Fill Dam

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Fill Dam

Background

Dams are among the oldest structures built by humans for collective use. A dam is a barrier that is constructed across a river or stream so the water can be held back or impounded to supply water for drinking or irrigation, to control flooding, and to generate power. The main kinds of dams are earth fill, rock fill, concrete gravity, concrete arch, and arch gravity. The last three types are all made of concrete, reinforced concrete, or masonry. (The term masonry can mean concrete, bricks, or blocks of excavated rock.) Fill dams include all dams made of earth materials (soil and rock) that are compacted together. One type of fill dam called a tailings dam is constructed of fine waste that results from processing rock during mining; at mine sites, this soil-like waste is compacted to form an embankment that holds water for the mining and milling processes or to retain the tailings themselves in water.

Of the main categories of dams listed above, all have been built since ancient times although many refinements were developed in the nineteenth and twentieth centuries with improved engineering technology. Dams that leak have failed to do their job, either because they simply can't hold water or because the water seeping through them eats materials away from the inside of the dam causing it to fail structurally. In modern times, most fill dams are also built with zones including a clay center or core, filter and drainage layers, coarser materials sandwiching the clay core, and rock on the upstream (water) face to prevent erosion. These zones can be seen clearly when a cross section is cut from the upstream to the downstream side of the dam. All fill dams depend on weight to remain stable.

Fill embankments are usually less expensive to construct than concrete dams. Soil or rock are present at the site, and construction techniques, though complex, are also less costly than for concrete construction. For these reasons of available materials, low cost, and stability with mass, fill dams are often built across broad water courses. They also are more flexible than concrete structures and can deform without necessarily failing if foundation materials under the dam compress with the weight of the dam and the water.

History

Quite naturally, early dam builders began by using plentiful materials like sand, timber and brush, and gravel. Their construction method consisted of carrying the materials by the basketful and loosely dumping the fill, so many of these dams may have survived only a few years. Scientists have not been able to pinpoint dates for the earliest dam construction, but they do know dams were needed where food was grown and in areas prone to flooding.

Design of fill dams is based on experience; while failures are unfortunate and sometimes catastrophic, they are also the best teachers, and many engineering advances have been founded on careful study of earlier failures. The engineers of ancient India and Sri Lanka were the most successful pioneers of fill dam design and construction, and remains of earth dams can still be seen in both countries. In Sri Lanka, long embankments called tanks were built to store irrigation water. The Kalabalala Tank was 37 mi (60 km) long around its perimeter.

The most famous earth fill dam recently constructed is the Aswan High Dam that was built across the Nile River in Egypt in 1970-1980. An earth fill dam was also the victim of a spectacular failure in June 1976 when the Teton Dam in Idaho eroded from within due to incorrect design of the zones inside the dam that allowed seepage, failure, and flooding of the valley downstream. Although earth dams tend to be short and broad, Nurek Dam in Tajikistan is 984 ft (300 m) high.

Raw Materials

The materials used to construct fill dams include soil and rock. Soil is classified by particle size from the smallest, submicroscopic particles called clay; silt, which is also very fine; sand ranging from fine to coarse, where the fine grains are the smallest soil particles our eyes can see; and gravel. Coarser fragments called cobbles and boulders are also used in dam construction but usually as protective outer layers.

Specific soil types and size ranges are needed to construct the zones within the dam, and explorations of the dam foundation area, the reservoir where the water will be stored, and surrounding areas are performed not only for design of the dam but to locate construction materials. The costs of fill construction rise dramatically with the distance materials are hauled. Samples of potential construction materials are tested in a soil laboratory for grain size, moisture content, dry density (weight), plasticity, and permeability. Clay is not only very fine in size but has chemical characteristics that cause it to stick together. The combination of fine size and plastic behavior also causes the clay to be less permeable to water. If clay is available near the site, the dam can be built with an impermeable core or central zone that prevents water from passing through the dam; otherwise, the dam must be designed so water can seep slowly and safely through a different combination of materials in its zones.

