A Two-Way Slab “TWS” is simply a slab carrying loads in both directions (length and width) to its supports (Beams or Columns). This is because it’s length to width ratio is (Long/Short) not big enough for loads to travel in the short direction only (width).
Theoretically speaking:
Loads are lazy, they travel in the shortest path possible, they like to spend the least effort.
Slabs have 4 supports to transfer its loads to (2 in each direction).
Slab: A structural element that has large width and length compared to its thickness. Designed to carry whatever loads the building is meant to carry, and transfer it to its the supporting elements.
Loads: Slab’s own weight, Slab covering and finish, live loads (people, furniture, machines…etc) etc.
Supports: Beams or Columns that the Slab is resting on at its 4 edges (could be on . The slab transfers its loads to the supports > Supports transfer its loads to the foundations on the ground > Foundations transfer all these loads to the grounds below the building.
That being said, let’s assume a (10 x 2) Rectangular Slab (width=2m and length=10m) the ratio is large enough (Long/Short = 10/2 = 5) in this case all the loads will travel much faster through the Short Direction which is the 2m width to the nearest supports.
This makes a One Way Slab “OWS”. On this slab loads will not travel in the Long Direction (the 10m length). The slab will carry 100% of the loads to half of its supports through one direction only. (supports on the other direction will not receive any loads from the slab, therefore these slabs can have 2 supports only at the short direction).
The Length is larger than Double the Width (Long/Short > 2)
100% of the Loads Travel in the Short Direction only.
Only the 2 Supports on this Short Direction will carry the slab and 100% of its the loads.
We need Reinforcement (steel bars or other) in this direction only to resist the straining actions caused by loads traveling through this direction to the supports.
We will study and design based on one direction only.
Slab thickness is design based on short direction only.
Now assume a (5 x 5) Square Slab with ratio (Long/Short = 5/5 =1), in this case loads will travel in both directions equally, since Long=Short they will spend almost equal effort. Each direction will carry 50% of the loads to its supports.
This makes a Two Way Slab “TWS”. In this case all 4 supports are working and they receive almost equal loads from the slab. (The Slab must be supported from all directions).
The Length is equal to the Width (Long/Short = 1)
Loads Travel in Both Directions. 50% of the total loads through each direction.
All supports (2 pairs in each direction) will carry half of the slab and its loads.
We need equal reinforcement in both directions.
We will study two directions, but since they are equal, we can design one direction and apply the same to the other.
Slab thickness is design based on either direction, since they are equal.
If we have something in between the above two cases, assume a (4×6) Rectangular slab (width=4m, length=6m) Which direction will the loads travel through?
The ratio is (Long/Short = 6/4 = 1.5) which is smaller than double the width (1.5 < 2) which means this a “TWS” as well.
The number we compare to the length to width ratio (2) might be different depending on the Application and the Code we are following (which is based on studies and experiments), but here we assume that the breakpoint is the length being over or under double the width.
So this is also a “TWS” where we predict loads travel in both directions, but will each direction carry 50% of the loads? We know that one direction is shorter than the other, therefore more of our loads will prefer to travel through the easier-shorter direction, and less will travel through the longer direction. Supports will receive a percentage of the Slab loads based on the distance it has to travel.
The Length is smaller than Double the Width (Long/Short <2)
Loads Travel in Both Directions. The short direction transfers a larger percentage of the loads, the long direction transfers a smaller percentage of the loads.
All support (2 pairs in each direction) will carry the slab, but each pair will receive different percentage of the loads.
We need more reinforcement in the short direction (larger percentage of loads > larger bending moment) than we need in the long direction (smaller percentage of loads > smaller bending moment)
We will study and design each direction separately (different loads, different bending moments, different reinforcement)
Slab thickness is designed based on the worst case, the larger straining actions we calculate from the Short or Long direction (most probably the short direction)
The exceptional durability of portland cement concrete is a major reason why it is the world’s most widely used construction material. But material limitations, design and construction practices, and severe exposure conditions can cause concrete to deteriorate, which may result in aesthetic, functional, or structural problems.
Concrete can deteriorate for a variety of reasons, and concrete damage is often the result of a combination of factors. The following summary discusses potential causes of concrete deterioration and the factors that influence them.
Corrosion of reinforcing steel and other embedded metals is the leading cause of deterioration in concrete. When steel corrodes, the resulting rust occupies a greater volume than the steel. This expansion creates tensile stresses in the concrete, which can eventually cause cracking, delamination, and spalling (Figs. 1 and 2).
Fig. 1. Corrosion of reinforcing steel is the most common cause of concrete deterioration.
Fig. 2. The expansion of corroding steel creates tensile stresses in the concrete, which can cause cracking,
delamination, and spalling.
Steel corrodes because it is not a naturally occurring material.Rather, iron ore is smelted and refined to produce steel. The production steps that transform iron ore into steel add energy to the metal. Steel, like most metals except gold and platinum, is thermodynamically unstable under normal atmospheric conditions and will release energy and revert back to its natural state — iron oxide, or rust. This process is called corrosion.
For corrosion to occur, four elements must be present: There must be at least two metals (or two locations on a single metal) at different energy levels, an electrolyte, and a metallic connection. In reinforced concrete, the rebar may have many separate areas at different energy levels. Concrete acts as the electrolyte, and the metallic connection is provided by wire ties, chair supports, or the rebar itself.
a – Concrete and the Passivating Layer
Although steel’s natural tendency is to undergo corrosion reactions, the alkaline environment of concrete (pH of 12 to 13) provides steel with corrosion protection. At the high pH, a thin oxide layer forms on the steel and prevents metal atoms from dissolving. This passive film does not actually stop corrosion; it reduces the corrosion rate to an insignificant level. For steel in concrete, the passive corrosion rate is typically 0.1 μm per year.
Without the passive film, the steel would corrode at rates at least 1,000 times higher (ACI 222 2001). Because of concrete’s inherent protection, reinforcing steel does not corrode in the majority of concrete elements and structures.
However, corrosion can occur when the passivating layer is destroyed. The destruction of the passivating layer occurs when the alkalinity of the concrete is reduced or when the chloride concentration in concrete is increased to a certain level.
b – The Role of Chloride Ions
Exposure of reinforced concrete to chloride ions is the primary cause of premature corrosion of steel reinforcement. The intrusion of chloride ions, present in deicing salts and seawater, into reinforced concrete can cause steel corrosion if oxygen and moisture are also available to sustain the reaction (Fig. 3). Chlorides dissolved in water can permeate
through sound concrete or reach the steel through cracks.
Fig. 3. Deicing salts are a major cause of corrosion of reinforcing steel in concrete.
Chloride-containing admixtures can also cause corrosion. No other contaminant is documented as extensively in the literature as a cause of corrosion of metals in concrete than chloride ions. The mechanism by which chlorides promote corrosion is not entirely understood, but the most popular theory is that chloride ions penetrate the protective oxide film easier than do other ions, leaving the steel vulnerable to corrosion.
c – Carbonation
Carbonation occurs when carbon dioxide from the air penetrates the concrete and reacts with hydroxides, such as calcium hydroxide, to form carbonates. In the reaction with calcium hydroxide, calcium carbonate is formed:
Ca(OH)2 + CO2 → CaCO3 + H2O
This reaction reduces the pH of the pore solution to as low as 8.5, at which level the passive film on the steel is not stable. Carbonation is generally a slow process. In high-quality concrete, it has been estimated that carbonation will proceed at a rate up to 1.0 mm (0.04 in.) per year. The amount of carbonation is significantly increased in concrete with a high water-to-cement ratio, low cement content, short curing period, low strength, and highly permeable or porous paste.
Carbonation is highly dependent on the relative humidity of the concrete. The highest rates of carbonation occur when the relative humidity is maintained between 50% and 75%. Below 25% relative humidity, the degree of carbonation that takes place is considered insignificant. Above 75% relative humidity, moisture in the pores restricts CO2 penetration (ACI 201 1992).
Carbonation-induced corrosion often occurs on areas of building facades that are exposed to rainfall, shaded from sunlight, and have low concrete cover over the reinforcing steel (Fig. 5).
Fig. 4. Carbonation-induced corrosion often occurs on building facades with shallow concrete cover.
d – Dissimilar Metal Corrosion
When two different metals, such as aluminum and steel, are in contact within concrete, corrosion can occur because each metal has a unique electrochemical potential. A familiar type of dissimilar metal corrosion occurs in an ordinary flashlight battery. The zinc case and carbon rod are the two metals, and the moist paste acts as the electrolyte. When the carbon and zinc are connected by a wire, current flows. In reinforced concrete, dissimilar metal corrosion can occur in balconies where embedded aluminum railings are in contact with the reinforcing steel.
