Ask the Experts: Understanding the Conceptual Hydrogeology Model

Name: Ask the Experts: Understanding the Conceptual Hydrogeology Model
Uploaded: 2024-08-05T21:52:12.000Z
Duration: 2 h 58 min 13 s

Introduction to the GCE Ask the Experts Webinar

Overview of the Webinar Series

The webinar is part of a series focused on water and mining operations, specifically addressing slope stability.

This session will last 90 minutes, including an introduction followed by a Q&A segment for both pre-submitted and live audience questions.

About the Geotechnical Center of Excellence (GCE)

The GCE is based at the University of Arizona, Tucson, and operates as an industry-funded, member-led center.

It focuses on interdisciplinary collaboration between academia and industry to enhance geotechnical research and education.

Course Details

The related course covers various topics from site characterization to numerical modeling applications in water management.

Development partners include Pau Associates, which provides resources such as guidelines for evaluating water in pit slope stability.

Acknowledgments and Introductions

Recognition of Supporters

Acknowledgment is given to GCE member companies that support webinars and educational projects.

Introduction of Panelists

Jillian Nunan serves as moderator; Julia Potter is the director; Christian Ortman is a computing sciences researcher with GCE.

Expert panel includes Jeff Beel (Global Technical Adviser), Jeremy Dowling (Hydrogeology Adviser), Simon Shaw (Principal Hydrogeologist), Christian Cacy (Lead for Water for Americas), Lauren Loric (Principal Consultant), Yos Ryel (VP Mine Water Management), and John Rup (Principal Hydrogeologist).

Participant Engagement Guidelines

Communication During the Webinar

Participants are muted but can engage via chat or Q&A features.

Questions can be submitted anytime through the Q&A button; participants can also vote on questions they want prioritized.

Recording Information

Understanding the Conceptual Hydrogeological Model

Introduction and Context

Participants are encouraged to verify information and consult qualified professionals for specific concerns.

The webinar aligns with a course on water and mine operations, specifically focusing on the conceptual hydrogeological model as part of a series of five webinars covering various topics.

Key Components of the Conceptual Model

Important questions related to the conceptual hydrogeological model include understanding components like stratigraphy, structure, recharge, hydraulic properties, groundwater flow paths, surface-groundwater interaction, discharge, and mining operation influences on site hydrology.

Participants are asked to focus their questions on these topics; future webinars will address numerical modeling applications and slope depressurization programs.

Question & Answer Session Begins

The Q&A session starts with pre-submitted questions. Participants can also submit live questions during this segment.

The first question addresses constructing a conceptual hydrogeological model using 2D geological information.

Discussion on 2D vs. 3D Models

A panelist states that constructing a model solely from 2D data is not feasible; one must consider a 3D perspective for accurate representation.

Another panelist emphasizes that while 2D cross-sections illustrate aspects of models, they do not constitute complete conceptual geological models.

Importance of Comprehensive Data

Panelists agree that while 2D sections are useful for displaying data, they should be derived from comprehensive 3D geological information.

In some cases where only limited data is available (e.g., magnetic surveys), it’s crucial to utilize all accessible information to construct an effective conceptual model.

Revisiting and Updating Models

The discussion shifts towards the necessity of revisiting conceptual models whenever new information arises or if system behavior changes unexpectedly.

Hydrogeological Modeling and Updates

Importance of Updating Models

The scale of the model influences its updates; localized slope models can be updated without altering larger district models unless significant changes occur.

Continuous revision of the big picture is essential as new data emerges, with suggested updates every couple of years unless major changes arise.

New instabilities or redesigns in slopes may necessitate updates to slope-scale understanding, while district scales are less frequently revised.

Agency Requirements and Data Gathering

Some environments mandate annual updates for numeric models due to agency requirements, driving the need for timely data integration.

While annual updates are not common practice, they do exist in certain contexts, highlighting variability in monitoring practices.

Hydrogeological Unit Definition

Combining hydrogeological units should start with analyzing hydraulic stress responses through pore pressure and water level data.

Key features that define hydrogeological boundaries include lithological contacts and structural boundaries like fault zones.

Geotechnical Characteristics

Geotechnical assessments focus on fracture frequency and orientation to understand how these factors influence hydrological behavior at a smaller scale.

Blockiness and joint condition significantly impact the definition of conceptual units for further development in geotechnical studies.

Mining-Induced Effects on Geological Units

Three distinct zones are identified around mining activities: blast damage zone (mechanical breakage), zone of relaxation (stress relief), and an unaffected zone beyond mining influence.

