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APPURTENANT STRUCTURES 259 8 Appurtenant Structures INTRODUCTION Appurtenant structures are other structures around a dam that are necessary to the operation of the dam project. These include spillways, outlet works, power plants, penstocks, gates, valves, trash racks, diversion works, and switchyards. Generally, these are smaller structures than the dam, but they can be of considerable importance to the project because they control the flow of water and power. Incidents of failure or near-failure of all types of dams that were attributed to defects in the appurtenant structures have been well documented in the literature. Often, the defects in appurtenant structures, when identified in the early stages, can be corrected by taking preventive maintenance measures without endangering the integrity of the dam. In cases where more extensive repair work is required, it may be necessary to lower the reservoir level to provide a sufficient factor of safety during repairs. In extreme instances, defects in appurtenant structures can be of such magnitude that they lead to complete failure and subsequent abandonment of the dam. This chapter describes some problems common to appurtenant structures, together with suggested solutions. Table 8-1 summarizes defects, causes, effects, and remedies. DEFECTIVE SPILLWAYS The main appurtenant structure of a dam is usually the spillway. The primary defect most often indicated is inadequate discharge capacity. Inade
APPURTENANT STRUCTURES 260
APPURTENANT STRUCTURES 261 Type of Defect Causes Effects Remedies Differential Gates becoming Repair foundation foundation inoperable settlement Gate frames crack Trash and debris Vibration Install trash racks Trash can knock gates from frames Galvanic corrosion Corrode Provide cathodic and/or mineral moveable parts; protection deposits makes gates Exercise gate to inoperable prevent formation of deposits Poor design and/or Vibration Revise operating inadequate procedures operational procedures Unbalanced flow Provide adequate (can cause other air vents problems to occur, such as buckling of steel liners and concrete erosion) Defective Surface Cavitation Grinding surface conduits irregularities (offset erosion Piping to smoothness joints, voids, that will prevent transverse grooves, cavitation erosion roughness) Air vents at irregularities Require close construction tolerances Provide aeration grooves to draw air into flowing water Sealing in conduit Unsteady flow Perform conduit prototype studies and modify Structural Adequate air vents vibrations Unsymmetrical flow Cavitation Repair concrete Install guide vanes Erosion in stilling Baffle blocks at basins terminal structure Adequate air vents Settlement of Joint separation Stabilize foundations Structural foundations cracking Replace joint Piping collars Replace joint seals Corrosion Piping of Replace or repair embankment conduit material through holes
APPURTENANT STRUCTURES 262 Type of Causes Effects Remedies Defect Defective Inadequate design Uncontrolled Investigate and drainage Improper seepage modify system installation Piping Install new or Boils improve existing drain field Provide relief wells Reduce reservoir pool level Inadequate filter Saturated conditions Improve filter layer layer Seepage of fines from foundation Mineral deposition Clogging Ream drains. Drill supplemental drains Erosion Inadequate design Fluctuating Increase thickness of spillways and positive to negative of concrete slabs stilling basins or uplift pressures Impose tailwater can develop on elevation that will spillways and force hydraulic stilling basins (can jump cause cracking of Provide floor concrete slabs in drain openings in stilling basins and locations to avoid subsequent subjecting them to removal of fluctuating embankment pressures material); this fluctuation of pressure can demolish a spillway or stilling basin Structural cracks in Water seepage Pressure grout concrete slabs of through slab and cracks in slab spillways and eroding of Replace with stilling basins embankment thicker slab materials Development of Evaluate voids under slab effectiveness of Loss of slab support energy dissipators Breakup of slab and replace if necessary Fill voids under concrete slabs Anchor invert Unsymmetrical Unsymmetrical Operate gates operation of outlet loading of spilway symmetrically gates Scour actions in Repair with discharge area erosion resistant aggregate and high-strength concrete
APPURTENANT STRUCTURES 263 quate capacity can lead to overtopping of the dam, which is particularly critical in earth or rockfill dams because overtopping can cause failure. In the evaluation of older dams a determination of inadequate spillway capacity is generally the result of new criteria and updated hydrological procedures and records rather than design or construction faults. Type of Defect Causes Effects Remedies Excessive Abrasion and Repair with discharges cavitation erosion special concretes Abrasive objects in of concrete in and steel plates stilling basin spillway and Line dissipators (rocks, construction stilling basins with steel plates debris, etc.) Damage to chute Install rip-rap blocks and energy dissipators Breakup of slabs and destruction of spillway a Overtopping is more critical on earth or rockfilled dams. Concrete dams can