SEEPAGE CONTROL MEASURES
- The safety of earth dam depends almost entirely on seepage control through dam and its foundation.
- Hence measures of seepage control are very important for the success of an earthen dam.
- Seepage control through the dam.
- Seepage control through the foundation.
- Various measures of seepage control under each categoray have been discussed in brief.
Seepage Control Through the Dam
1. Rock Toe.
- Rock toe keeps seepage line well within the dam section.
- It also helps a great deal for the drainage purposes.
- The height of the rock toe is kept about one fourth of the height of the dam.
- Rock toe should be designed like filter.
2. Horizontal Drainage Filter.
- It is provided at the base of the dam, starting from down stream end of the dam and extending backwards into the dam.
- Backward extension of the filter depends upon so many factors.
- But this extension may at the most be up to centre line of the dam.
- This filter controls seepage line and does not allow it to get exposed on D/S face of the dam.
- It also accelerates the process of consolidation.
- It also causes drainage of foundation.
- If seepage pressure at the D/S end of the dam is still excessive, the horizontal filter drain may be continued even beyond the D/S toe of the dam.
- Sometimes rock toe and horizontal filter drains are dispensed with and entire D/S portion of the dam may be made from coarse-grained soil.
3. Chimney Drains.
- Under conditions of large stratification, the permeability in the horizontal direction is more than in the vertical direction.
- This causes a greater speed of horizontal seepage than vertical seepage.
- Chimney drain or filter, if correctly built, intercepts all the seepage from the dam regardless of the stratification in the dam.
- Chimney drains also render earth dam earthquake resistant.
Seepage Control Through the Foundation
1. Impervious Cut-off.
- The cut-off is a wall of relatively impervious material.
- It is used to prevent seepage through the foundation.
- It is always led down into the foundation from the ground surface.
- If possible, cut-off should extend upto the impervious strata lying below the ground level.
- Partial cut-offs do not prove much effective in preventing seepage.
- A 90%depth of cut-off reduces about 25% seepage.
2. D/S Seepage Berms.
- Additional berms may be built on the D/S side of the dam in continuation of the D/S end.
- Such a bean is useful in controlling seepage where D/S top strata is relatively thin and uniform or even top strata is absent.
3. Drainage Trenches.
- This measure is adopted when top stratum is pervious and thin.
- Porous drains remain enclosed in gravel filters.
4. Relief Well.
- It is such a well which if not constructed would cause formation of sand boils and possibly sub-surface piping.
- They reduce the sub surface uplift pressure D/S of the dam.
- They intercept the seepage through the foundation and control the outlet for seepage.
- Relief wells become necessary when impervious layer overlies a pervious layer and the thickness of overlying impervious layer is less than the depth of water impounded.
- Relief wells consist of 10 cm to 15 cm diameter holes filled with filter material.
5. Upstream Impervious Blanket.
- Such a blanket when constructed over a pervious foundation reduces the quantity of seepage on D/S side.
- It also causes reduction in uplift pressure throughout the D/S side.
- The provision of U/S blanket is found economical and more effective when the depth of previous overburden is large and the provision of the cut-off wall is uneconomical.
- Blankets are particularly effective when there are cracks and fissures in the foundation beneath the dam structure. In such cases they seal such openings and reduce the seepage considerably.
- The blanket should be composed of such material which is at least 100 times less pervious than the foundation material.