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:
- Coarse sand and gravel
- Coarse sand, gravel and clayey silt
- Only silt clay is available.
Foundation conditions at the site may be any of the following types:
- Impervious of large depths.
- Pervious for moderate depth and thereafter impervious.
- Pervious for indefinite depths.
In all the three conditions of foundations earth dams of different materials can be construct show in Figs. 12.10 to 12.14.
Case1. 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 andgravel but central core is made from the available clayey silt.
- The central core may be vertical or inclined. See Fig. 12.10.
2. Pervious Foundation for Moderate Depth and then Impervious.
- Construction of this dam is similar in case (1) except that central core is carried down to the level of impervious layer. See Fig. 12.11.
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.
- 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 extend beyond the upstream end of the dam. See Fig. 12.12 (upper).
- 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 install the centre of the dam. See Fig. 12.13 (upper).
2. Foundation Pervious for Some Depth and then Impervious.
- Construction is similar to (1) except that core is start from the level of the impervious foundation, Fig. 12.13 (middle).
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. See Fig. 12.13 (lower).
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 adopt for (1).
- A sheet pile cul-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).