current Overburden Weight - Shaft Area

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Figure 4-9. Modeled Dimensions for the Five Mine Designs Selected for Further Analysis.

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• Chain-pillars model: an offset and repeating array of long chain pillars that are 48 ft wide and 150 ft long, with all rooms 40 ft wide. This particular design orients the primary mining direction parallel to the maximum horizontal in situ stress and all of the intersections are three-way designs. A schematic of a larger array of the chain-pillar design is provided in Figure 4-10 to better illustrate the design. • The large-pillars model: 200- × 400-ft pillars and all rooms 40 ft wide (similar to the shaft-pillar area).

Figure 4-10. Schematic of a Larger Array of the Chain-Pillar Design (Not to Scale).

4.3.1 Floor Heave Floor heave has been determined to be a large component in room closure and, therefore, a major goal of the proposed mine-design evaluation is to reduce the potential for floor heave. Evaluations for the five proposed mine designs selected for further study were based on the predicted amount of failed floor shale. Again, these results were obtained by simulating brittle failure of the floor shale, while not allowing the floor beam to fail. Figure 4-11 compares the predicted amount of floor-shale failure for the

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current and proposed mine designs. All four of the newly proposed mine designs show a noticeable decrease in the amount of floor-shale failure as compared to the existing design. Note that the same relative improvement is predicted by both changes in mine design (while maintaining the same ER) and changes that reduce the ER. For example, the reduction in crosscut widths to 40 ft while pillars remain at 95×95 ft (modeled ER = 48 percent) is predicted to result in the same percentage of failed floor shale as the 70×70 pillars model (modeled ER = 57 percent).

Figure 4-11. Predicted Percent of Failed Floor Shale Below the Beltway for the Selected Mine Designs.

4.3.2 Salt and Roof-Shale Conditions In addition to the potential for floor heave, it is also important to consider that changes to reduce the potential for floor heave do not induce other problems. Metrics that were also evaluated for the five mine designs include the potential for roof failure, the potential for salt dilation in the roof and pillars, and the predicted closure rates attributed solely to salt creep. The potential for roof failure was evaluated using the M-C factors of safety for the roof shale. An FS of less than 1.0 is indicative of failed rock. Generally, an FS greater than 1.0 is preferable to account for normal geological variations and other uncertainties. This study focused on comparing the predicted factors of safety to the existing mine design as a means to evaluate whether or not the changes are better, worse, or similar to the current design. Figure 4-12 displays factors-of-safety contours within the roof shale for the five mine designs after 2 months of creep has occurred. Figure 4-13 provides a similar plot after 5 years of creep has occurred. In Figures 4-12 and 4-13, the current mine-design model indicates areas where the FS is near 1.0, which indicates that the potential for roof falls exists and does not change significantly over time. The two alternative designs that maintain the same ER (the 70×70 pillars and the chain-pillars models) both predict roof conditions that are similar to the current design.

0%

5%

10%

15%

20%

25%

30%

35%

40%

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Per

cen

t o

f F

aile

d F

loo

r S

hal

e B

elo

w B

eltw

ay (

%)

Time After Excavation (yrs)

Current Design

70x70 Pillars

40w Rooms

Chain Pillars

Large Pillars

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Figure 4-12. Predicted Factors of Safety in the Roof Shale at 2 Months for the Selected Mine Designs.

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Figure 4-13. Predicted Factors of Safety in the Roof Shale at 5 Years for the Selected Mine Designs.

