Grout curtains are used under dams where the foundation would otherwise pass too much seepage, or pass dangerous seepage. 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.
Exaggerated example showing wide cracks in a foundation emptying a dam where there is no grout curtain. The seepage passes underneath and emerges downstream.
This is an animation. It runs itself.
An additional, subtle safety threat from seepage. It can reduce stability in this type of dam.
This is also an animation running itself.
Grout penetrates out from the holes to provide a more or less continuous 'curtain' in the foundation.
The grouting is often done before constructing the dam
When slight seepage passes through the curtain it should be picked up by a row of drainage holes. These discharge to a low pressure region such as a gallery in the dam. They provide a 'safety valve' for the dam.
Foundation problems can cause dams to collapse:
This dam was partly overturned; other sections were washed downstream.
Click on the picture for an enlargement and details.
Typical layout of grout holes in a grout curtain
If the surface rock requires strengthening, blanket grouting could be appropriate.
The purpose of the grout curtain is to limit the quantity of underground seepage and to reduce its pressure. The drainage curtain is intended to pick up any seepage which passes through the grouting and then to dissipate remaining pressure.
The section shown is for the 'thin' type of arch dam. Pressures from the stored water are transferred by arch action to the abutments in addition to those carried to the base rock by cantilever action.
The grout curtain has a similar purpose to that at arch dams. Drainage holes are often omitted because seepage can emerge between buttresses without affecting stability.
Buttress dams have a concrete face retaining the water; the face is supported by buttresses at intervals.
The purpose of the 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.
The types of dam shown are zoned internally, with an 'impervious' core usually made from clay placed in layers and compacted carefully. The small amount of seepage passing through the core is picked up in filters immediately downstream of it; these prevent the seepage from carrying core material away. There are also filters on the upstream side of the core to protect the core from sloughing if the storage is drawn down rapidly. The core is held in position by large shells of rockfill or similar; these give the dam stability.
The purpose of the grout curtain at this type of dam is to limit the quantity of underground seepage. Drainage provisions are not necessary if the fill material is sufficiently pervious.
The water-retaining membrane on the upstream face is usually concrete, but may be bitumen or plastic. The rockfill supports it.
The curtain limits the amount of seepage passing under the dam. The filter provides drainage.
Homogeneous dams are built almost entirely with the one material - there is no zoning. The material selected has to provide a barrier to water and is therefore unable to provide safe dissipation of any seepage passing the grout curtain. Accordingly a filter is often provided on the foundation surface downstream of the grouting to guard the base of the fill from erosion by seepage.
This is a pictorial view of part of a weak foundation requiring grouting for strengthening purposes
The foundation is stripped down to firm rock and then grouted by means of (relatively) short holes.
The grouting commences with widely spaced holes called 'Primary' holes. The little man is grouting one of them.
The primary holes have been completed, but penetrations of grout from them along open cracks are unlikely to have met, leaving ungrouted cracks.
Holes midway between primary holes are then drilled and grouted to fill these remaining cracks. The fresh holes are called 'Secondary' holes.
This type of grouting is usually laid out on a grid. An inadequately grouted area is likely to remain in the centre of the group of holes. This is grouted with a further hole called 'Tertiary'.
If penetrations have been insufficient to give adequate grouting, additional holes (not shown) are placed between the earlier holes