Cave Mine Hydrogeology Notes

Key Concepts

  • Significant hydrologic changes occur over time in most cave mines.
  • Interconnection to adjacent groundwater or surface water increases.
  • Main operational concerns:
    • Amount of groundwater inflow over time.
    • Sudden changes in inflow rates.
    • Surface water inflow and recharge.
    • Potential for mine inundation.
    • Moisture content of the ore.
    • Mud rush risk.
    • Preferential versus attenuated flow.

Stages of Study for Caving Projects

  • Hydrogeological characterization and active mine drainage are required for:
    • Construction of access tunnels or shaft sinking.
    • Underground tunnels for access to ore deposits.
    • Construction of underground infrastructure levels.
    • Caving Mining Operations
  • Levels of study:
    • Level 1: Concept study.
    • Level 2: Prefeasibility.
    • Level 3: Feasibility.
    • Level 4: Detailed design and mine development.
    • Level 5: Operations.

Considerations for Cave Mines

  • Interactions with the regional groundwater system.
  • Hydraulic properties of materials change over time (linked to geotechnical evolution).
  • The thickness of materials changes over time.
  • Surface water recharge from above increases over time.

Regional Groundwater Interception

  • Peripheral Drainage System:
    • Provides an alternative pathway for water, rather than to the draw points
    • Maximizes mine retention (attenuation) potential.
    • In some cases, it can help reduce cooling and ventilation requirements.
    • Minimizes water pressure above extraction points.

Variation in Hydraulic Properties with Time

  • "Attenuation": 'New' porosity will be created as the Block Cave evolves, attenuating recharge.
  • "Short-circuits": Rapid vertical flow paths through porous materials and major subvertical structures.
  • Flow is more rapid when pathways are saturated.
    • Expanding global database, but extremely difficult to predict

Precipitation Over Subsidence Zone

  • As deformation reaches the surface, infiltration begins as the subsidence and fracture zone increases.
  • The percentage of precipitation converted to recharge will be higher for more intense events.

Infiltration from Surface Waters

  • Infiltration by extreme rain events:
    • Water is "channeled" to the top of the sinking zone.
    • Moisture content of sinking zone material is key.
    • Surface water diversions are important.
    • Extreme water ingress must be addressed in infrastructure design.
  • Stagnation on the surface of the subsidence zone:
    • Concern: fines dragged down slopes, causing a pool and rupture.
  • Dynamic porosity increases as the sinking zone develops.

Rainfall and Runoff Frequency Model

  • Distribution of flow frequency of water reaching the bottom of the pit.
  • Increased infiltration through fragmented slopes.
  • Loss of detours on the upper level.
  • Flow distribution at the ore extraction level.

Operational Water Input Data to the Block Cave

  • Argyle:
    • The first storm was larger, but produced relatively low water inputs at the ore extraction level.
    • The second storm was smaller but produced more water inflows because the sinking zone had already been "pre-wetted."
  • Palabora:
    • There is a "tipping point" above which the inflow increases significantly.
    • Small rainfall events (<15 mm/d) alone do not create a noticeable change in inflow.
    • Rainfall of up to about 5050 mm/day increases flows between 2,0002,000 and 5,0005,000 m\textsuperscript{3} /d.
    • Recessions that return to baseline are relatively long

Basic Requirements for the Water Management Program

  • A transient hydrogeologic conceptual model of the block cave mine.
  • Understanding of underground water inflows.
  • A phased underground inflow management system including pumping, storage and contingency.
  • A surface water management program that considers the expanding mine, the need for diversions, and, extreme weather events.
  • A ore moisture monitoring and mudrush risk management protocol.
  • Water program monitoring that informs risk management procedures, and, refinement of predicted future conditions.
  • Mine wide water discharge or disposal for dewatering flows and diverted surface water.

Why Cave Properties Matter for Water Control

  • Attenuation and potential saturation above fragmented fine-grained layer.
  • Preferential flow down cave margins and along relict structures.
    *Mobilised material (porosity 0.15−0.30.15 -0.3)
    *Void space (porosity 1)
    *Yielded material (porosity 0.02−0.080.02 -0.08)
    *Yielding (seismogenic) material (porosity 0.01−0.030.01 -0.03)
    *Elastic material (porosity 0.005−0.020.005 -0.02)

Hydraulics of the Cave Area

  • El Salvador provides a unique opportunity to observe the hydraulic properties of the block cave system
  • Increase in porosity and hydraulic conductivity as the cave propagates.
  • Most flow is preferential in margins and lateral fractures.

El Salvador

  • The main structures are not evident within the mobilized zone.
  • The materials are very homogeneous and porous.
    Vertical Cave Propagation and Changes in Material Properties
  • Initial condition: Secondary porosity.
    • Fracture porosity varies between 0.00010.0001 and 0.010.01.
  • Mobilized Condition: Primary or "Intergranular" Porosity.
    • Porosity may vary between 0.050.05 and 0.30.3.
      Relationship between porosity change and hydraulic conductivity change in crystalline rocks
  • Utilizando Bernabe et.al

General Conceptual Model

  • Groundwater levels are mostly near the ground surface, with seasonal fluctuation.
  • Most active groundwater flow occurs within the weathered zone.
  • The only source of groundwater is infiltration and recharge in the wet season.
  • All groundwater movement is localized.
  • There is no regional-scale groundwater flow.

Vertical Flow Attenuation: Illustrative Transient Model

  • A simplified test model to illustrate possible attenuation and saturation of water in block caving over time.
  • It uses the transient properties of the material defined for block caving for RCM.
  • It includes unsaturated flow to allow for 'new porosity' as block caving evolves.

