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How Dams are Built

Construction work on a dam project is carried out at the dam site itself and also at other locations such as spillway site, rock fill quarry, clay and gravel borrow areas and road works which may be near the dam site or, in other cases, may be considerable distances from the dam site.

From a construction point of view the dam site consists of the river bed area which is normally submerged by the river and the abutments or valley sides above the river bed. No work can be carried out in the river bed area until the river has been diverted, usually through a tunnel or channel. After diversion the river bed area is dry and construction of the dam wall can commence.

Before the dam wall can be built two important preliminary steps have to be completed: the dam foundation area must be stripped of overburden (ie material, usually soil, which is unsuitable for dam construction must be removed and disposed of), and the foundation for the dam must be prepared. Foundation preparation usually consists of two quite different activities: removal of pockets and seams of weak material and replacement of this material with "dental" concrete, so called because the work resembles filling cavities in teeth; the second aspect of foundation preparation is grouting. Grouting involves drilling holes (often to quite great depths, equal to the height of the dam in some cases) and then pumping these holes full of cement grout (a mixture of cement and water). The purpose of grouting is to fill up open cracks in the rock foundation on which the dam is to be built so that water will not leak out of the reservoir after the dam is finished.

Once the foundation preparation is complete the construction of the dam wall itself can commence by hauling, dumping and compacting construction materials such as clay and rock fill. Compaction is carried out to ensure that the fill, as placed, will have the maximum possible density. Properly compacted fill will have the desirable engineering properties of high strength and low settlement over time, and, in the case of clay fill for a dam core, low permeability, which is important to reduce the likelihood of failure by piping or internal erosion.

A typical construction sequence for a fill dam is:

Stage 1: Excavate diversion tunnel and build coffer dams. The end of this stage is marked by the milestone of diverting the river through the diversion tunnel.

Stage 2: Strip dam foundation of overburden. Carry out foundation treatment and grouting. Excavate and haul fill construction materials from their sources and place and compact in the dam embankment. The end of this stage is marked by the milestone of closure of the diversion tunnel to start the storage of water in the dam reservoir. Excavation of the spillway will also be under way during this stage.

Stage 3: Complete outlet works, spillway and all other parts of dam project.

Management of floods during construction is one aspect always given considerable attention by the engineers designing the dam. Before the design of the dam can be completed engineering geologists have to find sources of suitable clay, gravel and rock which can be used as construction materials with which the dam wall can be built.

In fact, a dam project passes through four phases during its life;

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