Water is also a raw material. The various soil types have compaction characteristics that can be determined in the laboratory and used during construction. Soil can be compacted to its best functional density by adding moisture and weight and impact, called compactive effort. Large vibrating rollers press thin layers of soil into place after an optimal amount of water has been added. The water and weight bond the soil particles together and force smaller particles into spaces between larger particles so voids are eliminated or made as small as possible to restrict seepage.

Increasingly, fill dams also include geotextiles and geomembranes. Geotextiles are nonwoven fabrics that are strong and puncture-resistant. They can be placed between lifts as the dam is raised to strength weak materials. They are also used as filter fabrics to wrap coarser drain rock and limit the migration of fine soil into the drainage material. Geomembranes are made of high-density polyethylene (HDPE) plastic and are impermeable. They can be used to line the upstream face of a fill dam or even to line the entire reservoir.

Feasibility and
Preliminary Design

A specific need for a dam, whether it is water supply, storage of tailings or other materials, or flood control, stimulates the process of designing and building a fill dam. The need and the location are usually closely connected, so several sites may be considered. During feasibility studies, engineers identify these sites, make preliminary cost comparisons, decide on a probable design, and chose the best site for exploration. Feasibility certainly refers to the cost of building the dam, but it also includes the technical practicalities of site suitability, design, construction, and long-term maintenance and safety.

After a feasible site is chosen, a preliminary design of the dam is developed. The location of the dam is superimposed on a topographical map so the dimensions of the top of the dam relative to the tops of the adjacent hills and the proposed water level can be shown as well as the extent of the base of the dam in the stream channel. The proposed water level elevation shows the extent of the reservoir and determines—along with the shape of the basin—the quantity of water that the reservoir will hold. Quantities of water stored and materials used in constructing the dam help determine the value of the project and its costs. Sometimes multiple iterations of site selection, pre-design, and cost estimating are needed. Ideally, the foundation area under the dam will not require much excavation or grouting to prevent seepage, and the materials inside the reservoir area can be excavated and used to build the dam so that more reservoir storage is gained at the same time as soil or rock are excavated to construct the embankment.

When the optimal site is chosen on paper, an exploration program is developed and performed. During the exploration, test borings are drilled along the line of the axis of the dam across its proposed width, along or near the proposed upstream and downstream toes of the dam, at the site of the proposed spillway, and in the reservoir area. The borings are excavated deep into the foundation to evaluate its strength and permeability (potential for seepage) properties. As the borings are drilled through the overlying soil, it is also sampled and tested in the laboratory so it can be evaluated as potential dam construction material. Field tests of permeability are also performed at the site of the dam and in the reservoir area. If it is the source for construction materials, test pits are also dug in the reservoir area so that the volume of available soil (and related costs) can be estimated.

Design

After the field exploration and laboratory testing are complete, the engineering team begins final design of the dam based on the preliminary assumptions, the findings in the field, and any changes in design or economics that are based on field findings. In designing a fill dam, engineers look at five critical considerations: the mass of the dam that will make it stable; design of a core and other interior zones to prevent seepage through the dam; design of a cutoff wall or other seepage prevention under the dam; erosion protection on the upstream face; and economics.

Fill dams are typically shaped like triangles with the apex or point at the top or crest of the dam and the broad base on the floor of the creek channel. The width of the base in cross section provides friction to prevent sliding, and the total mass of the dam makes it strong enough to resist the weight of water behind it. The foundation area is cleaned of soft, permeable, and compressible soil; and a cut-off wall is cut down to rock or firm soil. The cut-off wall can be constructed of steel sheet piling or concrete, but, for most fill dams built since about 1960, the cut-off wall is simply an extension of the clay core. Where foundation rock or soil contains voids or fractures, a series of holes may be drilled into the foundation, and concrete grout is injected in the holes to seal the fractures and help cut off seepage.

The zones of a fill dam may consist of a number of distinct layers from the center of the dam and moving upstream toward the water and a different set of layers from the center moving downstream. Materials for the zones are selected for strength properties and permeability characteristics, and the placement of one zone next to another is carefully governed by sets of calculations based on these properties. Filter and drainage zones are included so that any water succeeding in reaching the inside of the dam is channeled around the core and out through drainage layers at the base of the dam.

