(2) design of small earth dams

8/13/2019 (2) Design of Small Earth Dams

1/6

Design of Small Earth Dams

Y.C.Agarwal*

SYNOPSIS:On the earth, most of the dams constructed so far are earthen dams. Failure of a number ofearthen dams is due to faulty design. The design of different components of earthen damhas been narrated in this paper. This will help in better understanding of design of damsand dam failures.Definition:A dam exceeding 15m in height above deepest river bed level is defined aslarge dam. Also a dam in between 10 m to 1 m height is termed as large dam if volume ofearth dam exceeds 0.! million cubic metre and storage exceed one million cubic metre orthe maximum flood discharge exceeds "000 m#$ sec.A dam not satisfying the above criterion of large dam is termed as %mall dam.An earthen dam may be homogeneous or &oned type.

'omogeneous earth dam( A purely homogeneous type of dam is

composed of a single )ind of material. The purely homogeneoustype of section, has now been replaced by a modified homogeneoussection, in which small amount of carefully placed perviousmaterial control the action of seepage so as to permit much steeperslopes as compared to pure homogenous dam. *Fig. 1+.

oned earth dam( oned earth dams are composed of a central

impervious core flan)ed by &ones of materials considerably morepervious called shells. *Fig. "+

The main components of the earth dam are described below(COMPONENTS OF EARTH DAM

1. -ut off". -ore#. -asing. /nternal drainage system and foundations. %lope protection. %urface drainage!. /mpervious blan)et

FUNCTIONS AND DESIGN REQUIREMENTS

1. Cut off:

The cut off is reuired,o To reduce loss of stored water through foundations and

abutmentso To prevent sub2surface erosion by piping.

The type of cut off should be decided on the basis ofdetailed geological investigations. /t is desirable to providepositive cut off. 3here this is not possible, partial cut offwith or without upstream impervious blan)et may beprovided. /n any case, adeuate drainage arrangementsmay be provided on the down stream.The following guidelines may be adopted for design of cutoff.


2/6

a. The cut off shall be located such that its centre line should

be within the base of impervious core and should beupstream of centre line of dam.

b. The positive cut off should be )eyed at least to a depth of

0. metre into continuous impervious sub stratum orinerodable roc) formation.

c. A minimum bottom width of .0 metre is recommended.d. %ide slopes of at least 1(1 or flatter may be provided in

case of over burden while 1$"(1 and 1$(1 may be providedin soft roc) and hard roc) respectively.

e. The bac) fill material for cut off trench shall have same

properties as those specified for impervious core.f. The cut off in the flan)s on either side should normally

extend upto the top of impervious core.g. /f cut off trench is terminated in roc) formation which is

weathered or have crac)s, 4oints and crevices5 and ifpercolation test exhibit a lugeon value of more than10*refer /% 02167+, then roc) foundation below the bed

of cut off trench should be grouted.". Core:The core provides impermeable barrier within the body of the dam./mpervious soils are generally suitable for the core */% 167216!0+.'owever soils having high compressibility 8 liuid limit, and havingorganic contents may be avoided, as they are prone to swelling 8formation of crac)s.Following guidelines are recommended for design of core.

a. The core may be located either centrally or inclined

upstream.b. The minimum top width should be )ept # metres.

c. The top level of the core should be fixed at 0. m above

93:.

d. The side slopes may be )ept 0.(1 and 1(1.e. Thic)ness of core at any section shall not be lesser than #0;

*preferably not less than 0 percent+ of maximum head ofwater acting at that section.

#. Casing:The function of casing is to impart stability and protect the core.The relatively pervious materials, which are not sub4ected tocrac)ing on direct exposure to atmosphere, are suitable for casing.Top width of dam should be provided as . m *minimum+. Theberms may be provided for the dam, which are more than 10 m inheight. 9inimum berm width may be )ept as # m.

. Internal drainage system:To ensure safety of dam, it is very important to handle the seepage

water in the dam so as to maintain the original particles of soils intheir place. The measures commonly adopted for safe disposal ofseepage water through emban)ment dams are5

a. /nclined or vertical filter *chimney filter+

b. 'ori&ontal filter

c.


3/6

soil *casing or foundation+ should be satisfied ./n case of damportions, where the head of water is # m or less it is not reuired toprovide chimney filter or hori&ontal filter. Adeuate toe protectionshall however be provided. The height of roc) toe is generallyprovided as 0." ', where ' is the height of emban)ment. 'oweverminimum height of roc) toe be )ept as 1.0 metre.


4/6

suitability of soils for blan)et. *Table "+ A #00mm thic) layer ofrandom material over the blan)et is recommended to preventcrac)ing due to exposure to atmosphere. The impervious blan)etmay be designed in accordance with /%( 71216!!. As a generalguideline, impervious blan)et with a minimum thic)ness of 1.0metre and a minimum length of times the maximum water head

measured from upstream toe of core may be provided.

