Dam engineering i 3

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3 Site Investigation and Selectionof Dam Types
3.1 Introduction
A dam and reservoir site must satisfy certain functional and technical requirements.
Whether these requirements are satisfied can be found out through site investigations
and technical evaluations.
Functional: the balance between its natural physical characteristics and the purpose
of the dam/reservoir governs the functional suitability of a site. The catchment
hydrology, available head and storage volume, etc, must be matched by the
operational parameters needed of the project.
Technical: Technical suitability is associated with the presence or absence of suitable
site for a dam, material of construction, and integrity of reservoir basin with respect to
leakage. Hydrological, geological/geotechnical characteristics of the catchment and
the site are the principal determinants establishing the technical suitability of a
reservoir site.
In addition, assessment of the anticipated environmental consequences of
construction and operation of the dam is needed to be evaluated to select site for
storage and dam construction.
The following are major considerations:
1. Major design inputs: geotechnical, structural, hydraulic, hydrologic, and
environmental impacts/effects;
2. Optimum design solutions: solutions of appropriate type of dam (no clear-cut-
rule) is derived from the above inputs with economic factors including
construction constraints;
3. As 2 (above) implies, there are frequently several alternative solutions, which
are of equal technical, but of different economic validity. Note that, both relative
economic validity and to a lesser extent technical validity are subject to change
as technology develops;
4. Each and every dam is quite unique solution to the problems of the site in
question, in terms of the balance of technical and economic factors at the time
of consideration.
Principal stages involving site appraisal and leading to selection of optimum type of
dam are indicated schematically in Figure 3.1 (after Novak, et al).
In order to meet the requirements of dam site investigation, design and construction, a
fully coordinated team of specialists is needed. A team of hydraulic, structural,
materials, and geotechnical engineers, geologists, and hydrologists should ensure
that all engineering and geological considerations are properly integrated into the
overall design.

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Figure 3.1 Stages in dam site appraisal and project development
Some of the principal aspects of the analysis and design process that require
coordination are (US Army Corps of Engineers):
a. Preliminary assessment of geological data, sub-surface conditions and rock
structure;
b. Selection of material properties, design parameters, loading conditions, loading
effects, potential mechanisms, and other related features of the analytical models;
c. Evaluation of the technical and economic feasibility of alternative type structures;
d. Constructibility reviews to see whether design assumptions and construction
procedures are compatible;
3 - 20
Time span (years)
1 - 3
2 - 4
Strategic planning:
Project initiation
Field reconnaissance Mapping, surveys, data
Collection
Field studies & technical resources,
Report options, etc.
Phase 1 dam site reservoir site
Evaluation evaluation
Confirmation of
Dam type
Phase 2 dam site
Investigation
Dam design
1 - 2
Construction foundation feedback 2 - 6

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e. Refinement of the preliminary structure configuration to reflect the results of
detailed site explorations, material availability studies, laboratory testing, and
numerical analysis;
f. Cofferdam and diversion layout, design and sequencing requirements;
g. Size and type of outlet works and spillways;
h. Modification to the structure configuration during construction due to unexpected
variations in the foundation conditions.
3.2 Collection of data
3.2.1 Physical and Topographic Data
Selection of dam and reservoir sites requires presence of suitable topography. The
topographic information can be obtained through site visit and from large-scale
topographic maps or aerial photographs. The criteria for the choice of the site include
investigation of:
Valley form
- Canyon (gorge)and V-shape: due to erosion,
- U-shape: due to glacier cut,
- Wide valley: due to strong bank erosion,
- Box valley: due to fluvial deposit on the other shapes.
The valley width at the dam site is required to be narrow and wide in the storage part.
Slope: upstream of the dam site, the possible small slope and downstream of the
dam site the possible large slope (by hydropower scheme).
In the collection of relevant topographic information, the following may be followed:
General Plan:
- Obtain a general plan of the catchment and project area from relevant sources
(e.g. EMA),
- Carry out limited survey to include additional information in this plan (aerial
reconnaissance, physical surveys, walkovers),
- It must include: the dam site, irrigable area/power house site, catchment area of
the stream, locality to be supplied with potable water, if any. Scale may vary from
1:1000 to 1:10,000.
The following features should also be included:
i Contours at 0.5 to 1.5 m interval,
ii Location of existing works, if any, affected by the proposed development,
iii Proposed relocation of roads, railways, transmission lines, etc.,
iv Additional transportation facilities such as access roads, cable ways, etc.,
required for the execution of the project,
v Locations of the stream gauging stations, water sampling and meteorological
stations, if any in the area.

