In this article article we will discuss about how to design earth dam.
Design of the Section of an Earth Dam:
Design of the section of an earth dam evolves fixing the sizes of the following elements:
1. Top Width of the Dam:
There are several other factors that affect the top width of the dam but height of the dam is the most important factor. Nature of soil and use of the top of the dam for passing highway, are also the important factors.
Top width of the dam can be found out by the following empirical formulae:
Width of the top of the dam, should not be less than 3 m for any height of the dam.
2. Free Board:
The vertical height lying between the top of the dam and maximum reservoir level is known as the free board. Free board may further be stated as normal free board and minimum free board. Difference between top of the dam and normal reservoir level is known as normal free board.
The difference between top of the dam and the H.F.L. when all the spillways and other outlets are working as planned, is known as minimum free board. Amount of free board depends upon the wage height of the reservoir. The free board should be at least 1.5 times the wave height.
The wave height (hw) can be found out by formula given by Molitor Stevension and reproduced as follows –
Where
F = Fetch in kilometres
V = Velocity of wind in km/hr
hw = Wave height in metres measured between trough and crest of the wave.
Settlement allowance of 2% should be considered for the dams of heights upto 30 m and 3% for larger heights and earthquake prone areas.
U.S.B.R. has recommended minimum 2 m and maximum 3 m as the free board, for any dam when spillways are not controlled by gates. If spillways are provided with gates and height of the dam is say up to 60 m, the value of free board should be 2.5 m above the top of gates. For larger height dams and gated spillways free board should be 3 in above the top of gates.
3. U/S and D/S Slopes:
The U/S and D/S slopes of the earth dam depend upon following factors:
(i) Type of soil used in the construction of the dam,
(ii) Foundation conditions and
(iii) Height of the dam.
Flatter slope should be provided on U/S side so as to remain stable under the action of sudden drawdown conditions. If soils are quite permeable steep slopes can withstand the action of sudden drawdown as pore pressure gets dissipated almost immediately.
Terzaghi gave following slopes for fixing a tentative section of the earth dam:
The dimensions fixed by Strange for earth dam are as follows:
4. Central Core of the Dam:
The thickness of the impervious core depends upon the following factors:
(i) Rate of seepage through the core.
(ii) Type of material available for the construction of the core.
(iii) Minimum width required, that will permit proper construction.
(iv) Design of proposed filter that is to be provided on the D/S side of the core.
It should be understood clearly that shear strength of core material is always smaller than the rest of the dam section. It is due to the fact that core is made of clayey material. Hence from stability point of view thinner core is preferable. But thinner core walls are more likely to suffer from piping trouble.
The thickness of the core wall should not be less than the height of the dam above the point under consideration. Width of the core at the top should be minimum 3 m so that construction of core can be carried out by heavy earth machines. Minimum width of the core at base is when foundation is either impervious or pervious but having positive cut-off. The slope of the core is decided automatically.
In case foundation is pervious and there is no positive cut off trench, the core size is fixed by giving 1:1 slope on both of its sides and adopting 3 m as the minimum thickness of the core at the top.
Maximum size of the core may be fixed by giving side slope of (x – 1/2):1 on both the sides where x is the side slope of the dam itself. If side slope of the dam is 3:1, the value of x is 3 and maximum slope of the core will be (x – 1/2): i.e. (3- 1/2): 1 or 2½:1.
Sloping Core:
The core in the earth dam may have its axis inclined rather than vertical.
Advantages:
(a) The pressure at the contact surface of foundation and the dam is maximum. This reduces the possibilities of leakage along the contact.
(b) Height of the vertical core being less than the inclined length of the inclined core, thicker vertical core is constructed with the same quantity of core soil. This reduces the possibilities of piping failure of the core.
Advantages:
(a) D/S part of the dam can be constructed first and inclined core may be laid later.
(b) Grouting of foundation and construction of embankment can be done simultaneously.
Slope of the sloping core is made 1:1 on the upstream face and 1/2:1 on the D/S face. The slope of the downstream face is not opposite to the slope of U/S face, but in same direction.
5. Drainage System from Downstream Side of the Dam:
Proper drainage of the earth dam is essential for its successful working.
Drainage of the dam can be provided by following methods:
(i) By providing a Rock Toe, enclosed in a filter at the D/S end of the dam. This measure is adopted to prevent softening of D/S Toe. By rock toe sloughing of the D/S slope is prevented.
(ii) Horizontal Blanket Filters:
This measure is wisely used for the drainage of earth dams of moderate heights. It has been found that horizontal filter materials stratify with time and thus flow of seeping water through the filter is prevented.
Due to this difficulty, the seeping water may start flowing horizontally at some higher level above the filter level. This causes seeping water to come out, at some higher level on D/S side than at rock toe level. This may ultimately lead to sloughing.
(iii) Provision of Chimney Drains or Filters:
It is a measure to eliminate the trouble of stratification in the dam. In this case high inclined or vertical filter drains are provided in continuation of horizontal filter drains, to intercept the seeping water along any horizontal surface before embarking at the D/S slope of the dam. These inclined filters are known as chimney drains. Chimney drains are used in high earth dams, to cause effective drainage for full height of the dam.
For very high dams the spread of the dam on D/S side becomes quite large and system of inclined and horizontal filter drains may not perform their intended function effectively. Effectiveness of the drainage system can be increased by constructing one or more longitudinal drains and connecting these drains with horizontal system of filter drains.