Below is a list of metals in order of electrochemical activity:
Zinc / Aluminum / Steel / Iron / Nickel / Tin / Lead / Brass / Copper / Bronze / Stainless Steel / Gold
When the metals are in contact in an active electrolyte, the less active metal (lower number) in the series corrodes.
2. FREEZE-THAW DETERIORATION
When water freezes, it expands about 9%. As the water in moist concrete freezes, it produces pressure in the capillaries and pores of the concrete. If the pressure exceeds the tensile strength of the concrete, the cavity will dilate and rupture. The accumulative effect of successive freeze-thaw cycles and disruption of paste and aggregate can eventually cause significant expansion and cracking, scaling, and crumbling of the concrete (Fig. 5).
The resistance of concrete to freezing and thawing in a moist condition is significantly improved by the use of intentionally entrained air. Entrained air voids act as empty chambers in the paste for the freezing and migrating water to enter, thus relieving the pressure in the capillaries and pores and preventing damage to the concrete.
Concrete with low permeability is also better able resist the penetration of water and, as a result, performs better when
exposed to freeze-thaw cycles. The permeability of concrete is directly related to its water-to-cement ratio—the lower the water-to-cement ratio, the lower the permeability of the concrete.
Fig 5. Freeze-thaw cycles can cause scaling of concrete surfaces
a – Deicer Scaling
Deicing chemicals used for snow and ice removal, such as sodium chloride, can aggravate freeze-thaw deterioration. The additional problem caused by deicers is believed to be a buildup of osmotic and hydraulic pressures in excess of the normal hydraulic pressures produced when water in concrete freezes. In addition, because salt absorbs moisture, it keeps the concrete more saturated, increasing the potential for freeze-thaw deterioration. However, properly designed and placed air-entrained concrete can withstand deicers for many years.
In the absence of freezing, sodium chloride has little to no chemical effect on concrete. Weak solutions of calcium chloride generally have little chemical effect on concrete, but studies have shown that concentrated calcium chloride solutions can chemically attack concrete. Magnesium chloride deicers have come under recent criticism for aggravating scaling. One study found that magnesium chloride, magnesium acetate, magnesium nitrate, and calcium chloride are more damaging to concrete than sodium chloride (Cody, Cody, Spry, and Gan 1996). Deicers containing ammonium nitrate and ammonium sulfate should be prohibited because they rapidly attack and disintegrate concrete.
b – Aggregate Expansion
Some aggregates may absorb so much water (to critical saturation) that they cannot accommodate the expansion and
hydraulic pressure that occurs during the freezing of water. The result is expansion of the aggregate and possible disintegration of the concrete if enough of the offending particles are present. If a problem particle is near the surface of the concrete, it can cause a popout.
D-cracking is a form of freeze-thaw deterioration that has been observed in some pavements after three or more years of service. Due to the natural accumulation of water in the base and subbase of pavements, the aggregate may eventually become saturated. Then with freezing and thawing cycles, cracking of the concrete starts in the saturated aggregate at the bottom of the slab and progresses upward until it reaches the wearing surface. D-cracking usually starts near pavement joints.
Aggregate freeze-thaw problems can often be reduced by either selecting aggregates that perform better in freeze-thaw cycles or, where marginal aggregates must be used, reducing the maximum particle size.
Fig 6. D-cracking is a form of freeze-thaw deterioration that has been observed in some pavements after three or more
years of service.
3. CHEMICAL ATTACK
Concrete performs well when exposed to various atmospheric conditions, water, soil, and many other chemical exposures. However, some chemical environments can deteriorate even high-quality concrete. Concrete is rarely, if ever, attacked by solid, dry chemicals. To produce significant attack on concrete, aggressive chemicals must be in solution and above some minimum concentration.
a – Acids
In general, portland cement concrete does not have good resistance to acids. In fact, no hydraulic cement concrete, regardless of its composition, will hold up for long if exposed to a solution with a pH of 3 or lower. However, some weak acids can be tolerated, particularly if the exposure is occasional.
Acids react with the calcium hydroxide of the hydrated portland cement. In most cases, the chemical reaction forms water-soluble calcium compounds, which are then leached away by aqueous solutions (ACI 201 1992).
The products of combustion of many fuels contain sulfurous gases which combine with moisture to form sulfuric acid. Also, certain bacteria convert sewage into sulfuric acid. Sulfuric acid is particularly aggressive to concrete because the calcium sulfate formed from the acid reaction will also deteriorate concrete via sulfate attack (Fig. 7).
Fig. 7. Bacteria in sewage systems can produce sulfuric acid, which aggressively attacks concrete
In addition to individual organic and mineral acids which may attack concrete, acid-containing or acid-producing substances, such as acidic industrial wastes, silage, fruit juices, and sour milk, will also cause damage.
Animal wastes contain substances which may oxidize in air to form acids which attack concrete. The saponification reaction between animal fats and the hydration products of portland cement consumes these hydration products, producing salts and alcohols, in a reaction analogous to that of acids. Acid rain, which often has a pH of 4 to 4.5, can slightly etch concrete, usually without affecting the performance of the exposed surface.
Any water that contains bicarbonate ion also contains free carbon dioxide, a part of which can dissolve calcium carbonate unless saturation already exists. This part is called the “aggressive carbon dioxide.” Water with aggressive carbon dioxide acts by acid reaction and can attack concrete and other portland cement products whether or not they are carbonated.
Calcium-absorptive acidic soil can attack concrete, especially porous concrete. Even slightly acidic solutions that are lime-deficient can attack concrete by dissolving calcium from the paste, leaving behind a deteriorated paste consisting primarily of silica gel.
To prevent deterioration from acid attack, portland cement concrete generally must be protected from acidic environments with surface protective treatments. Unlike limestone and dolomitic aggregates, siliceous aggregates are acid-resistant and are sometimes specified to improve the chemical resistance of concrete, especially with the use of chemical-resistant cement. Properly cured concrete with reduced permeability experience a slightly lower rate of attack from acids.
b – Salts and Alkalis
The chlorides and nitrates of ammonium, magnesium, aluminum, and iron all cause concrete deterioration, with those of ammonium producing the most damage. Most ammonium salts are destructive because, in the alkaline environment of concrete, they release ammonia gas and hydrogen ions. These are replaced by dissolving calcium hydroxide from the concrete. The result is a leaching action, much like acid attack. Strong alkalies (over 20 percent) can also cause concrete disintegration (ACI 515 1979).
c – Sulfate Attack
Naturally occurring sulfates of sodium, potassium, calcium, or magnesium are sometimes found in soil or dissolved in ground-water. Sulfates can attack concrete by reacting with hydrated compounds in the hardened cement. These reactions can induce sufficient pressure to disrupt the cement paste, resulting in loss of cohesion and strength.
Calcium sulfate attacks calcium aluminate hydrate and forms ettringite. Sodium sulfate reacts with calcium hydroxide and calcium aluminate hydrate forming ettringite and gypsum. Magnesium sulfate attacks in a manner similar to sodium sulfate and forms ettringite, gypsum, and brucite (magnesium hydroxide). Brucite forms primarily on the concrete surface, consumes calcium hydroxide, lowers the pH of the pore solution, and then decomposes the calcium silicate hydrates.
Environmental conditions have a great influence on sulfate attack. The attack is greater in concrete exposed to wet/dry
cycling (Fig. 8). When water evaporates, sulfates can accumulate at the concrete surface, increasing in concentration
and their potential for causing deterioration.
Fig 8. The bases of these concrete posts have suffered from sulfate attack
Porous concrete is susceptible to weathering caused by salt crystallization. Examples of salts known to cause weathering of field concrete include sodium carbonate and sodium sulfate (laboratory studies have also related saturated solutions of calcium chloride and other salts to concrete deterioration). Under drying conditions, salt solutions can rise to the surface by capillary action and, as a result of surface evaporation, the solution phase
becomes supersaturated and salt crystallization occurs, sometimes generating pressures large enough to cause cracking and scaling (Mehta 2000).
Sulfate attack is a particular problem in arid areas, such as the Northern Great Plains and parts of the Western United States. Seawater also contains sulfates but is not as severe an exposure as sulfates in groundwater.
