Ask the Experts: Implementing Slope Depressurization Programs

Name: Ask the Experts: Implementing Slope Depressurization Programs
Uploaded: 2024-10-03T16:23:05.000Z
Duration: 2 h 54 min 7 s

Introduction to the Webinar

Overview of the Event

The webinar is an "Ask the Experts" format, allowing live questions from participants.

Focus topic: Water and Mining implementation of slope depressurization programs; duration is 90 minutes until 9:30 a.m. or until all questions are addressed.

Hosted by the Geotechnical Center of Excellence (GCE) at the University of Arizona, which collaborates on research and educational projects in geotechnics.

Course Information

Related to GCE's professional development course on water in mining operations and slope stability, available through their learning portal.

Course developed in partnership with Pau Associates; funding provided by a large open pit project.

Panel Introduction

Meet the Panelists

Jillian Nunan introduces herself as Senior R&D Engineer at GCE, along with James McNab, also a Senior R&D Engineer.

Jeff Beal introduced as Global Technical Adviser with Pau Associates and lead technical adviser for the water course.

Jeremy Dowling is presented as Hydrogeology Adviser and President of Pau Associates, also a lead technical adviser for the water course.

Travis White introduced as Principal Hydrogeologist with Anglo American and presenter in the water course.

Lauren Loric introduced as Principal Consultant with Atasa Consulting, expert in geotech hydrology and mining.

Webinar Participation Guidelines

Interaction Instructions

Participants encouraged to ask questions throughout; two communication methods available: chat and Q&A functions.

Questions should be submitted via Q&A for prioritization; participants can upvote questions they want answered first.

Recording Notice

The session will be recorded for later posting on both the Watercourse site and GCE's YouTube page. Previous webinars are also accessible there.

Webinar Content Focus

Topics Covered Today

Understanding Dewatering and Depressurization in Mining Operations

Introduction to Topics

The discussion will cover various topics including slope depressurization, strategic planning, surface water management, real estate access, and general sitewide water management.

Participants are encouraged to submit questions throughout the session regarding these topics.

Clarifying Dewatering vs. Depressurization

A question arises about the difference between dewatering and depressurization; specifically if they are synonymous terms.

Jeremy explains that while both terms relate to managing water in mining operations, they serve different purposes: dewatering involves removing large volumes of water inflow, while depressurization focuses on reducing pore pressure for slope stability.

Dewatering typically requires significant pumping efforts (e.g., sump pumping), whereas depressurization may involve targeted drainage measures like horizontal drains or underground galleries.

There is an overlap where dewatering can positively impact pore pressures in pit slopes by lowering the water table.

Another panelist adds that "unwatered" could be a useful term for situations where water is removed from sumps but not from within rock or soil masses.

Importance of Fracture Characterization in Drilling

Isabelle asks about strategies for characterizing fractures to enhance the success rate of drilling dewatering wells.

Understanding the Importance of Pilot Holes in Fractured Aquifers

The Role of Pilot Holes

Drilling a pilot hole is essential for understanding fracture systems, especially when significant investment is made in production wells.

Fractures are highly variable; a fault zone may yield water at one location but not at another just 20 feet away, emphasizing the need for pilot holes.

Advanced drilling techniques are crucial to characterize the subsurface effectively, particularly in initial site investigations where fractures are not well understood.

Case Study and Lessons Learned

A past project involved $3 million wells that consistently encountered fractures until they decided to skip pilot holes, resulting in a dry well.

This example underscores the critical nature of pilot holes in ensuring successful drilling outcomes.

Structural Models and Geophysical Techniques

Understanding the structural model of fractured aquifers provides insight into high-potential areas before detailed field investigations.

Utilizing geophysical methods like sub-aerial magnetics can enhance resolution on specific targets, although challenges arise with vertical structures and overburden.

Drilling Practices and Indicators

During drilling, observing drill chip size changes can indicate encountering large fractures; air loss may precede water returns on surface.

Certain indicator minerals (e.g., ion carbonates, calcites) can signal water presence during drilling operations.

Monitoring Flow Rates During Pilot Hole Drilling

Logging flow rates while drilling pilot holes helps identify fracture locations based on fluctuations in flow rate.

