dam site selection

1. EARTHEN DAM
TOPIC :– IMPORTANCE OF SOIL SURVEY AND SELECTION
OF SUITABLE DAM SITE
Presented by:
Rashmi Dhote MT21WRE010
Gaurav Gautam MT21WRE012
Harshit Sharma MT21WRE013
Aditya Jaiswal MT21WRE014
Submitted to:
Dr. Anirban Mandal

2. Contents:
• Soil survey
• Site selection of dam
 Topography
 Geology
 Material availability
 Hydrology
 Spillway
 Earthquake
 Height of dam

3. Soil Survey
Introduction:
• A soil survey is the systematic description of soils present in the field,
their grouping into well-defined mapping units like soil series, phases
etc. to identify their best use and show their location on the map.
Objective:
• The main objective is to get information about the soil genesis and
provide information about the soil resources of the area.

4. Types of Soil Survey:
1. Detailed Survey : involves elaborate mapping
2. Reconnaissance Survey : less elaborate mapping and larger
areas can be covered
3. Detailed - Reconnaissance Survey : constitutes of both
detailed and reconnaissance surveys

5. Methods of soil survey:
There are three main stages of soil survey -
• Pre – field preparation : includes study of existing data i.e. maps, reports
and toposheets.
• Field surveys : includes soil mapping and land evaluation operation.
• Post – field operations : analysis of samples is done and reports are
prepared.

6. Components of soil survey:
• a brief review on how to use the survey
• a general soil map for comparing the sustainability of large sections of the
county
• a detailed map with specific soil series outlined and indexed
• a section on the use and management of soils
• tables describing the physical features and environment of the county

7. Fig: Flowchart representing steps for selection of
dam site

8. Importance of soil survey:
• Used by farmers to help them determine suitability of soil for different crops
• Engineering properties of soil
• To estimate the potentials and limitations of soils for specific purposes
• Basis for judgement of land use and management
• Suitability of land for dam construction

9. Dam site requirement criteria
• Area having gentle slope rather than steep slope i.e, slope stability .
• Area receiving adequate rainfall .
• Area away from settlement and agricultural field.
• Type of soil i.e, permeability of soil.
• WaterTable
• Effects on forest and wildlife.
• Earthquake faults.
• Stream flow /Watershed

10. Dam site issue
• Catchment
• Reservoir
• Slope stability
• Foundation rock
• Silting

11. Selection of suitable dam site
1. Small river channel width with steep side gorge will ensure a stable and economical dam design as well as short
dam crest length.This will also lead to large storage for small dam length.
2. A wide and gently sloping valley at upstream of the dam site for storage dams & narrow and steeply sloping
valley for hydropower dams. So, that the flowing water has the necessary potential energy and speed to
generate electricity.
3. Dam site selection should be such that where a deep reservoir is possible. A deep reservoir requires less area,
lesser land costs, less surface evaporation and many benefits. It also means lesser compensations to be given to
the land owners and lower land cost.
4. Enough water flownshould be available to meet the demand of the area
5. High sediment load tributaries are excluded from the dam flow.
6. Geology should be favorable for foundation (foundation can be designed at any site, but it increases costs),
competent hard rock is most suitable.
7. Abutments are water tight, for minimum percolation and seepage losses.
8. The dam river should have a small 'river sediment rate' so that the dam could have a longer life span

12. 10. Site Selection for Dams should ensure that no seismic and tectonic activities or active faults in and near the
site should be present to avoid any disaster and save costs on design of dam.
11. The dam being constructed should have socio-political stability i.e. it should not cause any political or social
controversies.
12. Dam area should be thinly populated to pose minimum risk / loss to local population.
13. Site must have adequate stream flow record and flood frequency.
14. Reservoir area should not be very sensitive to environment i.e. construction of dam should not much disturb
the local ecosystem, natural habitats of plants and animals, wild life parks, endangered species, historical
places, monuments etc.
15. Site is easily accessible; approach road is present or can be developed easily.
16. Site Selection for Dams should be such that construction material are available nearby easily.
17. No mineral resources in reservoir area (present or future).

13. Stages of investigation in selection of dam site
Preliminary Investigation
• Lithology: It provides details of rock type present, their nature and extent of
weathering, occurrence of rock and soil debris etc.. in that area
• Structure: It provides information on strike, dip of beds and also details of folds,
faults, joints and unconformities.
• Topography: It provides information on surface features like valley, hills, trend of
river, stability of slope, scope for occurrence of landslide. The rough assessment
of depth of bed rock
• GroundWater Conditions: It provides information on springs, seepages, wells
etc., which provides information on scope for leakage and present of any cavities.

14. Detailed Investigation
• Surface Investigation: preparation of geological map of the area, Important
Engg. properties of rocks such as compressive & tensile strength, porosity,
permeability, durability etc.
• Sub-Surface Investigation to know the sub-surface profile. Drilling of bore holes
will gives detailed information on cavities & fractures present and also helps in
verifying the Geo-physical investigations

15. Topography
Most suitable place must be chosen for construction. Ideally , it must be
• Narrow gorge .
• Enough catchment area behind.
• Location of spillway.
• Possibility of river diversion during construction.
• Sedimentation possibilities.

