INTERNAL EROSION Associated with Conduits through Embankments
Transcript of INTERNAL EROSION Associated with Conduits through Embankments
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INTERNAL EROSIONAssociated with Conduits
through Embankments
by
Hal Van AllerMaryland Dam Safety
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Embankment Dam Schematic
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Medford Quarry Wash Pond CMP failure
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Velvet Hills sinkhole
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Sinkhole in crest of new MD dam
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WCC (Joe Kula) Testpit along pond CMP spillway
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AACounty CMP with void
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Loveton Dam Failure (1989) Breach
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Anti-seep collars do
not prevent seepage failures!
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Loveton 78” CMP viewed from downstream
78 inch CMP viewed from downstream
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Failed PVC pipe with anti-seep collar
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Annap Mall collapsed CMP d.s. of Riser
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Annap Mall u.s. view
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What caused these failures?• Design• Construction• Inspection• All of the
above??
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Spillways are “Confined Space”
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Ray Martin cave
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Typical Problems schematic
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Compaction in Haunches
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Pipe Installation in DamDam Embankment
Filter Diaphragm
Flowable FillPipe in vertical trench
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Excavation through dam
• Side slopes must be 2(H):1(V) or flatter
15 ft
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FILTER DIAPHRAGM
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Centennial Park dam chimney drain construction
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Filter Diaphragmflooding with water for
compaction
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Merkle Filter Diaphragm with vibrator
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CONSTRUCTIONExperienced contractor desired
• must realize that ponds are different from roads and utilities
• must work with design engineer• compaction along spillway is critical--
don’t construct embankment and then cut trench
• spillway joints must be watertight
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Alternative Designs• Avoid large diameter CMP• User weir or drop box structure• If pipes unavoidable, use:
– Concrete pressure pipe (min. ASTM C-361)– Complete concrete cradle or flowable fill– Watertight joints with o-ring gaskets– Filter diaphragm
• Avoid combining pond with road embankment
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Chute Spillways
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Concrete flumes on embankment have problems too
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CMP spillway on steep slope through dam embankment
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Tasker’s Chance concrete weir
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Crooksville, OHIO-COMPLETED WEIR
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Parker Pond new sheet pile spillway
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Do's and Don’ts for Pond Design and Construction
• Don’t use anti-seep collars to “stop” leakage• Do use filter diaphragms• Don’t allow spillway installation in trench• Do require watertight joints• Do inspect each pipe joint as it is put together
so it can be redone if necessary• Do consider alternative spillway design that
eliminates pipe through dam or foundation
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InspectionMore than just testing Compaction!
• Inspector should:! work for the design engineer, not the
contractor! understand design assumptions and effects
of unanticipated conditions! not allow deviation from design without
design engineer’s approval! observe and document: pipe mfg marks, joint
type, gasket material and size, and installation procedure
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Foundation
• Proof rolling• Inspection by Geotechnical Engineer
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Compaction
• Use STANDARD Proctor (ASTM D-698 or AASHTO T-99)
• Do Not use MODIFIED Proctor (ASTM D-1557 or AASHTO T-180)
• Nuclear gage readings need to be corrected to match lab moisture & density
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Utility Conduits(water, sewer, gas)
• Pipes through the dam must meet spillway requirements
• No granular backfill for pipes parallel to the dam axis
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Pre-cast riser structures
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Anchor Plates
Movement
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Leaky Joints
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Concrete Pipe Installation
• Pipe supports• Clean and Lubricate joints• Lubricate Gasket• Tension Gasket• Full concrete cradle, not just bedding• If not o-ring gaskets then owner and
engineer must approve
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Prepare foundation and supports
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Mudmat is a good idea
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Clean joints
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Lubricate joints
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Lubricate o-ring
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Tension o-ring withround-shaft screwdriver
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Mis-aligned o-ring ...
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...causes broken bell
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Gaskets for Concrete PipeTraditional O-ring joint
•Deep joint allows for substantial extensibility
•Symmetric design accommodates internal or external pressure
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Newer “Profile Gaskets”
Press-Seal Gasket Company states these “advantages” over o-rings:
•Stepped joint is cheaper to manufacture than groove for confined o-ring gasket
•Easier to install, less force needed to “home” joint
•Less bell breakage
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Gaskets for Concrete PipeNew style “Profile Joint”
•Smaller joint separation allowance
•Designed for internal pressure
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Gaskets for Concrete PipeNew style “Profile Joint”
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Formed concrete cradle
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Cradle poured against
soil
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Poor installation
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Dam on Soft Foundation
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