In this article we will discuss about:- 1. Design of Embankment 2. Seepage through Embankments 3. Stability.
Design of Embankment:
The design of the embankment for the farm pond is based on the same principles used in the case of the design of earthen dams.
The following aspects need to be considered:
1. Foundation Conditions:
The foundation for the embankment should provide stable support as well as resistance to the passage of water. Subsoil conditions at the proposed site should be studied by means of auger hole borings.
Good foundation materials should provide both stability and imperviousness. A mixture of coarse and fine textured soils like gravel-sand-clay mixtures, gravel- sand-silt mixtures, sand-clay mixtures and sand-silt mixtures will be good foundation materials.
Coarse textured materials like gravel and sand provide good support but are highly pervious to water. If such materials exist, steps should be taken to prevent seepage under the embankment.
Fine textured materials such as silts and clays are comparatively impervious to water but not very table materials for the foundations. For low embankments (upto 5 m high) the nature of foundation materials is not critical.
For, higher dams, if the foundation materials are not satisfactory the base width is to be increased such that the load per unit area decreases.
2. Cross-Section:
The design of the cross-section depends upon the nature of the foundation material as well as the fill material available at the site. Unless the material used for the embankment is fine material and will be impervious to water, the earthen embankments for retaining water should be provided with an impervious core.
If the foundation material is porous in nature, a cutoff trench is to be provided to control seepage. The cutoff trench should extend up to an impervious soil stratum.
The side slopes are dependent upon the height of the dam, nature of the foundation material and the nature of the fill material. For earthen dams of height 15 m and below and with average materials, the side slopes should not be steeper than 3 : 1 on the upstream side, and 2 : 1 on the downstream side. Soils which are relatively coarse and cannot be compacted well need flatter slopes like 3 : 1 or 4 : 1.
In case of soils which are well, graded and can be compacted well, side slopes 2 : 1 can be adopted on both upstream and downstream sides. On embankments higher than 10 m berms are provided on the downstream side of the dam.
The berms are of 1 m to 3 m width and have a mild inward slope for drainage. The recommended side slopes for different soil types are given in Table 27.1.
The top width of the dam varies depending on the height of the dam and use of the top surface. Upto 5 m height of the dam a minimum top-width of 2.5 m is recommended. If the top is to be used as a road, a width of 5 m or more is to be adopted.
The earthen dam will settle after construction and allowance for this settlement should be made. The amount of settlement depends upon the method of construction used, and foundation material.
Settlement upto 20 to 25 per cent can occur when the construction is done by a drag line. Settlements could be very small (of the order of one per cent) when the fill material is compacted well.
The following formula is used for estimating settlement in case of earthen embankments-
Seepage through Embankments:
Seepage through the earthen embankment has to be controlled to ensure the safety of the embankment as well as to minimise the loss of water. When water is standing against the earthen embankment a seepage line or saturation line (also known as phreatic line) is established.
It is the line below which there are positive hydrostatic pressures. On the line itself the pressure is atmospheric i.e., the hydrostatic pressure is zero. Above the seepage line in the capillary fringe, the pressures are negative. Even though some flow occurs in the capillary fringe, in the analysis of seepage through embankments, the flow in the capillary fringe is usually neglected.
In a given embankment section, it is necessary to predict the position of the seepage line in order to –
(1) Ensure that the seepage line does not cut the downstream face of the dam and cause softening or sloughing of the toe,
(2) Obtain the dividing line between the wet and dry soil for the purpose of stability computations, and
(3) Obtain the top boundary line for drawing the flow-net for seepage computations.
It has been generally observed that the seepage line is affected by –
(1) The permeability of fill materials and the foundation,
(2) The type and design of the core wall or cut-off within the embankment, and
(3) Drainage conditions in the downstream portion of the embankment.
If an embankment section is homogeneous the seepage line cuts the downstream face. The position of the seepage line depends on the geometry of the cross-section and not on the permeability of the material as long as the section is homogeneous.
A relatively less pervious material will take longer time to attain the final steady position of the seepage line than a pervious material, but the ultimate position in the two cases will be the same, provided that the flow is within Darcy’s law in both the cases.
Several solutions for the determination of the discharge and the free surface through homogeneous earthen embankment have been developed. Each of these procedures makes use of Dupuit’s assumptions.
