EARTH DAMS

EARTH DAMS

INTRODUCTION:-

Dams which are constructed with earthen materials such as gravel, sand, silt and clay are called as earthen dams. Earthen dams are the most ancient type of embankments as they can be built with the natural and locally available materials with minimum processing. Earthen dams are preferred mostly since they are economical in construction and also the locally available materials are utilised and less skilled labour is required.

Classification of Earthen Dams:-.

Homogeneous embankment dam: The simplest type of an earthen embankment consists of a single material and homogenous throughout. A Purely homogeneous section is used when only one type of material is economically or locally available. These sections are used for low to moderately high dams.

Zoned embankment dams: Zoned embankments are usually provided with a central Impervious core, covered by comparatively pervious transition Zone, which is finally Surrounded by a much more pervious outer zone.

Diaphragm type embankment dam: Diaphragm type embankments have a thin Impervious core, which is surrounded by earth or rockfill.The impervious core,called diaphragm, is made of impervious soils,concrete, steel,timber or any other materials.

Causes of Failure of Earthen Dams:-

Earthen dams are less rigid and hence more susceptible to failure. They may fail due to improper designs, faulty construction and lack of proper maintenance etc. The various causes leading to failure of earthen dams are

1. Hydraulic failure.
2. Seepage failure.
3. Structural failure.

Hydraulic failure:- About 40% of earth dam failure have been attributed to hydraulic failure under this category is due to

1. a. By over topping : Due to in sufficient free board
2. b. Erosion of upstream face : Due to lack of U/S stone pitching or rip rap
3. C. Cracking due to frost action.
4. d. Erosion of d/s face by gully formation.

Seepage failure:-Uncontrolled or concentrated seepage through the dam body or through the foundation may lead to piping or sloughing and subsequent failure of the dam. More than 30% of the dams have failed because of these reasons.

Structural failure:- About 25% of dam failures have been attributed to structural failure. Structural failures are generally caused by shear failures, causing slides.

Design Criteria of Earth Dam:-

1. Low permeability fill of the available material
2. Borrow pits should be as close as possible.
3. Sufficient spillway and outlet capacities shall be provided, so as to avoid over topping.
4.   The seepage line (Phreatic line) should remain  well within the d/s face of the dam.
5. There should be no possibility of free flow of water from u/s to d/s.
6. U/S face should be properly protected from wave action.
7. U/S & D/S slopes should be so designed as to be stable under worst conditions of loading.
8. Functions and design requirements.

Functions and Design Requirements of Various Components Of Earth Dam:

1. Top width: The width of the dam at the crest should be fixed according to the working space required at top. No dam should have a crest width of less than 6.0m.
2. Free Board: Free Board is a vertical distance between the horizontal crest of embankment and reservoir level. As per IS:10635-1993,Minimum free board should not be less than 2.0m over FRL and 1.5m over MWL. Sufficient free board must be provided so that there is no possibility of the embankment being over topped.
3. Casing of outer shell: The function of casing or outer shells is to impart stability and protect the core. The relatively pervious materials which are not subject to cracking on direct exposure to atmosphere are suitable for casing. Suitability of soils for construction of earth dam shall be as per appendix-A of IS: 8826-1978.Thedesign slopes for u/s & d/s embankment may vary widely depending on the character of materials available, foundation condition and the height of the dam. The slopes also depend upon the type of dam (i.e. Homogeneous /zoned).
4. Central Impervious core: The core provides impermeable barrier within the body of the dam. Impervious soils are generally suitable for core. Core may be located either centrally or inclined upstream. The location will depend mainly on the availability of material, topography of site, foundation condition. The main advantage of a central core is that it provides higher pressure at the contact between the core and the foundation reducing the possibility of leakage and piping. The top level of core should be fixed at least 1m above the maximum water level to prevent seepage by capillary syphoning.

The soils having high compressibility and liquid limit are not suitable as they are prone to swelling and formation

of cracks. Suitability of soils for construction of core for earth dam shall be as per IS:8826-1978.

5. Cut-off trench: Cutoff is required to i) reduce loss of stored water through foundations and abutments ii) Prevent Sub-surface erosion by piping. The minimum width of 4m is recommended for Cut off trench. The side slopes for COT will depend upon substrata. Side slopes of 1:1 or flatter may be provided in case of over burden, while 0.5:1 and 0.25:1 may be proposed in soft rock and hard rock respectively.
6. Stability analysis: The slopes of the embankment shall be stable under all loading conditions. They should also be flat enough so as not to improve excessive stresses on foundations.Embankment slopes shall be designed in accordance with IS:7894-1975 a)Stability of d/s slope during steady seepage:-The most critical condition for which the stability of d/s slope must be examined, when the reservoir is full and the seepage is taking place at full rate.

b)Stability of u/s slope during sudden drawdown: The critical condition for u/s slope can occur when the reservoir is suddenly emptied.

