Ask the Experts: Numerical Model Applications

Introduction to the Webinar

Overview of the Webinar

• This webinar focuses on "Water in Mining: Numerical Model Applications" and is structured as an "Ask the Experts" session, allowing attendees to engage with a panel of experts.

• The event will last for 90 minutes, providing ample time for discussions and questions.

About the Geotechnical Center of Excellence (GCE)

• The GCE at the University of Arizona is industry-funded and member-led, aiming to bridge gaps in geotechnical research through collaboration across academia and industry.

• Interested individuals can learn more about GCE via email, LinkedIn, or their website.

Course Details

Water and Mine Operations Course

• Today's topic relates to Section Three of a course on water and mine operations focusing on numerical model applications. Other sections cover various aspects relevant to mining operations.

• A QR code is provided for those interested in enrolling in the course, with additional links shared in the chat by Allison Brucey.

Acknowledgments

Support from Partners

• Thanks are extended to Pau Associates and the Large Open Pit Project for their support in developing this course alongside GCE members who fund research projects and webinars.

Panel Introduction

Meet the Panelists

• Jillian Nunan serves as moderator; she is a senior R&D engineer with GCE. She introduces other team members including program coordinator Allison Brucey and computing sciences researcher Christian Ortman.

• Julia Potter, director of GCE, also joins as part of the team facilitating today's discussion.

Expert Panelists

• Jeff F., Global Technical Adviser at Pau Associates, expresses enthusiasm for engaging discussions during the webinar.

• Jeremy Dowling (Hydrogeology Adviser & President at Pau Associates) shares his excitement about participating in today’s session.

• Phil de Grath (Director & Principal Engineer at Giraff Mining Consultant) highlights his involvement as a presenter in related courses.

Additional Panel Members

• Jim Wiel (Principal Hydrogeologist with Rio Tinto Kennecott Copper) greets attendees from Bingham Canyon Mine while Travis White (Principal Hydrogeologist with Anglo American) joins from South Africa expressing gratitude for being part of this event.

Engagement Guidelines

Interaction During Webinar

• Attendees are muted but can communicate via chat or submit questions through a Q&A feature available on their control panel throughout the session. Questions can be submitted anonymously if desired.

Voting Mechanism

• Participants have an option to vote on submitted questions using a thumbs-up feature which helps prioritize popular inquiries during discussions later on in the webinar.( t = 363 s )

Recording Information

Educational Purpose Disclaimer

Numerical Model Applications in Geotechnical Analysis

Introduction to the Webinar

• The webinar focuses on numerical model applications related to water and mine operations, particularly slope stability.

• Participants are encouraged to ask questions about topics such as planning numerical models, water input for geotechnical analysis, and coupled groundwater and pore pressure models.

Importance of Modeling

• A question is posed regarding the significance of modeling in geotechnics, specifically how water pressure affects geotechnical parameters and slope stability.

Influence of Water Pressure on Geotechnical Parameters

• Jeff F explains that effective stress is a key concept; it describes how water pressure counteracts the weight of material above a surface.

• Increased water pressure reduces frictional resistance between surfaces (e.g., two plates of glass), making them easier to move.

Mechanisms Affecting Slope Stability

• Water creates an upward force that diminishes downward stress, leading to reduced shear strength on surfaces affected by water.

• This principle applies not only to flat surfaces but also porous media like sand, where water can reduce frictional resistance among grains.

Driving Force of Water in Slope Stability

• The driving force exerted by water can push slopes outward, especially when tension cracks fill with water.

• This effect becomes more significant when structures are parallel to slopes, as they may exacerbate instability due to increased hydraulic pressures.

Conclusion and Further Learning

• Understanding these fundamental concepts is crucial for assessing risks associated with mining slopes or underground mining operations.

Groundwater Conditions and Ru Factor Analysis

Understanding Groundwater Dynamics

• Groundwater conditions are rarely static; they fluctuate due to transient pore pressure changes from rainfall, snowmelt, and other factors.

• Long-term fluctuations in groundwater can lead to ongoing rock mass damage, as demonstrated by PhD studies on cyclical pore pressure loading and unloading affecting joint strength.

• The temporal aspect of groundwater conditions is crucial for stability analysis, emphasizing the need for continuous monitoring.

Ru Factor in Stability Analysis

• A participant asks about the appropriate range of Ru factors for stability analysis concerning water infiltration from rain and perched water.

• Phil references Lauren's paper presented at a conference that details how to estimate Ru factors based on calibrated data from case studies.

• Overestimation of Ru numbers is common; values greater than 0.2 for hard rock conditions are considered overly pessimistic.