Understanding Hydrogeological Models and Their Components

Importance of Composite Logs and Hydraulic Testing

Emphasizes the necessity of using composite logs at a consistent scale, highlighting that hydraulic testing is crucial for defining geological units and understanding hydrogeologic behavior.

Enhancing Hydrogeologic Conceptual Models

Discusses how consulting various geotechnical data sets can enhance hydrogeologic conceptual models, particularly in identifying faults that provide valuable information about hydrogeologic characteristics.

Relationship Between Geotechnical Data and Hydrogeology

Notes that the properties of faults, such as thickness and fill material, often correlate with important hydrogeologic properties like barrier or conduit types.

Structural Fabric's Role in Permeability

Highlights the significance of structural fabric orientation in introducing anisotropy into permeability directions within rock masses, stressing the need for site-specific interpretations.

Integrating Data into 3D Frameworks

Advocates for integrating geotechnical and hydro data sets into a 3D framework (e.g., Leap Frog), allowing for a comprehensive understanding of three-dimensional relationships among geological components.

Simplifying Conceptual Models for Specific Applications

Suggests keeping conceptual models simple by aligning detail levels with specific questions being addressed, such as determining appropriate cell sizes for regional groundwater models.

Initial Focus on Water Level Data

Recommends prioritizing water level data over permeability data when assessing mine sites to understand connectivity between boreholes based on water level similarities or differences.

Correlating Water Levels with Structural Models

Stresses the importance of correlating variations in water levels with structural features to refine definitions of hydrogeological models and flow systems before analyzing permeability numbers.

Excavation Damage Zones in Underground Operations

Addresses whether there are equivalent excavation damage zones in underground operations compared to open pits, noting their smaller scale but still significant impact on water flow around tunnels or stopes.

Case Studies on Tunnel Convergence and Stability

Shares examples where mining parallel to structures led to high convergence rates causing failures due to unexpected water ingress, illustrating the critical relationship between excavation practices and hydrology.

Challenges in Dewatering Systems for Stopes

Excavation Damage Zones and Pore Pressure Dynamics

Understanding Excavation Damage Zones (EDZ)

The term "excavation damage zone" (EDZ) originated in the nuclear waste isolation industry, focusing on preventing water from entering or leaving storage facilities.

Confinement is crucial; structures parallel to slopes experience minimal confinement, leading to easier openings compared to highly confined circular tunnels.

Characteristics of Damage Zones

The excavation damage zone around a tunnel may only extend 1-2 meters, while in sloped areas, it can reach tens of meters.

An example discussed involved slab failure due to structural positioning behind a decline, illustrating how proximity affects stability.

Pore Pressure and Stability

A question raised about characterizing the nucleus of the damage zone relates to pore pressure changes potentially triggering instability.

If pore pressure drops significantly after excavation, it indicates that the structure may be dilated and not receiving recharge.

Hydrogeological Data Importance

Hydrogeological data is emphasized as more critical than geological characterization; pore pressure data ultimately dictates stability outcomes.

Damage zones are heterogeneous with variations due to blast damage and stress relief; thus, a robust monitoring network is essential for pit slope design.

Testing Relationships Between Pore Pressure and Safety Factors

To test relationships between pore pressure and safety factors effectively, understanding fault properties is necessary for sensitivity analysis in slope stability models.

Lab tests on faults are challenging due to variability along strike and dip; field tests also face limitations in measuring permeability accurately.

Conclusion on Fault Behavior

The response of faults can be monitored through pore pressures which indicate whether water ingress occurs.

Characterizing Fault Strength and Pressure Gradients in Mining

Importance of Material Testing

Discusses the significance of testing clay materials to characterize their strength, particularly in relation to faults.

Highlights the practical challenges of placing pressure sensors (Peters) on major structures but suggests assessing orientation against the sensor data for insights.

Variability in Geological Structures

Notes that geological structures can exhibit significant variability over short distances, leading to unexpected issues like water inflow when passing through faults.

Emphasizes that adjacent headings may not reveal fault presence, indicating local scale variability.

High Pressure Gradients Observed

Introduces a question about realistic pore pressure gradients encountered in mining operations.

Shares an example where static pressure increased dramatically from 2m to 65m due to the weight of a waste dump on clay material.

Understanding Pore Pressure Dynamics

Describes how pore pressures can differ significantly between layers, with one sensor showing no change while another experiences high pressure due to load compression.

