Transcript Erosion control how practical is it – really? Gordon McPhail

Erosion control
how practical is it – really?
Gordon McPhail
Director and Principal, Metago Environmental Engineers (Australia) Pty Ltd
Lvl 2, 14 Ventnor Avenue, West Perth, Western Australia
[email protected]
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Introduction
• “Mining is a giant waste generation
exercise”
• Most of the waste is erodable due to:
– Material type
– Landform
• Design experience is short but liabilities
are long term
• Many measures common today are
sub-optimal
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Introduction
• The simple fact is erosion is inevitable;
we can only slow it down
• But what works best?
• Good news – long term modeling tools
that allow engineering optimisation are
coming of age
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Outline
1. Demonstrate that erosion modeling
tools can clarify our thinking
2. Use modeling to evaluate the potential
effectiveness of mitigating options and
develop guiding principles
3. Show the benefits of the principles if
applied to new “greenfields” structures
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Modeling erosion of tailings
storage structures
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Typical TSF configurations
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How do they erode?
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Modeling erosion of Waste
Rock Dumps
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Features of a typical dump
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Modeling erosion of Heap
Leach Pads
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Heap leach pad
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Typical projected profile to
maximise capacity
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10 years
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100 years
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Typical erosion
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Modeling erosion of dumps of
dispersive materials
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Dispersive materials are
particularly intractable:
• Poor bonding of clay minerals
• Highly erodable
• Slightest water ponding creates
tunneling, cavity formation and
gulleying
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A word on calibration
Three main methods:
• Full scale calibration using 3D data
• Flume tests
• Short term rainfall and erosion tests
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Full scale calibration
• Laser and stereo photography
• Long enough for natural armouring to
emerge (not less than 5 years
depending on rainfall patterns)
• Match total erosion
• Match depth and spacing of gulley
formation
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Total erosion
Hypsometry
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Flume calibration
• Suitable when have freshly placed
materials
• Can evaluate armouring
• Convenient
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Short term tests
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Mitigation
•
•
•
•
Armouring
Re-shaping
Containment
Promote erosion into a pit
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Armouring
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200 years unarmoured
200 years armoured
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Armouring implies smaller
erosion parameters
Design Option 3 b1 modification
110
105
Per year
100
95
90
85
80
75
Cumulative
Modelled Erosion
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
0
50
100
150
Time (years)
200
250
Design Option 3 (x5) Eroded Volume (x100,000 m3)
Design Option 3 (x5) Erosion Rate (x1000 m3/year)
Design Option 3 (x2) Eroded Volume (x100,000 m3)
Design Option 3 (x2) Erosion Rate (x1000 m3/year)
Design Option 3 Eroded Volume (x100,000 m3)
Design Option 3 Erosion Rate (x1000 m3/year)
300
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Considerations - Armouring
•
•
•
•
•
Availability of armouring materials
Access and constructability
Segregation during construction
Need for layering
Cost
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Re-shaping
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Dozing (and repeatedly redozing) the whole slope
• Constructability
• Cost
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Containment
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Containment trench
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Promoting erosion
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What about this in regard to
dispersive materials:
Since tunneling and cavity formation is
practically impossible to stop regardless
of armouring type,
if there is a pit nearby,
why not promote erosion as a means for
relocating the material into the pit?
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Slope the top surface
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Slope and gulley top surface
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More gulleys on top surface
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What do we learn?
• Use flatter side slopes
• Avoid step-backs
• Prevent drainage off the top surface
down the slopes
• Use gentle gradients to get water off the
top surface
• Avoid sub-dumps
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Greenfields
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Pit-side view
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Erosion:
20 years
50 years
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Back view
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Erosion views from back
20 years
50 years
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Concave slope
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Motivation:
• Nature tends towards minimisation of
energy
• Natural analogues concave
• Long term modelling confirms that
profiles tend to become concave
• Concave slopes will minimise erosion
volumes
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Stepped to concave profile
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Two-slope profile
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Construction
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Erosion from a concave slope
• 40% reduction in erosion compared with
stepped
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Conclusions
• Erosion control is indeed practical but it
is limited to slowing down erosion not
eliminating it
• A combination of armouring, re-shaping
and containment will optimally limit the
dispersion of solids
• Erosion modeling provides a practical
engineering design tool for optimising
landform geometry
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Conclusions
• There are significant cost benefits to
planning out the optimum landform
profile prior to construction as opposed
to modifying a landform at closure
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Thank you
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