Safety of Existing Dams Evaluation and Improvement (1983) / Chapter Skim
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8. Appurtenant Structures
8. Appurtenant Structures
Pages 259-271
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From page 259... ...
The primarv defect most often indicated is inadequate discharge capacity. Inade259
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From page 260... ...
260 TABLE 8-1 Evaluation Matrix of Appurtenant Structures SAFETY OF EXISTING DAMS Type of Defect Causes Effects Remedies Defective Insufficient Overtoppinga Reevaluate spillway spillways analysis Erosion or capacity usingpresent washout on day hydrologic downstream techniques side Design error Use watershed model simulation and prototype studies in design New criteria Erosion along and Institute major repairs: established around spillway Increase spillway chute capacity Construction of auxiliary or emergency Major or Breach Alternate methods:& unpredicted Revise reservoir events operating procedures Restrict reservoir utilization Require attendance of dam personnel during flood events Establish well-defined emergency procedures Obstruction to Excess trashb Overtopping Install log booms or trash spillways and burden racks based on use of outlet works reservoir, anticipated trash burden, etc. Erosion Perform maintenance as required to remove excess trash buildup - Damage to trash racks Defective gates Mechanical Upsets normal Perform regular and hoists breakdown operation maintenance on characteristics of mechanical equipment dam Inadequate gate Vibrations seals Fatigue cracking Cavitation around gate guides Damage to gate frames and operating shaft Check bottom gate seals for damage Provide for sharp clean flow breakoff Repair cavitated areasC with steel liners; check that all gate frames are securely mounted
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From page 261... ...
Defective Surface Cavitation erosion Grinding surface to conduits irregularities Piping smoothness that will (offset joints, prevent cavitation voids, erosion transverse Air vents at irregularities grooves, Require close construction roughness) tolerances Provide aeration grooves to draw air into flowing Sealing in conduit Unsteady flow conditions Structural vibrations Unsymmetrical Cavitation flow Settlement of foundations Corrosion water Perform prototype studies and modify Adequate air vents Repair concrete Install guide vanes Baffle blocks at terminal structure Adequate air vents Stabilize foundations Replace joint collars Replace joint seals Replace or repair conduit Erosion in stilling basins Joint separation Structural cracking Piping Piping of embankment material through holes
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From page 262... ...
Drill deposition supplemental drains Erosion Inadequate Fluctuating positive Increase thickness of design of to negative or concrete slabs spillways and uplift pressures Impose tailwater elevation stilling basins can develop on that will force hydraulic spillways and jump stilling basins Provide floor drain (can cause openings in locations to cracking of avoid subjecting them to concrete slabs in fluctuating pressures stilling basins and subsequent removal of embankment material) ; this fluctuation of pressure can demolish a spillway or stilling basin Structural cracks Water seepage Pressure grout cracks in in concrete through slate and slab slabs of eroding of Replace with thicker slab spillways and embankment stilling basins materials Development of Evaluate effectiveness of voids under slab energy dissipators and Loss of slab support replace if necessary Breakup of slab Fill voids under concrete slabs Anchor invert Unsymmetrical Unsymmetrical Operate gates operation of loading of symmetrically outlet gates spillway Repair with erosion Scour actions in resistant aggregate and discharge area high-strength concrete
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From page 263... ...
bLarge trash, such as logs, etc., can damage spillways, stilling basins, and energy-dissipating blocks as it is carried over the spillway. CNew techniques for repair: polymer-impregnated concrete has been used to repair cavitation in concrete tunnels and stilling basins.
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From page 264... ...
Some spillway stilling basins originally constructed with the basin invert elevation incorrectly set in relation to tailwater and the conjugate depth of design discharge have been destroyed or severely damaged. It is sometimes possible to terminate the spillway discharge channel as a bucket at an elevation above tailwater.
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From page 265... ...
A frequent deficiency in the outlet works of embankment dams relates to the elevation at which the intake structure was placed when originally constructed. If inadequate dead storage capacity was provided, the intake structure may be in danger of becoming obstructed by a mixture of waterlogged trash, sediment, and debris.
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From page 266... ...
Unsymmetric flow conditions through conduits caused by bends or irregular gate operation can result in cavitation in the conduit and erosion in the stilling basin. Guide vanes installed in the conduit and adequate air vents
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From page 267... ...
In corroded metal conduits, embankment material can be piped through the corroded holes and may require total lining/grouting or replacement of the conduit. Bare metal conduits are frequently found in and beneath embankment dams especially in smaller, older, privately owned dams storing water for farm use and recreation.
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From page 268... ...
, oscillations due to fluid-induced structural loadings caused the collapse of a radial gate. This resulted in a sudden rise in the water level downstream, with a subsequent loss of human lives.
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From page 269... ...
They should be rubber bar seals rigidly attached. Differential foundation settlement of the gate structure can crack gate frames or skew the frames, so that the gates become inoperable.
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From page 270... ...
Structural cracks in spillways and stilling basins or poorly constructed joints allow seepage through the slab; this can cause piping of embankment material with a subsequent loss of slab support. If corrective measures are not taken, complete breakup of the slabs may result.
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From page 271... ...
Brazil Blames Earth Dam Collapses on Failure to Open Spillway Gates, February 2.
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