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The two models that have a reduced ER indicate slightly better roof conditions as compared to the existing design. The potential for salt dilation was evaluated by using RD factors of safety. An FS against dilation of less than 1.0 does not indicate immediate salt failure. An FS rating of less than 1.0 does, however, indicate that the salt is dilating and, therefore, weakening over time. Some degree of dilation around mined openings is unavoidable but should be confined to near openings. Pillar cores with a low potential for dilation are typically indicative of stable pillars. A comparative approach is again used in evaluating design changes. Figure 4-14 provides factors-of-safety contours against dilation for the entire B6 Salt for the five mine designs after 2 months of creep has occurred. Figure 4-15 shows a similar plot after 5 years of creep. After 2 months of creep, as illustrated in Figure 4-14, all of the designs display a similar potential for dilation, which is confined primarily to the pillar ribs and the immediate roof and floor salt. The exception is the large-pillar, low-ER design, where the potential for dilation is slightly reduced. The different models display more variability in potential for dilation after 5 years of creep, as depicted in Figure 4-15. Over time, creep has generally reduced the potential for dilation in all of the models; however, the potential for dilation remains relatively higher in the chain-pillars model. Additionally, the potential for dilation in the pillar core of the chain pillar is greater than the other models, which presents some concern for pillar degradation over time. A comparison of the closure rate of the five models is provided in Figure 4-16. The floor shale was not allowed to fail in this comparison so that the impact of salt creep can be isolated. Both of the models that maintain the current ER (the 70×70 pillars and the chain-pillars models) are predicted to have faster salt-creep closure rates than the current design because the pillar size has been reduced (thereby increasing the stress difference at the center of the pillar, which drives creep). However, the predicted closure rates from salt creep are still generally less than the current measured rates in the mine (generally, 1 to 2 inches per year or more). Therefore, if floor heave can be reduced, the additional closure from salt creep is still likely to result in a net reduction in the closure rate. As would be expected, both of the models with a lower ER were predicted to have a slower salt-creep closure rate than the current design.

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Figure 4-14. Predicted Salt Dilation Factors of Safety in the B6 Salt at 2 Months for the Selected Mine Designs.

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Figure 4-15. Predicted Salt Dilation Factors of Safety in the B6 Salt at 5 Years for the Selected Mine Designs.

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Figure 4-16. Comparison of Salt-Creep Closure Rates Versus Time for the Selected Mine Designs (Floor Heave From Floor Shale Failure is Not Included).

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 1 2 3 4 5 6 7 8 9 10

Clo

sure

Rat

e (i

n/y

r)

Time (yrs)

Current Design

70x70 Pillars

40w Rooms

Chain Pillars

Large Pillars

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5.0 SUBSIDENCE

Subsidence and subsidence rates at the surface are controlled by several factors, which include the mining extent, mining height, mining depth, ER, mining-advancement rate, and room-closure rate. Proposed mining-advancement rates and locations were provided for the western mine expansion based on 31 years of mining. The mining blocks and the proposed sequence of advancement are indicated in Figure 1-1. 5.1 SUBSIDENCE-MODEL DESCRIPTION The subsidence comparison was analyzed using the basis of the subsidence analysis software program SALT_SUBSID [Nieland and Van Sambeek, 2010]. Inverse fitting of SALT_SUBSID input variables has been previously performed for the Hampton Corners Mine by Van Sambeek and Groff [2014] based on measured subsidence data. This fitting was able to successfully replicate the measured subsidence data at the surface and, therefore, accounts for all of the contributing factors (i.e., salt creep and floor heave). SALT_SUBSID’s theoretical basis is that mined excavations in salt rocks will slowly close with time because of the deformation process of salt creep. Subsidence is estimated in SALT_SUBSID using influence functions to relate the closure of underground openings to surface displacements. The ultimate subsidence from complete closure of an underground opening is estimated by integrating the influence function over the volume of the opening. The subsidence at a specific time is determined as the ultimate subsidence multiplied by the fractional closure of the opening at the desired time. The solutions for each individual opening are superimposed to obtain an approximation for the total displacements at the ground surface. The surface displacement, jS , at time t at a given horizontal location ( ),P x y with respect to the global coordinate system, can be expressed as follows: ( ) ( )1, , ( ) ' , ' , '

i

n

j i i j i i iAi

S x y t C t h f x y z da=

= × (5-1)

where:

( )

number of underground openings( ) fractional closure of underground opening at time height of opening horizontal area of opening ultimate displacement from complete closuri

i

i

j i i i

n

C t i t

h i

A i

f x , y ,z

=

=

=

=

′ ′ ′ =

( )

e of opening relative to the opening centerdisplacement component , , or , , elative coordinates from ( , ) to center of underground opening .i i i

i

j x y z

x y z r P x y i

=

′ ′ ′ =

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The influence function used to simulate the Hampton Corners Mine is based on the closure of openings in an isotropic elastic half-space, as described by Maruyama [1964]. The influence function for vertical surface displacements calculated with this model is given by: ( )( )2.52 3, , 2 1zf x y z −′ ′ ′ =