The upstream (water) face of the dam is sometimes protected with a concrete slab or an asphalt face. More commonly, cobble- and boulder-sized stones are placed on this face near the water surface; this facing is called riprap and prevents wave action at the water surface from eroding the dam construction materials. Other facilities for controlling the water level and any water movement through or over the dam, like an emergency spillway, are also designed specifically for the dam's location, uses, type and materials of construction, and water inflows into the reservoir.

The economics of dam construction are considered throughout the design process. Construction materials must be available at or near the site. Rock can be placed at steeper angles than soil, and it weighs more; so a dam built mostly of rock can be smaller in design section. Excavating and moving rock can be more expensive than soil, however, so the design engineers must consider cost factors. Other materials like asphalt, concrete, steel, and cement for grouting are also expensive. The proper balance of safety and economy must be determined by the engineers. Large earthmoving machines have made construction of zoned, fill dams more economical than construction of concrete dams at many sites.

The Construction Process

Fill dams are constructed in the dry season when water levels in the river or stream are lower, rainfall on sources of fill material is less likely, and conditions are better for operating large construction equipment. Before construction actually begins, the site is surveyed to locate the dam alignment on the existing ground, the areas that will be excavated, and the borrow areas or sources for the soil or rock used in construction. Construction management facilities are set up; usually, the construction manager (a field engineer with years of similar experience) will work out of a trailer on site. Depending on the site, it may be necessary to install instruments to monitor the effects of dam construction on adjacent hillsides or other features and to measure groundwater levels throughout construction in the foundation and surroundings. And, of course, the flow of the stream that is being dammed through the site must be stopped. This can be done by a variety of methods including diverting the stream, perhaps to flow through a neighboring channel, or stopping it upstream with a temporary dam or cofferdam.
Before construction of the dam begins, the foundation area must be prepared. In rare cases, dams can be constructed directly on the existing materials in the channel floor; at most sites, these materials are compressible (and would cause the dam to settle irregularly) and permeable (allowing water to pass under the dam). The foundation area also includes the abutments, which are the hillsides forming the two ends of the dam. Soil and soft or highly fractured rock are excavated, sorted by type, and stockpiled for later use in dam construction. The surface of the foundation bedrock is cleaned to a surprising degree; it is broomed and hosed with water so that any voids or irregularities are visible and cleaned of soft soil. The foundation is carefully inspected before any construction work; additional exploratory drilling may be done if there are any questions about the foundation's condition. If the rock is fractured or contains voids or holes, these are sealed with cement grout that is injected through small diameter drill holes in a process called dental work.
The base of the dam must go down into the ground before it rises above it. A trench that is the full width of the dam (across the channel) is cut into firm rock. The trench is called a keyway or cutoff wall and may have several benches or notches into rock. It prevents the dam from sliding along a smooth foundation and also creates a longer path for any seepage to try to flow under the dam. The impervious clay that will make up the core of the dam is placed in the keyway and compacted and raised, layer by layer, until the top of the keyway or base of the majority of the foundation is reached.
The soil in the keyway and all the zones of the dam are raised to the same levels at the same time. Ramps may have to be cut into the keyway area for the construction equipment, and then they must be built up to the working surface of the rising top of the dam. Whenever possible, roads are cut in from the two sides (abutments) of the dam for the easiest access; eventually, an access road will be built on the crest of the dam and extending onto these abutments.
Large earthmovers haul the specific type of soil needed to raise the zone of the dam they are working on. The soil is spread in thin layers, usually 6-8 in (15.2-20.3 cm) thick, sprayed with water to the correct moisture content, and compacted with sheepsfoot rollers (compactive rollers with prongs resembling animal hooves mounted in rows around the roller that press and vibrate the soil firmly in place). If gravel is used in construction, a vibrating roller is used to vibrate the grains together so their angles intermesh and leave no openings.
Throughout the compaction process, inspectors approve the soil that is hauled on site and hauled to the particular zone of the dam. They reject material that is contaminated with grasses, roots, trash, or other debris; and they also reject soil that does not appear to be the proper grain size for that zone of the dam. For quality control, samples are collected and tested in the laboratory (for large dams, an on-site soil lab is installed in a construction trailer) for a variety of classification tests. Meanwhile, the inspector uses a nuclear density gauge to test the soil for density and moisture content when it has been placed and compacted. The nuclear density gauge uses a very tiny radioactive source to emit radioactive particles into the soil; the particles bounce back onto a detector plate and indicate the moisture and density of the soil in place. The process is not harmful to the environment or the operator (who wears a badge to monitor radioactive exposure) and provides data without having to excavate and sample. If the compaction requirements are not met, that layer of soil is excavated, placed again, and recompacted until its moisture and density are suitable.
Construction of the fill dam proceeds layer by layer and zone by zone until the height of each zone and, eventually, the crest of the dam are reached. If the entire dam cannot be built in one construction season, the dam is usually designed in phases or stages. Completing a construction stage (or the entire dam) is often a race against time, the weather, and the project budget.
Some earth dams have instruments installed in them at the same time as fill placement is done, and the instruments are constructed to the surface in layers and zones, just like the fill. The condition of the dam is monitored throughout its lifetime, as required by federal, state, and local laws and by standards of engineering practice. Types of instruments vary depending on the location of the dam; almost all dams have settlement monuments that are surveyed to measure any settlement in the surface or zones of the dam, slope indicators to show if the sloping faces inside or on the surface of the dam are moving, and water-level indicators to monitor the water level in the dam's zones. Dams in seismically active areas may also be equipped with instruments to measure ground shaking.
Fill dams may have a variety of other facilities, depending on their, size, use, and location. An emergency spillway is required at all dams to allow for flood waters to flow over an escape route, rather than over the top of the dam. Other spillways for production of hydroelectric power may be designed and constructed at power-generating dams, and inlet and outlet tunnels are needed to release water for irrigation and drinking-water supplies at embankments built for those purposes. At fill dams, it is usually desirable to place these other facilities in excavations through the foundation or abutment rock; the process of compacting earth against structures that actually pass through the fill is tricky and allows for seepage paths.
Sometimes the reservoir area is also cleared when it is to be filled with water, particularly if lumber can be harvested. It is not necessary (and it is much too expensive) to clear it of all shrubs and grass. The process of filling the reservoir is relatively slow, so most wildlife will move as the water level rises; areas of concern include habitats for rare or endangered species, and drowning of these habitats has been a concern in the construction of a number of dams.