4. ALKALI-AGGREGATE REACTIVITY
In most concrete, aggregates are more or less chemically inert. However, some aggregates react with the alkali hydroxides in concrete, causing expansion and cracking over a period of years. This alkali-aggregate reactivity has two forms—alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). ASR is of more concern than ACR because aggregates containing reactive silica materials are more common.
a – Alkali-Silica Reactivity
Aggregates containing certain forms of silica will react with alkali hydroxide in concrete to form a gel that swells as it draws water from the surrounding cement paste or the environment. In absorbing water, these gels can swell and induce enough expansive pressure to damage concrete:
1. Alkalies + Reactive Silica → Gel Reaction Product
2. Gel Reaction Product + Moisture → Expansion
Typical indicators of alkali-silica reactivity are map (random pattern) cracking and, in advanced cases, closed joints and spalled concrete surfaces (Fig. 9). Cracking usually appears in areas with a frequent supply of moisture, such as close to the waterline in piers, from the ground behind retaining walls, near joints and free edges in pavements, or in piers or columns subject to wick action.
Because sufficient moisture is needed to promote destructive expansion, alkali-silica reactivity can be significantly reduced by keeping the concrete as dry as possible. The reactivity can be virtually stopped if the internal relative humidity of the concrete is kept below 80%. In most cases, however, this condition is difficult to achieve and maintain. Warm seawater, due to the presence of dissolved alkalies, can particularly aggravate alkali-silica reactivity.
Fig. 9. Typical indicators of alkali-silica reactivity are map cracking and, in advanced cases, closed joints and spalled
concrete surfaces
b – Alkali-Carbonate Reactivity
Reactions observed with certain dolomitic rocks are associated with alkali-carbonate reaction (ACR). Dedolomitization, or the breaking down of dolomite, is normally associated with expansive alkali-carbonate reactivity. This reaction and subsequent crystallization of brucite may cause considerable expansion.
The deterioration caused by alkali-carbonate reaction is similar to that caused by alkali-silica reaction (Fig. 10); however, alkali-carbonate reaction is relatively rare because aggregates susceptible to this reaction are less common and are usually unsuitable for use in concrete for other reasons, such as strength potential.
Fig. 10. Map cracking pattern caused by alkali-carbonate reactivity.
5. ABRASION/EROSION
Abrasion damage occurs when the surface of concrete is unable to resist wear caused by rubbing and friction. As the outer paste of concrete wears, the fine and coarse aggregate are exposed and abrasion and impact will cause additional degradation that is related to aggregate-to-paste bond strength and hardness of the aggregate.
Although wind-borne particles can cause abrasion of concrete, the two most damaging forms of abrasion occur on vehicular traffic surfaces and in hydraulic structures, such as dams, spillways, and tunnels.
a – Traffic Surfaces
Abrasion of floors and pavements may result from production operations or vehicular traffic. Many industrial floors are subjected to abrasion by steel or hard rubber wheeled traffic, which can cause significant rutting.
Tire chains and studded snow tires cause considerable wear to concrete surfaces (Fig. 11). In the case of tire chains, wear is caused by flailing and scuffing as the rotating tire brings the metal in contact with the concrete surface.
Fig 11. Tire chains and studded snow tires can cause considerable wear to concrete surfaces
b – Hydraulic Structures
Abrasion damage in hydraulic structures is caused by the abrasive effects of waterborne silt, sand, gravel, rocks, ice, and other debris impinging on the concrete surface. Although high-quality concrete can resist high water velocities for many years with little or no damage, the concrete may not withstand the abrasive action of debris grinding or repeatedly impacting on its surface.
In such cases, abrasion erosion ranging from a few millimeters (inches) to several meters (feet) can result, depending on flow conditions. Spillway aprons, stilling basins, sluiceways, drainage conduits or culverts, and tunnel linings are particularly susceptible to abrasion erosion. Abrasion erosion is readily recognized by its smooth, worn appearance, which is distinguished from the small holes and pits formed by cavitation erosion.
As is the case with traffic wear, abrasion damage in hydraulic structures can be reduced by using strong concrete with hard aggregates. Cavitation is the formation of bubbles or cavities in a liquid. In hydraulic structures, the liquid is water and the cavities are filled with water vapor and air. The cavities form where the local pressure drops to a value that will cause the water to vaporize at the prevailing fluid temperature. Cavitation damage is produced when the vapor cavities collapse, causing very high instantaneous pressures that impact on the concrete surfaces, causing pitting, noise, and vibration.
6. FIRE/HEAT
Concrete performs exceptionally well at the temperatures encountered in almost all applications. But when exposed to fire or unusually high temperatures, concrete can lose strength and stiffness (Fig. 12).
Fig. 12. When exposed to fire or unusually high temperatures, concrete can lose strength and stiffness
Numerous studies have found the following general trends:
• Concrete that undergoes thermal cycling suffers greater loss of strength than concrete that is held at a constant temperature, although much of the strength loss occurs in the first few cycles. This is attributed to incompatible dimensional changes between the cement paste and the aggregate.
• Concrete that is under design load while heated loses less strength than unloaded concrete, the theory being that
imposed compressive stresses inhibit development of cracks that would be free to develop in unrestrained concrete.
• Concrete that is allowed to cool before testing loses more compressive strength than concrete that is tested hot. Concrete loses more strength when quickly cooled (quenched) from high temperatures than when it is allowed to cool
gradually.
• Concrete containing limestone and calcareous aggregates performs better at high temperatures than concrete containing siliceous aggregates (Abrams 1956). One study showed no difference in the performance of dolostone and limestone (Carette 1982). Another study showed the following relative aggregate performance, from best to worst: firebrick, expanded shale, limestone, gravel, sandstone and expanded slag.
• Proportional strength loss is independent of compressive strength of concrete.
• Concrete with a higher aggregate-cement ratio suffers less reduction in compressive strength; however, the opposite is true for modulus of elasticity. The lower the water-cement ratio, the less loss of elastic modulus.
• If residual water in the concrete is not allowed to evaporate, compressive strength is greatly reduced. If heated too quickly, concrete can spall as the moisture tries to escape.
7. RESTRAINT TO VOLUME CHANGES
Concrete changes slightly in volume for various reasons, the most common causes being fluctuations in moisture content and temperature. Restraint to volume changes, especially contraction, can cause cracking if the tensile stresses that develop exceed the tensile strength of the concrete.
a – Plastic Shrinkage Cracking
When water evaporates from the surface of freshly placed concrete faster than it is replaced by bleed water, the surface concrete shrinks. Due to the restraint provided by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks of varying depth (Fig. 12). These cracks are often fairly wide at the surface.
Fig. 13. Plastic shrinkage cracks can occur when water evaporates from the surface faster than it is replaced by bleedwater
Plastic shrinkage cracks can be prevented by taking measures to prevent rapid water loss from the concrete surface. Fog nozzles, plastic sheeting, windbreaks, and sunshades can all be used to prevent excessive evaporation.
Because almost all concrete is mixed with more water than is needed to hydrate the cement, much of the remaining water evaporates, causing the concrete to shrink. Restraint to shrinkage, provided by the subgrade, reinforcement, or another part of the structure, causes tensile stresses to develop in the hardened concrete. Restraint to drying shrinkage is the most common cause of concrete cracking.
In many applications, drying shrinkage cracking is inevitable. Therefore, control joints are placed in concrete to predetermine the location of drying shrinkage cracks. Drying shrinkage can be limited by keeping the water content of concrete as low as possible and maximizing the coarse aggregate content.
c – Thermal Cracking
Concrete expands when heated and contracts when cooled. An average value for the thermal expansion of concrete is about 10 millionths per degree Celcius (5.5 millionths per degree Fahrenheit). This amounts to a length change of 5 mm for 10 m of concrete ( 2 ⁄ 3 in. for 100 ft of concrete) subjected to a rise or fall of 50°C (90°F).
Thermal expansion and contraction of concrete varies with factors such as aggregate type, cement content, water-cement ratio, temperature range, concrete age, and relative humidity. Of these, aggregate type has the greatest influence.
Designers should give special consideration to structures in which some portions of the structure are exposed to temperature changes, while other portions are partially or completely protected. Allowing for movement by using properly designed expansion or isolation joints and correct detailing will help minimize the effects of temperature variations.