Surface Recharge and Inflow Events: Key Considerations

  • One-off events versus long-term weather.
  • Increasing topographic extent of the surface recharge area.
  • Mechanisms for the vertical downward movement of water.
    • Preferential flow in lateral cracks.
    • Attenuation in porous materials.
    • Chance of standing water pooling, with sudden downward release.

Concentrated Water

  • Surface water run-on to the cave zone footprint area.
  • Concentration and ponding of surface water runoff within the footprint area.
  • Melting snowbanks.
  • Sustained "point-sources" of groundwater inflow.
  • Recharge typically increases as the footprint of the surface disturbance (subsidence) zone increases.
  • Recharge events can cause transient or permanent saturated flow pathways through the cave zone

Sources of Surface Water

  • Runoff from precipitation onto the pit itself.
  • Runoff from natural drainages up-gradient of the open pit or surface mining footprint.
  • Run-off from above the crest of the wall.
  • Uncontrolled groundwater seepage.
  • Groundwater from wells and horizontal drains that have no reticulation.
  • Leakage from pipes, tanks or other Mine facilities.

Components of Characterization

  • Rainfall and other ambient climatic variables.
  • Upgradient surface water channels that may need to be diverted around the pit.
  • Overland flow (run-on) that may occur to the pit area.
  • Frequency, magnitude and routing of in-pit surface water runoff.
  • Areas inside the pit where surface water may accumulate, and the flow rate to those areas.
  • Quantifying flow contributions from natural river channels that may cross the subsidence zone.
  • Peak incident rainfall, local runoff, and infiltration to the surface of the cave zone.
  • Planning, design, and maintenance requirements for any surface water diversions around the margins of the cave zone.

Surface Water Infrastructure Types

  • Diversions - defined by a single peak event.
  • Storage ponds – defined by a series of events and or wet season requirement.
  • Conveyance infrastructure - defined by a water balance
  • Typical Design Criteria
    • Construction (short-term) – 2 yr to 20 yr return period
    • Operational (long-term) – 20 yr to 100 yr return period
    • Closure (permanent) – >200 yr return period
    • Critical infrastructure (e.g. spillways) – 1000 yr to PMP event

Mudrush (wet muck) Events: Water may contribute in a number of ways:

  • When consolidation of fine-grained material occurs immediately above one or more draw point with a subsequent increase in head above the “bridge” as water continues to percolate downward, or when transient saturated flows occur.
  • When continuous concentrated water inputs to the cave zone create permanent saturated flow paths that coincides with fine grained materials being extracted from draw point.
  • Progressive "soaking up" of water and increase in the overall moisture content in the fine-grained layers higher in the cave zone, causing perched water zones and rapid collapse and drainage of the saturated “bridge”.
  • Fine-grained layers with high moisture content progressively moving downwards with the draw until they are within or immediately above the draw points.

The contribution of water to mudrush potential may occur from either:

  • Single rapid recharge events which move rapidly down preferential flow paths and increase the moisture content of fine-grained materials lower in the cave zone.
  • A sustained preferential saturated flow path spatially coinciding with fine grained material above the extraction level.
  • A general increase in water conditions within the cave zone, leading to higher overall moisture content of fine-grained materials which may eventually enter the draw points.

Mudrush TARP: Considerations

  • the rate of draw.
  • the time the draw point has been stood down.
  • the nature (grain size) of the material above the draw point, and its ability to release water.
  • how the fines content of the material is changing with time (short term and long term).
  • the water content of the fine-grained material, and changes with time (short term or long term).
  • the rate of water flow moving down to the active mining panel, and
  • the timing of peak recharge events.

Factors contributing to Mudrush events

*Consolidation of fine-grained materials above the extraction point
*Water accumulation on consolidated materials
*Material can potentially "flow" with a delivery height of ~55 m
*Aggravated if extraction points have been idle
*Exacerbated by large recharge events

Mitigation of Mudrush

  • Operation of peripheral dewatering system to minimize flow to DPs.
  • Steady mining, with good distribution across DPs.
  • Identification of “at risk” DPs based on material (fines), structures, location, timing.
  • Increased inspection of higher risk DPs.
    The goal is to reduce potential mudrush.

Uncertainty: Practical Management

  • Peer review validates predictions and bounds ranges for design.
  • Industry data and analogues are very important.
  • The initial monitoring plan is critical.
  • Risk based zoning of draw points: Peripheral, near major structures, inactive, proximity to argillic alteration.

Surge Water & Rapid Inflow Potential surge water sources:

  • Surface recharge events entering through the cave and lateral cracks
  • Single or multiple extreme precipitation events
  • Ice melt / snow thaw
  • Development of new connections to large volumes of water
    Aquifer units
    Pit lakes or surface reservoirs
    Historic underground workings that are flooded

Water Management on Extraction Level

Underground handling of extreme flow events
Water management at the mineral extraction level
Water treatment points need to be evaluated
Underground Sludge Removal Needs Consideration Effects on on-site water management and any off-site discharges

Management of water ingress at extraction points

Underground handling of extreme flow events: design underground storage areas and pumping infrastructure based on risk assessment and mine operating experience
Changes in water chemistry: Affects pumping infrastructure, water supply, and off-site discharge

Monitoring: Objectives

  • To build confidence in the future inflow predictions.
  • To provide empirical data on water content which will feed into the analysis and model calibration.
  • To identify and develop risk mitigation procedures.
  • To provide operational control and management for the dewatering system