Some of these procedures are discussed below:
1. Dupuit’s Solution:
The discharge per unit width through any vertical section of the dam in Fig. 27.5 is given by–
This equation specifies a parabolic free surface, commonly known as Dupuit’s parabola. In the derivation above, no consideration has been taken of the entrance on exit conditions of the line of seepage or of the development of a surface of seepage.
In fact, in the absence of tail water (h2 = 0), the line of seepage is seen to intersect the impervious base. Also, it should be noted that both the discharge quantity and the locus of the free surface are independent of the slopes of the dam.
2. Solution of Schaffernak and Van Iterson:
The first approximate method that accounts for the development of free surface was proposed by Schaffernak and van Iterson in 1916.
Considering an earthen embankment on an impervious Sbase (Fig. 27.6) with no tail water and applying Eq. 27.4 to triangle CAB, we obtain for the discharge per unit width (with x taken as +ve to the left)
Unlike Dupuit’s solution the parabolic free surface for this case is tangent to the downstream slope, as is required. For the entrance conditions correction at the upstream slope, Casagrande recommended that point D0 (Fig. 27.7) instead of point D be taken as the starting point of line of seepage (D0 is 0.3 m from point D at upstream reservoir surface). The actual entrance condition is then obtained by sketching in the area DF normal to the upstream slope and tangent to the parabolic free surface.
In a homogeneous embankment, located on impervious foundation where the discharge slopes are flatter than 1 : 1 (Fig. 27.7) it can be assumed that the point where the seepage line intersects the downstream face is given by Example 1.
Example:
For an earthen embankment with water standing at a height of 10 m and side slopes of 2:1, calculate the value of “e” in Fig. 27.7.
Solution:
When pervious shells are provided in the upstream and downstream sides, the position of the seepage line will be as shown in Fig. 27.8. The upstream portion will have little effect on the position of the line whereas the downstream portion will act as drain.
If, instead of the impervious foundation, a considerable layer of relatively pervious material overlies the impervious layer, discharge takes place through the pervious stratum down to the impervious stratum in the foundation.
In such cases a hydraulic gradient line is assumed and atleast 1 m to 1.2 m of cover over the hydraulic gradient is provided. The gradient line may be assumed as a, straight line with slope of 1 : 1 in an impermeable clay varying to 1 : 12 in sandy soil. In order to safely dispose the seepage water, toe drains or filters are provided.
Freeboard and Wave Protection:
Freeboard is the additional height of the dam provided as a safety factor to prevent overtopping due to unexpected runoff or by wave action. It is the vertical distance between the designed water elevation and the elevation of the top of the dam after settlement.
This is also referred to as the net freeboard. The vertical distance between the crest of the mechanical spillway and the top of the dam is referred to as the gross freeboard. The net free-board should be sufficient to prevent the waves in the pond from reaching the top portion of the embankment where they can cause damage. Wave heights for moderate sized reservoirs are determined using Hawksley’s formula –
In addition to providing enough freeboard from wave height point of view, precautions are taken to protect the dam from wave damage. There are several methods for the purpose and the choice of the method depends on whether the water level remains fairly constant or fluctuates.
Vegetative methods, berms, and rip-rap are the methods used for controlling erosion by waves. Vegetative methods consist of establishing a thick grass cover on the slope. This is useful when wave action is not severe.
If the water level in the pond remains fairly at a constant level, a berm of 2 to 3 m wide at this level controls wave action. Booms consist of a single or double line of logs chained together and suitably anchored is also used.
They float on water very near the embankment and break the wave action. Rip-rap consists of loose stones or concrete blocks dumped or hand placed on the side of the embankment facing the water. Rip-rap is an effective method of protection from wave action.
The layer of stones should be about 30 cm thick and are to be placed on a layer of gravel or crushed stone of about 25 cm thick. Rip-rap could be made continuous up to the toe of the dam or a berm is to be provided at the place where the rip-rap terminates below the water level.
Stability of Embankments:
The various design factors considered in the preceeding paragraphs will ensure safety of the embankment. However, when the height of the embankment exceeds 15 m, the section of the embankment should be carefully designed.
Additional analysis regarding the stability of the slopes needs to be carried out. Such procedures are outlined in literature on soil mechanics.