DD HOOVER DAM is a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the

border between the US states of AMERICA.

DAM AND DAM SAFETY EARTH SCIENCE PROJECT .

RIPU DAMAN SINGH , 12215008, CIVIL IDD

4/5/2014

DAM AND DAM SAFETY

INTRODUCTION

Dam is a solid barrier constructed at a suitable location across a river valley to store flowing water.

REVIEW OF LITERATURE

First dam was constructed by the Egyptians in 2950-2750 B.C, using stone/ brick ma-sonry.

Earth dam was built first in Mesopotamia around 2000 B.C.

Romans used concrete and mortars around 100 AD.

Due to large size and amount of building material need to construct earth and gravity dams

Grand Anicut (Kallanai)

The oldest dam in the world

Built by Chola king Karikalan around the 2nd Century AD

To divert the waters of the Kaveri across the fertile delta region for irrigation via canals.

It is constructed from unhand stone spanning the Kaveri and is 329 m (1,079 ft.) long, 20 m (66 ft.) wide and 5.4 m (18 ft.) high

OBJECTIVES

Irrigation Water for domestic consumption Drought and flood control For navigational facilities Hydroelectric power generation Recreation Development of fish & wild life Soil conservation

THEORETICAL ASPECTS

The following factors shall be considered when selecting the site of a dam.

Topography: the dam should be located where the river .Has a narrow gorge which opens

out upstream to create a large reservoir. The dam should be preferably located where the river bed is high, to reduce the height and cost of the dam.

Suitable Foundation: Suitable foundation should exist at the site for the particular type of

dam. For gravity dams of great height, sound rock is essential. Good Site for reservoir: site should have the following characteristics to make a good site

for a reservoir: (i) Large storage capacity: The topography of the site should be such that the reservoir has a large capacity to store water. (ii) Shape of reservoir basin: The reservoir basin on the upstream of the dam should preferably be cup-shaped, with a flat bottom but steep slopes. (iii) Watertightness of the reservoir: The geological conditions of the reservoir site should be such that the reservoir basin is watertight. The reservoir sites having pervious rocks are not suitable. The reservoir basins having shales, slates, schists, gneiss, granite, etc. are generally suitable. (iv) Good hydrological conditions: The hydrological conditions of the river at the reservoir site should be such that adequate runoff is available for storage. The catchment area of the river should give high yield. There should not be heavy losses in the catchment due to evaporation, transpiration and percolation. (v)Deep reservoir: The site should be such that a deep reservoir is formed after the construction of the dam. A deep reservoir is preferred to a shallow reservoir because in the former the

evaporation losses are small, the cost of land acquisition is low and the weed growth is less. (vi) Small submerged area: The site should be such that the submerged area is a minimum. It should not submerge costly land and property. It should not affect the ecology of the region. Monuments of historical and architectural importance should not be submerged. (vii) Low siltin flow: The dam site should be such that the reservoir would not silt up quickly. The life of the reservoir depends upon the rate of silting. The site should be selected such that it avoids or excludes the water from those tributaries which carry a high percentage of silt, i.e. if any tributary carries relatively large quantity of sediments, the dam should be constructed upstream of the confluence of that tributary with the river. (viii) No objectionable minerals: The soil and rock mass at the reservoir site should not contain any objectionable soluble minerals which may contaminate the water. The stored water should be suitable for the purpose for which the water is required.

Earthquake hazards: If the dam site is located in a seismic zone, the most

suitable type of the dam is one which can resist the earthquake shock without much damage. Earth dams and rock fill dams are generally more suitable for such sites, provided suitable modifications are made in the design. However, by adopting suitable measures and considering various forces and factors affecting the seismic design, other types of dams can also be provided.

Climatic conditions: Climatic conditions should also be considered while

selecting the type of dam. In extremely cold climates, buttress and arch dams should be avoided. These dams have thin concrete sections and are easily damaged due to

spilling of concrete which occurs due to alternate freezing and thawing. Similarly, if there are frequent rains and the climate is extremely wet, it will be difficult to control water content of the soil and compaction in an earth dam. Therefore, earth dams should be avoided.

