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Seepage Control on Dams with Sand/Gravel Filters
Recent Industry Advances
Michele Lemieux, P.E. Montana Dam Safety Program
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•Background – Seepage and Dams
•Filter / Drain Design Steps – Overview
•Top 10 List of Recent Industry Advances
Presentation Outline
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Stability Saturation of portions of an embankment causing loss of soil strength
Piping
Movement of soil through an unprotected exit
Seepage and Dams
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Stability
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Piping
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Stability
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Piping
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Sand Filter Drain
Rock
Drain Pipe
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Filter sand
Drain Gravel
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Embankment Soils – usually fine grained (clay silt)
Gravels
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Reservoir WaterSurface
UnprotectedExit
Void caused by Piping
Sinkhole
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Filter / Drain Design Steps
Embankment Soils
Sand FilterGravel Drain
Seepage Flow
1. Design Sand Filter to be compatible with embankment soils
2. Design Gravel Drain to be compatible with sand filter
3. Design drain pipe to be compatible with gravel drain
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Embankment Soil or “Base Soil”
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Base Soil
Filter
Embankment Soils
Sand FilterGravel Drain
Seepage Flow
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Filter soils must fit in between these 2 lines
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These on site gravels will not be a good filter – too coarse
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Commercial concrete sand fits pretty good
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Filter Sand
Drain Rock
“Base Soil”
“Filter”
Embankment Soils
Sand FilterGravel Drain
Seepage Flow
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Drain Rock
“Base Soil” Drain Pipe
Embankment Soils
Sand FilterGravel Drain
Seepage Flow
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Background – Seepage and Dams Filter / Drain Design Steps – Overview
•Top Ten List of Recent Industry Advances
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TOP TEN LIST 10. Don’t use on-site soils for critical filter / drain applications
Tend to be gap graded, prone to segregation during placement
Generally, have too many clay/silt size particles Washing on-site can be problematic for fall construction in Montana
Uncertainty – variations in gradations and quantity of material
It is rare to find natural materials that can satisfactorily serve as filter
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9. Keep fines content down: < 3% in stockpile, < 5% in place
Sand and gravel tend to breakdown during placement
Permeability goes down dramatically with increasing clay and silt. A soil with as little as 7% clay can be essentially impermeable
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8. ASTM C33 concrete sand is an excellent filter for most embankment dams
Readily available
Perfect filter for MOST clay silt soils
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7. Don’t overestimate permeability of your concrete filter sand
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7. Don’t overestimate permeability of your concrete filter sand
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Prevents problems from construction defects (loose lifts, poor bond between lifts, pervious layers, desiccation, and dispersive soils)
6. Put in a chimney filter when repairing your embankment
Overwhelming evidence of their effectiveness
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5. If there is a chance of concentrated flows, use 2 stages in your chimney filters
Filter Drain
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4. Design toe drain pipes to accommodate an inspection
Plentiful cases where drain pipes get damaged during construction
A pipe that fits a video camera generally has adequate capacity.
BONUS!!!
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3. Make an informed choice of the plastic pipe in your toe drain
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Single Wall Corrugated HDPE
Dual Wall Corrugated HDPE
Solid Wall HDPE
10X Strength double wall corrugated HDPE
4X Strength double wall corrugated HDPE
Solid Wall PVC pressure pipe
$
Joint Separation & Collapse
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Sand can clog pipe perforations Must have properly sized drain rock adjacent to perforations
2. Always design your toe drains with 2 stages
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1. Consider using method based specifications
Over-compaction of filters and drains, breaks down particles, causing loss of permeability
Type of equipment used for compaction, number of passes of equipment, moisture application is often left up to the contractor
For critical filters and drains, specify exactly how you want them constructed
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References will be posted at: http://dnrc.mt.gov/wrd/water_op/dam_safety_technical_ref.asp