Incorporation of Natural Slope
Features into the Design of
Final Landforms for
Waste Rock Stockpiles
B. Ayres, B. Dobchuk, D. Christensen, and
M. O’Kane – O’Kane Consultants Inc.
M. Fawcett – Fawcett Mine Rehabilitation
Services Pty Ltd.
Seventh International Conference on Acid Rock Drainage
St. Louis, MI, USA – March 27-30, 2006
Presentation Outline
• Key Elements of
Natural Slopes
• Historic Stockpile
Reclamation
• Need for Improved
Landform Designs
• Final Landform Design Methodology
• Short-term vs. Long-term Slope Stability
• Contour-Terraced Waste Stockpile Design
• Key Points
Key Elements of Natural Slopes
(near Salt Lake City, UT)
•
•
Variety of shapes
•
Drainage systems
follow natural drop
lines in slopes
•
Vegetation adjusted to
hillside hydrogeology
Convex summits and
concave at base
Natural Soil-Mantled
Slope Profile
Historic Reclamation Practices
• Uniform slopes
conforming to neat
lines and grades
¾ Simpler to design and
construct, but …
•
•
Steep slopes
•
Revegetation efforts
typically follow
artificial configurations
Drainage structures
oriented along
contours and highly
engineered
Need for Improved Stockpile
Final Landform Designs
•
•
Greatest physical risk to reclaimed mine landforms
is associated with gully erosion and re-established
surface water drainage courses (McKenna &
Dawson, 1997)
Steep unarmored slopes will flatten, planar slopes
will gully, straight drainage courses will meander,
and linear or convex slopes will become concave
Need for Improved Stockpile
Final Landform Designs (cont’)
• Incorporation of natural
slope features not only
improves aesthetics, but
emulates slopes that are in
equilibrium with local
conditions of rainfall, soil
type, and vegetation
•
•
Time and resources are NOW available!!
Two key components to design a sustainable F / L:
1) Follow geomorphic principles
2) Holistic view of mining operations
Design Approach for Waste
Stockpile Final Landforms
1)
Determine the final land use through consultation
with all stakeholders and assess potential
geologic or structural control elements
2)
Observe and collect data on the natural landscape
(e.g. hillslope forms and gradients, vegetation,
watershed characteristics)
Design Approach for Waste
Stockpile Final Landforms (cont’)
3)
Source: Greg Hancock
4)
Determine the longterm eroded profile
for all slopes of the
final landform
through erosion and
landform evolution
numerical modeling
Determine a footprint design for construction of
the stockpile based on natural landform contours
(for post-mining visual blending) and potential
re-grading to construct the final landform
Design Approach for Waste
Stockpile Final Landforms (cont’)
5)
6)
Design a surface water
management system to
safely convey water off
the landform, and ensure
runoff reaches final
discharge pts. in volumes
and at velocities that will
not cause unacceptable
sedimentation
Develop a waste management plan / stockpile
design taking into consideration the storage of PAF
and NAF materials
Design Approach for Waste
Stockpile Final Landforms (cont’)
7)
Develop a revegetation plan suitable for various
locations on the final landform based on data
collected from natural landscapes (Step 2)
8)
Review the preferred design with stakeholders for
general acceptance prior to implementation
Design Guidelines for Waste
Stockpile Final Landforms
•
•
Design using natural analogues
•
Preferred landform is a “spur-end” shape in plan
with a concave-convex profile (multi-angled slope)
•
•
•
Predict landform evolution over a min. 100-yr period
Visually soften steeper areas by avoiding
“engineered” ridges and sharp changes of angles
Avoid use of imported substrate, gabions, concrete
Design conservatively!!
Performance Monitoring of
Rehabilitated Mine Landforms
•
•
Watershed is ideal unit size to evaluate behavior …
major building block of landscapes
Key elements to track or monitor are revegetation
and erosion developments, defining the water
balance, and evolution of cover soils
MEND, 2006. Macro-scale cover design and performance
monitoring manual. (in progress)
Short-term vs. Long-term
Slope Stability
•
Various measures can provide short-term stability
¾ E.g., contour banks, cross-slope ripping, dozer basins or
“moonscaping”, erosion control blankets
•
Prone to failure after 1 to 10 years
¾ May aid in establishing a good stand of grasses & legumes
•
Moonscaping has been of
limited success due to:
¾ Poor construction
¾ Sediment build-up
¾ Overtopping leading to a
cascading failure
¾ High visual impact!!
Source: Mike Fawcett
Evaluation of Original Landform
Design for Whistle Mine Pit
Original Landform Design – Output
from SIBERIA Model (after 100 yrs)
Significant Gully / Rill
Development and
Interrill Erosion
Ultimate Final Landform for the
Whistle Mine Pit Cover
Contour-Terraced Stockpile
•
Stockpile design w/
closure in mind
•
Favors creation of
swales and ridges
•
Reduced erosion
during life of mine
compared to
conventional
stockpiles
Key Points
•
Waste stockpile final landforms w/ uniform slopes
are simpler to design and construct, BUT …
¾ Does not achieve mine closure objectives of minimum
erosion, long-term sustainability, and limiting long-term
maintenance liabilities
•
Incorporation of natural slope features not only
improves aesthetics, but emulates slopes that are in
equilibrium with local conditions of rainfall, etc.
•
Two key elements for designing final landforms:
¾ Incorporate geomorphic principles … sustainability!!
¾ Holistic view of mining operations … cost issue!!
•
Time and resources are NOW available!!