Environmental considerations: The dam and its appurtenant works should

be aesthetically acceptable and they should not have any adverse effect on ecology and environment. Generally, earth dams are more suitable than concrete dams for aesthetical consideration. They merge easily with the natural environment in the valley

INVESTIGATION FOR DAM

The following investigations are usually conducted locate the most suitable site for a dam

1. Engineering surveys: Engineering surveys are conducted for the dam, the reservoir and other associated works. Generally, the topographic survey of the area is carried out and the contour plan is prepared. The horizontal control is usually provided by triangulation survey and the vertical control by precise levelling. For the area in the vicinity of the dam site, a very accurate triangulation survey is conducted. A contour plan to a scale of 1/250 or 1/500 is usually prepared. The contour interval is usually 1 m or 2 m. The contour plan should cover an area at least upto 200 m upstream and 400 m downstream and for adequate width beyond the two abutments. For the reservoir, the scale of the contour plan is usually 1/15,000 with a contour interval of 2 m to 3 m, depending upon the size of the reservoir. The area-elevation and storage-elevation curves are prepared for different elevations upto an elevation 3 to 5 m higher than the anticipated maximum water level (MWL).

2. Geological Investigations: Geological Investigations of the dam and reservoir site are done for (a) Suitability of foundation for the dam, (b) Watertightness of the reservoir basin, and (c) Location of the quarry sites for the construction materials. Subsurface explorations are carried out to determine the depth of overburden to be removed for laying the foundation of the dam, the type of rock, the nature and extent of the fault zones, if any, present in the rock. The information obtained from the geological investigations is used for devising a

suitable programme of foundation treatment and grouting if necessary. Geological investigations are conducted to detect the presence of faults, fissures, and cavernous rock formations which have cavities and are porous. If such formations exist in small areas, they may be treated and made watertight. However if they are wide spread, the site may have to be abandoned. Geological investigations are also conducted for location of suitable quarries for stones and borrow areas for soils. The quality and the quantity of the available construction materials are also ascertained.

3. Hydrological Investigations: Hydrological Investigations are conducted (a) to study the runoff pattern and to estimate yield and (b) to determine the maximum discharge at the site. The most important aspect of the reservoir planning is to estimate the quantity of water likely to be available in the river from year to year and seasons to season. For the determination of the storage capacity of a reservoir, the runoff pattern of the river at the dam site is required. The spillway capacity of the dam is determined from the inflow hydrograph for the worst flood when the discharge in the river is the maximum. Flood routing is done to estimate the maximum outflow and the maximum water level reached during the worst flood. The methods for the fixation of reservoir capacity, for the estimation of the maximum flood discharge, and for flood routing are already learnt in Hydrology course.

4. Sub-surface exploration: Sub-surface exploration programme usually includes one or more of the following methods.

(i) Geophysical method, (ii) Sounding and penetration methods, (iii) Open excavations, (iv) Exploratory boring, (v) Rock drilling.

Foundations of a dam are usually of the following two types: (a) Rock foundation, and (b) Alluvial foundation.

High dams are usually constructed on rock foundations. Moreover, the spillways and outlets are normally built on sound rock. Thorough subsurface explorations of rocks are carried out to de-termine the depth of overburden, location of fault zones, extent of jointing, existence of solution cavities, presence of soluble materials, disintegration of rock, etc. The presence and nature of clay or any other material in the seams of the jointed or fractured zones of the rock should be specially investigated. Such materials create problems and may even lead to failure when the reservoir is filled and very high hydrostatic pressure develops. The depth of exploratory investigations should be taken upto the bed rock. If the bed rock is at great depth, there should be at least one boring upto the bed rock. Investigations must be done through all soft, unstable and permeable strata of the overburden. For large dams, the foundations should be thoroughly investigated to great depths. Alluvial foundations consist of sand, gravel, silt and clay. Generally, earth dams and low gravity dams are constructed on such foundations. For all other types of dams, rock foundations are required. Geophysical methods and sub-surface soundings are used to determine the depth of bed rock. The properties of soils such as permeability, density, consolidations characteristics and shear strength are determined.

GEOPHYSICAL INVESTIGATION

1. Seismic Refraction Surveys.- Seismic refraction

surveys are performed to determine the compressional- wave velocities of materials from the ground surface down to a specified depth. the objective of a seismic refraction survey is to determine the configuration of the bedrock surface and the compressional-wave velocities in the surficial deposits. Bedrock may be defined in terms of compressional-wave velocity.

Applications.-Seismic refraction surveys

have been used in many types of exploration programs and geotechnical investigations. Seismic refraction surveys are routinely used in foundation studies for construction projects and in siting studies, fault investigations, dam safety analyses, tunnel alignment studies, and rippability studies.

Equipment.-The basic equipment used for

seismic refraction work consists of a seismic amplifier, a recorder (oscillograph or an oscilloscope) and a transducer (geophone). Depending on the scope of work, a single channel (one geophone) to a multichannel system may be required.