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Larger Plans of the Dam and Spillway Sites:
These should be in the scale of 1:500 to 1:1000 with contours as close as possible.
These plans should show:
- Over banks,
- Location and elevation of all features such as buildings, roads, etc.,
- Location and numbering of test pits and borings.
3.2.2 Geotechnical and Geological Data
A geological map of the entire catchment and project area is essential. Investigation of
geological and geotechnical information on the origin, deposition, formation and
physical characteristics of the dam foundation and reservoir area are needed. As a
basis for the investigation, if there is no accurately describing geological map, such
maps are produced on large scale for the dam site and on small scale for reservoir
site.
Dam and Spillway Site
Subsurface investigation should be carried out by experienced geologist to obtain the
following:
- Geological section of the selected dam site,
- Quality of the overburden if an earth dam is to be built,
- Shearing strength of the material of overburden and of the dam material,
- Quantity and quality of the overburden material for construction purposes,
- Presence of joint planes, caverns, solution channels,
- Quality of the rock if concrete dam is to be built,
- Depth to which rock is weathered,
- Presence and extent of seams and joint planes (and orientations)
- Strength of the rock (hardness and durability),
- Availability of aggregate.
Reservoir Site
- Check the existence of cracks which are potential leakage sources,
- Banks should be checked for possible zones of landslide.
Earthquake
Information on seismic activity of the area should be obtained.
Here it is assumed that adequate knowledge of the relevant engineering geology, soil
mechanics and geotechnical parameters are acquired.

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3.2.3 Water Resources Data
Data regarding the following are required for water resources planning and reservoir
design:
- Catchment area;
- Discharge; daily/monthly volume of flow in the stream and peaks of stream flow at
or near dam site;
- Sediment carried by the stream;
- Maximum observed flood, report on damage caused by flood (extent of flood);
- Data establishing water demand (number of people to be served, approximate
maximum and minimum daily requirement, irrigation water requirement, other
requirements for industries, livestock, etc.);
- Meteorological data such as average temperature, average monthly rainfall,
maximum recorded storm intensities, rate of evaporation;
- Ground water level;
- Data on minimum downstream water requirements.
3.2.4 Project Development Data
Data such as agricultural, hydropower and other relevant data to the project purpose.
For irrigation purpose, for instance, the following data are essential for the
determination of water requirements:
- Size of the area to be irrigated,
- Soil structure;
- Possible types of crops;
- Types of soils along the conveyance canals.
3.2.5 Miscellaneous Data
- Materials data: soil, gravel and stone (for aggregate and riprap) are needed in
good quantity and quality for dam construction (possible borrow sites for these
materials should be identified), the borrow area should be within a reasonable
distance from the site. Selection of suitable borrow area is identified by:
 Thickness of the top organic soil which has to be discarded;
 Content of organic matter in the rest of the soil;
 Quantity of oversized cobbles, which would have to be removed from the soil.
- Rock for aggregate and riprap has to pass the standard tests of specific gravity,
absorption, abrasion, soundness, etc.;
- Erosion in the catchment area – identify sources of erosion;
- Transport – existing facilities and rates;
- Local labour availability and rate.
3.3 Investigations
The purpose of site or material investigation in the context of dam engineering is to
determine the suitability of the selected site for dam construction and reservoir storage
as well as to describe the geotechnical parameters necessary for the design and
construction of the structures.

