A dam is a barrier across flowing water that obstructs, direct or slows down the flow, often create a reservoir, lake or impoundment. The foundation for most dams more than 15m high, and for some which are smaller, are treated by grouting. Grouting consists of a drilling a line or lines of drill holes from the cutoff level of the dam into the dam foundation, and forcing cement slurry, or chemicals into the foundation under pressure. Grouting is often done before constructing the dam.
Curtain grouting is design to create a thin barrier (or curtain) though an area of high permeability. It usually consists of a single row of grout holes which are drilled and grouted to the base of the permeable rock, or to such depth that acceptable hydraulic gradients are achieved. For large dams on rock foundations, or dams on very permeable rock or soil foundations, 3 or 5 or even more lines of grout holes maybe adopted.
The holes are drilled and grouted in sequence to allow testing of the permeability of the foundation (by parker testing) before grouting and allow to a later check on the effectiveness of grouting from the amount of the grout accepted by the foundation.
Figure 1The purpose of the grout curtains are used under dams where the foundation would otherwise pass too much seepage, reduce the seepage erosion potential and reduce leakage through the dam foundation. Grout curtain is restrict seepage to such an amount that it does not cause too much loss of storage, and does not dislodge the foundation downstream or erode the base of the dam.
These curtains are never completely water-tight; the reduced amount of seepage passing through them should be picked up in drainage holes downstream of the curtain. A grout curtain in the rock are provided for under seepage control. These discharges to a low pressure region .They provide a 'safety valve' for the dam. Existing of curtain grouting could reduce the permeability of the bedrock thus minimize the infiltration of groundwater and the exfiltration. Besides that, it can reduce uplift pressures (under concrete gravity dams when used in conjunction with drain holes) and strengthen the dam foundation and reduce settlements in the foundation.
Figure 2
Wide cracks in a foundation emptying a dam where there is no grout curtain. The seepage passes underneath and emerges downstream.
Warragamba dam is one for the Australian Dams which constructed before 1962. The dam is on Triassic Sandstone with intervening shales in a gorge with a chord/height ratio of 3 1/4 .
The grouting of the Triassic sandstones for the dam is extensive:
a) Intermediate pressure curtain – When the concrete of the dam had reached a height of about 35ft across the valley holes 10ft apart, were drilled diagonally under the heel of the dam to reach a depth of 75ft below foundation level.
b) High pressure curtain – This curtain is to provide a high permeable zone to reduce leakage and uplift under the dam.
Reference :
a) Dam Geology by R.C.S. Walters ( 2nd Edition )
