Safety of Existing Dams Evaluation and Improvement (1983) / Chapter Skim
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7. Embankment Dams
7. Embankment Dams
Pages 213-258
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From page 213... ...
The two primary types are the earthfill dam, an embankment dam in which more than one-half of the total volume is formed by compacted or sluiced fine-grained material, and the rockfill dam in which more than one-half of the total volume is formed by compacted or dumped pervious natural or quarried stone. Earthfill Dams Homogeneous Earthfill Dams Homogenous earthfill dams are composed of materials having essentially the same physical properties throughout the cross-section.
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From page 214... ...
1978~. Zoned Earthfill Dams Zoned earthfill dams are composed of an impervious zone or core of finegrained soils located within the interior of the cross-section and supported by outer zones or shells of more pervious sand, gravel, cobbles, or rock fragments.
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From page 215... ...
The walls are usually dry rubble but may occasionally be mortared. Rockfill Dams Faced Rockfill Dams Faced rockfill dams consist of a pervious rock embankment with an impermeable membrane on the upstream face.
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From page 216... ...
Impervious Core Rockfill Dams Impervious core rockfill dams consist of an interior impervious zone or element supported by zones of dumped or compacted rock. The interior element controls the retention of the water and is usually a compacted impervious soil protected by filter or thin transition zones.
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From page 217... ...
The foundation requirements for earthfill dams are less stringent than those for rockfill dams (Engineering Foundation 1974~. Foundations for embankment dams must provide stable support under all conditions of saturation and loading without undergoing excessive deformation or settlement.
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From page 218... ...
Equally important threats to the overall structural or seepage stability of the dam are defects in appurtenant structures, such as spillways and conduits, and associated outlet works, such as gates, hoists, and valves. The following sections include discussions of common defects that can cause partial or total failure of the dam, indicators of these defects, possible causes of each defect, effects on the dam, methods of investigating the defects, and potential remedial measures, with brief examples of actual applications on existing dams.
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From page 219... ...
If a dam is found to be unstable for steady seepage at high pool level, the most common remedy is to install drains, relief wells, or other seepage control measures to reduce the magnitudes of the pore pressures within the embankment and/or its foundation. Rapid drawdown has caused instability in the upstream slopes of many dams, including Pilarcitos Dam and San Luis Dam in California and many others.
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From page 225... ...
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From page 226... ...
226 eel o C)
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From page 227... ...
Methods of Assessing Stability of Existing Dams The fact that a dam has been subjected to the most severe conditions (drawdown, sustained high reservoir level, or earthquake) expected during its life
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From page 228... ...
If the investigations indicate lower strength or higher pore pressure than used in design analyses, additional analyses will be needed to determine if the stability of the embankment will be adequate. Analyses used to determine the stability of embankment dams are discussed in the Stability Analyses section.
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From page 229... ...
use of geofabric reinforcing in the embankment to permit construction of a less massive dike imposing smaller loads on the foundation, together with an upstream seepage cutoff. Transverse vertical cracks in homogenous earthfill dams have resulted from differential settlement caused by foundation profile irregularities and by consolidation upon saturation of the embankment or of a granular foundation.
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From page 230... ...
Older dumped rockfill dams have characteristically settled relatively large amounts, especially from the initial application of the water load during first filling. Where the facing is reinforced concrete, this settlement has often cracked the slabs, closed and spelled the central vertical joints and the horizontal joints, torn waterstops, and opened the terminal vertical joints and any perimetric joints.
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From page 231... ...
On the other hand, upstream slope erosion may or may not be readily recognizable, depending on the level of the reservoir as related to the eroded area. Improperly protected upstream slopes can erode rapidly and severely from periods of heavy wave action, but it is possible that such erosion could temporarily go undetected, particularly if the reservoir is rising, until the water level later drops to expose the damaged area.
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From page 232... ...
For some subsoil conditions a filter layer may be required below the fabric. Other common means of upstream slope protection in areas where rockfill is not readily available are asphalt or Portland cement concrete and soil cement.
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From page 233... ...
In some cases the seepage may be entirely harmless; in others, it may be extremely serious and immediate treatment becomes imperative. Whether such leakage can or will lead to serious stability problems or will require expensive repairs depends on many factors, some of which may be related to the original embankment design (for example, inadequate seepage cutoff measures)
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From page 234... ...
Accordingly, the fundamental achievement of a remedy must be the control of the hydraulic gradient within tolerable limits. The hydraulic gradient can be controlled by barriers of low permeability, adjacent zones of increasing permeability in the direction of flow, lengthened travel paths that increase friction losses, and forced flow through anisotropic foundation formations in the direction of lesser permeability.
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From page 235... ...
The treatment for zoned earthfill dams will usually be limited to added zones of higher permeability at the downstream face because the impervious zones are buried and unreachable, although the addition of barriers of low permeability within the upper elevations of an existing impervious zone may sometimes be feasible. Seepage through so-called homogeneous earth dams, where permeability is relatively high or where leakage may concentrate through anomalous regions or transverse cracks, can be controlled by treatment either, or both, upstream or downstream.
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From page 236... ...
Anchored butyl rubber sheets have been used successfully on the surface of the panels to waterstop the panel joints. A leaky rockfill dam can be modified to include an inclined earth core by using the existing dam for the downstream shell and constructing transition zones, filter zones, impervious zones, and shell elements upstream.
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From page 237... ...