Free topography Data Sources – Digital Elevation Models
1. Space Shuttle Radar Topography Mission (SRTM)
NASA only needed 11 days to capture Shuttle Radar Topography Mission (SRTM) 30-meter digital elevation model. Back in February 2000, the Space Shuttle Endeavour launched with the SRTM payload.
Using two radar antennas and a single pass, it collected sufficient data to generate a digital elevation model using a technique known as interferometric synthetic aperture radar (inSAR). C-Band penetrated canopy cover to the ground better but SRTM still struggled in sloping regions with foreshortening, layover and shadow.
In late 2014, the United States government released the highest resolution SRTM DEM to the public. This 1-arc second global digital elevation model has a spatial resolution of about 30 meters. Also, it covers most of the world with absolute vertical height accuracy of less than 16m.
Below, shaded relief images of deeply eroded volcanic terrain in northeast Tanzania demonstrate the improved nature of the highest-resolution SRTM data now being released. The image at left has data samples spaced every 90 meters (295 feet); the image at right has samples spaced every 30 meters (98 feet).
2. ASTER Global Digital Elevation Model
ASTER GDEM is an easy-to-use, highly accurate DEM covering all the land on earth, and available to all users regardless of size or location of their target areas.
Anyone can easily use the ASTER GDEM to display a bird’s-eye-view map or run a flight simulation, and this should realize visually sophisticated maps. By utilizing the ASTER GDEM as a platform, institutions specialized in disaster monitoring, hydrology, energy, environmental monitoring etc. can perform more advanced analysis.
The ASTER Global Digital Elevation Model (ASTER GDEM) is a joint product developed and made available to the public by the Ministry of Economy, Trade, and Industry (METI) of Japan and the United States National Aeronautics and Space Administration (NASA). It is generated from data collected from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), a spaceborne earth observing optical instrument.
The first version of the ASTER GDEM, released in June 2009, was generated using stereo-pair images collected by the ASTER instrument onboard Terra. ASTER GDEM coverage spans from 83 degrees north latitude to 83 degrees south, encompassing 99 percent of Earth’s landmass.
The improved GDEM V2 (released October 17, 2011) adds 260,000 additional stereo-pairs, improving coverage and reducing the occurrence of artifacts. The refined production algorithm provides improved spatial resolution, increased horizontal and vertical accuracy, and superior water body coverage and detection. The ASTER GDEM V2 maintains the GeoTIFF format and the same gridding and tile structure as V1, with 30-meter postings and 1 x 1 degree tiles.
Version 2 shows significant improvements over the previous release. However, users are advised that the data contains anomalies and artifacts that will impede effectiveness for use in certain applications. The data are provided “as is,” and neither NASA nor METI/Japan Space Systems (J-spacesystems) will be responsible for any damages resulting from use of the data.
3. JAXA’s Global ALOS 3D World
ALOS World 3D is a 30-meter resolution digital surface model (DSM) captured by the Japan Aerospace Exploration Agency’s (JAXA). Recently, this DSM has been made available to the public.
The neat thing about is that it is the most precise global-scale elevation data now. It uses the Advanced Land Observing Satellite “DAICHI” (ALOS) based on stereo mapping from PRISM.
JAXA has been processing about 100 digital 3D maps per month as part of our engineering validation activities of DAICHI so for. As we conducted research and development for full automatic and mass processing map compilations, we now have a perspective to process 150,000 maps per month. By applying our research and development results, we will start the 3D map processing in March 2014 to complete the global 3D map in March 2016. JAXA will commission the compiling work and service provision to NTT DATA Corporation and Remote Sensing Technology Center (RESTEC), Japan.
In order to popularize the utilization of the 3D map data, JAXA will also prepare global digital elevation model (DEM) with lower spatial resolution (of about 30 meters under our current plan) to publish it as soon as it is ready. Its use will be free of charge. We expect that data from Japan will become the base map for all global digital 3D maps, and contribute to the expansion of satellite data utilizations and the industrial promotion, science and research activities as well as the Group on Earth Observations.
4. Light Detection and Ranging (LiDAR)
You might think that finding LiDAR is a shot in the dark.
But it’s not anymore.
Slowly and steadily, we are moving towards a global LiDAR map.
With Open Topography topping the list at #1, we’ve put together a list of some of the 6 best LiDAR data sources available online for free.
Because nothing beats LiDAR for spatial accuracy. After you filter ground returns, you can build an impressive DEM from LiDAR.
And if you still can’t find anything in the link above, try your local or regional government. If you tell them what you are using it for, they sometimes hand out LiDAR for free.
Construction Equipment Earthwork & Soil Compaction
1.Cable Excavator
Cable excavators are large earthmoving machines used for heavy excavation of materials with the use of cables or wire ropes. Almost obsolete, cable excavators were once used in mining and some construction applications and usually consisted of a variety of attachments that could transform it into a backhoe, skimmer, dragline, and more. Aside from a few companies that still manufacture them, cable excavators have been replaced by hydraulic excavators due to their cheaper costs, easier operation and faster mobility.
2. Hydraulic excavators (slewing excavators)
A hydraulic excavator (digger) is a large vehicle that is designed for excavation and demolition purposes. Hydraulic excavators consist of a chassis, boom, and bucket, and move via tracks or wheels. They range in size and function, an example of which is the similar but smaller “mini excavator.” All versions are generally designed for the same purposes. Hydraulic excavators weigh between 3,000 and 2 million pounds and their speed ranges between 19 HP and 4,500 HP.
3.Backhoe excavators
Backhoe is another widely used equipment which is suitable for multiple purposes. The name itself telling that the hoe arrangement is provided on the back side of vehicle while loading bucket is provided in the front.
This is well useful for excavating trenches below the machine level and using front bucket loading, unloading and lifting of materials can be done.
4. Bulldozers (dozers)
A bulldozer is a crawler (continuous tracked tractor) equipped with a substantial metal plate (known as a blade) used to push large quantities of soil, sand, rubble, or other such material during construction or conversion work and typically equipped at the rear with a claw-like device (known as a ripper) to loosen densely compacted materials.
Bulldozers can be found on a wide range of sites, mines and quarries, military bases, heavy industry factories, engineering projects and farms.
5. Scrapers
Scraper, in engineering, machine for moving earth over short distances (up to about two miles) over relatively smooth areas. Either self-propelled or towed, it consists of a wagon with a gate having a bladed bottom. The blade scrapes up earth as the wagon pushes forward and forces the excavated material into the wagon. When the wagon is filled, the gate is closed, and the material is carried to the place of disposal. The scraper is the dominant tool in highway construction.
6. Graders
Grader, in excavation, precision finishing vehicle for final shaping of surfaces on which pavement will be placed. Between its front and rear wheels a grader carries a broad mechanically or hydraulically controlled blade that can be extended from either side. Either end of the blade can be raised or lowered. Graders may be used for shallow ditching, but most models are used to assist other earth-moving equipment and to smooth roads, fills, and cuts.
7. Compactors
A compactor is a machine or mechanism used to reduce the size of material such as waste material or bio mass through compaction. A trash compactor is often used by a home or business to reduce the volume of trash it produces.
A landslide is defined as the movement of a mass of rock, debris, or earth down a slope due to gravity. The materials may move by falling, toppling, sliding, spreading, or flowing.
Landslide Animation:
What causes a landslide?
Almost every landslide has multiple causes. Slope movement occurs when forces acting down-slope (mainly due to gravity) exceed the strength of the earth materials that compose the slope. Landslides can be triggered by rainfall, snowmelt, changes in water level, stream erosion, changes in ground water, earthquakes, volcanic activity, disturbance by human activities, or any combination of these factors.
What are submarine landslides?
Earthquake shaking and other factors can also induce landslides underwater. These landslides are called submarine landslides. Submarine landslides sometimes cause tsunamis that damage coastal areas.
Where do landslides occur?
Landslides in the United States occur in all 50 States. The primary regions of landslide occurrence and potential are the coastal and mountainous areas of California, Oregon, and Washington, the States comprising the intermountain west, and the mountainous and hilly regions of the Eastern United States. Alaska and Hawaii also experience all types of landslides.
How fast do landslide travel?
Landslides can move slowly, (millimeters per year) or can move quickly and disastrously, as is the case with debris flows. Debris flows can travel down a hillside at speeds up to 200 miles per hour (more commonly, 30 – 50 miles per hour), depending on the slope angle, water content, volume of debris, and type of earth and debris in the flow. These flows are initiated by heavy periods of rainfall, but sometimes can happen as a result of short bursts of concentrated rainfall or other factors in susceptible areas. Burned areas charred by wildfires are particularly susceptible to debris flows, given certain soil characteristics and slope conditions.