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For such purposes, thus, general site exploration and investigation, in addition to the
above data collection, involve the following:
 Field investigation
 In situ tests/field tests
 Laboratory tests
3.4 Location of Dam
Influencing factors:
- Plan for the use of water,
- Width and shape of the valley,
- Load carrying capacity and impermeability of the foundation,
- Seepage loss in the reservoir area,
- Quantity, quality and transport distance of the construction materials,
- Suitability for appurtenant structures (bottom outlets, intakes, spillways,
powerhouses),
- Danger due to slide, avalanche, etc.,
- Influence on environment and landscape,
- Recreational value,
- Available storage area,
- Dam heightening and capacity augmentation possibilities,
- Cost,
- Social and political implications.
3.5 Height of Dam
The selection of the height of a dam is influenced by:
- Local topographic conditions,
- Dam type,
- Required storage,
- Finance.
3.6 Selection of Dam Type
Influencing factors:
- Topography (valley form),
- Foundation (suitability, impermeability),
- Geology (layers, fishers),
- Required height,
- Purpose of the dam,
- Climate,
- Flood spillway,
- Availability, quality (nature, state) of construction materials,
- Construction (supply, transport, equipment, qualification of personnel),
- Landscape,
- Cost (economy).

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It is necessary to make open the possible alternative solutions until an optimum
solution is found with respect to cost, construction program and available resource.
Novak, ET al., consider four cardinally important points in selection of dam type as:
1. Hydraulic Gradient: the nominal value of hydraulic gradient i for seepage
under, around or through the dam vary by at least one order of magnitude
according to type. The ability of softer and weaker or more erodible
foundations to resist high hydraulic gradients safely is very limited. Notional
values of gradient range from about 0.5 for a homogeneous embankment to
10 or more for a buttress or cupola dam,
2. Foundation Stress: nominal stresses transmitted to the foundation vary
greatly with dam type. The notional foundation stress values for 100m high
dam, for instance, varies from 1.8 MN/m² for embankment to 10 MN/m² for
arch dams (see Table 3.1),
3. Foundation Deformability: certain types of dams are better able to
accommodate significant foundation deformation and/or settlement without
serious damage,
4. Foundation Excavation: economic considerations dictate that the excavation
volume and foundation preparation should be minimized.
Table 3.1 Notional foundation stresses; 100 m height dam (Novak, et al, 1996)
Dam type Notional maximum stress
(MN/m²)
Embankment 1.8 – 2.1
Gravity 3.2 – 4.0
Buttress 5.5 – 7.5
Arch 7.5 – 10.0
Figure 3.2 illustrates examples of valley profiles with suggested dam type and
Table 3.2 provides type characteristics with respect to choice of dam (after Novak,
et al., 1996).

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Figure 3.2 Illustrative examples of dam type in relation to valley profile
Table 3.2 Dam selection: type characteristics.
Type Notes and characteristics
Embankment
Earthfill Suited to either soil or rock foundation and wide valleys; can accept
limited differential settlement given relatively wide and plastic core. Cut-
off to sound, i.e. less permeable, horizons required. Low contact
stresses. Requires range of materials, e.g. for core, shoulder zones,
internal filters, etc.
Rockfill Rock foundations preferable; can accept variable quality and limited
weathering. Cut-off to sound horizons required. Rockfill suitable for all
weather placing. Requires material for core, filters, etc.
Concrete
Gravity Suited to wide valleys, provided that excavation to rock is less than
about 5 m. Limited weathering of rock acceptable. Check discontinuities
in rock with regard to sliding. Moderate contact stress. Requires
imported cement.
Buttress As gravity dam, but higher contact stresses require sound rock.
Concrete saved relative to gravity dam 30 – 60%.
Arch and Cupola Suited to narrow gores, subject to uniform sound rock of high strength
and limited deformability in foundation & most particularly in abutments.
High abutment loading. Concrete saving relative to gravity dam is
50 – 85%