ELEMENTS OF DAM ENGINEERING

LECTURE NOTES - I

« WATER

RESOURCES »

Prof. Dr. Atıl BULU

Istanbul Technical University College of Civil Engineering Civil Engineering Department Hydraulics Division

To Blacks and Indians of Republic of South Africa

CHAPTER 1

ELEMENTS OF DAM ENGINEERING

1.1. INTRODUCTION The primary purpose of a dam may be defined as to provide for the safe retention and storage of water. The structural design life for dams depends upon the reservoir siltation. Reservoirs are readily classified in accordance with their purpose; a) b) c) d) e)

Irrigation, Water supply, Hydroelectric power generation, Flood control, Recreation.

Dams are numerous types. An initial broad classification into two generic groups can be made in terms of the principal construction material employed. a) Embankment Dams: Constructed of earthfill and/or rockfill. Upstream and downstream face slopes are similar and of moderate angle, giving a wide cross-section and a high construction volume relative to height. b) Concrete Dams: Constructed of mass concrete. Face slopes are dissimilar, generally steep downstream and vertical upstream, and dams have relatively slender profiles dependent upon the type. The concrete dams can be considered to include also older dams of appropriate structural type constructed in masonry. Embankment dams are numerically dominant for technical and economic reasons, and account for over 85% of all dams built. The embankment utilized is locally available and untreated material. As the embankment dam evolved it has proved to be increasingly adaptable to a wide range of site circumstances. In contrast, concrete dams are more demanding in relation to foundation conditions. They also proved to be dependent upon relatively advanced and expensive construction skills.

1

A.Bulu

Possible time span (years)

strategic planning: project initiation 3 - 20 mapping, surveys field reconnaissance data collection

feasibility studies report

technical resources, options, etc.

phase 1 dam site evaluation

reservoir site evaluation

1-3

2-4 confirmation of dam type

phase 2 dam site investigations 1-2 dam design

construction

Foundation feedback

2-6

Figure 1.1. Stages in dam site appraisal and project development

1.2. THE PLANNING PROCESS Rivers are sources of energy (hydroelectric power) and water supply for municipalities and agriculture. Many rivers also serve as transportation arteries and are sources of recreation. Flooded rivers cause property damage and loss of life. Rivers are also often used for sewage disposal. We design and construct water resources projects to control the rivers to our advantage as our population increases and demand for food, water, power,

2

A.Bulu

recreation, and land grows. Careful planning should be done to achieve optimum utilization of river basins as whole, as well as specific projects within them. Planning means determining the best way to accomplish a particular objective by evaluation various alternatives. For example, in the context of water resources, a problem may exist of not having enough water for the demands of a large city during drought periods. Careful planning should be done to bring about a solution to the problem. Planning involves evaluating several possible solutions. Some solutions entail building structures such as dams and supply pipes. Planning also involves designing these features and their cost, cost comparisons often determine the best alternative.

1.3. GENERAL SITE APPRAISAL A satisfactory site for a reservoir must fulfill certain functional and technical requirements. The balance between its natural physical characteristics and the purpose of the reservoir governs functional suitability of a site. Catchment hydrology, available head and storage volume must be matched to operational parameters set by the nature and scale of the project served. Technical suitability is dictated by the presence of a site (or sites) for a dam, the availability of materials suitable for dam construction, and by the integrity of the reservoir basin with respect to leakage. The hydrological and geological or geotechnical characteristics of catcment and site are the principal determinants establishing the technical suitability of a reservoir site. To these must be added an assessment of the anticipated environmental consequences of construction and operation of the dam. The principal stages involved in site appraisal, and leading to the selection of the optimum dam site and type of dam are as indicated in Fig. (1.1).