Why study landslides?
Landslides are a serious geologic hazard. It is estimated that in the United States they cause in excess of $1 billion in damages and from about 25 to 50 deaths each year. Globally, landslides cause billions of dollars in damages and thousands of deaths and injuries each year.
Who is most at risk for landslides?
As people move into new areas of hilly or mountainous terrain, it is important to understand the nature of their potential exposure to landslide hazards, and how cities, towns, and counties can plan for land-use, engineering of new construction and infrastructure, and other measures which will reduce the costs of living with landslides. Although the physical causes of many landslides cannot be removed, geologic investigations, good engineering practices, and effective enforcement of land-use management regulations can reduce landslide hazards.
Do human activities cause landslides?
Yes, in some cases human activities can be a contributing factor in causing landslides. Many human-caused landslides can be avoided or mitigated. They are commonly a result of building roads and structures without adequate grading of slopes, of poorly planned alteration of drainage patterns, and of disturbing old landslides.
Where can I find landslide information for my area?
The USGS National Landslide Information Center (NLIC) is a part of the U.S. Geological Survey Landslide Hazards Program that collects and distributes all forms of information related to landslides. The NLIC is designed to serve landslide researchers, geotechnical practitioners engaged in landslide stabilization, and anyone else concerned in any way with landslide education, hazard, safety, and mitigation. Every state in the US has a geoscience agency and most have some landslide information. The Association of American State Geologists provides links to the State Geologist for every state.
What was the most expensive landslide to fix in the United States?
The Thistle, Utah, landslide cost in excess of $200 million dollars to fix. The landslide occurred during the spring of 1983, when unseasonably warm weather caused rapid snowmelt to saturate the slope. The landslide destroyed the railroad tracks of the Denver and Rio Grande Western Railway Company, and the adjacent Highway 89. It also flowed across the Spanish Fork River, forming a dam. The impounded river water inundated the small town of Thistle. The inhabitants of the town of Thistle, directly upstream from the landslide, were evacuated as the lake began to flood the town, and within a day the town was completely covered with water. Populations downstream from the dam were at risk because of the possible overtopping of the landslide by the lake. This could cause a catastrophic outburst of the dam with a massive flood downstream. Eventually, a drain system was engineered to drain the lake and avert the potential disaster.
How many deaths result from landslides?
An average of between 25 and 50 people are killed by landslides each year in the United States. The worldwide death toll per year due to landslides is in the thousands. Most landslide fatalities are from rock fall, debris-flows, or volcanic debris flows.
What should I know about wildfires and debris flows?
Wild land fires are inevitable in the western United States. Expansion of human development into forested areas has created a situation where wildfires can adversely affect lives and property, as can the flooding and landslides that occur in the aftermath of the fires. There is a need to develop tools and methods to identify and quantify the potential hazards posed by landslides produced from burned watersheds. Post-fire landslide hazards include fast-moving, highly destructive debris flows that can occur in the years immediately after wildfires in response to high intensity rainfall events, and those flows that are generated over longer time periods accompanied by root decay and loss of soil strength. Post-fire debris flows are particularly hazardous because they can occur with little warning, can exert great impulsive loads on objects in their paths, and can strip vegetation, block drainage ways, damage structures, and endanger human life. Wildfires could potentially result in the destabilization of pre-existing deep-seated landslides over long time periods.
How do landslides cause tsunamis?
Tsunamis are large, potentially deadly and destructive sea waves, most of which are formed as a result of submarine earthquakes. They may also result from the eruption or collapse of island or coastal volcanoes and the formation of giant landslides on marine margins. These landslides, in turn, are often triggered by earthquakes. Tsunamis can be generated on impact as a rapidly moving landslide mass enters the water or as water displaces behind and ahead of a rapidly moving underwater landslide.
What are some examples of landslides that have caused tsunamis?
The 1964 Alaska earthquake caused 115 deaths in Alaska alone, with 106 of those due to tsunamis generated by tectonic uplift of the sea floor, and by localized subareal and submarine landslides. The earthquake shaking caused at least 5 local slide-generated tsunamis within minutes after the shaking began. An eyewitness account of the tsunami caused by the movement and landslides of the 1964 Alaska earthquake.
Research in the Canary Islands concludes that there have been at least five massive volcano landslides that occurred in the past, and that similar large events may occur in the future. Giant landslides have the potential of generating large tsunami waves at close and also very great distances and would have the potential to devastate large areas of coastal land as far away as the eastern seaboard of North America.
Rock falls and rock avalanches in coastal inlets, such as those that have occurred in the past at Tidal Inlet, Glacier Bay National Park, Alaska, have the potential to cause regional tsunamis that pose a hazard to coastal ecosystems and human settlements. On July 9, 1958, a magnitude M 7.9 earthquake on the Fairweather Fault triggered a rock avalanche at the head of Lituya Bay, Alaska. The landslide generated a wave that ran up 524 m on the opposite shore and sent a 30-m high wave through Lituya Bay, sinking two of three fishing boats and killing two persons.
How soon does the danger of landslides end after the rain stops?
It’s not possible to exactly predict the number of days or weeks that landslides remain a danger after heavy rain. Residents near mountain slopes, canyons, and landslide prone areas should stay alert even after heavy rain subsides.
Why is southern California vulnerable to landslides?
Areas that have been burned by recent wildfires are highly susceptible to debris-flow activity that can be triggered by significantly less rainfall than that which triggers debris flows from unburned hill slopes.
What was the biggest landslide in the world?
The world’s biggest historic landslide occurred during the 1980 eruption of Mount St. Helens, a volcano in the Cascade Mountain Range in the State of Washington, USA. The volume of material was 2.8 cubic kilometers (km).
What was the biggest prehistoric landslide?
The world’s biggest prehistoric landslide, discovered so far on land, is in southwestern Iran, and is named the Saidmarreh landslide. The landslide is located on the Kabir Kuh anticline in Southwest Iran at 33 degrees north latitude, 47.65 degrees east longitude. The landslide has a volume of about 20 cubic kilometers, a depth of 300 m, a travel distance of 14 km and a width of 5 km. This means that about 50 billion tons of rock moved in this single event!
Some single landslide events have killed numbers in excess of the populations of small countries.
Landslides are life-threatening events that can make it seem as though the world we live upon is crumbling around us. Those landslides listed below are some of the deadliest in recorded human history, each taking away human life by the thousands.
10. Diexi Slides, Sichuan, China, August 1933 (3,000+ deaths)
On August 5, 1933, a strong earthquake triggered a massive landslide in Diexi, Mao County, Szechwan, China. The event, known as the Diexi Slides, claimed more than 3,000 lives, and destroyed many villages within the affected region. The old town of Diexi suffered the worst fate of all as it sank into the landslide-created dam below.
9. Khait Landslide, Tajikstan, July 1949 (4,000 deaths))
For centuries, the mountainous belt running through Central Asia has witnessed a large number of disasters involving earthquake-triggered landslides. One such natural catastrophe occurred in July of 1949, when the 7.4 magnitude Khait Earthquake triggered hundreds of landslides near the southern limits of the Tien Shan ranges in central Tajikistan. The adjacent valleys of Yasman and Khait were the most affected by these earthquake-induced landslides. The Khait Landslide involved rockslides with saturated loess travelling at an estimated average velocity of around 30 meters per second. Approximately 4,000 people were killed in this tragic natural disaster.
8. 62 Nevado Huascaran Debris Fall, Ranrahirca, Peru, January 1962 (4,500 deaths)
Mount Huascarán is a famous Peruvian mountain with a snowcapped peak that rises to a height of 22,205 feet. In January of 1962, a thaw triggered the breaking off of a portion of the north summit of the mountain, leading to a landslide/avalanche that led to the tragic death of nearly 4,500 people. The avalanche, locally referred to as ‘Huayco’, involved a massive ice sheet that was estimated to be about 1 kilometer wide and 40 feet high. As the ice sheet moved rapidly down the slopes, it gathered rock and debris from the mountain and strengthened in force, completely burying several villages in Ranrahica underneath it.