Practical Applications of Ru Factors

• The ratio of downward force to upward force (Ru factor) is simplified as the weight of rock being approximately twice that of water under hydrostatic conditions.

• Typical Ru values are around 0.35 for hydrostatic conditions; sub-hydrostatic conditions usually yield lower values while artesian conditions may exceed this threshold.

Instrumentation and Data Collection

• Operators are encouraged to install piezometers at various depths to monitor shallow water conditions effectively during transient events like rainfall or snowmelt.

• More than half of global slope instability cases relate to surface water infiltration, highlighting the importance of understanding shallow slope conditions.

Site-Specific Considerations

• Jeff shares insights from their site regarding increases in Ru during snowmelt periods, with average changes around 0.8 but localized areas reaching up to 1.2 or higher.

Groundwater Modeling in Excavation Damage Zones

Understanding Variability in Groundwater Presence

• The amount of water in shallow, transient parts of the pit slope can vary significantly due to fracture distribution and local recharge sources.

• A sensitivity-based approach has been utilized, incorporating a three-dimensional model with calibrated groundwater flow data to assess the excavation damage zone.

Assessing Geotechnical Vulnerability

• The geotechnical model includes a range of Ru values to identify which sectors of the slope are most vulnerable to stability issues related to shallow and transient water.

• Future monitoring efforts will focus on areas identified as having high geotechnical vulnerability.

Numerical Codes for Fractured Crystalline Media

Optimal Numerical Code Selection

• A question arises regarding the best numerical code for modeling fractured crystalline media like copper deposits, emphasizing that the choice may not be as critical as previously thought.

• The effectiveness of a model is more dependent on geological understanding, quality data, and calibration rather than solely on numerical code selection.

Importance of Conceptual Models

• Poor conceptual models contribute significantly to errors in numerical modeling; thus, assumptions made during modeling are crucial.

• DFN (Discrete Fracture Network) models assume homogeneity in fracture properties but may not accurately represent variability at different scales.

Fracture Flow Dynamics

Key Insights into Fracture Behavior

• Flow dynamics are influenced by "pinch points" within fractures rather than average aperture sizes; these pinch points control overall flow rates.

• While DFN models provide insights into anisotropy, they should not be relied upon for precise details due to inherent variability among fractures.

Practical Applications and Limitations

• In practice, many fracture systems exhibit negligible hydraulic conductivity due to being clay-filled or mineralized, limiting their utility in certain contexts.

• Emphasis is placed on using codes that practitioners are comfortable with to avoid unnecessary learning curves and errors.

Conclusion: Model Implementation Strategies

Critical Factors for Successful Modeling

• A successful model relies heavily on a robust conceptual framework supported by comprehensive datasets rather than just code selection.

Understanding Hydrogeology and Modeling in Mining

Importance of Conceptual Models

• The effectiveness of hydrogeological models relies on understanding which fractures are significant for groundwater flow, necessitating extensive data collection and site comprehension.

• A robust conceptual model is crucial; the quality of inputs directly influences the utility of any model used in mining contexts.

Model Selection and Practicality

• Different modeling codes have varying practical applications, particularly in mining scenarios like block caving; however, the choice should align with specific modeling objectives.

• Assumptions made during modeling significantly impact outcomes more than the choice of code itself.

Utilizing Models for Data Validation

• Models serve as tools to identify discrepancies between predictions and actual field data, prompting further investigation into potential geological features such as unmapped faults.

• Continuous refinement of conceptual models is necessary based on real-world observations to enhance accuracy.

Geological Insights and Limitations

• Many instabilities observed in slopes can be attributed to unexpected geological conditions, emphasizing that geology often defies statistical analysis.

• Ground truthing is essential; numerical models should complement rather than replace fieldwork and analytical methods.

Challenges with Numerical Models

• There exists a tendency among regulators and corporate management to over-rely on numerical models without fully understanding their limitations, leading to potential misjudgments.

Modeling Pore Pressure and Groundwater: Insights and Tools

Overview of Software for Modeling Pore Pressure

• Discussion on software packages available for modeling pore pressure, emphasizing that any numerical code can provide answers regarding pore pressure.

• Mention of MODFLOW and FIFLOW as standard codes capable of determining pore pressure; clarification needed on what is meant by "coupled groundwater."

• Introduction to CW (Coupled Water) and Sigma W as effective tools in 2D groundwater modeling, particularly for capturing geotechnical parameters.

Coupled Solutions in Tailings Modeling

• Importance of coupled solutions in tailings modeling; Sigma W is highlighted as a suitable 2D code for this purpose.

