"Who builds dams ???"
NATIONAL TECHNICAL UNIVERSITY OF ATHENS LABORATORY OF MATERIALS OF CONSTRUCTION Dams- materials- environmental impacts Emil. Koroneos, Professor G. Poulakos, Associate Professor G.-Fivos Sargentis, Civil Engineer December 2004 Hydrologic cycle Dams Dam failure Basic forces on a concrete dam Dam types Material testing for concrete dams Environmental impacts-case study What is a dam: A dam is a structure built across a stream, river or estuary to retain water. There is no unique way to retain water, which is why we see many different shapes for dams. Some dams are tall and thin, while others are short and thick. Dams are made from a variety of materials such as rock, steel and wood. We will concentrate on dams made from concrete, a complex material, because it is important for the construction of large dams. Who builds dams Dams are really the product of many people, but civil engineers are usually key players. What is a Societal Structure Societal structures are those whose performance - including catastrophic failure - has a large impact on a significant population. An obvious example of a societal structure is a bridge. A bridge provides a large number of people with a convenient way to cross a river. Another example is a highway, something that isn't usually thought of as a structure but also needs to be built and maintained because it can affect a large population. HYDROLOGIC CYCLE The earth's water supply remains constant, but man is capable of altering the cycle of that fixed supply. Population increases, rising living standards, and industrial and economic growth have placed greater demands on our natural environment. Our activities can create an imbalance in the hydrologic equation and can affect the quantity and quality of natural water resources available to current and future generations. DAMS Dams gather drinking water for people Dams help farmers bring water to their farms Dams help create power and electricity form water Dams keep areas form floating Dams create beautiful landscapes and space for pleasure Dams failure Over 100 years ago, a catastrophic dam failure caused thousands deaths in Harrisburg (Pennsylvania). Before the failure: and after A Tragic Display of Strength The great reserves of strength inherent in an arch dam were dramatically displayed in 1963 when the reservoir behind Vaiont Dam in Italy was virtually destroyed by a landslide. Vaiont, at that time the second highest dam in the world, was built across a narrow gorge on limestone foundations so that the crest 858 feet above the valley bottom was only 623 feet in length. Some large-scale instability in the mountainside above the reservoir had been observed earlier by the engineers during filling. They were allowed to proceed very slowly, and three years later, on Oct. 9, 1963, with filling still incomplete, about 314 million cubic yards of soil and rock slid down into the reservoir, sending a tremendous volume of water to a height of 853 feet on the opposite side of the valley. The flood overtopped the dam to a depth of 328 feet and surged down the valley, causing a major tragedy, the destruction of several villages with a large loss of life. Yet, only superficial damage was caused to the dam, which, at its crest, is about 11.2 feet thick. Basic forces on a concrete dam The main forces on a dam. • forces of the reservoir water • uplift force • weight of concrete Beside these main forces, there are many other forces that may act on a gravity dam: • there may be water on the downstream side of the dam as well; this water will have the same sort of vertical and horizontal forces on the dam as the water on the upstream side • internal hydrostatic pressure: in pores, cracks, joints, and seams • temperature variations • chemical reactions • silt pressure; silt will build up over time on the upstream side; silt provides about 1.5 times the horizontal pressure of water and twice the vertical pressure of water • ice load on the upstream side • wave load on the upstream side • earthquake loads • settlement of the foundation or abutments • other structures on top of the dam -- gates, a bridge, cars • creep of concrete: deformation of the concrete when under a constant load for a long period of time. Dam types Gravity dams Arch dams An Introduction to Gravity Dams Massive dams (material-concrete) that resist the thrust of water entirely by their own weight. Most gravity dams, like the Grand Coulee Dam in Washington, are expensive to build because they require so much concrete. Still, many people prefer its solid appearance to the thinner arch dams. Embankment dam They are massive dams made of earth and rock. Like gravity dams of concrete, embankment dams rely