stand a limited amount of overtopping. b Large trash, such as logs, etc., can damage spillways, stilling basins, and energy-dissipating blocks as it is carried over the spillway. c New techniques for repair: polymer-impregnated concrete has been used to repair cavitation in concrete tunnels and stilling basins. d New technique for repair: for spillway repair, rollcrete has been used as an alternative repair method. The subjects of spillway design floods; the ability of the spillway and reservoir acting together to control safely the design flood; and the general types of mitigating measures where that ability is lacking are discussed at length in Chapter 4. Some of the specific defects and poor hydraulic behavior that have been observed and remedies that have been used are discussed below. Siphon spillways have been constructed at a few earthfill dams, usually as towers or imbedded risers in combination with the outlet works conduit. In some instances subsequent performance has demonstrated that the discharge capacity is much less than what was theoretically predicted. Where topography permits, a supplemental open channel spillway can be constructed beyond one end of the dam with the control elevation above that
APPURTENANT STRUCTURES 264 of the siphon. Vegetated linings may suffice depending on the frequency and duration of discharge in excess of that which can be handled by the siphon spillway (Cortright 1970). 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. The bucket is supported on a foundation level at or below the expected depth of the eroding plunge basin. The support can be provided by cast-in-place piling in drilled or cased holes in granular formations or by a reinforced concrete substructure on a hard rock formation. Nonsuperelevated horizontal curves have been built in spillway discharge channels where flow velocity is supercritical. As a result, flows have overtopped the outer wall with consequent erosion and structural damage. It may be possible to raise the outer wall and accept the transverse slope of the water surface provided the erratic wave pattern created is safely contained beyond the curve. In one instance the curved portion of the channel was compartmented with several vertical walls so that the outer rise in the transversely sloping water surface was diminished sufficiently for containment within the available freeboard. Spillways with converging training walls are often susceptible to having an actual capacity less than theoretical. This is caused by water piling up along the converging walls and overtopping, often with serious results. Hydraulic model testing is often the optimum and only effective way of determining actual capacity. In recent years diversion facilities during construction of some rockfill dams (both impervious core and faced) have included crest and downstream face reinforcing. Floods have been successfully passed over the top and down the slope of the uncompleted embankment with minimal damage. This suggests the possibility of a less costly way of increasing the spilling capability at an existing rockfill dam where the spillway capacity is too small. In some cases such treatment would be a temporary betterment until a permanent solution could be financed. In other cases the treatment might be justifiably considered permanent, for example, where the required spilling capability was expected to operate rarely, if ever, during the project life. A decision to adopt this remedy would depend on full consideration of the quality and size of the rock in the top and face layers of the dam, the character of the foundation rock along the toe, a limiting dam height, the length of dam to be so treated, anticipated depth and duration of overtop-ping, river channel characteristics immediately downstream, and a nearby
APPURTENANT STRUCTURES 265 source of additional rock (original abutment or downstream quarries, perhaps). The face reinforcing commonly consists of a heavy square steel mesh retained by an orthogonal pattern of spaced horizontal and sloping reinforcing bars. The bar network is retained against the face with bent anchor bars embedded in the rock mass. The horizontal and sloping bars are anchored to bedrock along the toe. During original construction the anchor bars are embedded as the rock lifts are being placed. At an existing dam the embedment would have to be made in a sliver fill of rock placed against the existing face. Spillway capacities can be increased by constructing vertical concrete parapet walls on the tops of dam embankments. This can be feasible both for rockfill and earthfill dams. Usually the parapet wall is considered to provide residual freeboard only and the effective head on the spillway is thus measured from dam top elevation (Cortright 1970). OBSTRUCTIONS IN SPILLWAYS AND OUTLETS In conjunction with spillway capacity, obstructions in the spillways and outlet works also can affect the stability and desired operating characteristics of dams. These obstructions can be caused by faulty design, structural defects, excessive reservoir trash burden, siltation, landsliding, or a combination of these factors. One documented incident that illustrates the results of obstructions occurred at the Nacimiento Dam near Bradley, California (ASCE/ USCOLD 1975). After several intense storms the high-level outlet slide gate clogged with trash and failed. Where trash racks are used, their proper design and placement plus regularly scheduled maintenance and cleaning of debris from the racks can help prevent such incidents. The design of trash racks generally must consider such factors as the intended use of the reservoir (recreation, water supply, flood, etc.), types of gates, and maintenance requirements (U.S. Bureau of Reclamation 1974). Where log booms are used to prevent obstructions to spillways and intakes, accumulated debris should be continuously removed and inspection made for damaged, corroded, or inadequate log booms. 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. Loss of withdrawal capability is of great concern when a more serious dam defect appears. Permanently sub-
