Overview of Mine Management Houmao Liu Itasca Denver, Inc. [email protected]

● ● GEOMECHANICS ● GEOMECHANICS● MICROSEISMICS ● HYDROGEOLOGY ● MINING ● CIVIL ● MICROSEISMICSENERGY ● MINING ● CIVIL ● ENERGY 1

Introduction

● ● GEOMECHANICS ● HYDROGEOLOGY GEOMECHANICS● MICROSEISMICS ● HYDROGEOLOGY ● MINING ● CIVIL ● MICROSEISMICSENERGY ● MINING ● CIVIL ● ENERGY 2

What is Mining Hydrology?

● GEOMECHANICS ● HYDROGEOLOGY ● MICROSEISMICS ● MINING ● CIVIL ● ENERGY 3

Impacts of Water on Mining • Pore pressures – reduce slope stability • Inflows and “muck rushes” in underground mines • Direct costs of dewatering (, pumps, pipelines, power) • field design for in situ recovery • Blasthole instability and need for waterproof explosives • Higher maintenance of mining equipment (e.g., tires) • Periodic slowdowns/shutdowns due to pump failure (mechanical or electrical) and water freeze-up • Conduction of heat in underground mines • Engineering costs (specialized staff and consultants)

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Impacts of Mining on Water Resources • Disposal (and possible prior treatment) of mine water

• Impacts on water rights

• Environmental impacts

● GEOMECHANICS ● HYDROGEOLOGY ● MICROSEISMICS ● MINING ● CIVIL ● ENERGY 5

What’s Your Problem? Quantities of Inflow • When? • Where? • How much?

Geotechnical Concerns • Pore pressures • Swelling or slaking of rock • Mud rushes

Environmental Impacts • Loss of water resources • Degradation of water quality

…or all of the above!

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Mine Water Issues – You Just Can’t Win

K Issue(s) Mitigation Pros Cons 1) Relatively easy and 1) High cost (wells, power, pipelines, Lots of water fast to dewater water treatment facilities) High to pump and Dewatering wells 2) Water can be used for 2) Potentially major environmental discharge water supply impacts 1) Sub-horizontal High pore 1) Reduces factor of safety in slopes drainholes Can target specific Low pressures in 2) Depressurizing requires relatively 2) Drainage problematic areas slopes long period of time to be effective galleries

● GEOMECHANICS ● HYDROGEOLOGY ● MICROSEISMICS ● MINING ● CIVIL ● ENERGY 7

Quote from Read and Stacey (2009) • “Both groundwater pressure and surface water flow aspects … may have significant negative effects on the stability of a slope,…”

• “These aspects are usually the only elements in a slope design that can be readily modified by artificial intervention…”

● GEOMECHANICS ● HYDROGEOLOGY ● MICROSEISMICS ● MINING ● CIVIL ● ENERGY 8

How It Fits into the “Big Picture”

Mine Planning

Geotechnical Hydrogeology

DESIGN FACTORS

● GEOMECHANICS ● HYDROGEOLOGY ● MICROSEISMICS ● MINING ● CIVIL ● ENERGY 9

Methods of Mine Dewatering

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Why Dewater? • Eliminate/minimize inflows to pits and underground mines (you don’t want to mine under water!) • Improve slope stability • Eliminate/minimize inflow and “muck rushes” in underground mines • Eliminate/minimize need for waterproof explosives • Minimize maintenance of mining equipment (e.g., tires) • Improve cooling in hot mines • Eliminate/minimize ice problems in cold environments

Increase Safety and Minimize Risk! Reduce Costs!

● GEOMECHANICS ● HYDROGEOLOGY ● MICROSEISMICS ● MINING ● CIVIL ● ENERGY 11

Example of Drainage vs. Depressurizing

● GEOMECHANICS ● HYDROGEOLOGY ● MICROSEISMICS ● MINING ● CIVIL ● ENERGY 12

Example of Highwall Depressurization

SUB-HORIZONTAL DRAIN

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Dewatering – Terminology

Draining Dewatering…………... Depressurizing Passive inflow – seepage

Active dewatering – wells, drains, galleries, trenches

Residual passive inflow – seepage during active pumping

● GEOMECHANICS ● HYDROGEOLOGY ● MICROSEISMICS ● MINING ● CIVIL ● ENERGY 14

Methods of Dewatering Active Methods • Perimeter wells • In-pit wells • In-pit sub-horizontal drainholes • Drainage galleries Passive Methods • In-pit sumps Alternatives or Methods (Combined with Dewatering) • Grouting • Freezing

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Methods of Dewatering (cont.)