Conducting prolonged airlift tests post-drilling assesses sustainability of flow from fractures; sustained flow suggests connectivity to broader networks.

Geological Analysis and Fracture Characterization

Understanding Geological Controls on Fractures

The process begins with mapping pilot holes into the geological and structural model using software like Leap Frog, which helps correlate fractures with lithologies and major structures.

This correlation aids in understanding geological controls that influence productive fractures, allowing for better predictions of favorable zones for dewatering drilling.

Methods for Characterizing Fractures

A follow-up question addresses the best methods to characterize fractures, emphasizing the importance of fault kinematic analysis in understanding fault systems.

Drilling a hole and conducting hydraulic tests is deemed the most effective method, as hydraulic characteristics of fractures can vary significantly over short distances.

Choosing Between Standpipe Piezometers and Vibrating Wire Piezometers

A live question arises regarding when to use standpipe piezometers versus vibrating wire piezometers (VWPs), prompting a discussion among panelists.

Standpipe piezometers are defined as open holes that allow water from various units to flow into a composite water level, while VWPs are typically grouted in at specific horizons for more precise measurements.

Application Scenarios for Piezometer Types

In unsaturated environments or shallow aquifers, standpipe piezometers may suffice; however, VWPs are recommended when dealing with multiple lithologies or deeper depths beyond 50 to 100 meters.

An example illustrates using both types: grouted VWPs were used on upper slopes with clay while standpipes were employed in fractured rock aquifers where conditions allowed.

Importance of Water Level Measurements

The discussion highlights that in banded iron formations, standpipes may be adequate due to minimal vertical head differences; however, combinations might be necessary based on site-specific conditions.

Water Level Measurement and Installation Techniques

Importance of Proper Installation

Emphasizes the significance of quality installation for water level measurement tools, particularly vibrating wire piezometers (VWPs). Proper installation is crucial to ensure data reliability.

Highlights that poorly installed instruments yield worthless data. If VWPs cannot be installed correctly, alternatives like standpipe piezometers may be more effective.

Data Interpretation Challenges

Discusses the utility of VWPs in low hydraulic conductivity environments, where they provide valuable insights compared to open standpipes in high flow conditions.

Suggests installing multiple-level VWPs even in high permeability areas to confirm that composite water levels from open standpipes are accurate.

Observations on Regional Recharge

Shares an experience from a site with high permeability where regional recharge was detected earlier in dolomite layers than other units, providing early warning signals for mining operations.

Permeability Thresholds for Pumping Wells

Understanding Permeability Limits

Introduces a discussion about the threshold permeability (10^-5 m/day), questioning its viability for pumping wells. This level is considered low and may not yield significant flow rates.

Application Scenarios

Explains that while low permeability limits flow rates, specific applications like depressurizing clay layers can still benefit from closely spaced pumping wells despite minimal yields.

Practical Experiences with Low Yield Wells

Describes projects utilizing closely spaced pumping wells to manage silts and clays effectively, even when individual well yields are low.

Key Takeaways on Well Performance

Fractured Rock and Hydraulic Conductivity

Understanding Permeability in Fractured Rock

Initial high permeability values can be misleading; sustained pumping may reveal limitations in fracture networks, leading to decreased production rates.

The interconnectedness of fractures is crucial for assessing well performance, emphasizing the need to consider hydromechanical coupling when permeability is low on a large scale.

Connectivity is prioritized over conductivity; characterizing flow into wells requires understanding how water enters—through porous media or fracture systems.

Assigning a single hydraulic conductivity value from one test oversimplifies complex hydraulic systems; a comprehensive view of fracture interactions is necessary for accurate assessments.

The rock matrix typically has lower permeability than fractures, highlighting significant contrasts that affect water extraction capabilities.

Challenges with Low Hydraulic Conductivity Materials

Weak lithologies with low hydraulic conductivities pose challenges for dewatering and depressurization due to material instability during drilling.

Efficient hydrogeological data collection in weak materials requires specialized drilling techniques like symmetrics or sonic drilling to maintain hole stability.

Using appropriate drilling methods allows for effective installation of support structures within boreholes, essential for maintaining open holes in challenging conditions.

