The Dam 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.

1. 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.
2. 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.
3. 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.
4. 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.


Read more: How fill dam is made - material, making, history, used, processing, dimensions, product, History, Raw Materials, Feasibility and Preliminary Design, Design, The Construction Process, Quality Control http://www.madehow.com/Volume-5/Fill-Dam.html#ixzz1MP8iIdym
Read more: How fill dam is made - material, making, history, used, processing, dimensions, product, History, Raw Materials, Feasibility and Preliminary Design, Design, The Construction Process, Quality Control http://www.madehow.com/Volume-5/Fill-Dam.html#ixzz1MP8ZXmss