on their heavy weight to resist the force of the water. But embankment dams are also armed with a dense, waterproof core that prevents water from seeping through the structure. In 1850, French engineer J. Augustin Tortene de Sazilly (1812-1852) showed in a lecture that the most advantagous profile for a gravity dam is triagle with a verticle upsteam face. Sazilly also analyzed three recent French navigation dams in a paper published posthumously in 1853. He used the cross sections of these three dams to illustrate the confusion and uncertainty in the design of gravity dams. In fact, two of The Triangular Gravity Dam the dams were wrongly inclined on the upstream side! The parts of a gravity dam Gravity dams use their weight to hold back the water in the reservoir. Gravity dams can be made of earth or rock fill or concrete. In this section we will concentrate on concrete gravity dams. These dams can be very expensive because of how much material they use. Generally, the base of a concrete gravity dam is equal to approximately 0.7 times the height of the dam: base = 0.7 * height The shape of the gravity dam resembles a triangle. This is because of the triangular distribution of the water pressure. The deeper the water, the more horizontal pressure it exerts on the dam. So at the surface of the reservoir, the water is exerting no pressure and at the bottom of the reservoir, the water is exerting maximum pressure. The shape can vary slightly; any of these shapes can be used for a simulation of a gravity dam if the exact shape is not known. An Introduction to Arch Dams An arch dam is a curved dam which is dependent upon arch action for its strength. Arch dams are thinner and therefore require less material than any other type of dam. Arch dams are good for sites that are narrow and have strong abutments. The parts of the Arch dam Arch dams are usually made of concrete and are very suitable for narrow gorges with strong abutments. The gorge is often in the shape of a V; less often it is a U- shape. Arch dams use much less concrete than gravity dams. The best design is a double-curved arch. Materials: The Return of Concrete Concrete -now on the basis of Portland cement- was used for the first time since the Romans for the Boyds Corner gravity dam completed in 1872 in New York . The Lower Crystal Springs Dam in California, completed in 1890, was the first dam in which the water content of the concrete was specifically controlled. Because the cement industry was nonexistent in California, the cement had to be imported from England. In 1892 in France and in 1918 in the United States it was scientifically determined that by decreasing the amount of water and increasing the amount of cement in concrete, it became stronger. However, a minimum amount of water must be there to ensure the workability of concrete while the amount of cement may need to be limited on account of the heat it develops during the hardening process. MATERIAL TESTING Fracture Brittle Duractile Schematic representation of tensile stress-strain behaviour for brittle and duractile materials Stress loaded to fracture. Strain Many structures are made of steel and/or concrete, and both of these materials can fracture. Civil engineers use both steel and concrete, so they must be concerned with fracture. In steel, fracture is often ductile and may be due to fatigue. A fatigue crack is caused when a cyclic load is applied. You are applying a cyclic load to a soda can tab when you push it back and forth to break the tab off of the soda can. "Ductile fracture" means that the part of the material which is cracking is also deforming (its shape is changing). Think of pulling a piece of silly putty into two pieces: as you pull it apart, it deforms until you have created two new pieces. On the other hand, concrete fracture is brittle. Brittle fracture means there is no deformation around the crack; the very grains of the material are separating, or cracking themselves. Concrete is the material of choice for many, many structures: buildings, bridges, dams, to name a few. Concrete offers many advantages: low cost, good weather and fire resistance, good compressive strength, and excellent formability. Concrete is bad in tension and therefore prone to cracking. Also, it's behaviour is not so predictable. Many dams are made of concrete and have suffered from cracking, which is caused by many different forces Concrete is not found in nature the way we would find aluminium, nickel or iron. Concrete is formed from combining water, a special cement and rock: PORTLAND CEMENT + H2O + ROCK = HARDENED CONCRETE + ENERGY(HEAT) Heat? Yes, and lots of it if your concrete structure is big. The heat, and temperature variations