7. Huaraz Debris Flows, Ancash, Peru, December 1941 (5,000 deaths)
In December of 1941, the residents of Huaraz, a Peruvian city in the Ancash region, were completely unaware that a retreating glacier tongue above their city would soon be responsible for wreaking havoc its people and claim thousands of the lives living within. Just before dawn on December 13, 1941, disaster struck the Peruvian city when a landslide resulted in glacial ice crashing down into Lake Palcacocha, generating huge waves that completely destroyed the dam on the lake. This released large volumes of water, itself laden with mud, rock, and ice, into the valley below with an unimaginably high force. Another dam in the nearby Lake Jircacocha was also broken by the flowing glacial water, resulting in the furious waters of both of the two lakes emptying themselves onto the city of Huaraz, claiming more than 5,000 lives in the process.
6. Kelud Lahars, East Java, Indonesia, May 1919 (5,000+ deaths)
Mount Kelud, in Eastern Java, Indonesia, is quite infamous as an extremely active, hazardous volcano, and one which has erupted about 30 times in the past killing thousands of people in its volcanic disasters. One of the deadliest eruptions of this volcano occurred on May 19, 1919, when over 38 million cubic meters of water were expelled from the crater lake of the volcano, which had accumulated large amounts of sediment and volcanic material to form lethal lahars. The lahars moved down the mountains with high velocity and swept away and drowned all that were unfortunate enough to be in its path.
5. North India Flood mudslides, Kedarnath, India, June 2013 (5,700 deaths)
One of the worst natural disasters in the history of India occurred in June of 2013, when powerful flash floods killed around 5,700 people in the Himalayan state of Uttarakhand. Consistent cloudbursts and incessant monsoon rainfall were primarily held responsible for the disaster, which has been officially termed as a natural calamity. However, a section of environmentalists, scientists, and the educated public think otherwise. According to them, thoughtless human intervention in the Himalayan mountain ecosystem had rendered the ecosystem extremely fragile and prone to disaster. The unchecked tourism in the region had promoted the rapid growth of hotels, roads, and shops throughout the region without paying heed to the environmental laws and demands of the ecosystem. The mushrooming of hydroelectric dams in Uttarakhand was also another important factor held responsible for the environmental damage. Heavy rainfall had been previously recorded in the region which had also led to flash floods, but the devastation produced in 2013 was comparable to no earlier data. It is believed that floodwaters had no outlets this time, as most of the routes taken by the water previously were now blocked by sand and rocks. Hence, the lethal waters, laden with debris from dam construction and large volumes of mud and rocks, inundated towns and villages and buried all forms of life that came in its way.
4. 70 Nevado Huascaran Debris Fall, Yungay, Peru, May 1970 (22,000 deaths)
In May of 1970, an earthquake triggered a massive series of landslides and avalanches of rock and snow that buried the towns of Yungay and Ranrahirca. Nearly 22,000 people perished in this natural disaster. The avalanche travelled a distance of 16.5 kilometers. It ended up carrying 50-100 million cubic meters of water, mud, and rocks, which reached the village of Yungay and smothered all life forms therein under its deadly cover.
3. Armero Tragedy, Tolima, Colombia, November 1985 (23,000 deaths)
A dormant volcano, the Nevado del Ruiz in Tolima, Colombia, suddenly came to life on November 13, 1985, wreaking havoc on the nearby villages and towns, and killing as many as 23,000 people. A pyroclastic flow from the crater of the volcano had melted the glaciers in the mountain and sent deadly lahars, saturated with mud, ice, snow, and volcanic debris, rushing down the mountain at killer speeds towards the residential areas directly below it. The lahars soon engulfed the town of Armero, killing thousands there, while casualties were also reported in such other towns as Chinchiná
2. Vargas Tragedy, Vargas, Venezuela, December 1999 (30,000 deaths)
The Winter of 1999 witnessed unusually heavy rainfall in the Vargas State of Venezuela. The rainfall triggered a series of large and small flash floods and debris flows that claimed around 30,000 lives in the region. As per estimates, approximately 10% of the population of Vargas perished in the disaster. The entire towns of Carmen de Uria and Cerro Grande completely vanished under the mud bed, and a large number of homes were simply swept away into the nearby ocean.
1. Haiyuan Flows, Ningxia, China, December 1920 (100,000+ deaths)
The 8.5-magnitude Haiyuan Earthquake was the world’s second deadliest earthquake of the 20th Century. It generated a series of 675 major loess landslides causing massive destruction to lives and property. The natural calamity which struck the rural district of Haiyuan on the evening of December 16, 1920 claimed over 100,000 lives, and severely damaged an area of approximately 20,000 square kilometers. The worst affected areas included the the epicenter of the earthquake in the Haiyuan County in what is now the Ningxia Hui Autonomous Region, as well as the neighboring provinces of Gansu and Shaanxi. Haiyuan County alone lost more than 50% of its population in the disaster. One of the landslides buried an entire village in Xiji County as well.
Whether its builder is a beaver or a person, a dam is always used for the same purpose: to manage, direct and prevent water flow. There an estimated 845,000 dams in the world; here are our picks for the 18 strangest.
Three Gorges Dam
Where: Sandouping, China–Yangtze River
Why It’s Unique: China’s Three Gorges Dam is not only the world’s largest hydroelectric dam, it’s also the world’s single largest source of electricity. The construction of the dam has been convoluted: Preliminary plans began as far back at 1932 but construction but didn’t start until late 1994; the dam isn’t scheduled to be completely finished until 2011. The structure’s estimated life is as short as 70 years; that was deemed long enough to justify the displacement of 1.24 million people.
Itaipu Dam
Where: On the border of Brazil and Paraguay–Parana River
Why It’s Unique: The Itaipu Dam, a partnership between Brazil and Paraguay, generated over 90,000 gigawatt hours of power in 2000—then a world record for hydroelectric generation. With a height of more than 196 meters, the dam stands as tall as a 65-story building. Its construction used enough steel to build 380 Eiffel Towers, along with 12.3 million cubic meters of concrete.
Guri Dam
Where: Bolivar State, Venezuela–Caroni River
Why It’s Unique: The Guri Dam in Venezuela not only boasts sky-high walls and powerful generators, it also has artistic flair. Artist Carlos Cruz Diez decorated one of the plant’s machine rooms in mind-bending pattern of colorful vertical bars, while Alejandro Otero built an enormous rotating kinetic sculpture nearby. The dam produces the energy equivalent of approximately 300,000 barrels of oil per day.
Grand Coulee Dam
Where:Grand Coulee, Washington–Columbia River
Why It’s Unique: Washington state’s Grand Coulee Dam is the largest in the United States. Nearly a mile long and 503 meters wide, its base area is large enough to hold all the pyramids of Giza. At 115 meters high, the dam is more than twice the height of Niagara Falls. The dam also has a memorable role in folk music history—a governmental energy organization commissioned Woody Guthrie to write songs about the dam in the early 1940s, including “Roll On, Columbia, Roll On” and “Grand Coulee Dam.”
Sayano-Shushenskaya Dam
Where: Khakassia, Russia–Yenisei River
Why It’s Unique: Russia’s Sayano-Shushenskaya Dam may not hold any records for its electricity generation, but other dams are no match for its sheer strength—the structure’s stated ability to withstand 8.0-magnitude earthquakes has earned it a spot in the Guinness Book of World Records. Still, not even the world’s strongest dam is immune to problems—a 2009 accident in which a turbine exploded resulted in the deaths of 75 people and 40 tons of oil spilled into the river.
Krasnoyarsk Dam
Where: Divnogorsk, Russia–Yenisey River
Why It’s Unique: Although the Krasnoyarsk dam has operated without the notoriety of its Russian neighbor, this concrete gravity dam has troubles of its own. The plant and its reservoir have apparently wrought changes on the local climate, causing the area to experience warmer and more humid weather conditions than the norm, and reducing ice cover in the area, which is in Siberia. Russia shows off the engineering feat on its 10-ruble bill.
Robert-Bourassa Dam
Where: Quebec, Canada–La Grande River
Why It’s Unique: Situated over Canada’s La Grande River, the Robert-Bourassa dam reaches 140 meters below the surface, making it the world’s largest underground plant. The dam’s centerpiece is a unique “giant’s staircase”—each step is the size of two football fields—that sweeps water downward.
Sand Dams
Where: Kenya
Why It’s Unique: Since 1995, Kenya has constructed more than 500 sand dams, which are usually about 50 meters long and 2 to 4 meters high. Unlike larger dams, which usually are used for hydroelectric power, these smaller structures are designed to store water during the wet season so dry communities have a water reservoir when the rain stops. These dams, which store water buried in silt, do a better job than surface water dams of keeping water from evaporating and maintaining water quality.