• Discussion on the limitations of Flack for groundwater simulations but acknowledges its capability to simulate coupled geotechnical conditions.

Open Source Codes and Resources

• Clarification that there are no widely recognized open-source codes specifically for coupled hydro-geomechanical simulation.

• Suggestion to contact USGS, specifically Rich Nicewonger, for updates on open-source developments related to groundwater flow models.

Comparing Fan-Shaped vs. Parallel Drain Holes in Dewatering

Advantages and Disadvantages of Drain Configurations

• Introduction to the question about comparing fan-shaped versus parallel drain holes; emphasis on drilling techniques based on structural orientation.

• Explanation that drilling parallel holes may be more effective when targeting subparallel structures behind slopes.

Practical Considerations in Drilling Techniques

• Discussion on the practicality of using fans when access is limited or when variable structural orientations exist.

• Emphasis on installing horizontal piezometers before drain holes to monitor pressure reduction effectively.

Real World Applications and Experiences

Drainage Strategies in Mining Operations

Overview of Drainage Techniques

• Discusses the potential benefits of using a common collar for drilling, which simplifies water management from drains by reducing the number of collector pipes.

• Highlights the practicality of dedicated underground cubbies for parking rigs and drilling fans, emphasizing that orientation is dictated by structural considerations.

• Suggests that when drilling into pit slopes, it's crucial to consider both small-scale fabric and larger structures in rock to optimize drainage effectiveness.

Importance of Geological Framework

• Stresses the need for designing drain programs based on geological frameworks to ensure effective depressurization and drainage.

• Jim adds that orienting drains favorably can lead to significant cost savings while improving depressurization outcomes.

Operational Insights

• Jim shares experiences from Bingham pit where fan configurations allowed more efficient drainage with fewer drill pads needed due to operational constraints.

• Describes instances where unexpected high-angle features were encountered during probing, leading to significant water flow rates that were not anticipated.

Insurance Policies in Drainage

• Emphasizes the role of horizontal drain drill rigs as an insurance policy against unforeseen geological conditions affecting large walls.

• Notes the importance of physical observations over models in guiding drainage strategies while still attempting to incorporate additional monitoring points.

Alternative Drilling Configurations

• Travis mentions using vertical and subvertical wells when access issues prevent horizontal drilling, highlighting cost-effectiveness through cheaper drilling methods and low-cost pumping solutions.

Modeling and Predictive Scenarios

Drain Modeling and Practical Applications

Importance of Field Data in Drain Planning

• The model allows for easy comparison of different drain configurations, but it is essential to rely on practical field data and petric responses for effective planning of drain drilling programs.

• While the model provides a general sense of potential outcomes with drains, actual field data is crucial for planning expanded drain programs.

Complexity vs. Simplicity in Modeling

• A balance must be struck between complexity and capability; only include necessary complexity to capture key processes without adding unnecessary details that do not influence outcomes.

• Excessive complexity can obscure understanding; it's vital to maintain clarity about hydrogeology and validate models against real-world conditions.

Model Verification and Data Collection

• Emphasizing the importance of being "approximately right" rather than "precisely wrong," one should avoid unjustifiable complexities in modeling.

• If insufficient data exists, acquiring more data is critical; convincing clients of this need can be challenging but necessary for accurate modeling.

Benchmarking as a Validation Tool

• In situations lacking large-scale pumping trials, benchmarking against similar sites can provide valuable insights into expected conditions at the project site.

Monitoring Over Complex Modeling

• Emphasis should be placed on monitoring rather than overly complex numerical modeling; simple groundwater assumptions integrated into geotechnical models are often more effective.

Current Approaches to Quantifying Uncertainty in Model Predictions

Overview of Uncertainty Quantification

• A question is raised about current methods for quantifying uncertainty in model predictions and how this information is communicated to decision-makers, including regulators.

Common Methods for Uncertainty Analysis

• Two prevalent approaches are identified: stochastic methods, which involve statistical distributions on inputs, and deterministic bracketing of uncertainties.

• Stochastic methods may dilute key insights; thus, understanding the main drivers of uncertainty through deterministic analysis is often more effective.

Communication of Risk and Trade-offs

• Effective communication involves outlining risk levels, likelihood, consequences, and trade-offs associated with mitigation measures. Decision-makers prefer clear cost-benefit analyses related to risks.

Importance of Geological Data

• The geological context significantly influences model predictions; field data should be used to validate models rather than relying solely on stochastic analysis.

