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An embankment dam is a gravity dam formed out of loose rock, earth, or a combination of these materials. The upstream and downstream slopes of embankment dams are flatter than those of concrete gravity dams. In essence, they more closely match the natural slope of a pile of rocks or earth. Of the many different kinds of embankment dams that exist, rock-fill embankment dams and zoned-embankment dams are among the most common. Rock-fill embankment dams consist of a mound of loose rock covered with a waterproof layer on the upstream side to prevent excessive seepage and erosion. The waterproof layer may be made of concrete, flat stone panels, or other impervious materials. Zoned-embankment dams include an impervious core surrounded by a mound of material that water can penetrate. The supporting mound is usually made of loose rock or earth. The core might be built from concrete, steel, clay, or any impervious materials. Like concrete gravity dams, embankment dams hold back water by the force of gravity acting upon their mass. Embankment dams require more material because loose rock and earth are less dense than concrete. Despite the huge volumes of material required to build an embankment dam, engineers often choose to build them if the materials are readily available. The Tarbela Dam, which crosses the Indus River in Pakistan, contains more than 126 million cubic meters (more than 165 million cubic yards) of earth and rock. This amounts to more than 15 times the volume of concrete used in the Grand Coulee Dam.
Arch dams are concrete or masonry structures that curve upstream into a reservoir, stretching from one wall of a river canyon to the other. This design, based on the same principles as the architectural arch and vault, transfers some water pressure onto the walls of the canyon. Arch dams require a relatively narrow river canyon with solid rock walls capable of withstanding a significant amount of horizontal thrust. These dams do not need to be as massive as gravity dams because the canyon walls carry part of the pressure exerted by the reservoir. For example, the Glen Canyon Dam, which spans the Colorado River in Arizona, is the highest arch dam in the United States. It is 216 m (710 ft) high and 475 m (1,560 ft) long but contains less than 4 million cubic meters (under 5 million cubic yards) of concrete. Because they require less material than gravity dams, arch dams can be less expensive to build. More from Encarta Not all concrete and masonry dams that curve into a reservoir qualify as arch dams. In some cases, engineers choose to use an arched shape even if it is not a structural necessity. For example, Hoover Dam features a prominent curve but the structure is actually thick enough to stand as a gravity dam. In many ways, the massive dam’s curvature comprises more of an aesthetic effect than it does a structural necessity.
A buttress dam consists of a wall, or face, supported by several buttresses on the downstream side. The vast majority of buttress dams are made of concrete that is reinforced with steel. Buttresses are typically spaced across the dam site every 6 to 30 m (20 to 100 ft), depending upon the size and design of the dam. Buttress dams are sometimes called hollow dams because the buttresses do not form a solid wall stretching across a river valley. Buttress dams fall into two basic categories: flat slab and multiple arch. Flat slab buttress dams have a flat upstream face. These dams are sometimes called Ambursen dams in recognition of Nils Ambursen, the Norwegian-born American engineer who popularized them in the early 20th century. An example of a flat slab buttress dam is the Stony Gorge Dam, which crosses Stony Creek near Orland, California. It stands 42 m (139 ft) tall, stretches 264 m (868 ft) long, and contains 33,000 cubic meters (43,100 cubic yards) of concrete. Multiple arch buttress dams feature an upstream face formed by a series of arches. The arches rest on top of buttresses that extend down to the foundation. Bartlett Dam, on the Verde River near Phoenix, Arizona, is a multiple arch dam. It stands 94 m (309 ft) high, stretches 244 m (800 ft) long, and contains nearly 140,000 cubic meters (182,000 cubic yards) of concrete. Like arch dams, buttress dams require less concrete than comparable gravity dams, but they are not necessarily less expensive to build than concrete gravity dams. Costs associated with the complex work of forming the buttresses or multiple arches may offset the savings in construction materials. Buttress dams may be desirable, however, in locations with foundations that would not easily support the massive size and weight of gravity dams.
The complexity of dam construction varies with the size of the dam. A small dam across a stream might be built in just a few weeks. But a large structure—such as Hoover Dam, which towers 221 m (726 ft) above the riverbed—can take several years to plan and several more years to actually build. Dams of this scale require extensive site testing to verify that the underlying rock can withstand the pressure exerted by the dam and the reservoir. The river must be temporarily diverted and the foundation cleared of earth and loose rock. In some cases roads and other distribution systems must be constructed to transport millions of tons of material and equipment as well as thousands of personnel to the site. Project planners may also have to construct temporary housing for workers to live in while they work at the site.
Before construction begins, engineers survey the geology of a proposed site to ensure that it will provide a foundation strong enough to support the weight of the dam. They evaluate the structural condition of the bedrock by drilling core samples that can be further studied in geological laboratories. The engineers must also determine if excessive amounts of water from the reservoir will seep into the rock, which could undermine the foundation and cause the structure to collapse or wash away. Engineers also consider the possible stresses that the foundation will be subjected to, as it must withstand the weight of both the dam and the water in the reservoir. Larger dams exert greater stresses, which means that a foundation adequate for a dam 30 meters high may be inadequate for a structure 120 meters high. Much scientific study and analysis goes into the process of designing and building a dam and every dam site presents a unique set of conditions that engineers must evaluate individually.
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