Redridge Steel Dam
Where: Redridge, Michigan–Salmon Trout River
Why It’s Unique: Located in Houghton County, Mich., this flat slab buttress dam is one of only three steel dams in the United States. Built in 1894, the dam’s spillway broke in 1941 and was partially repaired in 2001.
Timber Dams
Where: Japan
Why It’s Unique: To limit carbon dioxide emissions from steel and concrete dam construction, northern Japan’s Akita Prefecture started a project to build small-scale dams out of the country’s abundant supply of cedar. The dams serve mainly to minimize the effects of landslides and mud flows in the mountains.
Inguri Dam
Where: Jvari, Georgia–Inguri River
Why It’s Unique: At 892 feet in height, the Inguri Dam is the world’s tallest concrete arch dam. Completed in 1978, it was repaired in 1999 at a cost of 116 million euros.
New Cornelia Mine Tailings Dam
Where: New Cornelia Mine Tailings Dam
Why It’s Unique: In terms of sheer volume, the 7.4 billion cubic foot New Cornelia MineTailings Dam is the country’s largest dam structure. But this dam isn’t used for water—it’s used for mining. Mine tailings (loose collections of crushed rock left over from the mining process) were dumped here before the mine was shut down in 1983.
Syncrude Tailings Dam
Alberta, Canada
The Syncrude Tailings Dam holds the highest volume of material of any dam in the world: 540,000,000 cubic meters. This dam holds tailings from oil sands extraction; 500,000 tons of tailings are produced each day.
Verzasca Dam
Where: Ticino, Switzerland
Why It’s Unique: The Verzasca Dam, completed in 1965, is renowned for its beauty and its slender concrete arch. The design used less concrete than comparable dams, resulting in lower construction costs. When its reservoir was filled, small earthquakes were triggered.
Santee Cooper Dam System
Where: Pinopolis, South Carolina—Santee River
Why It’s Unique: Built to create jobs in the region during the Great Depression, the Santee Cooper Dam system boasts a reservoir area of 186,000 acres. The dam system, 42 miles in total, survived the third worst earthquake in U.S. history and was subsequently redesigned and stabilized for future quakes. The Pinopolis Dam, which is part of the Santee Cooper system, has the highest single-lift lock in the world for raising and lowering boats between different levels of water.
Roosevelt Dam
Where: Phoenix, Arizona—Salt River
Why It’s Unique: Italian stonemasons crafted this dam, hand-cutting all the stones for the project. In recent years, the dam’s height was raised 23 meters to increase water storage space by 20 percent, and it was completely resurfaced in concrete, changing its appearance.
Chalk Hills Dam
Where: On the Border of Wisconsin and Michigan—Menominee River
Why It’s Unique: The power house connected to this dam resembles a cathedral, complete with stained-glass windows celebrating the engineers and bankers involved in the original construction, and small multi-colored terrazzo tile. The structure was completed in 1927.
World’s Largest Beaver Dam
Where: Wood Buffalo National Park—Alberta, Canada
Why It’s Unique: Google Earth found the largest beaver dam in Alberta, Canada at 850 meters long–the closest size relative exists in Montana at 652 meters. Viewers think two beaver families constructed this massive piece of architecture, which contains two separate beaver lodges inside. The entire dam is surrounded by wetlands, common of more sizable beaver creations.
Dam types can be classified in different categories according to the material used in construction and how they withstand the thrust of water:
homogeneous drained earthfill dams, either zoned or with a man-made impervious element;
gravity dams, whether concrete or RCC;
arch dams;
and buttress or multiple arch dams (not dealt with here).
Fill dams are flexible structures while the other types are rigid.The main parameters to be taken into account in choosing a dam site and type are the following:
topography and inflow in the catchment area;
morphology of the river valley;
geological and geotechnical conditions;
climate and flood regime.
In many cases, after consideration of all these aspects, several types of dams will remain potential candidates. Economic considerations are then applied to rank the available alternatives.
1. TOPOGRAPHY AND INFLOW IN THE CATCHMENT AREA
If we ignore the case of lakes for recreational purposes and small dams for hydroelectric power generation, reservoir storage is the main factor influencing the entire dam design. The objective is in fact to have a volume of water available for increasing dry weather river flow, irrigation or drinking-water supply, or free storage capacity to attenuate flooding.
The first task therefore consists in calculating the volume of water that can be stored in a basin, possibly at several different sites. A first approximation can be achieved using a 1/25 000 scale map with contour lines every 5 or 10 metres, except for reservoirs with storage of several tens of thousand cubic metres. The second task will then be to check whether conditions in the catchment area are such that the reservoir will be filled and to calculate the risk of shortfall.
2. MORPHOLOGY OF THE RIVER VALLEY
A dam is by nature linked to an environment. The morphology of the river valley therefore plays a vital role in the choice of a dam site and the most suitable type of dam.
Of course, the ideal and most economical location will be a narrow site where the valley widens upstream of the future dam, provided that the dam abutments are sound (i.e. a narrowing with no zones prone to rockfall or landslide).
Such a site is rarely found, either because the natural structure of a valley does not offer any point of narrowing or because the choice of the site is not solely dependent on engineering considerations.
As a first approach, a wide valley will be more suitable for construction of a fill dam.
A narrow site will be suitable for a gravity dam as well, and a very narrow site will be suitable for an arch. In every case, of course, provided that the foundation is acceptable.
3. GEOLOGY AND FOUNDATION CONDITIONS
The nature, strength, thickness, dip, jointing and permeability of the geological foundations at the site are a set of often decisive factors in selection of the dam type.
ROCK FOUNDATIONS
Except for severely jointed rock or rock with very mediocre characteristics, rock foundations are suitable for construction of any type of dam, provided that suitable measures are taken to strip off severely weathered materials and, if necessary, treat the foundation by grouting. Fill dams will always be suitable. For the other types, requirements are more severe for RCC, still more for conventional concrete, and finally most stringent for arch dams. The most important aspect is cracking (faults, joints, schistosity).
GRAVEL FOUNDATIONS
Provided that they are sufficiently compacted, gravel foundations are generally suitable for earth or rockfill dams, at least in terms of mechanical strength. Leakage must be controlled by suitable impervious barriers and drainage systems. In practice however, this type of foundation essentially is found on rivers with high flows. The dam must
therefore be able to discharge high floods, which precludes earthfill dams. Very small concrete dams may also be suitable provided precautions are taken with leaks and seepage (risk of piping) and with differential settlement.
SANDY-SILT FOUNDATIONS
Silt or fine sand foundations can be suitable for construction of earthfill dams, and even, in exceptional cases, for very small concrete gravity dams provided strict precautions are taken.
CLAY FOUNDATIONS
Clay foundations almost automatically impose the choice of a fill dam with slopes that are compatible with the mechanical characteristics of the geological formations.
4. AVAILABLE MATERIALS
Availability, on the site or near it, of suitable materials to build the dam has a considerable influence and one that is often decisive in choosing the type of dam:
soil that can be used for earthfill,
rock for rockfill or slope protection (rip-rap),
concrete aggregate (alluvial or crushed materials),
cementitious materials (cement, flyash, etc.).
If it is possible to extract the materials from the reservoir itself, reservoir storage can be increased. This also usually keeps the cost of transport and restoring borrow areas to a minimum.
As a general rule, if silty or clay soil of satisfactory quality (fines content, plasticity, condition) and quantity (1.5 times or twice the volume of fill required) is available, a dam construction alternative using homogeneous earthfill or quarry-sorted materials – setting aside the coarsest materials for the downstream shoulder – will be the most economical provided that the flood flows to be discharged are moderate.
If only a limited quantity of impermeable materials, and coarse or rockfill materials as well, is available, the possibility of a zoned earthfill dam or a rockfill dam with a watertight core can be considered. The disadvantage of this alternative is placement in zones, which is all the more complicated when the site is narrow, hindering movement of the machinery.
If only coarse materials are available, they can be used to build a homogeneous embankment with a watertight diaphragm wall built in the centre of the dam, by grouting after the fill has been placed or by an upstream watertight structure (concrete or bituminous concrete facing).
If only rockfill is available, a compacted rockfill dam with external watertight structure (geomembrane, hydraulic concrete or bituminous concrete facing) on the upstream face, will be suitable. A concrete alternative, especially RCC, can also prove to be competitive provided the foundation is good enough (rock or compact ground) with no need for excessive excavation.