• Hydrogeological uncertainties can lead to significant deviations from predicted outcomes due to operational assumptions that may not hold true during implementation.

Sensitivity Analysis Techniques

• One method discussed involves running sensitivity analyses by varying pressure conditions (e.g., increasing head by 50 feet or more) to observe effects on geotechnical models.

• For long-term projects, it’s crucial to update models regularly based on new data as geological conditions evolve over time.

Best Practices for Model Updates

• Continuous updates are recommended throughout a project's lifecycle; regular iterations every one to two years help align predictive models with real-world changes.

Deep Mining and Hydrogeological Modeling

Differences in Modeling Approaches

• The distinction between modeling for pit slopes and water resources is highlighted, emphasizing that the approach to sensitivity complicates the process but makes it more understandable and practical.

Addressing Deep Mine Challenges

• A question arises regarding the adequacy of current numerical modeling methodologies as mines reach depths of 1,000 to 1,500 meters or more, noting a lack of precedence for such high slopes.

Importance of Historical Data

• John suggests Jim should address how deep mining impacts modeling. Jim emphasizes reliance on historical data from their operations to monitor slope behavior effectively.

Monitoring Techniques in Deep Pits

• Jim discusses using natural pump tests during downtime of large pumps to gather data on pore pressures around underground workings, which aids in understanding water movement across structures.

Methodology Consistency Despite Depth Changes

• It is asserted that hydrogeologic characterization methods remain crucial even as pits deepen. The importance of accumulating historical data from instruments over time is reiterated.

Conceptual Models and Drainage Solutions

• Maintaining consistent methodologies is essential; understanding conceptual models and monitoring them closely are key. For large pits with steep slopes, drainage tunnels can mitigate risks associated with slope failures.

Proactive Measures for Slope Management

• Jim's proactive implementation of drainage tunnels allows flexibility in mine planning without compromising overall slope performance due to water issues. This strategy enhances confidence in managing deeper pits effectively.

Examples from Major Mining Operations

Understanding Slope Management in Mining

Importance of Access and Monitoring

• The necessity for periodic wide catch benches is emphasized to maintain permanent access for monitoring, preventing a "blind" situation where data cannot be collected.

• Implementation of 75 to 100-foot-wide benches allows for the installation of hydrogeologic and geotechnical instrumentation, significantly enhancing monitoring capabilities.

• Plans are underway to install wells deeper than the current mine bench for pre-depressurization ahead of upcoming mining slices.

Benefits of Drainage Galleries

• A drainage gallery or tunnel can serve as an access drive for underground mining, providing early and permanent access as open-pit operations transition underground.

• Developing a drainage gallery at the right time can facilitate resource characterization through underground drilling, improving understanding of geological conditions.

Numerical Modeling Considerations

• Transitioning to high slopes necessitates a change in mindset regarding management and design; numerical modeling becomes crucial for stability analysis.

• Various numerical models (2D and 3D) should be employed instead of limit equilibrium analyses to capture modes of instability effectively.

Challenges in Underground Mining Operations

Addressing Water Ingress from Drill Holes

• Ungrouted exploration drill holes pose significant challenges by allowing water ingress into underground mines, complicating hydrogeological assessments.

• Historical issues with water pressure due to surface connections via old drill holes highlight the need for proactive measures like cementing these holes.

Importance of Monitoring Programs

• Effective monitoring programs are essential during exploration phases; operators must balance operational goals with necessary data collection efforts.

Discussion on Water Management in Mining Operations

Importance of Analyzing Material Properties

• The transient changes in material properties are crucial when assessing block caves, emphasizing the need for thorough analysis.

• A diamond drill hole analysis conducted by Anglo American is highlighted as a valuable methodology that helps mitigate risks associated with mining operations.

Learning from Experience

• Personal experiences shared about managing significant water inflow (1500 liters per second) during early career stages illustrate the importance of preemptive exploration drill holes.

• Reflection on a past mine closure reveals that groundwater recovery can take time, underscoring the long-term impacts of water management decisions.

Approaches to Modeling Water Storage

• Two main approaches for modeling water storage are discussed: equivalent storage and discrete feature methods, each suited for different geological contexts.

• The choice between these methods depends on factors like drift size and rock mass characteristics, impacting model accuracy.

Capturing Hydraulic Responses

• It’s essential to accurately capture void evolution from native rock to void space and any backfill processes within predictive simulations.

• Ensuring that hydraulic responses align with applied models is critical; verification of model functionality is necessary after applying different choices.

Closing Remarks and Acknowledgments

• Appreciation expressed towards panel experts for their insights and contributions during the discussion.