Mentions observing major pore pressure gradients across structures, especially under stress loading or discrete geological features.

Diagnostic Tools for Gradient Analysis

Explains that while low permeability media typically do not show high gradients, they can still provide diagnostic insights into geological conditions.

Discusses how vertical gradients can indicate flow barriers or recharge areas within natural systems.

Hydromechanical Coupling Insights

Addresses hydromechanical coupling's role in slope stability and its association with geotechnical impacts.

Hydromechanical Coupling and Slope Stability

Understanding Hydromechanical Coupling

Hydromechanical coupling involves the interaction between mechanical stress or loading and water pressure, affecting slope stability.

Typically, water pore pressure is integrated into mechanical models without considering how mechanical changes affect permeability.

In areas of excavation or relaxation zones, assumptions are made about increased permeability due to structural unloading.

Impacts on Slope Stability

The dilation of joints or structures parallel to a slope can allow water infiltration, which may significantly impact force distribution and lead to instability.

Common triggers for instability include dilated structures receiving water from weathered zones, rainfall, or leaks from pipes and tanks.

Assumptions in Analysis

Often, analyses assume that water will infiltrate dilated structures without detailed assessments of the extent or effects of this infiltration.

This indirect coupling suggests that while we recognize potential risks from water ingress, specific modeling is rarely conducted.

Variability in Hydromechanical Responses

Different types of hydromechanical coupling exist; near-surface structures may dilate under geomechanical stress allowing more water entry which can drive instability.

Conversely, impermeable rock masses may experience volumetric expansion leading to reduced pore pressures as fractures slightly open.

Geotechnical Performance Considerations

Changes in geomechanical states can either improve or reduce the geotechnical performance of slopes depending on how they interact with hydraulic conditions.

Determining Dimensions and Distribution of Relaxation Zones

Methods for Assessment

To determine dimensions and distribution of relaxation zones, examining P-wave velocities (Pomers) is suggested as a robust method.

Understanding Damage Zones in Mining

Conceptual Models and Measurements

The discussion emphasizes the importance of developing a conceptual model for understanding damage zones in mining, suggesting that empirical measurements should be aligned with theoretical frameworks.

It is noted that the depth of relaxation zones correlates with mining depth; deeper pits lead to larger relaxation zones, estimated to be about one-quarter to one-third of the mining depth.

The current understanding of these relationships is described as crude, highlighting a lack of comprehensive data on mechanical properties and connectivity within rock masses.

Rock Characteristics and Their Impact

The characteristics of rock—such as brittleness, ductility, and strength—significantly influence the extent of relaxation zones. Stronger rocks may show minimal relaxation even at great depths.

Joint orientation also plays a critical role; if joints are parallel to slopes, they can increase the extent of relaxation observed.

Empirical Evidence and Monitoring Techniques

Various methods like horizontal drain drilling can provide insights into rock mass quality at depth based on drilling performance.

Microseismic studies have been utilized to analyze wave velocity around pits, supporting theories regarding seismic velocity drops indicating relaxation zones.

Advances in Seismic Monitoring

Bingham Canyon's microseismic network demonstrated lower seismic velocities around pits due to joint openings, reinforcing concepts about damage zones.

Current research includes cross-hole seismic monitoring before and after slope excavations to better understand changes in seismic velocity related to permeability.

Numerical Modeling in Hydrogeology

Importance of Prior Information

A good hydrogeological model is essential for effective numerical modeling; prior information such as long-term pumping trials helps validate models against stress responses.

Fracture Models vs. Equivalent Porous Media (EPM)

Validating numerical models requires variable head data from responses during pumping or excavation activities. This data is crucial for accurate calibration.

The scale at which fractures are analyzed affects modeling approaches; smaller scales may not support detailed fracture flow models effectively.

Limitations of Fracture Flow Models

Groundwater Flow and Thermal Transport Modeling

Overview of Groundwater Temperature and Flow Models

The discussion begins with the importance of using an EPCM model for groundwater flow analysis, particularly in cases involving high temperatures (over 100°).

A focus is placed on thermal transport through rock masses, emphasizing the need to understand how quickly water mixes and cools in fracture flow situations.

The challenge arises when a porous media model misrepresents a fracture flow scenario, leading to inadequate cooling predictions due to incorrect assumptions about contact areas.

Model Comparisons and Methodology

To address the limitations of current models, a specific study will utilize both a 2D PCM model and a 2D fracture flow model to compare cooling differences rather than seeking absolute results.