2. Seismic Reflection Survey –P Seismic reflection

surveys provide information on the geological structure within the earth. The information obtained from seismic reflection surveys can be used to define the geometry of subsurface layers and, thereby, provide information on faulting.

Applications-High-resolution seismic reflection

surveys have been used in a large number of engineering investigations to provide definitive information on the locations and types of faults and the locations of buried channels. In some cases where it is not practical to use seismic refraction reflection surveys is very similar to that used for seismic refraction surveys. In some cases, the equipment may be identical. For civil engineering investigations and ground-water studies, small portable equipment of up to 24 channels may suffice.

3. Shear- Wave Surveys.-Shear waves travel

through a medium at a slower velocity than compressional

waves. Therefore, shear-wave arrivals occur after compressional-wave arrivals on seismograms, or they are recorded as secondary arrivals.

Application.-In engineering investigations

shear-wave velocities are important because they can provide information on the inplace dynamic properties of a material. The relationship between compressional-wave velocity, shear-wave velocity, unit weight, and the inplace dynamic properties of a material is shown on figure 534.

4. Surface Waves.-Surface-wave surveys are

designed to produce and record surface waves and their characteristics. Surface waves, which travel along the boundaries between different materials, are the slowest seismic waves. Because there are

different types of surface waves, not only must the waves be recorded, but their characteristics must also be determined.

Applications. -The principal application of

shear-wave surveying, for geotechnical investigations, is to determine the type and characteristics of surface waves that can exist at a given site. This information is useful for determining preferred site frequencies; and for earthquake design analysis.

Equipment.-The cables and amplifiers used

in normal refraction surveying can also be used in surface-wave surveying.

5. Vibration Surveys.-Vibration surveys

measure the vibrational levels produced by mechanical or explosive sources. Once these levels are determined, procedures can be designed to reduce the possibility of vibrational damages.

Applications. - Vibrational surveys have

been performed in conjunction with quarrying and mining operations, during excavations, to measure the effects of traffic on sensitive equipment, and to measure the effects of aircraft (sonic vibrations) on urban areas and on historic buildings.

Equipments-Many manufacturing and research facilities

Use geophones are three-component, low frequency geophones, similar to (or the same as) those used in surface wave surveys. Most equipment can record ground motion in terms of particle displacement, velocity, or acceleration. Some equipment only records on magnetic tape,

6. Electrical-Resistivity Profiling Surveys.- Electrical-resistivity profiling is based on the measurement of lateral changes in the electrical properties of subsurface materials. The electrical resistivity of any material depends

Applications.-Electrical-resistivity profiling

is used to detect lateral changes in the electrical properties of subsurface material, usually to a specified depth. This technique has been used to map the lateral extent of sand and gravel deposits, to provide information for cathodic protection of underground

utilities, to map the lateral extent of contamination plumes (in toxic waste studies), and in fault exploration studies.

Equipment.-Most electrical-resistivity surveying

equipment consists of a current transmitter, a receiver (to measure the resulting poten-tial), two (or more) current electrodes and two (or more) potential electrodes.

7. Electrical-Resistivity Soundings.-Electrical-

resistivity sounding is based on the measurement of vertical changes in the electrical properties of subsurfe.ce materials. In contrast to resistivity profiling, in which the electrode separation is fixed, the electrode spacing used for resistivity sounding is variable, while the center point of the electrode array remains constant. The depth of investigation increases in a general sense as the electrode spacing increases, thus resistivity soundings are used to investigate variations of resistivity with depth.

Applications.-Electrical-resistivity soundings,

often referred to as VES (vertical electrical soundings), are commonly used for aquifer and aquaclude delineation in ground-water investigations.

They have been used for bedrock delineation studies, where there may not be enough contrast in velocity to permit seismic surveying.

Equipment.-The equipment used in resistivity

sounding is identical to the equipment used in electrical-resistivity profiling. For shallow investigations a d-c (direct current) transmitter is normally sufficient; whereas, for deeper investigations an a-c (alternating current) transmitter may be required.

8. Electromagnetic-Conductivity Sounding Surveys.- The basic principles involved in EM surveying have been discussed in the previous paragraphs. Electromagnetic sounding surveys are used to determine vertical changes in the conductivity of surface materials.

Applications.-Electromagnetic sounding

surveys have been applied to delineate areas of permafrost, to locate gravel deposits, to map bedrock topography, and to provide general geologic information. The EM sounding and profiling surveys have also been applied to fault studies.