BASIC DESIGN REQUIREMENTS:

1. Safety against overtopping:a. %ufficient spillway capacity should be provided to prevent

overtopping.b. The free board should be sufficient to prevent overtopping

by waves and should be provided as per /% 10#2166#. Theminimum free board of 1.m should be provided.

c. The free board should be sufficient to ta)e into account thesettlement of emban)ment and foundation.

". Stability analysis:The design of small emban)ment dam sections may be divided intothe following three categories based upon the height of theemban)ment in its deepest portion.

a. where the height of emban)ment is m or lessb. where the height of emban)ment is 10 m or less, but more

than mc. where the height of emban)ment is 1 m or less, but more

than 10 m

For small dams under category *a+ and *b+ above, the stabilityanalysis may not be necessary. Beneral guidelines and therecommended slopes are given in Table 1 for guidance of the

designer. The minimum top width may be )ept as .0 m.'owever the designer with his experience and 4udgement maydecide the adeuate side slopes where special technical oreconomic considerations may have to be ta)en into account.%tability analysis may be carried out in accordance with /% !76216! based upon the detailed foundation 8 borrow areainvestigation and laboratory testing if the soil strata below the damseat consist of wea) foundation and $ or the height of emban)mentis more than 10 m.3ea) foundation conditions include fissured clay, expansive soils,shales, over consolidated highly plastic clays, soft -lays, dispersivesoils etc. within the substratum in the dam seat.9ain problem of silt and clay foundations is stability. /n addition to

the obvious danger of bearing failure of foundations of silt and clay,the design must ta)e into account effect of saturation of thefoundations of the dam and appurtenant wor)s by the reservoir.9ethod of treatment*a+ To remove soils of low shearing strength*b+ To provide drainage of foundation to permit increase of strengthduring construction*c+ To reduce magnitude of average shearing stress along potentialsurface of sliding by flattening slopes of emban)mentCoc)ets of material substantially more compressible or lower instrength than the average, are usually removed.The most practicable solution for foundation of saturated fine2grained soils is to flatten the slopes of emban)ment.

%oils of low density are sub4ected to large settlements whensaturated by the reservoir, although these soils have high dry


5/6

strength in natural state. /f proper measures are not ta)en tocontrol excessive settlement, failure of dam may occur bydifferential settlement and foundation settlement .The reuiredtreatment of low2density foundation would be dictated by thecompression characteristics of the soil. Foundation consolidationwill be achieved during construction.

#. Seepage control and safety against internal erosion:The seepage through the dam emban)ment 8 foundation should besuch as to control piping, erosion, sloughing and excessive loss ofwater. %eepage control measures are reuired to control seepagethrough dam and foundation.=esign for control of seepage through dam shall be made inaccordance with provisions contained in /% 6"621666 D-ode ofpractice for drainage system for earth and roc)fill dams. =esign forcontrol of seepage through foundation may be made in accordancewith /% 71216!! DBuidelines for design of under seepage controlmeasures for earth and roc) fill damsD.!"#"g:/f only one type of suitable material is readily available nearby, ahomogeneous section is generally preferred. /f the materialavailable is impervious or semi pervious, a small uantity ofpervious material is reuired as casing for protection againstcrac)ing. On the other hand if it is pervious, a thin imperviousmembrane is reuired to form a water barrier.

S$a%#l#$& a$ '(")$#!"Eunctions of emban)ment dam with foundation, abutments, masonry structures li)eoverflow, non2overflow dams and outlets need special attention with reference to one orall of the following criteria.*a+ Bood bond between emban)ment dam and foundations

*b+ Adeuate creep length at the contact plane*c+ Crotection of emban)ment dam slope against scouring action*d+ asy movement of trafficFor details /% 1"162167! should be referred.

REFERENCES:

1. /% 1"16(167! -riteria for design of small emban)ment dams". /% 7"#!(167 -ode of practice for protection of slope for reservoir

emban)ment#. /% 6"6(1666 -ode of practice for drainage system for earth and

roc)fill dams. /% 71(16!! Buidelines for design of under seepage control

measures for earth and roc) fill dams. /% 0(166 Cressure grouting of roc) foundations in river valley

pro4ects 2


6/6

1. Type of section'omogeneous$9odifiedhomogeneous section

oned $ 9odifiedomogeneous$'omogeneoussection

oned $ modifiedhomogeneous$homogeneoussection

". %ide slopes $% =$% $% =$% $% =$%

a+ -oarse grained soil

*i+B3,BC,%3,%C Got %uitable Got %uitable Got suitable forcore, %uitable forcasing &one

*ii+B-,B9,%-,%9 "(1 "(1 "(1 "(1

%ection to bedecided basedupon stabilityanalysis

b+ Fine grained soil

*i+-:,9:,-/,9/ "(1 "(1 ".(1 "."(1 2do2

*ii+ -', 9' "(1 "(1 #.!(1 ".(1 2do2

#. 'earting &one Got reuired 9ay be Crovided Gecessary

a+ Top width 22 #m #m

b+ Top :evel 22 0.m above 93: 0.m above 93:

.

(2) design of small earth dams

Documents