Analysis of Buttress Dams - Coursework Example

Buttress Dams Introduction The idea to build buttress dams was driven by the scar of water and material in areas where labor was cheap. Hence these dams were mainly constructed for the purpose of minor irrigation. However, with the improvement in the designs of these dams, the apparent flaws have also surfaced, which are being tackled, at individual level……(Chadwick, Introduction to Buttress Dams) An American engineer used such type of dam construction in his projects during the 20th Century and hence, these dams are also called Ambursen dams. The terrain condition is also responsible for choosing building of this type of dams as the land at the particular site may not be able to support the size and weight of other types of dams. As the name suggest, the dam has the support of many buttresses at different intervals, while these are concrete slabs with reinforced steel that act as a strong seal against the flow of water from the water bodies like river. As buttress dams can be of many types, the two main types are flat slab and multiple arch dams. While in the flat slab type dams, the flat front faces the water upstream; in multiple type, the front of the dam has several arches which face the upstream. A fine example of multiple arch type buttress dam is the Bartlett dam constructed in United States, which happens to be the tallest Buttress concrete dam in the country. This dam is about 287 feet high, while it consists of 10 arches, 9 buttresses and 2 gravity wing dams. A picture of this dam is given in the Annexure, at the end of this paper, as figure 1. Buttresses Buttress normally is a structure, which has a composition of a system of curved beams. Each beam in such a composition is responsible for transmitting part of the water load and its dead weight to the dam foundation. The columns are proportioned in such a way that they develop uniform compressive stress, while they should be curved to avoid any load being eccentric at a particular point. Buttresses of large dams are normally built with contraction joints, which caters to the development of secondary tensile stress in the structure….(Buttresses) Simple design parameters While discussing about the physics involved in designing the buttress dams, the concerned engineers consider same forces here as they would do while building an arch or a gravity dam. Accordingly, three types of forces would come into play while building a buttress dam. These are the forces exerted by the water and concrete plus the normal force exerted in the upward direction by the earth. These forces are detailed below, individually. Force exerted by water would be proportional to the depth of the water, which also depends on the area of the dam. This can be calculated by multiplying the average pressure of water with the total area of the dam. Similarly, the downward force extended by the concrete would depend on the total weight of the dam. Hence, the specific weight of the concrete is used for this purpose, which means volume of concrete should be multiplied by the specific weight of the concrete, to determine the force exerted by the concrete in the dam. To determine the specific weight of the concrete, the ratio of concrete density to that of water at 4 degree Celsius is determined. Accordingly the specific gravity of concrete being 23.6N/cubic meter, the down ward force exerted by the concrete would be equal to the total weight of the dam multiplied by the specific gravity of concrete. The normal force applied by the earth would be in the opposite direction, as it is also called an upward force. The dam being a static one, the force exerted by the earth in upward direction is responsible for providing equilibrium to the dam weight. Accordingly, the normal force of the dam would be equal to the weight of the dam multiplied by the specific gravity, as given above. However, this force shall be acting in the opposite direction…..(Buttress Dams) General analysis of Buttress Dams Buttress dams use material economically as against the other types of dams, as they provide considerable reduction in the uplift load. Such dams also have the greater ability adjusting to the foundation deformation, without causing any major damage. However, the finished unit cost of building these dams is higher as this cost gets enhanced considerably due to the extensive use of non-repeatable formwork, which the building of such dams demands, although lesser quantity of concrete is used for constructing buttress dams. In addition, due to increased stresses being transmitted into the foundation of each buttress, the design parameters and the building of foundation quality concepts are stricter than those meant for building the other types of dams, like a gravity dam. However, the design parameters of buttress dam and their analysis are akin to that of gravity dams. The structural form of buttress dams should consider the confinement of uplift pressures to the buttress head, which would result in the modified uplift distribution. Accordingly, the necessity of providing pressure relief drains is minimized considerably, as they may be provided only for specific constructions. The other major advantage of such structural form would be the enhanced stability of the dam….(Nalluri, Novak & others, Hydraulic structures, pages 143-145) Building massive Buttress Dams The massive buttress dams are built with the structures of a series of independent units, where each unit has a buttress head joined with its supporting buttresses. Each unit, when measured along the axis of the dam would have a length of 12 to 15 meters. Accordingly, the structural analysis of the structure is done while considering the buttress unit as whole. Hence, sliding stability of the complete unit is determined, in terms of the shear or sliding factor. While conducting the stress analysis of the individual buttress unit, it is essential to make the finite element analysis, which would help is determining the correct shape for the buttress head as it approaches its junction with other supporting buttresses or the buttress web. The sides of this web are normally flared towards the base level, so that the contact area and, accordingly, the sliding resistance are increased. This also results in the moderation of the contact stress upon the dam foundation. The profile design of a buttress dam can have either the round head or the diamond head, as per the choice of the concerned designer. However, the profile designs should eventually meet the stress analysis results, and they may therefore require modifications, accordingly……( Nalluri, Novak & others, Hydraulic structures, pages 145-146) Cross section of a typical Buttress Dam The cross section and plan view of a typical buttress dam is shown in the figure-2, given at the end of this paper, in the Appendix. As shown in the figure, the buttress head could be flat or rounded. The buttresses would be made of concrete with reinforced steel bars, for the purpose of providing added strength and shear resistance, particularly for the big and massive buttress dams. While the particular buttress may be designed as hollow or solid, it is interesting to note that the highest concrete buttress dam in America has hollow buttresses…….