1、 The Design of Earth-Rock Dam and Spillways Part 1 Embankment Dams 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 roc
2、kfills 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 soft and will easily break down under the action of excavation and placement can be classified with earthfills. Rocks which are hard and
3、will not break down significantly are treated as rockfills. The selection and the design of 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 th
4、e engineers judgment. Freeboard ： 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
5、allowance must be made for settlement of the embankment and the foundation which will occur upon completion of the embankment. 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. In general, the top w
6、idth should not be less than 30 ft. If a danger exists of an overtopping wave caused either by massive landslides in the pool or by seismic block tipping, then extra top width of erosion resistive fill will be required. Alignment ： construction costs but such alignment should not be such as to encou
7、rage 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 t
8、he embankment under pool load will put the embankment in compression thus minimizing transverse cracking, Abutments ： Three problems are generally associated with the abutments of earth dams:seepage, instability, and transverse cracking of the embankment. If the abutment consists of pervious soils i
9、t may be necessary to construct an upstream impervious blanket and downstream drainage measures to minimize and control abutment seepage. Where steep abutments exist, especially with sudden changes of slopes or with steep bluff, there exists a danger of transverse cracking of the embankment fills, T
10、his can be treated by excavation of the abutment to reduce the slope, especially in the imperious and transition zones. The transition zones, especially the upstream, should be constructed of fills which have little or no cohesion and a well-distributed gradation of soils which will promote self-hea
11、ling should transverse cracking occur. Stage construction ： It is often possible and in some cases necessary, to construct the dam embankment in stages. Factors dictating such a procedure are : a wide valley permitting the construction of the diversion or outlet works and part of the embankment at t
12、he same time； a weak foundation requiring that the embankment not be built too rapidly to prevent overstressing the foundation soils； a wet borrow area which requires a slow cases it may construction to permit an increase in shear strength through consolidation of the fill. In some cases it may be n
13、ecessary to provide additional drainage of the foundation or fill by means of sand drain wells or by means of horizontal pervious drainage blankets. Embankment soils ： Most soils are suitable for use for embankment construction, however, there are physical and chemical limitations, soils which conta
14、in excessive salts or other soluble materials should not be used. Substantial organic content should not exist in soils. Lignite sufficiently scattered through the fill to prevent the danger of spontaneous combustion, is not objectionable. Fat clays with high liquid limits may prove difficult to wor
15、k and should be avoided. Compaction requirements ： The strength of the impervious and semi-impervious soils depends upon he compacted densities. These depend in turn upon the water content and weight of the compacting equipment. The design of the embankment is thus influenced by the water content of
16、 the fill or after placement practicable alternations to the water content either prior to placement of the fill or after placement but prior to rolling. If the natural water content is too high, then it may be reduced in borrow area by drainage, or by harrowing. If the soil is too dry it should be
17、moistened in the borrow area either by sprinkling or by ponding and then permitted to stabilize the moisture content before use. The range of placement water content is generally between 2 percent dry to 2 or 3 percent wet of the standard Proter optimum water content. Pervious soils should be conpac
18、ted to at least 80 percent of relative density. If necessary, test fills should be constructed with variations in placement water content, lift thickness, number of roller passes and type of rollers. For cases of steep abutment, the fill must be placed in thin lifts and compacted by mechanical hand
19、tampers. All overhangs should either be removed or filled with lean concrete prior to fill placement. Types of instruments ： The type of instrumentation depends upon the size and complexity of the project. The devices in common use are : piezometers； surface movements； settlement gages； inclinometer
20、s； internal movement and strain indicators； pressure cells； movement indicators at conduit joints and other concrete structures. Part 2 Spillways A spillway is the safety valve for a dam. It must be designed to discharge maximum flow while keeping the reservoir below a predetermined level. A safe sp
21、illway is extremely important. Many failures of dams have resulted from improperly designed spillways or spillways of insufficient capacity. Spillway size and frequency of use depend on the runoff characteristics of the drainage basin and the nature of the project. The determination and selection of
22、 the reservoir inflow design flood must be based on an adequate study of the hydrologic factors of basin. The routing of the flow past the dam requires a reasonably conservative design to avoid loss of life and property damage. Space limitations do not permit an adequate hydrologic treatment of floo
23、d flows. However, data are supplied for estimates of maximum flows for the initial project studies. A more detailed hydrologic analysis is necessary for the utilization of the annual and long-turn stream flow in a proper project formulation. The study of stream or river flows involves: the determina
24、tion of the amount of water available throughout a period of years； and the determination of the maximum volumes of water that must be handled for spillway design and dam safety. In the first aspect, the flow is studied for periods of drought and programmed into this study. A mass curve of the strea
25、m runoff over a period of years is developed to determine the available water. The mass curve is the accumulative total of the volume of flow past a given point on the stream over a period of time. Unfortunately, most of the small streams do not have sufficient records to develop the hydrologic info
26、rmation. The engineer usually develops synthetic curves from neighboring stream data and rainfall information. However, methods are available in various texts, journals, and reports to estimate stream flow. The second aspect involves estimating maximum flood flow to determine spillway requirement and dam safety. Studies show that flood flows are associated with frequency of the event of the risk of floods causing damage by exceeding the estimated design flow.