Installing vibrating wire piezometers (VWPs) through casing can be effective if the casing's permeability exceeds that of the surrounding material, facilitating reliable measurements.

Understanding Depressurization in Weak Materials

The Importance of Depressurization

Discusses the trade-off between steepening or lessening slope angles and the costs associated with deep depressurization, emphasizing that weak materials benefit significantly from this process.

Highlights the importance of identifying sources of recharge in high rainfall environments to minimize water infiltration into weak materials.

Surface Water Management Strategies

Stresses the necessity of effective surface water management on mine benches to redirect surface flows away from weaker rock domains.

Shares experiences from large open-pit coal projects where drain systems were implemented to achieve depressurization and improve slope stability.

Techniques for Effective Drainage

Mentions vertical drains as an alternative method for depressurizing slopes by drilling through weaker materials, which can be very effective if placed correctly.

Refers to a case study at H Las Crusis demonstrating successful depressurization due to mining activities, which is documented in a referenced "water book."

Monitoring Pore Pressure in Rainfall-Prone Areas

Challenges with Borehole Casing

Introduces a question regarding monitoring pore pressure using vibrating wire piezometers (VWPs) in cased boreholes, discussing whether grouting is possible without removing casing.

Explains that some boreholes are deliberately cased to create suites for sequential VWP installation, addressing both intentional and accidental collapses.

Ensuring Integrity During Installation

Emphasizes the critical need for grout integrity connecting the VWP sensor filter with surrounding formation material during installation.

Describes ideal conditions for VWP installation where grout is applied from the bottom up after flushing out water, ensuring proper contact with formation.

Alternative Installation Methods

Discusses using slotted casing as an alternative when maintaining hole integrity is challenging; this allows connection between formation and sensor filter through casing holes.

Notes that while installing VWPs in fractured environments may require slotting sections of screens, it poses reliability risks if not done carefully.

Grout Effects on Pore Pressure Measurement

Grout Composition and Functionality

Grouting and Drainage Techniques in Hydrogeology

Impact of Grout Mix on Pore Pressure Conversion

The grout mix's hydraulic conductivity should be lower than the formation to prevent preferential flow pathways. If the grout has higher conductivity, water will move through it instead of the formation, affecting pressure conversion.

Using a standard grout mix typically does not impact pore pressure conversion unless in extremely low hydraulic conductivity environments.

Drilling Angles for Drain Installation

Drains are more efficient when drilled slightly downward due to better hydraulics; this allows drainage points to be at the collar rather than high points.

In formations prone to clogging from precipitation, drilling down keeps drains saturated and prevents air entry, which can lead to blockages.

Conversely, if there is a risk of losing water into a blast damage zone, drilling upward reduces pressure in the drain and minimizes water loss.

Targeting Structures with Drain Design

When designing drains, consider what structures or lithological contacts you aim to target. Adjust both position and depth based on these objectives.

Avoid drilling too deep; while deeper drains may generally improve drainage, they can inadvertently draw water from further back into slopes if not carefully managed.

Optimizing Horizontal Drain Placement

To manage pore pressure effectively under variable hydrogeological conditions, start with empirical spacing based on trial installations around existing borehole piezometers (BWPs).

Optimal dimensions for horizontal drains depend on empirical responses observed during trials. Adjustments should be made based on measured effectiveness.

Implementation of Weep Drains

Weep drains are increasingly used alongside conventional horizontal drains in areas with dilated structures parallel to slopes. They help manage groundwater levels during rainfall events.

Drainage and Slope Stability Considerations

Importance of Planning Drain Drilling Programs

The concept of managing transient pressures in slopes is crucial when planning drain drilling programs to avoid unintended consequences.

Drilling too deep can inadvertently introduce deep water into shallow failure surfaces, potentially causing slope failures.

Managing Discharge from Drain Holes

Effective management of discharge from drain holes is essential; allowing water to cascade onto benches can lead to instability by recharging lower benches.

It’s important to direct drain flow away from the bench area using pipes or aligned ditches to prevent water accumulation and instability.

Understanding Pore Pressure Monitoring

Proper pore pressure monitoring (Pomers) is vital for understanding the impact of drains on slope stability; many projects lack adequate monitoring systems.

Optimization of drainage programs requires sufficient Pomers to assess how drains affect pore pressure effectively.