in general, can cause cracking problems. Typical Composition by Volume Cement 7-15% Water 14-21% Aggregate 60-80% Compressive Strength Test For this test, concrete is poured into a cylinder. The cylinders are usually steel. The cylinders filled with concrete are allowed to cure at 20 degrees °C for 28 days. Then each cylinder is loaded axially in a laboratory: the cylinder is stood up in a loading machine and an arm of the machine pushes down on the top of the cylinder until the cylinder breaks. The maximum amount of force the machine used on the cylinder is important. The compressive strength of the concrete, f'c is given by: f'c=P/A where P is the load at which the cylinder failed A is the cross-sectional area of the cylinder, in this case, π*r2 where r is the radius of the cylinder Split Cylinder Test This test uses the same type of cylinders as are used in the compressive test from above. But instead of standing up in the loading machine, the cylinder lays on its side. The machine pushes down on the free side of the cylinder. The cylinder will split in two halves. Based on the load at which the cylinder split, you can compute a tensile strength, fct, of the concrete. The equation is: fct=2P/(πdL) where P is the load at which the cylinder failed d is the diameter of the cylinder and L is the length of the cylinder Permeability of materials This test uses cubes. The upper side of cube gets Insulating water with hydraulic material pressure. After 3,6,12 hours of the test we are calculating the volume of the absorbed water in the material. With this test we can have relatively results about the behavior of the materials according to water. Volume of water Better material Time Materials corrosion The more constant form of matter, and consequently the maternal raw material of structural materials are the rocks, the minerals. With this form we can see the materials in nature in combination with the local natural environment. Raw materials are found in a situation of balance, both chemical and natural. They cannot be degraded through natural activities because they have already suffered all the changes imposed to them by nature, by the local environment and the various episodes (natural reasons) such as earthquakes, volcanos’ explosions, downpours, tides, heat, etc. If we treat maternal materials and turn them into structural materials, the structural materials are energetically upgrading. With the contribution of the environment in which they are placed, the conditions are created for the awakening and their inclination for natural, mechanical, chemical, and biological changes, or as we may say: their corrosion. Factors of corrosion Humidity, rain, ice Solar radiation Electrolytes, marine electrolytes, (salts) Wind, dust, soot and harmful gases, which are mainly: Carbon dioxide Sulphur dioxide Nitrogen oxides Changes of temperature Ice cycle Icing 75 min Freezing 30 min -20 º C 0ºC 20 º C shower shower defreezing 15 min heating 30 min Change of volume Temperature Gas cycle CO2, SO2 30 min Corroded air gas 20 º C 20º C 30 min Diphase material, liquid and air phase Calculate a crack The ways to calculate a crack, how it begins, how it grows, and if it stops, are all part of the field called fracture mechanics. There are many, many ways to try to calculate a crack. In fact, people are still fighting about which way is the right way. The right way really depends on what kind of material and what type of structure you're talking about. Here we'll discuss a version of fracture mechanics which can be applied to large concrete structures, like dams. Forces There has to be some sort of force acting on a material to make it fracture. There are three major forces to consider: Temperature variation -- this can be a big problem in concrete. As you learned earlier in the section on concrete, when concrete is mixed it gets very hot and expands. When it tries to cool off, the concrete wants to contract. If it can't contract (because it is connected to another wall or rock) then cracks form. Chemical reaction -- a reaction between the cement and the aggregate in the concrete which makes the concrete "grow” Live load -- the general pulling or pushing on a material. In a dam, loads could be caused by settling or sliding of the foundation or abutments, ice, silt, waves, earthquakes, or even bombs Modes These forces are simplified into three different "modes", basically three ways the forces can act and what kind of fracture they cause Mode I: the forces are perpendicular to the crack (the crack is horizontal and the forces are vertical), pulling the crack open. This is referred to as the opening mode. What would happen if both of the forces were pushing down on the