16. SELECTION OF SUITABLE DAM SITE
GEOLOGY
.The importance of geology in civil engineering may be briefly outlined as follows:
• Geology provides a systematic study of the structure and properties of construction
materials and their occurrence.The civil engineers need to know the properties of rocks
accurately to enable them to consider different rocks for any required purpose that is as a
foundation rock, as road metal, as concrete aggregate, as building stones, as the roofing
material for decorative purpose.
• The selection of a site is important from the viewpoint of stability of foundation and
availability of construction materials.Geology provides knowledge about the site used in
the construction of buildings, dams, tunnels, tanks, reservoirs, highways and bridges.
• Geology helps to identify area susceptible to failures due to geological hazards such as
earthquake, landslides, weathering effects, etc.
• The knowledge about the nature of the rocks is very necessary for tunneling, constructing
roads and in determining the stability of cuts and slopes.

17. • The foundation problems of dams, bridges and buildings are directly related to the geology of the
area where they are to be built.
• The knowledge of groundwater is necessary for connection with excavation works, water supply,
irrigation and many other purposes. Hydrological maps provide information about the distribution of
surface water channels and the groundwater depth.
• Geological maps help in planning civil engineering projects. It provides information about the
structural deposition of rock types in the proposed area.
• Geology helps in determining the earthquake-prone areas. If any geological features like faults, folds,
etc. are found, they have to be suitably treated to increase the stability of the structure.
• The knowledge of erosion, transportation and deposition (ETD) by surface water helps soil
conservation, river control, coastal and harbour works.
• A geological survey of a site before starting a project will reduce the overall cost.

18. Geology
These are some geological consideration for the selection of dam site:
• Narrow riverValley
At proposed dam site, if the river valley is narrow a small dam is required which reduces the cost of
construction
• Bedrock at shallow depths
Shallower bed rock aims lesser foundation cost. Some problems with it are:
a)This hilly terrains may not provide a suitable topography for larger reservoir basin
b)The flow of water may not be high, therefore only small dams can be constructed.
• Influence of weathering
Weathering reduces strength, durability of rock hence the extent of weathering should be carefully assessed
to ascertain whether a rock is suitable or not. Dull appearance , faded color and emitting a dull sound to a
hammer blow are simple indication of weathering.

19. Foundation Conditions. –
The suitability of the various types of rock and soil as foundation and construction materials are geologic
questions that must be considered.The foundation geology at a dam site often dictates the type of dam
suitable for that site.The strength, thickness, and inclination of strata; permeability; fracturing; and
faulting are all important considerations in selecting the dam type. Some of the different foundations
commonly encountered are discussed below.
(a) Rock Foundations
Competent rock foundations, which are free of significant geologic defects, have relatively high shear
strengths, and are resistant to erosion and percolation, offer few restrictions as to the type of dam that
can be built upon them.The economy of materials or the overall cost should be the ruling factor.Weaker
rocks such as clay shales, some sandstones, weathered basalt, etc., may present significant problems to
the design and construction of a dam.

20. (b) Gravel Foundations
Gravel foundations, if well compacted, are suitable for earth-fill or rockfill dams. Because gravel
foundations are frequently subjected to water percolation at high rates, special precautions must be
taken to provide adequate seepage control or effective water cutoffs or seals
(c) Silt or Fine Sand Foundations.-
Silt or fine sand foundations can be used for low concrete gravity dams and earth-fill dams if properly
designed, but they are generally not suitable for rock-fill dams. Design concerns include non-uniform
settlement, potential soil collapse upon saturation, uplift forces, the prevention of piping, excessive
percolation losses, and protection of the foundation at the downstream embankment toe from erosion.
(d) Clay Foundations.-
Clay foundations can be used for the support of earth-fill dams, but require relatively flat embankment
slopes because of relatively lower foundation shear strengths. Clay foundations under dams can also
consolidate significantly.

21. Because of the requirement for flatter slopes and the tendency for clay foundations to settle a lot, it is
usually not economical to construct a rock-fill dam on a clay foundation. Clay foundations are also
ordinarily not suitable for concrete gravity dams.Tests of the foundation material in its natural state are
usually required to determine the consolidation characteristics of the foundation strata and their ability
to support the superimposed load.
(e) Non-uniform Foundations.-
Occasionally, situations occur where reasonably uniform foundations of any of the types described
above cannot be found and where a non-uniform foundation of rock and soft material must be used if
the dam is to be built. Nevertheless, such conditions can often be counterbalanced by special design
features. Even dam sites those are not highly unusual present special problems requiring the selection of
appropriate treatment by experienced engineers.

22. CONCLUSION(As per geological point of view)
Earth dams are planned in such places where underlying materials is too weak to support
masonry dams or where competent rocks occur at greater depths as they are built of
clay,sand,gravel etc.
Geological and Foundation conditions should be thoroughly surveyed because the
foundations have to carry the weight of the dam.Various kind of foundations generally
encountered are
• Solid rock foundations such as granite have strong bearing power and almost every kind of
dam can be built on such foundations.
• Gravel foundations are suitable for earthen and rock fill dams.
• Silt and fine sand foundations suggest construction of earth dams or very low gravity dams.
• Clay foundations are likely to cause enormous settlement of the dam. Constructions of
gravity dams or rock fill dams are not suitable on such foundations. Earthen dams after
special treatments can be built.

23. Other factors
1. Material availability
2. Hydrology
3. Spillway
4. Earthquake
5. Height of dam