When the dam is complete, the water that was diverted from the stream channel is allowed to fill the reservoir. As the water rises, it is also rising in portions of the dam, and instruments within the dam are monitored carefully during the reservoir-filling period. Monitoring of the dam's performance, both by instruments and simple observation, is performed routinely; and safety plans are filed with local emergency services so that sudden changes in instrument readings or the appearance of the dam or its reservoir triggers actions to alert and evacuate persons living in the path of flood waters downstream. Repairs are also performed routinely.

Quality Control

Quality engineering is essential in the construction of a fill dam because the materials used have lower strength properties than the steel and concrete required for concrete dams and because placement ultimately determine strength, potential for problems like seepage and settlement, and finally performance and safety. The geotechnical project engineer occupies the key role of making sure the design and earth materials match to make a safe product; but many other professionals including geologists, construction technicians, other engineers, and the representatives of overseeing agencies are fully committed to the same purpose.

Byproducts/Waste

There are no byproducts in fill dam construction, although fill is sometimes generated for building access roads and other support structures. Waste is also minimal to nonexistent; excavation of excess soil and especially rock is very expensive as is hauling these materials so waste is engineered out of the design.

The Future

Primarily due to environmental concerns, design and construction of any dam in the future will be a much-studied and controversial process. Fill dams, however, tend to be perceived as more environmentally friendly because they are made of earth materials and blend into the scenery better than monolithic concrete structures. Fill dams have proven useful and less expensive solutions to meeting human needs for water supply, and vast improvements in engineering technology have improved their safety record in the late twentieth century. Although many costs and agendas must be considered in building dams, fill dams have and will continue to prove themselves allies in the needs to provide drinking water, irrigation supply, and flood control.