5. FLOODS AND FLOOD DISCHARGE STRUCTURES
The cost of flood discharge structures depends on the hydrological characteristics of
the catchment area.
When the catchment area is large and floods are likely to be high, it may be advantageous to combine the dam and spillway functions and build an overspill dam.
On the other hand, if the spillway can be kept small, a fill dam will be preferred if all other conditions are equal.
When construction of the spillway would require significant excavation, the possibility of using the excavated materials will also be a factor in favour of building a fill dam.
When a tunnel is required for temporary diversion of the river during the work, it can usefully be incorporated into the flood discharge structures, if necessary increasing its cross-section slightly.
The choice of an RCC dam can be attractive if it is a means of shortening construction lead time and removing the risk of damage from flooding of the site before construction is complete, a risk that, with any other alternative, would mean building costly diversion or protection structures.
6. ECONOMIC CRITERIA
In many cases, the considerations set out above will be sufficient to select several types of dam as potential alternatives. For example, if the foundation is rock, loose materials are available near the site and flood flows are high, the choice will be between an RCC dam and an earthfill dam with a costly spillway.
The studies must then be pursued for these two types of dam, taking care to refine the cost estimates as the studies progress. As soon as one of the dam types seems significantly more economical, it is preferable to waste no further time on the other option.
CONCLUSIONS ON SELECTING A TYPE OF DAM
The choice of a type of dam is imposed by natural conditions in many cases, with no need for in-depth investigations. For example, if the rock substratum is at a depth of more than 5 metres, the only reasonable alternative will be a fill dam, at least for any project less than 25 metres high. In some regions, the geological context is such that
only one type of dam is usually built.
In other cases, the choice of dam type will be a compromise between different aspects – type of foundation, availability of materials in the vicinity, hydrology – to arrive at the best option economically speaking.
However, it is always an advantage to make a decision as quickly as possible, as a rule after the feasibility studies.
Depending upon the position in a railway track, railway sleepers may be classified as:
Longitudinal Sleepers
Transverse Sleepers
1. Longitudinal Sleepers
These are the early form of sleepers which are not commonly used nowadays. It consists of slabs of stones or pieces of woods placed parallel to and underneath the rails. To maintain correct gauge of the track, cross pieces are provided at regular intervals.
At present this type of sleepers are discarded mainly because of the following reasons.
Running of the train is not smooth when this type of sleepers is used.
Noise created by the track is considerable.
Cost is high.
2. Transverse Sleepers
Transverse sleepers introduced in 1835 and since then they are universally used. They remove the drawbacks of longitudinal sleepers i.e. the transverse sleepers are economical, silent in operation and running of the train over these sleepers is smooth. Depending upon the material, the transverse sleepers may be classified as:
Timber/wooden sleepers
Steel sleepers
Cast Iron Sleepers
Concrete Sleepers
Timber or Wooden Sleepers
The timber sleepers nearly fulfilled all the requirements of ideal sleepers and hence they are universally used. The wood used may be like teak, sal etc or it may be coniferous like pine.
The salient features of timber/wooden sleepers with advantages and disadvantages.
Advantages of Timber Sleepers
They are much useful for heavy loads and high speeds
They have long life of 10-12 years depending upon the climate, condition, rain, intensity, nature of traffic, quality of wood etc
Good insulators and hence good for track circuited railway tracks
They are able to accommodate any gauge
Suitable for salty regions and coastal areas
Can be used with any section of rail
Can be handled and placed easily
They are not badly damaged in case of derailment
They are not corroded
Cheaper than any other types of sleepers
Disadvantages of Timber Sleepers
Liable to be attacked by vermin so, they must be properly treated before use
Liable to catch fire
They do not resist creep
They are affected by dry and wet rot
Become expensive day by day
Life is shorter compare to others
Steel sleepers
They are in the form of steel trough inverted on which rails are fixed directly by keys or nuts and bolts and used along sufficient length of tracks.
Advantages of Steel Sleepers
Have a useful life of 20-25 years.
Free from decay and are not attacked by vermins
Connection between rail and sleeper is stronger
Connection between rail and sleeper is simple
More attention is not required after laying
Having better lateral rigidity
Good scrap value
Suitable for high speeds and load
Easy to handle
Good resistance against creep
Disadvantages of Steel sleepers
Liable to corrosion by moisture and should not because in salty regions
Good insulators and hence cannot be used in track circuited regions
Cannot be used for all sections of rails and gauges
Should not be laid with any other types of ballast except store
Very costly
Can badly damaged under derailments
Way gauge is obtained if the keys are over driven
The rail seat is weaker
Having good shock absorber as there is not cushion between rail foot and ballast
Cast Iron Sleepers
They consist of two pots or plates with rib and connected by wrought iron tie bar of section of about 2″ ½” each pot or plate is placed below each rail. The pot is oval in shape with larger diameter 2′-0″ and smaller diameter 1′-8″ is preferred. Plate sleepers consist of rectangular plates of size about 2′ – 10′ x 1′ – 0″.
The relative advantages and disadvantages are given below.
Advantages of Cast Iron Sleepers
Long life upto 50-60 years
High scrape value as they can be remolded
Can be manufactured locally
Provided sufficient bearing area
Much stronger at the rail seat
Prevent and check creep of rail
They are not attacked by vermin
Disadvantages Cast Iron Sleepers
They are prone to corrosion and cannot be used in salty formations and coastal areas
Not suitable for track circuited portions of railways
Can badly damage under derailment
Difficult to maintain the gauge as the two pots are independent
Require a large number of fastening materials
Difficult to handle and may be easily damaged
Lack of good shock absorber
They are expensive
Concrete sleepers
R.C.C and pre-stressed concrete sleepers are now replacing all other types of sleepers except to some special circumstances such as crossing bridges etc here timber sleepers are used. They were first of all used in France round about in 1914 but are common since 1950. They may be a twin block sleepers joined by an angle iron. It may be a single block pre-stressed type.
Advantages Concrete Sleeprs
Durable with life range from 40-50 years
They can be produced on large quantities locally by installing a plant
Heavier than all other types thus giving better lateral stability to the track
Good insulators and thus suitable for use in track circuited lines
Efficient in controlling creep
They are not attacked by corrosion
Free from attacks of vermin and decay, suitable for all types of soils
Most suitable for welded tracks
Prevent buckling more efficiently
Initial cost is high but proves to be economical in long run
Effectively and strongly hold the track to gauge
Inflammable and fire resistant
Disadvantages Concrete Sleepers
Difficult to be handled
Difficult to be manufactured in different sizes thus cannot be used in bridges and crossing
It is a component of permanent way laid transversely under the rails and performing the following functions.
To support the rails firmly and evenly
To maintain the gauge of the back correctly
To distribute the weight common on the rails over a sufficiently large areas of the ballast
To act as an elastic medium between the rail and the ballast and to absorb the vibrations of the trains.
To maintain the track at proper grads
To align the rail properly
2. Characteristics of Ideal Railway Sleepers
Initial cost and maintenance cost should be low
They should resist weathering, corrosion, decay and other deterioration
They should bear the wheel load efficiently and satisfactorily
They should maintain the correct gauge
They should absorb shocks or vibrations due to moving vehicles
It should distribute the load properly and uniformly over the ballast
Fastenings of rail with sleepers should be strong and simple
They should not break while packing of ballast
Weight should not be low or high
3. Types of Railway Sleepers
Depending upon the position in a railway track, sleepers may be classified as:
1. Longitudinal Sleepers
2. Transverse Sleepers
i. Timber / wooden sleepers
ii. Steel Sleepers
iii. Cast Iron Sleepers
iv. Concrete Sleepers
4. Treatment of Wooden Sleepers
Untreated railway sleepers are prone to attack by decay and vermin. The life of untreated wooden sleepers is thus very less. The life of untreated sleepers can be prolonged considerably by treatment. An extra life of 30-50% is estimated for treated sleepers over untreated railway sleepers.
The fibers of wood contain millions of minute cells containing juices. When these juices ferment, they lead to decay of timber. In the treatment process these juices are removed as much as possible and cells are filled with some preserving solutions may be an oil or some salt solution.
5. Railway Sleeper Density
The number of sleepers required to be placed under the track per rail length is called as its sleepers density.
It is generally given by the formula Sleeper density = n + x