This approach highlights the rarity of needing a fracture flow model over traditional methods in certain unique scenarios.

Importance of Conceptual Models

Emphasis is placed on developing strong conceptual models based on data analysis; understanding key physical processes is crucial for realistic numerical modeling.

Challenges in Hydrogeological Characterization

Correlation Between Geotechnical and Hydraulic Parameters

A question arises regarding the impact of unclear correlations between hydraulic parameters and geotechnical characteristics on hydrogeological assessments.

It’s noted that lack of correlation can be acceptable; often, it reflects factors like fracture frequency or joint orientation rather than direct relationships.

Addressing Weak Geotechnical Domains

In cases where geotechnical domains are identified as weak or at risk, it's essential to characterize these domains hydrogeologically by installing test holes and monitoring pore pressures effectively.

Interpreting Data Sets for Accurate Assessments

Understanding Hydraulic Behavior Beyond Permeability

The conversation shifts towards recognizing that other geological structures may influence hydraulic behavior more significantly than permeability alone.

For instance, transient recharge can indicate underlying complexities affecting groundwater movement beyond simple permeability metrics.

Comprehensive Data Analysis Requirements

A thorough interpretation of all available data sets is necessary; high fracture frequency does not guarantee permeability if fractures are filled with clay or other impermeable materials.

Adequate characterization of both geotechnical parameters and hydraulic properties ensures confidence in understanding their interrelations within rock formations.

Characterizing Fault Impact on Groundwater Convection Cells

Fault Behavior in Hydrogeological Context

Faults and Hydrogeological Studies

Understanding Fault Dynamics

Faults can create barriers to water flow due to stratigraphic offsets, altered rock formations, and the presence of microfractures that direct water flow away from the fault line.

While some faults may allow localized preferential flow along their strike, this is typically limited in distance due to the nature of fault structures which often pinch and swell.

Enhanced flow along a fault does not extend indefinitely; it usually covers tens to hundreds of meters rather than kilometers.

Climate Change Analysis in Hydrogeology

Importance of Climate Change Considerations

Conducting climate change analysis for hydrogeological studies related to mining operations is debated; global climate models are complex and not always accurate.

Different global climate models yield varying predictions based on carbon loading scenarios, affecting long-term assessments of groundwater recharge.

Factors Affecting Groundwater Recharge

Groundwater recharge is influenced by multiple factors including rainfall intensity, evaporation rates, vegetation growth, and how these elements might be impacted by climate change.

In short-term assessments, climate change may have minimal impact on groundwater flow; however, longer-term closure assessments could reveal significant effects.

Sensitivity Analysis Recommendations

For closure studies, conducting a sensitivity analysis comparing wetter versus drier environments can provide insights into potential variations in groundwater levels and flows.

Case-Specific Considerations

Regional Impacts of Climate Change

The implications of climate change vary significantly by region; for instance, permafrost degradation in Arctic areas can affect foundation stability and enhance water flow dynamics.

In regions like the southwestern USA, increased frequency and intensity of storm events necessitate consideration for erosion impacts on pit slopes and operational safety.

Recent Observations

Recent extreme weather events highlight the importance of understanding surface water flows as they relate to mine performance; empirical data from such events aids in calibrating predictive models.

Hydrogeological Modeling and Machine Learning Insights

Rainfall Patterns and Their Impacts

The average annual rainfall in the southwest remains consistent year-to-year, but its distribution varies significantly.

Longer dry periods followed by intense bursts of rainfall can lead to increased destruction and damage.

This variability in rainfall patterns is becoming a critical consideration for various sites.

Future of Hydrogeological Models with Machine Learning

Discussion on the potential use of machine learning and geostatistical data analysis to enhance hydrogeological models.

While machine learning is being applied in geotechnical fields (e.g., slope stability), its application in hydro modeling is still developing.

Concerns are raised about the site-specific nature of hydraulic properties, making it challenging to apply generalized data effectively.

Challenges and Opportunities in Data Correlation

There’s a risk that applying machine learning could introduce additional uncertainty into already uncertain models.

Emphasis on the necessity for high-quality datasets as foundational elements for effective modeling using machine learning.

Current efforts include seeking correlations between hydrogeologic data sets and other geological or geotechnical datasets to improve conceptual understanding.

Closing Remarks and Future Directions

Panelists acknowledge the importance of considering these questions moving forward, highlighting areas for future research.