CONCRETE GRAVITY DAMS

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.

A vertical section through a concrete dam is shown here. The stored water is on the left and the grout curtain is shown in black, with the drainage curtain downstream of it also in black.

An exaggerated example is shown at a concrete gravity dam where wide cracks in a foundation permit seepage to pass under the dam and emerge downstream.

1. The stored water exerts a horizontal pressure against the upstream face of the dam.

2. The weight of the dam, acting downwards, counteracts against the water pressure to stop the dam overturning about the downstream toe.

3. If there is no grouting under the dam seepage can potentially pass along open cracks at its base and exert pressure upwards. This acts to reduce the effect of the dam's weight and therefore reduces its safety.
The illustration shows a dam tilting about its downstream toe because the dam has insufficient weight to withstand the combined effect of storage pressure and uplift pressure.

4. However, if a grout curtain intercepts the seepage and thereby cuts off most of the uplift pressure, the dam's stability is improved.

This dam failed because of foundation problems.

Blocks of it tilted around the downstream toe and were then washed downstream by the suddenly released water. One of them remained and can be seen lying on its back beyond the two blocks which did not fail.

Summary of the above.

This is a typical layout of grout holes in a grout curtain

ARCH DAMS

Similar considerations relate to the grouting of these.

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.

BUTTRESS DAMS

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

EMBANKMENT DAMS

The purpose of the grout curtain is to 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 type of dam shown is 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.

FACED DAMS

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.

HOMOGENEOUS DAMS

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

BLANKET GROUTING

This type of grouting strengthens an area rather than forming a curtain as shown above. The illustration shows its use at the foundation of a concret gravity dam. It is frequently used at all the other types of dam too and is usually carried out before the dam is built over it.

MINING and TUNNEL GROUTING

Grouting is sometimes carried out underground to strengthen the foundation ahead of the driving of tunnels, or mining drives, for the purposes of strengthening against collapse, and/or to control seepage inflows.
Steps in the processes are shown below.