APPURTENANT STRUCTURES 266 merged outlet gates in particular can be a problem because they are difficult to inspect or to maintain. They are assumed to function until sometime when they no longer do so. By then it may be too late to take corrective measures short of emergency measures. Where it is still possible to empty the reservoir, a vertical freestanding riser or possibly a sloping riser laid on the abutment or possibly the face of the dam can be constructed to a new and higher intake elevation. The existing gate control can be set at the new entrance to the structure, or it may be desirable to modify the type of control at the same time. Vertical risers or stub towers have been successfully installed on several dams owned by the Santa Clara Valley Water District in California. Where the reservoir cannot be drained and where extensive work by divers is impractical, a prefabricated riser can be added underwater. By making the riser watertight and by fitting it with connections for hoses through which ballast water or compressed air can be pumped, it is possible to tip up and position the riser vertically over the existing intake structure. A bottom end cover temporarily held in position by bolts and clamps is removed after the riser is in the vertical position and then blown off by compressed air. With the bottom cover off, the riser can be made to rise or sink by adding or releasing compressed air. The riser can then be joined to the existing intake structure and totally flooded. The top cover can then be removed, and a prefabricated trash rack arrangement can be installed by divers. This remedy was successfully made at Santa Felicia Dam in California by the United Water Conservation District (Bengry and Caltrider 1978). DEFECTIVE CONDUITS Surface irregularities such as offset joints, voids, and roughness create turbulence within a conduit, which can cause cavitation, leakage, and piping. Grinding surfaces to a smooth finish, applying a smooth coat of epoxy, and providing air vents and/or aeration grooves to draw air into the flowing water are some solutions for turbulence problems. Sealing or the transition from a free surface flow to full pipe flow in horizontal or inclined conduits can result in structural vibrations because of unsteady flow conditions and in an undesirable variation of the water flow (ASCE 1978). Sealing can be mitigated by providing adequate air vents in the conduit. In any case it is a condition to avoid through adequate design of the conduit and the use of prototype studies. 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
APPURTENANT STRUCTURES 267 can help streamline the flow. Baffle blocks and energy dissipators at the terminal structure can help control erosion. Settlement of conduit foundations can cause joint separation and structural cracking that can lead to leaking and piping. In such cases the foundations need to be stabilized and joint collars and seals repaired or replaced. 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. The conduits consist of welded or riveted steel or corrugated metal pipe. The transverse joints are welded, riveted, banded, or even slip- jointed. Bell and spigot east iron pipe have been used. The conduits were installed by bedding them either on embankment or granular foundation surfaces and surrounding them with the materials of the overlying embankment zones. Little or no compaction was achieved beneath the overhanging portions of the pipe sections. The overlying embankments themselves may not have been placed with controlled moisture and compaction procedures. The outlet discharge is often controlled only by downstream gates or valves, and the conduits are subjected to full reservoir head when the outlet is closed. The steel pipes are corroded by electrolysis and/or chemical action externally and are pitted internally. The rivets are no longer in intimate contact with the surrounding plate material. The banded joints are loose and rusted. Slip joints and bell and spigot joints have been opened by the base spreading forces of the embankment. These defects are reason for great concern and have caused a number of dam failures. An outlet conduit is subjected to full hydrostatic reservoir pressure when closed downstream and transmits that pressure directly to all portions of the embankment and foundation along its entire length. The conduit is subjected to the lesser pressure of the hydraulic grade line when flowing. If the point of free discharge is far beyond the downstream end of the conduit the pressure can approach reservoir head. Any leakage under pressure from the conduit into the surrounding embankment or foundation can cause failure by internal erosion. The existence of the defect can be determined by physical examination and reference to any reliable construction records. The deterioration of the conduit interior can be examined by closed-circuit television if the conduit is too small for entry. Inspectors can be pulled on wheeled dollies through dewatered conduits as small as 30 inches in diameter. Precautions should be taken to provide adequate air supply to the inspectors. Leakage appearing about the periphery of the conduit at the downstream face may have its