Minimum desired factor of safeties for:-

1. u/s Sudden draw down condition - 1.3
2. d/s Steady seepage with reservoir full – 1.5
3. Earth quake condition
4. 1. u/s Steady seepage -1.0
5. 2. d/s Reservoir full  -1.0
6. Seepage control:-The seepage through the embankment and foundation should be such as to control piping, erosion, sloughing and excessive loss of water. Seepage Control measures are required to control seepage through dam and seepage through foundation.
7. Special Design requirements:-

In addition to basic design requirement, special design requirements such as

1. Control of craking
2. Stability in earthquake
3. Stability at junction also should also be satisfied for earth dam.

Instrumentation:

Different types of measurements are needed for monitoring the behavior of earth of rock fill dam and IS : 7436:1993 provides guidelines for choice of instrumentation and their location.Instruments required in earth dam are

1. Piezometer
2. Vertical movement gauge.
3. Rain gauge.
4. Automatic wave height record.

 DESIGN OF EARTH DAM/ROCK FILL DAM:

1. Design proposal with basic parameters /Hydrology
2. Location of the proposed project ,Index plan/Layout plan(For Global co-ordinates, Latitude/Longitude, Maximum wind velocity at project site and Identified Seismic Zone of the Project)   Code 875(Part III ) 1987 and 1893-1984 (Reaffirmed 2002)
3. General Layout showing all the components of the project ,Geology of the project ,Recommendations of GSI along with Log of Bore holes analysis and Permeability test results
4. Free-board requirements-Fetch-Fore shore area plan with FRL and MWL contours ( Ref BIS code 10635-1993/Reaffirmed 1998)
5. maximum section of the proposed dam and cross sections for different heights of 5m along with recommended levels of C.O.T/Grounding pattern
6. L.S of dam indicating TBL ,Stripped level ,COT level and ground line
7. Certifying the suitability, availability of required quantities of soils ,Rip-Rap stone and other material.
8. Embankment soils –The soil test reports from the Laboratory  in accordance with the soil classification ( Ref- BIS code 1498-1970/Reaffirmed 2002)
9. Foundation soil properties based on the soil Test reports of the Laboratory  including Field permeability, Chemical analysis etc .in accordance with BIS Soil classification (Ref: BIS code no. 1498-1970/Reaffirmed 2002)
10.  Design parameters considered in the analysis of section ( Densities, Shear parameters at OMC/FMC and SMC conditions)
11. Stability of dam section ( U/S slope and Normal and Earth Quake conditions for Sudden Draw down and Down stream slope for Normal and Earth quake conditions for steady seepage ) (Ref- BIS code no 7894-1975 / Reaffirmed 1997)

Note : The relevant slope stability calculations shall be checked and furnished

12. Effect of TWL on D/S side of Erath dam and the measures proposed to protect D/S slope including adequate surplus arrangements proposed for the project up to safe distance based on the model studies
13. Slope protection to Earth/Rock fill dam.
    • Under seepage control measures (Cut-off trench, grouting ,diaphragm wall etc.) Ref .BIS code no 8414-1977/Reaffirmed 1999
14. • Drainage arrangements including Inclined /Horizontal filters within the dam and other drainage measures like Rock toe, Toe drain, Filters below UP stream rip-rap etc. Ref. BIS code no 9429-1999
15. • Slope protection measures (U/S rip-rap, D/S Turfing etc.) Ref BIS code no 8237-19985/Reaffirmed 1999.

• 14.Provision of additional measures of protection to earth/rock fill dams like Parapet, Guard stones at TBL, Chute drains on D/S slope profile walls on U/S, Measures of surface drainage like Longitudinal drains etc. Ref BIS code no 8237-1985/Reaffirmed 1999
• 15.Designed section of Erath /Rock fill dam in drawing to a suitable scale showing all the features pattern of grouting etc. drawn in LS of the dam  and Specification Notes
• 16.Necessity of Instrumentation in the dam ( Ref BIS 7436-(Part1) 1993/Reaffirmed 1998)

DESIGN OF HEAD SLUICE

The following basic parameters are required to take up the design of head sluices in Erath/Rock fill dams

1. Hydraulic particulars of the main canal in the head reach up to 1000m
2. MDDL of the reservoir to check the availability of minimum driving head
3. SILL level of the sluice in conformity with canal bed level
4. Highest command level to examine the suitability of SILL level of the sluice proposed
5. Location of the sluice to study whether the barrel is in cutting or not.
6. Discharge required to be taken for fixing vent size of the sluice
7. Trail pit details along the axis of sluice to study foundation stratification
8. Cross section  of Erath dam at Location of the sluice
9. Net-level plan at sluice location for  a distance 200m on either side of the sluice.

Reference IS Codes for Earth dams:-

REFERENCE OTHER BIS CODES

1. BIS Code no 875-(Part III) 1987and 1893-1984(Reaffirmed 2002 Design Loads other than Earh Quake for Buildings and Structures and Earth quake analysis of the structures
2. BIS Code no 10635-1993/Reaffirmed 1998-Guide Line for Free-board requirements in Embankment dams
3. BIS Code 1498-1970/ Reaffirmed 2002/ Classification and Identification of soils for General Engineering purpose
4. BIS Code no 7894-1975 ( Reaffirmed 1997)- Code of practice for stability analysis of earth dams
5. BIS Code 8414-1977 (Reaffirmed 1999) Guide Lines for Design of Under seepage control measures for earth and rock fill dams
6. BIS Code 9429-1999. Code of practice for Drainage system for Erath and Rock fill dams
7. BIS Code no 8237-1985/  (Reaffirmed 1999) Code of practice for Protection of Slope for Reservoir Embankments
8. BIS Code no 7436(Part I)-1993/ (Reaffirmed 1998) Guide for Types of measurements fro structures in River Valley projects and criteria for choice and location measuring instruments Part I – For Erath and Rock fill dams
9. BIS Code 7356(Part I) -2002/Reaffirmed 1998- Code of Practice Instrumentation maintenance and observations for pore pressure measurements in Earth dams and Rock fill dams Part I-Porous tube peizometers and Part II Twin tube hydrostatic peizometers
10. BIS Code no 7500-2000 – Code of Practice for installation and observation of cross drains for measurements of internal  vertical movements in Earth dam. 

OTHER IS CODES for SPILLWAY,NON-OVERFLOW DAMS

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