π ρ + (5-2)

where: ′ ′ ′ =

′ ′+ρ =′

2 22 2

, , coordinates to integration point in Equation 5-1 integralwith respect to the point at which subsidence is calculated.

x y z

x yz

The influence functions for the horizontal displacements for this model are given by: ( ) ( )

( ) ( )

, , , ,, , , ,

x z

y z

xf x y z f x y zz

yf x y z f x y zz

′′ ′ ′ ′ ′ ′=′

′′ ′ ′ ′ ′ ′=′

(5-3) For conventional dry mines, the fractional closure as a function of time, ( )C t , is modeled in SALT_SUBSID as: ( ) (1 )0 1 C

Nt

ERssC t y t y e

−β−

= + −

(5-4) where: 0, , , and ss cy y Nβ = model parameters ER = extraction ratio of the mine t = time since opening was created. The closure of an opening, ( )C t , is not allowed to exceed a value of 1. When ( )C t is equal to 1, the opening has completely closed. Vertical subsidence displacements are generally larger than the associated horizontal displacements. Tilt (the rate of change in vertical subsidence with respect to horizontal direction) is another measure that is used in evaluating the effects of surface subsidence. The horizontal surface strains and surface tilt are calculated from the surface displacements ( xS , yS , and )zS as follows: x

yHorizontal strain in Easting ( ) direction Horizontal strain in Northing ( ) direction x

y

x Sx

y Sy

∂= ε =∂∂= ε =∂

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xyxy

Surface shear strain Tilt in the Easting ( ) direction Tilt in the Northing ( ) direction

y x

z

z

S Sx dy

x T Sx

y T Sy

∂ ∂= γ = +∂∂= =∂∂= =∂

5.2 SUBSIDENCE COMPONENTS The most common subsidence effects include the following:

• Change in elevation • Differential settlement or angular distortion, also includes curvature • Horizontal strains (tension and compression) • Tilt or slope change. Figure 5-1 depicts these different subsidence components that result from the closure of a mined opening.

Figure 5-1. Components of Ground Movement as a Result of Subsidence. Vertical displacements of the ground surface are generally the most evident result of subsidence. For a given mined opening, the largest vertical displacement occurs over the center of the opening, as shown by the subsidence profile in Figure 5-1. The angle of draw defines the lateral extent of measureable effects of underground mining at the surface. As shown in Figure 5-1, angle of draw is the angle between a vertical line at the edge of the mined opening and a line extending from the edge of the mined opening to the edge of the subsidence bowl on the surface.

Ore Body

Original Ground Surface

Angle of Draw(Limit Angle)

Panel Width

SubsidenceProfile

Horizontal StrainProfile

Tilt Profile

Overburden

+ Strain (Compressive)

– Strain (Tensile)

