Seepage control measures in earth dams -

Seepage control measures in earth dams –

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.

Various measures of seepage control under each categoray have been discussed in brief.

1. Rock Toe.

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.

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.

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.

Er. Banwari Lal