Equipment.-The equipment used in EM sounding surveys is the same as that used in EM profiling surveys. For shallow investigations, less sophisticated equipment is required than for deeper investigations.

9. Ground-Probing Radar.-Ground-

probing radar surveys have the same general characteristics as seismic surveys. However, the depth of investigation with radar is much more shallow bhang that of a seismic survey. This disadvantage is partially offset, however, by the much greater size-resolution of radar techniques.

Applications-Ground-probing radar surveys

can be used for a variety of very shallow engineering applications, including locating pipes or other buried objects, high-resolution mapping of near-surface geology, locating near-surface cavities, and locating and determining the extent of piping

caused by sink-hole activity and leakage in dams. These applications are limited, however, by the very small depth of penetration usually possible with the very high frequencies involved in radar. Silts, clays, salts, saline water, the water table, and any other conductive materials in the subsurface will severely restrict or even prevent any further penetration of the subsurface by the radar pulses.

Equipment.-The equipment for ground probing

radar is manufactured by only two or three companies at this time, and only a few contractors offer these services. Therefore, the present sources for equipment and-contract services are limited. The equipment itself consists of an antenna/receiver sled, a control/signal processor unit, a strip chart recorder, a power supply, and various accessories, such as a tape recorder and special signal analyzers. This equipment would normally he operated from a vehicle, except for the antenna/receiver sled, which can be either towed behind the vehicle or pulled by hand. A schematic diagram of radar operations is shown on figure 5-37.

SAFETY

Dams are constructed to impound water for storage or to divert water for beneficial use. Unfortunately, the impoundment of water sometimes poses a potential hazard to public safety. The purpose of a dam safety program is to recognize potential hazards and re-duce them to acceptable levels.

Seismic refraction and MASW Seismic refraction Is a use full method for investigation geological structure and rock properties. The technique Involves the Observation of a Seismic signal That has Been refracted between layers of contrasting seismic veloci ty.

Hydrotechnical projects

The geophysical survey use non-destructive method to allow extensive investgations of various hydro technical projects

--‐ Land, rock or concrete dams --‐ Protection dykes --‐ Functional dykes

Geoelectrical- Using the Vertical Electrical Survey (SEV), Electrical Tomography This method Has proven

Most efficient For locating Areas of Water infiltrations And for Scanning the density Of the built in Material from The dykes Or dams.

Georadar (GPR)-It has a very high resolution and accuracy for areas made of concrete ,locating holes and

anomalies in the density of the material below.

Seismical -This Profiling method can accurately show data ab out the homogeneity of the built in ma-

terial from dams or dykes.

RESULTS AND DISCUSSIONS

Dams in the country represent a major investment and huge benefits to population in terms of irrigation, power and flood control. Most of the big dams are very old and regu-lar monitoring and maintenance of these dams is of utmost importance for continuing benefits. Unlike soil investigations, critical nature of dams, does not permit traditional invasive inspections by means of drilling, and such inspections are best avoided unless extremely important, and are done only when problem is too grave. Geophysical techniques, by virtue of their non-invasive and non-destructive nature, offer an excellent solution for investigation or regular monitoring of dams, and detection of anomalous conditions which might snowball into major problems if left untreated. Various geophysical methods are available to investigate the problems of earthen, ma-sonry, concrete or composite dams:

Seismic reflection and refraction surveys

Shear and surface wave surveys Vibration surveys Electrical resistivity and profiling surveys Ground probing radar

CONCLUSIONS

A number of geophysical techniques are utilised to evaluate conditions at existing dam sites. Most of these methods find application on earth or rockfill structures with some methods also effective on concrete dams. The main applications of geophysical methods at existing dams are related to seepage assessment, integrity evaluations, foundation conditions and seismic hazard estimation. Anomalous seepage is most readily identified with the self or streaming-potential method.

REFRENCES

For geophysical method Griffiths, D. H., and R. F. King, Applied Geophysics for

Engineers and Geologists, Pergamon Press, New York, NY, 1965.

Heiland, C. A., Geophysical Exploration, Hafner, New York, NY, 1968. Slide

wikipedia

For dam safety Committee on the Safety of Existing Dams, Water Science

and Technology Board, Commission on Engineering and Technical Systems, National Research Council, National Academy Press, Washington, D.C., 1983.

“Criteria for Selecting and Accommodating Inflow Design Floods for Storage Dams and Guidelines for Applying Criteria to Existing Storage Dams,” ACER Technical Memorandum No. 1, Bureau of Reclamation, Denver, CO, November 1981.

wikipedia