Dam Construction

Katse Dam

Situated in Lesotho, Katse Dam supplies water to South Africa. Photo by Christian Wörtz

In South African English, dam usually refers to both the dam wall as well as the store of water captured behind the wall. However, the term “dam”, in engineering jargon, usually refers to the manmade structures that block the flow of a river and resist the force that the water exerts on those structures (called hydrostatic pressure). The reservoir is the volume of water stored behind the dam.

Although the idea of storing large amounts of water through the capture and directing of surface run-off water is not new and can be seen as central to the sustaining of agricultural practices, the strength of dams has increased dramatically owing to technological advances made since the industrial revolution.

Large, powerful, natural rivers can be made into fresh water reservoirs by the intervention of a well-conceived, well planned and well executed engineering exercise. Many of us take our water supply for granted, but it has been made possible only by the concerted efforts of tens of thousands of labourers, designers, engineers and strategic planners. So important is our water supply that without it large-scale commercial farming would cease to exist, industry would grind to a halt, the economy would fail, even the property for sale in Sandton would become near worthless, and basic luxuries like hot showers and buying pet food online would become a relic of the past.

Below is a brief description of some of the predominant construction techniques used to build modern large and medium sized dams:

Arch Dams

As one can infer from its name, arch dams are highly reinforced concrete walls shaped with a curve that points in the up-stream direction. Viewed in plan (that is, from a bird’s eye view), the wall looks like an arch that is placed with the apex of its curve pointing towards the water reservoir. The arch is used in various structures and is an extremely useful form for distributing forces. In arched doorways the forces of the building material above the arch are transferred to the columns on which the arch rests. Arches were also used in the construction of Roman aqueducts. Arch dams are most often a feature of reservoirs situated in narrow, steep sided valleys. South Africa has several arch dams, and the Gariep Dam (to be discussed elsewhere) is an example of an arch-gravity dam. The Katse Dam situated in Lesotho forms part of the Lesotho Highlands Project, and is a 186m high reinforced concrete arch dam.

Buttress Dams

Buttress dams are constructed from either masonry or reinforced concrete, and essentially consist of two distinct groups of elements. The wall element is water tight and is in direct contact with the water in the reservoir; the wall is then buttressed by a series of buttresses which meet the wall at perpendicular angles. Forces placed on the wall by hydrostatic pressure are transferred to the buttresses which in turn transfer the force to the earth. The buttresses prevent the wall from being pushed over by the water. The engineering logic behind buttress dams is similar to that of gravity dams (see later), but whereas gravity dams rely purely on the mass of the dam wall to resist hydrostatic pressure, buttress dams use much less material. Buttress dams, like gravity dams, are often built in both narrow and wide valleys where the ground is of acceptable load bearing capacity.

Embankment Dams

Embankment dams are constructed from (predominantly) natural materials like earth fill and rock fill. The earth fill dams are constructed from compacted earth while rock fill dams are made from dumped rock or compacted rock fill. Viewed in cross section (i.e. a cut through the dam at a right angle to its length) both types of embankment dam are triangular in shape (with the broad edge at the bottom) and have a “core” made from clay to prevent the migration of water through the dam. These types of dam are best suited for long dams in shallow valleys.

Gravity Dams

Gravity dams can be constructed from either concrete or masonry, or from a combination of both. The name of the dam refers to the action by which it resists the forces placed on it by the water in the reservoir: the mass of the wall makes it rest firmly on its foundation, thus enabling it to counteract the hydrostatic pressure which would otherwise cause the dam to slide horizontally or even overturn. A cross section of the dam reveals that, like the embankment dam, it has the shape of a triangle, and is similarly suited to either wide, shallow valleys or deep and narrow valleys.

Tailings Dams

The term, “tailings dam” doesn’t refer to a construction technique, but rather to the use for which the dam has been built. Tailings dams (slime dams) store the waste products of industrial and mining practices. This is usually finely crushed sand, ash, and coal waste. Slime dams can receive up to 100 000 tons of waste per day over a period of two to three decades. This means that they really do have to be very big and are classified as large dams. Owing to the fact that South Africa has a large mining and industrial sector, slime dams are a common feature in the country. The environmental impact and waste capacity requirements of slime dams has made their design a highly specialised activity undertaken only by expert engineers and scientists.