crack? Nothing. This would close the crack. Mode II: the forces are parallel to the crack. One force is pushing the top half of the crack back and the other is pulling the bottom half of the crack forward, both along the same line. This creates a shear crack: the crack is sliding along itself. It is called in-plane shear because the forces are not causing the material to move out of its original plane. In this case, what would happen if both the forces were moving in the same direction, both forward or both backward? This would not cause the crack to grow, since all of the material would be moving in the same direction. Mode III: the forces are perpendicular to the crack (the crack is in front-back direction, the forces are pulling left and right). This causes the material to separate and slide along itself, moving out of its original plane (which is why its called out-of-plane shear). The forces could also be pushing left and right and the same effect would occur. But the forces have to be moving in opposite directions in order to grow the crack. Mode I cracking happens in concrete dams. If there is an existing crack on the upstream side, water can enter and open the crack: KI and KIc How do we describe what's happening during Mode I cracking? To help us do this, there is something called a stress intensity factor, which has the symbol KI. KI is basically a measure of the likelihood that the crack will grow when the opening forces are being applied. Think of KI as the crack's money: if the crack has enough money, then it can do what it wants, which is grow. How much money the crack has, KI is based on the load on the structure and the initial size of the crack. Now KIc is a material's resistance to crack growth. KIc tells you when the material will allow a crack to grow. This is a property which can be measured in a laboratory. Energy Another way to think about cracks is in terms of energy. What kind of energy and where does it come from? When a material is being pulled, like this one, the material gets some energy. When a dam is being pushed on by the reservoir, the dam must do work to hold the water back. This work is the stored energy in the dam. Cracks use up some of this stored energy when they grow in a dam Cracks in dams RCC dams The construction of dam using Roller Compacted Concrete (RCC) technology with high doses of fly ash. Approximately 60-70% cement is being replaced by fly ash. The dams constructed under this project using RCC technology with high doses of fly ash, includes dams of height about 90m. Coal Creek Station, owned by Great River Energy (Elk River) is located near Falkirk Mine in Underwood, N.D. It is one of the largest lignite-fired generating plants in North America with 650-foot tall chimney (almost as tall as the IDS Tower in downtown Minneapolis) CASE STUDY: N. Plastiras Lake Introduction The lake, situated in a mountainous part of Western Thessaly, is a multipurpose reservoir used for hydropower, irrigation, water supply and recreation. These competitive uses raise various conflicts between groups of different interests (farmers, residents, ecologists, hotel owners). Water basin The dam Characteristics of the water basin and the reservoir Basin area 161.3 km2 Maximum basin altitude 2140 m, mean altitude 1459 m Mean annual runoff 147 hm3 (1029 mm) Arch dam, height 83 m Reservoir capacity 362 hm3, net capacity 286 hm3, maximum area 25 km2 Intake level +776 m, spill level +792 m Installed power capacity 130 MW hydraulic head 577 m Annual water demand 160 hm3 (145 hm3 for irrigation, 15 hm3 for water supply) Reservoir operation analysis through a stochastic simulation approach Water quality analysis Landscape analysis Reservoir level at +783 m Level at +792 m (= spill level, photo digitally processed) Reservoir level at +783 m Level at +792 m (= spill level, photo digitally processed) Field research Photographs were taken systematically from specified observation positions so that we could estimate the changes of the landscape following the water- balance of the lake. Other problems to the landscape Road design Time programming of traveler Point of observation of traveler Residential development As the concept of the following statue presents: Nature will wriggle out of any norm. Environment-Ecology and morality “We must be the leaders and the owners of the nature” Rene Descartes (1596-1650) “We must learn to respect the earth of our children This respect must be our new morality” Friedrich Nietzsche (1844 – 1900) “Our society is abusive and bedrock foolish “ K. Kastoriadis (1922-1997) “It is rather immoral to wait for a building to take fire in order to have the criteria on how to study the new provisions of safety” D. Canter