APPURTENANT STRUCTURES 268 source in the conduit. Temperature and chemical comparisons of the water may verify that source. By varying downstream gate settings, corresponding changes in the leakage rate may help identify the source. If the conduit is of sufficient diameter and its full discharge capacity is not needed, a smaller-diameter pipe can be inserted and centered in the conduit, the annular space bulkheaded at both ends, and the space pressure filled with a sand- cement mixture. If the existing gate or valve was installed on the downstream end, it should be relocated to the upstream end of the conduit (Cortright 1970). Conduits can be taken out of service by filling them with a sand-cement mixture under pressure. A drain and filter system can be installed around the exterior of the conduit near the downstream end to protect the embankment against piping from any leakage that may tend to flow along the exterior surfaces of the conduit. If drawdown limitations allow, a new, shorter outlet works can be constructed and founded on an abutment by removal and replacement of a portion of the embankment. Although not favored as a permanent solution, an interim siphon outlet can be installed over the top of the dam until a permanent gravity-flow outlet works can be financed. If downstream releases are not required and the project has no other defects, the defective outlet can be taken out of service as described earlier and its replacement deferred temporarily until it can be financed. DEFECTIVE GATES AND HOISTS Defective gates and hoists, especially those under high head, can cause unexpected problems and threaten dam stability when malfunctions occur. Being mechanical devices, these gates and hoists are subject to breakdown. Generally, two types of gates can be found on dams, depending on the design: (1) spillway gates used to control flow releases over the spillway if reservoir storage above the spillway crest is desired and (2) gates that function as regulating and guard gates in conduits. One major problem that can occur with gates is induced vibrations from hydraulic forces during opening and closing. The problem has been most acute with radial gates on spillways. In Japan in 1967 (Journal of Fluids Engineering 1977), 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. In the United States it was reported that vibrations on spillway gates on the Arkansas River were severe enough to cause fatigue cracks (ASCE 1972). An investigation determined that to eliminate the vibrations a sharp, clean
APPURTENANT STRUCTURES 269 flow break-off point was required. Soft rubber seals should not be used on the bottom of the gates. 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. Cavitation erosion of concrete around gate frames can weaken the supports and cause subsequent failures. Cavitated areas can be repaired by using steel plates. Gate operation can be stopped by formation of ice in the guideways or by reservoir ice. Galvanic action and/or mineral deposits can corrode movable parts on gates, rendering them inoperable. Cathodic protection and regular exercising of gates can help prevent formation of deposits and eliminate the problem. If trash racks are damaged or not provided, large trash and debris also can knock gates from frames. Poor gate design and/or inadequate operational procedures can cause vibration and unbalanced flow. An unbalanced flow condition can lead to other problems such as cavitation and abrasive erosion and damage to gate frames. In such cases a revision of operating procedures may be all that is required to solve the problem. Providing adequate air ventilation behind the gate also will help mitigate such problems. Basic hydraulic design guides and criteria have been established by the U.S. Army Corps of Engineers and U.S. Bureau of Reclamation (ASCE 1973). Other problems related to gate structures have been caused by down-slope movement of riprap due to frost action/creep, causing the gate structure to tilt or crack. Where service bridges are used to gain access to the gate structure, any movement of the gate structure or any displacement of the bridge support foundation may induce stress or buckling of the structural elements of the bridge. DEFECTIVE DRAINAGE SYSTEMS Defective drainage systems are often a source of problems. Clogged or plugged drains can lead to saturated conditions and create uplift pressures on spillways. Inadequate filter systems also can cause saturated conditions and allow piping of fines from the foundation. When such conditions occur the problems can be temporarily lessened by a reduction of the reservoir pool level. Long-term solutions can include providing a new design with an improved drain field or installation of new field and/or relief wells. EROSION Erosion in and around dams is sometimes associated with defective appurtenant structures. Erosion can play a dual role in that it can be both the