1.3.1. Preliminary Study In the preliminary study phase, which may extend over a substantial period of time, the principal objective is to collect adequate topographical, geological, hydrological survey data. Large-scale maps and any records already available provide the starting point, but much more detailed surveys will inevitably be required. Aerial reconnaissance, employing modern sensors in addition to the traditional photogrammetric survey techniques, has a particular to play in the preparation of accurate and large-scale site plans (e.g. 1/5000 and larger). In the hands of a skilled interpreter, aerial surveys also provide valuable information on geology, on possible dam sites, and on the likely availability of construction materials. Hydrological surveys are directed to determining rainfall and runoff characteristics, and assessing historical evidence of floods.

3

A.Bulu

1.3.2. Feasibility Study The feasibility report prepared at the conclusion of the preliminary study phase assembles and interprets all available information, data, and records, and makes initial recommendations with respect to the technical and economic viability of the reservoir. Options with regard to the location, height, and type of dam are proposed, and comparisons drawn in terms of estimated costs and construction times. On the strength of this report a decision can be made with respect to further detailed investigations required to confirm the suitability of the reservoir basin and preferred dam site (or sites).

1.3.3. Final Study In the final phase of planning, detailed designs are made and plans and specifications are developed. Further investigations of the reservoir basin are principally directed to conforming its integrity with respect to water retention. A thorough geological assessment is necessary for this purpose, particularly in karstic and similarly difficult formations and in areas with a history of mining activity. The availability of construction materials, e.g. suitable fills, sources of aggregate etc., is also assessed in considerable depth. Hydrological studies are continued as necessary to confirm and extend the results of the initial investigations.

1.4. PLANNING CONSIDERATIONS OF PROJECTS INVOLVING A DAM AND RESERVOIR Whether it be for municipal water supply, irrigation, or hydropower several items must be considered in the planning and design of a dam and reservoir. • Hydrological data: Data of the stream that the dam is to be built on are analyzed to determine flood and drought flows and to determine the required capacity and operating procedure for the reservoir. Also, the required spillway capacity can be determined from the hydrological data. • Geological data: On-site inspection, geological mapping the drilling of exploratory holes and collection of core-sample data by geologists are usually required. These data reveal the structural ability of the foundation material to withstand the loads that may act on it and indicate the leakage and erosion problems that may be encountered. The data also reveal the availability of the fill and aggregate for the construction of the dam. • Reservoir data: A complete assessment of the area to be inundated by the reservoir must be made. This includes topographic maps, land ownership, land classification, and location of roads and public utilities. These data are used to estimate the cost of land acquisition and relocation of roads and utilities.

4

A.Bulu

1.5. TYPES OF DAMS Dams are classified according to the material (earth, rock, concrete) from which they are built and according to the configuration and the way in which they resist the forces imposed on them.

Figure 1.2. Earth dam, rockfill dam, and concrete dam Thus a gravity dam is one in which gravitational forces (such as the weight of the dam itself) are great enough to resist the overturning moment and sliding force of the hydrostatic forces imposed on it (Fig. 1.2.c)

Figure 1.3. Buttress and arch dams Another type of gravity dam is the buttress dam, in which reinforced concrete slabs constitute the face of the dam and are supported by vertical buttresses at intervals of 15 to 30 m. In contrast, an arch dam is designed to transfer the imposed loads to adjacent rock walls on either side of the canyon it is located. Both earthfill and rockfill dams are special types of gravity dams.

1.6. SELECTION OF TYPE OF DAM The optimum dam type for a specific site is governed by technical validity and by cost. In some situations the options are very limited on technical grounds, and the selection of type is correspondingly straightforward. In many situations options may exist between types of comparable technical validity, and the decision will rest upon the relative economics for that site.

5

A.Bulu

Four considerations of cardinal importance are detailed below: a) Hydraulic gradient: The nominal value of hydraulic gradient for seepage under a dam varies by at least one order of magnitude according to type. b) Foundation stress: Nominal stresses transmitted to the foundation vary greatly with dam type. c) Foundation deformability: Certain types of dam are better able to accommodate significant foundation deformation without serious damage. d) Foundation excavation: Economic considerations dictate that the excavation volume should be minimized. Notional stress values for 100 m high dams of different types are shown in Table (1.1). Table 1.1. Notional foundation stresses: dams 100 m in height Dam type