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Horizontal displacements of the ground surface result in tensile and compressive strains (expressed as a change in length per unit length). The strains are generally tensile just outside of the perimeter of the mined opening and compressive inside the perimeter, as shown in Figure 5-1. Tilt is the slope of the ground surface and is expressed as the change in elevation per horizontal-unit length. As shown in Figure 5-1, tilt values are generally greatest near the edges of mined openings. 5.3 SUBSIDENCE COMPARISON A subsidence analysis was performed using conservative estimates from the potential mine designs evaluated. When considering the impact of the potential mine-design changes on subsidence, a review of Equation 5-1 demonstrates that most input variables remain unchanged regardless of the potential design changes. The current reserves and ore height are established in the western expansion, and the proposed mining “blocks” and mining dates used to define the program have been established, as indicated in Figure 1-1. The distance from the mined blocks to the surface were estimated for each block based on geological maps provided by ARS and will not change based on variation in mine design. The only variable that could change significantly is the ER. Of the potential mine designs considered, the ER was either held constant with the existing mine design (approximately 60 percent) or reduced. The ER ratio of the main entries to access the yards was estimated to be 28 percent. This is slightly greater than the large-pillar design to account for the possibility for additional crosscuts. Therefore, the surface-subsidence impacts predicted from this approach represents an upper-bound for the western mine ER, which could potentially be reduced if design changes with a lower ER are implemented. The model parameter values of 0, , , and ss cy y Nβ control the volumetric room-closure rate and have been inversely fit from measured subsidence data at the Hampton Corners Mine. The fitted parameters include the impact of pillar punching and floor heave experienced through much of the mine and were used in the subsidence analysis of the western expansion. An additional consideration is that the potential mine-design changes have been developed to reduce floor heave in the mine, which if successful, would actually reduce the volumetric closure rate and, thereby, further reduce surface-subsidence rates. The subsidence modeling assumes a Year 1 exaction date in December of 2017. The proposed mining sequence was simulated over a 31-year period (2047) and additional subsidence was predicted until 2066 (50 years from the present year of 2016). The results of the subsidence modeling are described in the following sections. The subsidence modeling is focused on the western expansion area but includes the predicted impact of the existing mine. The predicted subsidence-related impacts in the western expansion are generally less than or similar to conditions predicted at the existing mine. 5.3.1 Change in Elevation Figure 5-2 shows contours of the estimated vertical subsidence predicted over the Hampton Corners western expansion at 5 years after mining was initiated in the western area. Figure 5-3 shows a similar plot after 10 years, and Figure 5-4 displays predicted subsidence contours at 50 years after mining. Figure 5-2 demonstrates that at no portion of the western mine expansion is predicted to have subsidence greater than 0.25 ft (3 inches) after 5 years, although the subsidence related to the existing mine can be seen. After 10 years, as illustrated in Figure 5-3, the western mine is predicted to have subsidence of less

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Figure 5-2. Estimated Vertical Subsidence (Feet) Over the Hampton Corners Western Expansion at 5 Years.

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Figure 5-3. Estimated Vertical Subsidence (Feet) Over the Hampton Corners Western Expansion at 10 Years.

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than 1 ft. The maximum vertical subsidence after 50 years of mining in the western expansion is less than 5 ft. To determine the angle of draw, Yao et al. [1991] suggests using 2 to 5 percent of the maximum subsidence to define the outer extent of the subsidence bowl. By using 0.25 ft as the outer extent (approximately 5 percent of the maximum subsidence value of 5 ft), the subsidence bowl can be defined as extending approximately 1,500 ft beyond the perimeter of the mineral rights. 5.3.2 Horizontal Strains Figure 5-5 shows contours of the predicted tensile strain (maximum principal strain) at 50 years after mining was initiated in the western area. Tensile strains are greatest around the mining area perimeter and around convex transitions to mined regions. The maximum predicted tensile-strain value is approximately 1.3 millistrain. Figure 5-6 illustrates contours of the ultimate compressive strain (minimum principal strain) at 50 years after mining was initiated in the western area. The compressive strains are greatest near the concave transitions to mined regions. The maximum predicted compressive strain is approximately -1.3 millistrain in the western mine area. 5.3.3 Tilt Tilting is caused by the nonuniform subsidence of a rigid foundation. Figure 5-7 illustrates contours of the predicted tilt over the mineral rights. Tilt values are greatest (approximately 0.2 to 0.3 percent) around the perimeter of the mining areas.

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Figure 5-4. Estimated Vertical Subsidence (Feet) Over the Hampton Corners Western Expansion at 50 Years.

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Figure 5-5. Estimated Tensile Strains (Feet/Feet) Over the Hampton Corners Western Expansion at 50 Years.

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Figure 5-6. Estimated Compressive Strains (Feet/Feet) Over the Hampton Corners Western Expansion at 50 Years.

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Figure 5-7. Estimated Tilt (Feet/Feet) Over the Hampton Corners Western Expansion at 50 Years.

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6.0 SUMMARY AND DESIGN OPTIONS

The primary objectives of this modeling effort were to evaluate potential mine-design options that could be used in the western expansion and evaluate the impact of these designs. The goals of the evaluated designs are to maintain or improve typical conditions in the mine, while also maintaining or improving far-field conditions (specifically, subsidence). The overarching findings from this study are: • Geological conditions west of the existing Hampton Corners Mine are considered suitable for mining and are, in fact, generally expected to be more favorable than conditions of the existing mine. This is because of the decreased overburden stress to the west, which improves in-seam performance metrics (i.e., floor heave, roof conditions, and salt conditions). • Several mine-design options have been identified that are predicted to further improve in-seam performance metrics. The potential mine-design models also predict subsidence and subsidence rates that are equivalent to or less than the current mine design. • Subsidence parameters have been determined by fitting the model to actual measured data from the existing mine. Within the time-period evaluated (50 years), the subsidence-related impacts (e.g., subsidence, subsidence rate, tilt, and strain) above the western expansion are predicted to be similar to or less than the impact predicted over the existing mine.