Geotechnical Criteria for Drain Placement

When defining optimal locations for subhorizontal drains, focus should be on areas where deformation is anticipated, ensuring maximum benefit from drainage efforts.

Drains should not be placed too deep; they must target specific failure surfaces at appropriate depths for effective pressure reduction.

Challenges with Raw Materials and Recommendations

Issues with raw materials include maintaining grout within boreholes; experiments are ongoing to find effective solutions.

Discussion on Drilling Techniques and Material Considerations

Importance of Material Selection in Drilling

The discussion highlights that the grout used in drilling does not need to be specialized, but considerations regarding deterioration are essential.

Emphasis is placed on using sulfate-resistant materials, especially in light of recent instabilities observed in conventional pads in Canada and Turkey.

Recommendations for Installation Practices

There are upcoming recommendations from the LP regarding installation practices for leach piles and preconditioning holes.

The use of L circulation material during or after drilling is suggested as a beneficial practice.

Advantages of Diamond Core Drilling

Travis discusses whether diamond core drilling is more advantageous than logging chips; he states it has its place but isn't necessarily superior.

Diamond core drilling is particularly useful for placement of vertical piezometers (VWPs), allowing for detailed downhole geophysics.

Planning and Execution with Diamond Core

Using diamond core allows precise planning for VWP placement by providing high-resolution data about subsurface structures.

It’s recommended to conduct diamond core drilling before larger diameter hole drilling to gather necessary geological information.

Limitations and Testing Methods

A limitation noted with diamond core drilling is the inability to perform airlift tests, which affects yield quantification at fractures.

Injection tests are highlighted as an alternative method for hydraulic testing, which can be easily conducted by lowering a transducer into the borehole.

Yield Measurement Techniques

In scenarios lacking a level logger, using a v-notch weir can serve as a last resort method to determine discharge rates effectively.

Injection Testing and Permafrost Management

Injection Testing Techniques

Discusses the importance of having a repertoire of boreholes (around 50) for reliable injection testing in green fields.

Emphasizes conducting temperature conductivity logs before and after injection tests to assess water movement into formations.

Highlights that this method helps identify productive fracture depths, showcasing practical applications from recent site work.

V-Notch Weirs Experience

Shares personal experiences with v-notch weirs, stressing the significance of using the correct conversion chart for different types (90° vs. 60°).

Mentions a specific v-notch weir displayed in the background, indicating practical knowledge in fieldwork.

Dewatering in Permafrost Environments

Effectiveness and Measurement

Explores experiences from various sites (e.g., Red Dog mine, Northwest Territories, Canada), noting that permafrost often does not require depressurization due to its inherent strength.

Describes methods used to measure freeze back behind pit walls, particularly on north-facing slopes which cool down post-mining.

Risks Below Permafrost

Warns about risks associated with sub-permafrost environments where high artesian pressures can create mining challenges if not pre-depressurized.

Advises on installing sufficient drainage systems when mining through permafrost to avoid encountering excess pressure conditions.

Negative Pressure Measurements in Unsaturated Zones

Realism and Relevance

Addresses concerns regarding negative pressure values measured in unsaturated mediums like damage zones; acknowledges they can be real but context-dependent.

States that negative pressures are common but should be approached cautiously; significant negative pressures (e.g., -30 m) may not be realistic or relevant.

Challenges with Measurement

Notes difficulties measuring negative pressure below certain depths due to cavitation effects as water transitions into gas states.

Pressure Dynamics in Tailings Dams

Overview of Pressure Measurements

The discussion highlights the common occurrence of negative pressures ranging from -1m to -3m around tailings dam walls, particularly where drainage is present.

It is noted that these pressure dynamics are significant and relevant for understanding the behavior of tailings dams.

Conclusion of Webinar Session

Acknowledgment of the expert panel's efforts in addressing numerous questions during the webinar, indicating a successful engagement with participants.

Mention of a final module titled "Nine Key Issues," which provides an overview by Jeff on challenges faced by mining hydrogeologists today; encourages students to explore this resource.

Appreciation and Future Engagement

Expression of gratitude towards the panelists for their contributions and to participants for their engaging questions, emphasizing the educational value gained from the session.