· A grout curtain can be installed beneath the impervious zone of an embankment dam by drilling through the dam. Care must be used to avoid hydraulic fracturing of susceptible fills with the drilling fluid.
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From page 238... ...
Since their overall record of performance worldwide is relatively short, the ability of the fabrics to endure and to retain their capabilities for a long time is not yet known. However, especially in remedial work on existing dams, where they may not have to be buried deeply and irretrievably, geotextiles may have suitable applications.
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From page 239... ...
The 12-inch drain pipe is connected to a series of round concrete sumps that collect drain flows for discharge by a float-controlled pump system (Civil Engineering-ASCE 1981~. This example demonstrates the application of several remedial measures for seepage control both in a foundation and in an embankment the slurry wall and the connecting impervious blanket upstream; the inclined embankment drain and foundation trench downstream.
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From page 240... ...
240 c 1 3 .— U' ~ Q O \ IU i,: _ lo_ _ ]
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From page 241... ...
It was unnecessary to line the upstream face of the earthfill dam so the membrane liner was terminated by planting it in an anchor trench in the dam embankment along the toe of the slope (USCOLD 1981~. Slurry Walls Excessive seepage in a dam may be remedied by installing a trench or slot filled with an impervious material along the dam's axis, working from or near the top.
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From page 242... ...
A slurry trench wall was installed in 1969 in this 30-foot-high dam and through about 30 feet of aeolian and fluvial foundation soils to remedy excessive seepage problems. The trench was excavated
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From page 243... ...
Other examples of successful remedial use of bentonite-cement slurry trenches are at the Eberlaste Dam, Austria; Kranji Dam, Singapore; Laguna Dam and others in Mexico (Japan Dam Foundation 1977~.
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From page 244... ...
Repair of Timber Facings A number of older rockfill dams were originally faced with tongue and grooved dimensioned lumber secured on timber sleepers set in vertical chases in the upstream slope of the rockfill that was usually hand or derrick placed. These facings gradually rotted or sometimes were quickly destroyed by fire when the reservoir stages were low.
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From page 245... ...
Overtopping In the engineering of permanent embankment dams, overtopping has been strictly avoided, for sound reasons. While the advantages of earthfills are well known, their vulnerability to erosion unless properly safeguarded is basic.
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From page 246... ...
Most critical is the protection of the downstream slope and especially its toe, which will be exposed to the potentially destructive velocities of both surface and seepage flow. In projects where the effectiveness of reinforced rockfill diversion dams has been demonstrated, essential elements have been (l)
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From page 247... ...
Malfunctioning Drains Adequate embankment and foundation drainage is one of the most important aspects of maintaining a stable embankment dam. If a proper, wellfunctioning drainage system is originally incorporated into an embankment dam, it is quite possible that this system will continue to perform adequately throughout the entire service life of the structure.
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From page 248... ...
This should be done over as wide a range in reservoir levels as possible, so that a relationship can be established between reservoir level and anticipated drainage flow rate. Any significant changes in the flows defined by this relationship may be cause for further investigation.
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From page 249... ...
For this type of problem, evaluation by an experienced engineer is essential, but even if the problem is properly defined, the cost of its solution may be very high. Trees and Brush Trees and brush are frequently allowed to grow on the slopes and tops of embankment dams.
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From page 250... ...
This would include trees that could damage upstream slope protection and trees on a crest where uprooting could leave less than a 10- or 12-foot width of undamaged embankment. Trees on or near a downstream slope should be removed if their root systems can penetrate significantly into the minimum necessary embankment cross section.
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From page 251... ...
Rodents and Other Burrowing Animals The burrowing of holes in earthfill dams by rodents is a widespread maintenance problem. This problem is known or suspected to have caused several failures of small dams.
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From page 252... ...
The configuration and positioning of the surface depend on the kind of embankment dam, the internal zoning, and the foundation's geologic structure. For example, connected plane surfaces are often used for an inclined or sloping core rockfill dam.
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From page 253... ...
Both embankment slopes are analyzed for the specific service conditions expected. Allied analyses are used during stability studies to determine seepage patterns and amounts, pore pressures, uplift forces, hydraulic gradients, and escape gradients in the embankment zones and the foundation by the application of the principles of flow through porous media and the graphical or mathematical modeling of flow nets (Cedergren 1967~.
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From page 254... ...
Steady Seepage Conditions The highest reservoir level that may persist over a significant period of time constitutes the most severe conditions of steady seepage, resulting in the lowest factor of safety for the downstream slope. A knowledge of water pressures within the various zones in a dam and its foundation is essential for a stability analysis.
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From page 255... ...
Generally, they are shallow slides within the upstream slope that pose no significant threat of loss of impoundment. In some cases (notably the slide at San Luis Dam)
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From page 256... ...
The shearing resistance of cohesionless soils may be evaluated by performing laboratory tests on samples compacted to the in situ relative density, by correlations between shearing resistance and relative density for similar soils, or by large-scale field direct shear tests. Large biaxial shear testing equipment developed in the past 30 years has enabled more accurate determination of strengths of rockfills.
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From page 257... ...
To evaluate the stability of loose cohesionless materials, more realistic dynamic analyses should be used, in conjunction with special laboratory tests to evaluate soil strength under cyclic loading. Although they generally perform well during earthquakes, dams of cohesive soils on stable foundations may suffer some permanent deformation and loss of freeboard due to earthquake shaking.
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From page 258... ...
(1978) "Seismic Stability of Hydraulic Fill Dams," Water Power and Dam Construction, October/November.
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