SUCCESSFUL DAMS IN NORTH AMERICA

SUCCESSFUL DAMS IN NORTH AMERICA.

THE HOOVER DAM Is a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the border between the U.S. states of Arizona and Nevada.It was constructed between 1931 and 1936. It has a height and length of 221.4m and 379m respectively.The reservoir maximum water depth is 180m, maximum length 180km and it creates Lake Mead.It has a two controlled drum-gate spillway type, and the spillway capacity is 11,000 cubic meter per second.The dam is mainly used for, Power generation, flood control, water storage and regulation.

The Hoover Dam(left), Hoover Dam Spillway(right)

Geologic conditions at the Dam siteThe Hoover Dam area contains exposures of Precambrian metamorphic rock, Tertiary volcanic and plutonic rock, and, Quaternary gravels. Paleozoic rocks are restricted to roof pendants in Tertiary plutons and xenolithic blocks in mafic lava flows. With rare exception, rocks of the Hoover Dam area have been broken along numerous late Miocene faults with complex slip components.The foundation and abutments are rock of volcanic origin which is andesite breccia. The rock is hard and very durable.

CONSTRUCTIONRiver diversion, Before the dam could be built, the Colorado river needed to be diverted away from the construction site. To accomplish this, four diversion tunnels were driven through the canyon walls, two on the Nevada side and two on the Arizona side.To protect the construction site from the Colorado River and to facilitate the river's diversion, two cofferdams were constructed.Once the cofferdams were in place and the construction site was drained of water, excavation for the dam foundation began.For the dam to rest on solid rock, it was necessary to remove accumulated erosion soils and other loose materials in the riverbed until sound bedrock was reached.

ContSince the dam was an arch-gravity type, the side-walls of the canyon would bear the force of the impounded lake, therefore the side-walls were excavated too, to reach virgin rock as weathered rock might provide pathways for water seepage.The cleared, underlying rock foundation of the dam site was reinforced with grout, called a grout curtain.This was done to stabilize the rock, to prevent water from seeping past the dam through the canyon rock, and to limit "uplift"upward pressure from water seeping under the dam.

How much cement and concrete was required?Cement of more than 5 million barrels. The daily demand during construction of the dam was from 7,500 to 10,800 barrels.Three and one-quarter million cubic yards. There are 4,360,000 cubic yards of concrete in the dam, powerplant, and outlet works.The first concrete for the dam was placed on June 6, 1933, and the last concrete was placed in the dam on May 29, 1935. Approximately 160,000 cubic yards of concrete were placed in the dam per month. Peak placements were 10,462 cubic yards in one day (including some concrete placed in the intake towers and powerplant), and slightly over 275,000 cubic yards in one month.

SpillwaysThe dam is protected against over-topping by two spillways. The spillway entrances are located behind each dam abutment, running roughly parallel to the canyon walls.The large spillway tunnels have only been used twice, for testing in 1941 and because of flooding in 1983. During both times, when inspecting the tunnels after the spillways were used, engineers found major damage to the concrete linings and underlying rock.The 1941 damage was attributed to a slight misalignment of the tunnel invert (or base), which caused cavitation, a phenomenon in fast-flowing liquids in which vapor bubbles collapse with explosive force.In response to this finding, the tunnels were patched with special heavy-duty concrete and the surface of the concrete was polished mirror-smooth.

ContThe 1983 damage, also due to cavitation, led to the installation of aerators in the spillways.Tests at Grand Coulee Dam showed that the technique worked, in principle.

OPERATIONSHydroelectric power generation, the Hoover Dam distributes power to the state of Nevada, the state of Arizona and about thirteen cities in the state of California.

Supplies water for Irrigation of over 1,000,000acres of land.

Supplies water for domestic purposes that serves about 8 million people in Arizona, Nevada and California.

THE W.A.C BENNETT DAMThe W. A. C. Bennett Dam is a large hydroelectric dam on the Peace River in northern British Columbia, Canada. At 183 m high, it is one of the worlds highest earth fill dams.It creates the Williston Lake, it is the third largest artificial lake in North America, with a surface area of 1,761 km square.Construction of the dam began in 1961 and culminated in 1968.Its main function is the production of hydroelectric power. It has the capacity to generate more than 13 billion kWh annually.

The W.A.C Bennett Dam

CONSTRUCTIONThe moraine provided the answer to constructing one of the largest earth filled dams of its kind. Three years of survey and study of data climaxed in plans to build a dam near the head of the canyon and to use the materials of the moraine to build it. A low coffer dam was constructed to direct the water into the tunnels allowing the dam to be built on the dry river bed.Excavating to create a solid base for building was the step that followed, and preparing the foundations and injecting grout into the ground to create a watertight seal.

ContOver the course of construction 55 million cubic yards of rock and dirt were taken from the nearby glacial moraine by conveyor belt to create the dam relying primarily on gravity to hold it together.A complicated conveyor system adjacent to the dam site mixed the moraine according to rigid specification for different zones of the dam.Dump trucks carrying 100 tons of earth rattled through the dam site. At the same time, work was being done to construct the power house deep within the east abutment out of the solid rock of the canyon wall. This massive chamber would house the hydroelectric generating facilities of the dam.

SpillwayAcross the valley the spillway channel had been gouged out of the rock to provide an escape route if the reservoir ever threatened to overflow.This discharge channel is lined with concrete and extends 2,300 feet from the crest of the dam to a point 250 feet above the downstream riverbed. When opened, it can allow 9,205 cubic meters of water per second to be launched into Dinosaur Lake below.

The W.A.C Bennett Spillway

OPERATIONSThe main function of Dam was the generation of Hydroelectric power.Gordon M. Shrum Generating Station at Bennett Dam has the capacity to generate over 13 billion kWh annually. The powerhouse, carved deep in bedrock under the east shoulder of Bennett Dam, was the largest underground powerhouse in the world when it began generating electricity in 1968.