Notional maximum stress (MN/m2)

Embankment Gravity

1.8 – 2.1 3.2 – 4.0

Buttress Arch

5.5 – 7.5 7.5 – 10.0

The situation of a wide valley with deep alluvial deposits is illustrated in Fig. (1.4.a). Considerations of foundation deformation and the depth of excavation required favor an earthfill embankment. The availability of competent rock at shallow depth as shown in Fig. (1.4.b), favors either a rockfill embankment or, secondly, a concrete gravity or buttress dam. Availability of rockfill, and thus cost, would dictate the final choice. A narrow and steep-sided valley in sound rock, as illustrated in Fig. (1.4.c) is suited to an arch dam. Economic considerations may favor the rockfill embankment. The situation shown in Fig. (1.4.d), with deep overburden under one half of the site, could well suggest the composite solution shown. An earthfill embankment is constructed where settlement may be significant, the spillway being conveniently accommodated on a concrete gravity section where the required excavation depth is reasonable.

6

A.Bulu

Figure 1.4.a. Wide valley with deep burden: morainic or alluvial deposits over 5-10 m: favors earth fill embankment

Figure 1.4.b. Valley with little overburden: suitable for embankment, gravity, or buttress dam

Figure 1.4.c. Narrow valley steep sides, little overburden: suitable for arch, or rockfill embankment dam

Figure 1.4.d. Deep overburden under one half of the site

1.7. LOADS ON DAMS The structural integrity of a dam must be maintained across the range of circumstances or events likely to arise in service. In all foreseeable circumstances the stability of the dam and foundation must be assured, with stresses contained at acceptable levels and watertight integrity essentially unimpaired. It is convenient to classify individual loads as primary, secondary, or exceptional. The classification is made in terms of the applicability and/or the relative importance of the loads: a) Primary loads are identified as those of major importance to all dams, irrespective of type, e.g. water and related seepage loads, and self-weight loads.

7

A.Bulu

b) Secondary loads are universally applicable although of lesser magnitude (e.g. silt load) or, alternatively, are of major importance only to certain types of dams (e.g. thermal effects within concrete dams). c) Exceptional loads are so designated on the basis of limited general applicability or having a low probability of occurrence (e.g. tectonic effects, or the inertia loads associated with seismic activity).

1.7.1. Schedule of Loads The primary loads and the more important secondary and exceptional sources of loading are identified schematically on Fig. (1.5) on a gravity dam.

1.7.2. Primary Loads a) Water load. Hydrostatic distribution of pressure with horizontal resultant force P1. (Note that a vertical component of load will also in the case of an inclined upstream face and that equivalent tailwater loads may operate on the downstream face). b) Self-weight load. Determined with respect to an appropriate unit weight of the material. The resultant, P2, is considered to operate through the centroid of the section. c) Seepage loads. Equilibrium seepage patterns will establish within and under a dam, e.g. in pores and discontinuities, with resultant vertical loads identified as internal and external uplift, P3 and P4, respectively.

1.7.3. Secondary Loads a) Sediment load. Accumulated silt etc. generates a horizontal thrust, considered as an additional hydrostatic load with horizontal resultant P5. b) Hydrodynamic wave load. Transient load, P6, generated by wave action against the dam (not normally significant). c) Ice load. Ice thrust, P7, may be significant in more extreme conditions. d) Thermal load. (concrete dams). Internal, not illustrated, generated by differentials associated with changes in ambient temperatures and with cement hydration and cooling. e) Interactive effects. Internal, not illustrated, arising from relative stiffness and differential deformations of dam and foundation.

1.7.4. Exceptional Loads a) Seismic load. Horizontal and vertical inertia loads are generated with respect to the dam and the retained water by seismic disturbance. For the water inertia forces are simplified equivalent static thrust, P8, is shown. b) Tectonic effects. Saturation, or disturbance following deep excavation in rock, may generate loading as a result of slow tectonic movements.

8

A.Bulu

Figure 1.5. Schedule of principal loads: gravity dam profile

9

A.Bulu