GROUT OR FREEZE WALL

ULTIMATE PIT BOUNDARY

“HORIZONTAL”SUB-HORIZONTAL DRAINHOLESDRAINHOLES

SUMP IN-PIT WELL

DRAINAGE GALLERY PERIMETER WELL WITH DRAINHOLES

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Summary of Dewatering • In order to understand the potential response of the groundwater system to mining, you must characterize the K of the rock mass to the extent possible.

• Choose an appropriate dewatering method and have specific dewatering targets.

◦ Best method is mine-specific – there is no “standard” method.

◦ Hybridization of methods is common and often required/effective.

• Cut off recharge to mine, if possible, by grouting, freezing, or intercepting run-off.

• Recognize your time constraint; in low-K materials, start soon!

• Monitor properly.

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Dewatering of an Open Pit • Dewatering Strategies Based on Conceptual Model: • Large dewatering wells for limestone • Small in-pit dewatering wells for saturated geologic units in the vicinity of mudstone • Plan of Dewatering Wells Based on Numerical Model: • Pumping rate • Commission schedule • Prediction of Chloride Concentration Based on Solute Transport Model: • Chloride concentration over the life of the mine

● GEOMECHANICS ● HYDROGEOLOGY ● MICROSEISMICS ● MINING ● CIVIL ● ENERGY 18

Depressurization of a Large Hardrock Open Pit • Depressurization Strategies • Scavenger wells to intercept shallow groundwater • In-pit subvertical drainholes to release the pressure at the toe of the slope • Sub-horizontal drainholes to depressurize the pit slope • Mobilized pumping will be implemented to further lower the water level below the pit floor

• Close Interactions between Geomechanical and Groundwater Flow Model • Target phreatic surfaces based on the factor of safety • Design the depressurization system based on the groundwater flow model • Provide guidance for the mine to implement the depressurization program

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Depressurization of a Large Mudstone Open Pit

• Massive Mudstone • Low yields • Challenges to depressurize the slope • Highly Permeable Mea • Unique opportunity of depressurizing the mudstone through underdrain • Groundwater Flow Model Is Used to Guide the Underdrain Program

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Inflow to Underground Mine

• 450-m-Thick Sandstone over Underground Mine • Potential inflow to mine workings under normal conditions • Potential inflow to mine workings if roof fails • Design Pumping Capacity for the Inflow with Roof Failure • Groundwater flow model was used to predict the inflow rates

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Groundwater Flow for Caving Operation

• Disturbed Zone • Increased permeability • Potential increase of flow rate • Mud Rush Potential • and saturation of caved materials

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Groundwater Flow for Room Pillar Operation

• Disturbed Zone • Increased permeability • Increase of flow rate • Inflow to Each Mining Zone • Pumping rate for each mining zone is required to maintain dry working condition

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Water Quality and Environmental Consideration

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Conceptual Diagram of Development of ARD in Open-Pit Mine

Pre-Mining Groundwater Table

Rainfall Infiltration

Surface-Water Runoff

ARD Runoff Groundwater Level Groundwater Flow ARD Seepage

Surface and Groundwater Post-Mining ARD Seepage Flow through Sulfide Groundwater Table Wall Rock and Rock Debris

Water That Collects in the Bottom of the Pit is Lost by a Combination of Seepage, Evaporation, and Surface Discharge

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Processes Involved in Evolution of Pit Lake

PRECIPITATION SEEPAGE FACE EVAPORATION ALLUVIUM

MINERAL OXIC ZONE BIOLOGICAL OXIDIZED PRECIPITATION ACTIVITY OXIDIZED AND DISSOLUTION

ADSORPTION UNOXIDIZED MIXING UNOXIDIZED

Limestone ANOXIC ZONE SLUDGE

Shale

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Environmental Components Related to Hydrogeology • Hydrogeologic and surface-water data collection and analysis (continued);

• Documentation of existing (pre-project) baseline hydrologic conditions;

• Geochemical characterization of waste rock (and ore) and groundwater;

• Evaluation of potential impacts to water resources; and

• Development of monitoring and mitigation plans.

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Construction, Operations, and Closure Water-related environmental considerations during these phases include:

• Regulatory compliance monitoring and reporting; • Mitigation and remediation activities; • Preparing for closure and long-term water management in mining voids (open pit and/or UG workings) and ancillary facilities; and • Assisting with bond release (e.g., plugging and abandoning wells or reclaiming RIB sites).

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Concluding Remarks

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Concluding Remarks • Plan ahead with involvement of other departments Now What??

• Conduct hydraulic investigation and install monitoring system at different project stages

• Groundwater is critical to mine operations

• Environmental considerations are important in all phases of the mining life cycle

• Do not underestimate the time or effort required to address environmental concerns

● GEOMECHANICS ● HYDROGEOLOGY ● MICROSEISMICS ● MINING ● CIVIL ● ENERGY