APPURTENANT STRUCTURES 270 cause and the effect of defects and, if left untreated, can lead to dam failure. Erosion is far more evident with spillways and stilling basins because the tremendous force of moving water makes the effects of erosion highly visible. An inadequate spillway or stilling basin design can lead to erosion with subsequent undermining of the dam itself. If uplift pressures on the spillway are not adequately controlled, the fluctuating positive to negative pressure can lead to cracking of the concrete slabs and removal of foundation materials (ASCE 1972). 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. Unsymmetrical operation of outlet gates and unsymmetrical loading of the spillway can result in scouring action in the discharge area. Excessive discharges due to major storm events likewise can cause abrasion, erosion, and cavitation of concrete on spillways and in stilling basins. Chute blocks and energy dissipators can be damaged. Remedial measures for erosion vary extensively depending on the size of the dam and severity of the erosion. Corrective action as simple as the placement of riprap in the discharge area may be all that is required to solve minor problems. For more complex problems a redesign and reconstruction of the spillway and stilling basin may be necessary. In cases where uplift pressures are determined to be the cause of erosion, slab thickness can be increased or rock anchors used to tie down the slabs to underlying rock. In addition, spillway and floor drains can be installed to relieve the excess pressure. Voids under spillway and stilling basin slabs need to be filled and the cracks grouted. In such grouting, care must be exercised to avoid having grout fill underlying filters and uplift the slab. If slabs are to be replaced, the use of erosion resistant aggregate and high-strength concrete is advantageous. Eroded concrete slabs and energy dissipators can be repaired and strengthened by use of steel plates. Nonstructural solutions to erosion can include better reservoir management through the controlled releases of water. EARTHQUAKES After an earthquake it is essential that appurtenant structures continue to function in order to keep water and power flow under control. The uncontrolled water flow could result in damage to all or part of the dam and the surrounding terrain. The location of these structures is significant in engineering studies because much more shearing energy may be transmitted to the base of one of these structures if it is located on a ridge than if it were
APPURTENANT STRUCTURES 271 located on firm, level ground. Likewise, its foundation may indicate the susceptibility to damage of the structure, e.g., a structure on alluvium may suffer considerably more damage than one located on bedrock. Freestanding structures, such as intake towers, tend to magnify ground motions. If they are submerged in the reservoir, the motion of such a tower causes a certain mass of the lake to move with it. This increases the apparent mass of the structure and, consequently, its response to the earthquake. For all these reasons the appurtenant structures must be studied for their response to earthquake. They must be structurally stable, i.e., the earth-quake- induced stresses in the concrete and steel must be within acceptable limits. Also a cheek must be made to see if the machinery will continue to function at the conclusion of the seismic incident. REFERENCES ASCE (1978) Size Determination of Partly Full Conduits. Proceedings 104 (HY 7 No. 13862). ASCE (1973) High Head Gates and Valves in the U.S. Proceedings 99:1727-75, October. ASCE (1972) Spillway Gate Vibrations on Arkansas River Dams. Proceedings (HY) 99:219-238, January. ASCE/USCOLD (1975) Lessons from Dam Incidents, USA, American Society of Civil Engineers, New York, pp. 259-61. Bengry, E. O., and Caltrider, W. T. (1978) ''Reservoir Outlet Extended Above Silt to Prevent Clogging,'' Civil Engineering-ASCE, September. Cortright, C. J. (1970) "Reevaluation and Reconstruction of California Dams," Journal of the Power Division, American Society of Civil Engineers, January, pp. 63, 65. Journal of Fluids Engineering (1977) "Instability of Elastically Suspended Tainter-gate System Caused by Surface Waves on the Reservoir of a Dam," Vol. 99, December, pp. 699-708. U.S. Bureau of Reclamation (1974) Design of Small Dams, Government Printing Office, Washington, D.C. Recommended Reading ASCE (1973) Reevaluation Spillway Adequacy of Existing Dams, Proceedings 99 (HY), pp. 337-382, February. ASCE Proceedings 98 (1972) Damage to Kannafuli Dam Spillway, (HY 12 No. 9452), December, pp. 2155-2170. Chopra, A. K., and Liaw, C-Y. (1975) "Earthquake Resistant Design of Intake-Outlet Towers," Journal of the Structural Division, ASCE, Vol. 101, No. 577 (July). Engineering News Record 200:11 (1978) Brazil Blames Earth Dam Collapses on Failure to Open Spillway Gates , February 2. U.S. Army Corps of Engineers (1964) Structural Design of Spillways and Outlet Works, EM 1110-2-2400. U.S. Bureau of Reclamation (1974) Safety of Dams, ASCE Proceedings 100 (HY), February, pp. 267-277.