(Parts of a buttress dam) Other external factors that may affect Buttress Dam design There could be any force acting on the dam from the water at downstream side, which can cause some vertical and horizontal forces on the dam, while they could be similar to such forces caused by the water on upstream side of the dam. Certain hydrostatic pressures can be experienced, internally, at the pores, cracks, seams or joints of the dam. There could be some problem with the hydration of concrete due to the temperature variations prevailing at the dam site. Due to serious temperature variations, the concrete can shrink, while it is cooling which can result in the changed dimensions of the concrete structure. However, certain remedial measures to overcome this can be taken that include pre-cooling and post-cooling treatment as well as addition of a specific retarding agent, which can cause reduced hydration or elimination of the same. Certain chemical reactions can take place in the concrete during setting or later on. These include alkali-silica and alkali-carbonate reactions, which can cause cracks in the concrete, particularly years after the dam has been constructed. As slit pressure plays an important role, while it builds up over a period on the upstream side of the dam, it can provide considerable horizontal and vertical pressure on the dam walls or sides. The designers of buttress dams should consider the ice loads, wave loads and earthquake loads, depending on the site environment, as they design these dams. The concrete can deform if it is under the pressure of any external load continuously for a longer period……………….(Cracking dams) Buttress Dams at sites prone to flood As flood waters require passing over the buttress dams, a “spill-over buttress dam” construction should be considered. In such cases, the nape of the dam should have sufficient provision of aeration, so that no vibrations or pulsations are transferred into the dam, which can cause its overstressing. At the same time transmission of such vibrations into the foundation can reduce its resistance to shear. In addition, a suitable paving like providing a concrete turbulence control wall can reduce erosion behind the buttress head or arches. In any case, the energy of the surcharge should not affect the foundation by reducing its shearing resistance to zero, while care should be taken that the surcharge does not rupture the river bed, downstream…………….(Spill-over buttress dams) Hollow buttress dams as reservoir dams Buttress dam is also called a hollow dam as the buttresses do not form a solid wall, while joining together, across the river valley. While the shape of the buttresses is either flat or curved, the upstream face of the dam is inclined by about 45 degrees. Most modern dams of this type are built with reinforced steel and concrete. A picture of “Sugar Hollow reservoir Buttress Dam“of White Hall, VA is given as figure-3 in the enclosed Annexure. Continuing with the idea of flat slab and multiple arch buttress dams, an engineer by the name of Noetzli designed the 115 feet high spillway section of the V.Carranza irrigation dam in Mexico in the year 1927, while using contiguous buttress dam design for the same. This design ensured thickening or widening of the upstream buttress heads or edges, which could make them contiguous……(contiguous Buttress Dam) Buttress dam examples While the Bartlett dam, being the tallest buttress dam built in United States is the fine example of such construction, the buttress dam built in Russia on river Zeya in the year 1978 is yet another example of large buttress dam construction as its reservoir has a capacity of holding 68,400,000 cubic meters of water. A Canadian example of buttress dam is visible at the multiple arches Daniel Johnson Dam built on river Manicougan in Quebec, which has a height of 703 feet. This dam was constructed in 1968 and it uses a total of 14 buttresses, while it has a crest length of 4297 feet…………(dam) Annexure Fig 1 (Source: http://physdams.tripod.com/physics24/id2.html ) Fig-2 (Source: http://www.simscience.org/cracks/advanced/butt_anat1.html ) Fig-3 (Source: http://www.ritchiewiki.com/wiki/index.php/Buttress_Dam ) Multiple arch dam (Source: http://www.dur.ac.uk/~des0www4/cal/dams/conc/concf22.htm ) BIBLIOGRAPHY In-text citation references and list of sources used ---(All accessed from 20to 22nd April 2010) Buttresses, available at: http://www.dur.ac.uk/~des0www4/cal/dams/conc/concf23.htm Buttress Dams, available at: http://physdams.tripod.com/physics24/id2.html Chadwick S. K.,”Introduction to Buttress Dams, Development of Dam engineering in US”, available at: http://www.dur.ac.uk/~des0www4/cal/dams/conc/concf19.htm contiguous Buttress Dam, available at: http://www.ritchiewiki.com/wiki/index.php/Buttress_Dam Cracking Dams, “What are the forces acting on a buttress dam”, available at; http://www.simscience.org/cracks/advanced/butt_fbd1.html dam. (2010). In Encyclopædia Britannica. Retrieved April 22, 2010, from Encyclopædia Britannica Online: http://www.britannica.com/EBchecked/topic/150337/dam Nalluri C, Novak Pavel & others, “Hydraulic structures, pages 143-145”, book available at: http://books.google.com/books?id=s9XSHsTctF4C&pg=PA144&lpg=PA144&dq=examples+of+Buttress+Dam&source=bl&ots=4YcGFc9bL6&sig=8x0JIVTjjQ-a_XJzmcK6Xbnwno4&hl=en&ei=maTIS7fZEceyrAex6-X1CQ&sa=X&oi=book_result&ct=result&resnum=9&ved=0CB4Q6AEwCA#v=onepage&q=examples%20of%20Buttress%20Dam&f=false Parts of a buttress dam, available at: http://www.simscience.org/cracks/advanced/butt_anat1.html Spill-over buttress dams, available at: http://www.dur.ac.uk/~des0www4/cal/dams/conc/concf25.htm Read More