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7.0 REFERENCES Arnold, R. D., 2015. Mechanical Properties Testing of Core From Well 1303, American Rock Salt, LLC, RSI-2511, prepared by RESPEC, Rapid City, SD, for American Rock Salt, Mount Morris, NY. Agapito Associates, Inc., 2006. 5-Year Performance Assessment of Hampton Corners Mine–Model Analysis Comparison to Measurements, prepared by Agapito Associates, Inc., Grand Junction, CO, for American Rock Salt, Mount Morris, NY. Agapito Associates, Inc., 1995. Supplemental Information Geotechnical Design Summary Report, prepared by Agapito Associates, Inc., Grand Junction, CO, prepared for Akzo Nobel Salt, Inc., Clarks Summit, PA. Evans, K. F., 1989. “Appalachian Stress Study 3. Regional Scale Stress Variations and Their Relation to Structure and Contemporary Tectonics,” Journal of Geophysical Research, Vol. 94, No. B12, pp. 17,619–17,645, December 10. Frayne, M. A., 2000. Numerical Modeling of Pillar Designs at the Shaft Bottom Area, RSI-1303, prepared by RESPEC, Rapid City, SD, for American Rock Salt Co. LLC, Groveland, NY. Itasca Consulting Group, Inc., 2014. FLAC3D: Version 5.01 Fast Lagrangian Analysis of Continua in 3 Dimensions, Minneapolis, MN. Kim, K., 1995. Magnitudes and Orientations of Principle Stresses From Hydraulic Methods, prepared for Akzo Nobel Salt, Inc., Clarks Summit, PA, January 6. Maruyama, T., 1964. “Statistical Elastic Dislocations in an Infinite and Semi-Infinite Medium,” Bulletin of the Earthquake Research Institute, Tokyo University, Vol. 42, pp. 289–368. Nieland, J. D. and L. L. Van Sambeek, 2010. SALT_SUBSID, Version 2.0 User’s Manual, Research Report RR2009-02, prepared by RESPEC, Rapid City, SD, and POD, Inc., Albuquerque, NM, for Solution Mining Research Institute, Clarks Summit, PA. Nopola, J. R., S. J. Voegeli, and L. L. Van Sambeek, 2016. Geomechanical Modeling of Alternative Mine Designs for American Rock Salt, Hampton Corners Mine, New York, RSI-2584, prepared by RESPEC, Rapid City, SD, for American Rock Salt, Mount Morris, NY. Norton, F. H., 1929. Creep of Steel at High Temperatures, McGraw-Hill Book Company, New York, NY. SRK Consulting, Inc., 2013a. 5-Year Performance Subsidence Assessment of the Hampton Corners Mine Comparison to Monitoring Data,” prepared by SRK Consulting, Inc., Lakewood, CO, for American Rock Salt, LLC, Mount Morris, NY SRK Consulting, Inc., 2013b. Results of Room Dimension and Stability Comparison,” memorandum prepared by SRK Consulting, Inc., Lakewood, CO, for American Rock Salt, LLC, Mount Morris, NY, February. Van Sambeek, L. and B. Groff, 2014. Hampton Corners Salt Mine Pillar-Design Review Based on Observed Mine Behavior and Surface Subsidence Analysis, RSI-2440, prepared by RESPEC, Rapid City, SD, for American Rock Salt, Mount Morris, NY .

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Van Sambeek, L. L., J. L. Ratigan, and F. D. Hansen, 1993. “Dilatancy of Rock Salt in Laboratory Tests,” International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, Vol. 30, No. 7, pp. 735–738. Yao X. L., D. J. Reddish, and B. N. Whittaker, 1991. “Influence of Overburden Mass Behavioural Properties on Subsidence Limit Characteristics,” Mining Science and Technology, 13, 167–173.