Filter Design:
Filter drains should be designed in such a way that all the seeping water through the dam is effectively drained off. The filter consists of more than one layer. The filter layer which comes in contact with the seeping water first is of fine material. Subsequent layers of filter are made of sand of increased coarseness.
The last layer from which water comes out of the filter is made of gravel. The filters of filter drains are also known as reverse or inverted filter. The soil of earth dam and foundation surrounding the filter is known as the base material.
According to earth manual of US Bureau of Reclamation Washington 1960 following four main requirements should be satisfied:
(a) Filter material should be fine and poorly graded so that the voids in the filter are small and thus prevent base material from entering the filter.
(b) The filter material should be coarse and pervious in relation to base material. This aspect facilitates rapid removal of seeping water without building up any seepage force within the filter.
(c) The filter material should be coarser than the perforations or openings in the drain pipes, so that filter material is not lost in the drains. The openings or perforation in the pipe drain should be adequate to admit all the seeping water safely.
(d) The thickness of filter material should be sufficient to provide a good distribution of all particle sizes, throughout the filter. The thickness should also be adequate to provide safety against piping.
According to Mr. Terzaghi, the filter material should fulfill the following two requirements:
1. The D15 size of filter material must not be more than 4 to 5 times D85 size of the base material. This prevents the foundation material from carrying through the pores of the filter material.
2. The D1S size of the filter material should be at least 4 to 5 times the D15 size of base material. This keeps seepage forces within the filter to permissible limit.
The two criteria can be expressed as follows also:
The requirements cited above should be satisfied between any two adjacent layers of the filter. Me Terzaghi’s criteria has been modified as per U.S. Bureau of Reclamation Design of Small Dams’.
The modifications are as follows:
Design of Earth Dams to Suit Available Materials:
In order to achieve economy, the design of an earth dam should be adopted to make full utilization of the materials available at or near the site. This will cause minimum transportation of other materials.
The available material, at or near the site may be any one of the following types:
1. Coarse sand and gravel
2. Coarse sand, gravel and clayey silt
3. Only silt clay is available.
Foundation conditions at the site may be any of the following types:
Case 1 – Gravel, Coarse Sand and Clayey Silt Available:
1. Impervious Foundation for Large Depth:
In this case no treatment of any type is required for foundation. The dam proper is made of coarse sand and gravel but central core is made from the available clayey silt. The central core may be vertical or inclined.
2. Pervious Foundation for Moderate Depth and then Impervious:
Construction of this dam is similar as in case (1) except that central core is carried down to the level of impervious layer.
3. Foundation Pervious for Large Depth:
The main problem in this case is seepage through foundation. The construction procedure above ground level is same as explained in (1).
Following measures may be taken to prevent seepage of water through the foundation:
(i) 1 to 3 m thick impervious blanket may be introduced at the upstream of the core at junction of earth dam and foundation. This blanket may be extended beyond the upstream end of the dam.
(ii) Vertical Cut-Off in form of sheet pile is provided below the dam.
Case 2 – Only Coarse Sand and Gravel is Available:
1. Impervious Foundation for Large Depth:
Dam is made of coarse sand and fine gravel but an impervious core of cement concrete or masonry is installed at the centre of the dam.
2. Foundation Pervious for Some Depth and then Impervious:
Construction is similar to (1) except that core is started from the level of the impervious foundation.
3. Pervious Foundation for Large Depth:
In this case main problem is seepage through the foundation. A horizontal impervious blanket is provided on upstream side in continuation of the central core. The horizontal impervious blanket prolongs the path of seepage and thus helps in reducing the resultant head of seeping water to zero, by the time it reaches the D/S toe of the dam.
Case 3 – Only Silty Clay is Available:
Silty clay is such a material which has less permeability than sand. In other words, this soil puts quite large resistance to seeping water.
1. Impervious Foundation to Large Depth:
The whole dam is a homogeneous construction with no central core. A deep horizontal filter is, however, provided along with rock toe to help in the drainage of seeping water.
2. Foundation Permeable to Some Depth and then it is Impervious.
In this case same construction is adopted as for (1). A sheet pile cut-off may be provided on the upstream side, to reduce the seepage through the permeable foundation. If foundation is pervious for large depth same construction is adopted as for (2).
Cut-Off Walls:
Cut-off wall is a device used to reduce or prevent the seepage of water through the pervious foundation. It is always provided below the upper surface of the foundation. If the cut-off wall extends through the foundation to impervious material below, it is called complete cut-off wall. If it penetrates only a part of the pervious foundation, it is known as partial cut-off. Partial cut-off walls are not very effective in reducing the seepage through a pervious foundation.
Following are the advantages of cut-off walls:
1. Danger of seepage along the contact surface of the foundation and the dam embankment is reduced.
2. It reduces the seepage through horizontal cracks, fissures and pervious seams in the foundation. Almost all the soils deposited by water are stratified or have layers of materials of various sizes. A vertical cut-off will penetrate a number of these layers or strata and thus force the seepage to pass through different strata.
The cut-off walls may be of any form from the following:
(a) Cut-off trenches back filled with impervious material.
(b) Masonry or concrete walls.
(c) Sheet piling — mostly made of concrete or steel.
(d) Grout curtain — A barrier is developed by grouting cement deep below the foundation.
Before choosing the type of cut-off to be used it would be appropriate if foundation conditions are fully investigated. If seepage is moderate, sheet pile treatment may be adequate.
If seepage is very heavy and foundation is pervious for large depth, masonry or concrete cut-off is found more appropriate. Grout curtain is a sort of preventive measure. It is adopted mostly when after construction it is observed that seepage is excessive and it has to be controlled.