THE DESIGN OF EARTH DAM
■PURPOSE
The purpose of this chapter is to discuss the basic concepts relative to the safe and economical design, construction and operation of earth dams .The following criteria must be complied with to provide a safe structure: ■GENERAL CRITERI
1. The foundations, abutment and the embankment must be stable for all conditions of construction and operations. While of a serious nature, some embankment and foundations distress can be tolerated during construction provided corrective measures are promptly taken.
2. Seepage through the embankment, foundation and abutment must not exert excessive forces on the structure nor must piping of material be permitted. Excessive seepage forces will result in instability of the dam or abutment while piping ,if not controlled, will eventually result in the release of the pool.
3. The top of the dam must be high enough to allow for settlement of the dam and foundation and also to provide sufficient freeboard to prevent waves from a maximum pool from overtopping the dam.
4. the spillway and outlet capacity must be such as to prevent overtopping of the dam. It is especially important that the spillway be capable of this alone because of the possibilities of the outlet being rendered inoperable.
5. The slope of the spillway and the outlet works must be stable under all operational conditions. It is especially important that the slopes adjacent to the inlet be stable under drawdown of the pool and that the slopes of the spillway cut be stable even under earthquake conditions.
The foundation, abutments and potential sources of borrow materials for construction of the dam must be studied in detail. This requires carefully made explorations as discussed in chapter 4.The studies will assess the strengths and seepage characteristics of the foundations: the stability of the abutments, especially during pool drawdown and the seepage potentials, and the dangers of piping through the joints, bedding planes, and solution cavities. Studies of the various borrow sources and distribution, variations of soil water contents and the water table with the seasons, thickness of required stripping and the various practical methods of borrow excavations to best use the available soils. Of vital
1
important to borrow usage is the variation of the climate and possibility of flooding on the use of borrow areas.
The influence must be considered of seismic action on the stability of the dam, the abutments, the cut slopes of the spillway and inlet and outlet works ,and especially the susceptibility of the foundation to seismically induced liquefaction. This is of importance if the dam is to retain a pool of significant volume.
■SELECTION OF EMBANKMENT TYPE
In general, there are two types of embankment dams: earth and rockfill. The selection is dependent upon the usable materials from the required excavation and available borrow. It should be noted that rockfills can shade into soil fills depending upon the physical character of the rock and that no hard and fast system of classification can be made. Rocks which are hard and will not break down under the action of excavation and placement can be classified with the earthfills. Rocks which are hard and will not break down significantly are treated in chapter The selection and the design of an earth embankment are based upon the judgment and experience of the designer and is to a large extent of an empirical nature. The various methods of stability and seepage analyses are used mainly to confirm the engineer’s judgment. Conditions which favor the selection of an earth dam for a site are:
1. Significant thickness of soil deposits overlying bedrock.
2. Weak or soft bedrock which would make the resistance to high stresses from a concrete dam difficult to obtain.
3. Abutments which are either of deep soil deposits or of weak rock. 4. Availability of a satisfactory location for a spillway.
5. Availability of sufficient and suitable soils from required excavation or nearby borrow
areas.
■ENVIRONMENTAL CONDITIONS
Federal and state laws now exist which require prevention of damage to the environment. These laws apply to design, construction and operation of the project.
Some problems which may require solutions are contamination of rivers by hydraulic fill operations, erosion from borrow pits, construction roads and quarries.
■GENERAL DESIGN CONSIDERATION ★FREEBOARD
2
All earth dams must have sufficient extra height known as freeboard to prevent overtopping by the pool. The freeboard must be of such height that wave action ,wind setup, and earthquake effects will not result in overtopping of the dam. In addition to freeboard ,an allowance must be made for settlement of the embankment and the foundation which will occur upon completion of the embankment .The hydraulic freeboard can be determined using the procedures given in ref.4 and ref.5.
An extra allowance for freeboard to counter earthquake induced settlement is required by the U.S.Army Corps of Engineers, either (1)the maximum pool plus normal freeboard or (2)the elevation of the flood control pool plus 3 percent of the height of the dam above stream bed. It should be noted that this allowance is for the settlement of the embankment under seismic action. If a compressible foundation exists, an extra allowance in the freeboard must be made .If there are dangers of a massive slide in the reservoir or a block tilting induced by earthquakes sending a wave over the top of the dam, then additional freeboard allowances and design provisions must be made .
★TOP WIDTH
The width of the earth dam top is generally controlled by the required width of fill for ease of construction using conventional equipment. To facilitate construction near the top of the dam the internal zonation should not be excessively complicated. If a highway is to cross the dam then this will control the top width. In general, the top width should not be less than 30 ft. If a danger exists of an overtopping wave caused either by massive landslide in the pool or by seismic block tipping, then extra top width of erosion resistive fill will be required. ★Alignment
The alignment of an earth dam should be such as to minimize construction costs but such alignment should not be such as to encourage sliding or cracking of the embankment. Normally the shortest straight line across the valley will be satisfactory, but local topographic and foundation conditions may dictate otherwise. Dams located in narrow valleys often are given an alignment which is arched upstream so that deflections of the embankment under pool load will put the embankment in radial compression thus minimizing transverse cracking . ★ABUTMENT
Three problems are generally associated with the abutments of earth dams (1)seepage,(2)instability, and(3)transverse cracking of the embankment. If the abutments consists of deposits of previous soils it may be necessary to construct an upstream impervious blanket and downstream drainage measures to minimize and
3
翻译03
