Types of dams: Introduction and classification

A dam is a hydraulic structure of fairly impervious material built across a river to create a reservoir on its upstream side for impounding water for various purposes. These purposes may be Irrigation, Hydro-power, Water-supply, Flood Control, Navigation, Fishing and Recreation. Dams may be built to meet the one of the above purposes or they may be constructed fulfilling more than one. As such, it can be classified as: Single-purpose and Multipurpose Dam.

Different parts & terminologies of Dams:

• Crest: The top of the dam structure. These may in some cases be used for providing a roadway or walkway over the dam.
• Parapet walls: Low Protective walls on either side of the roadway or walkway on the crest.
• Heel: Portion of structure in contact with ground or river-bed at upstream side.
• Toe: Portion of structure in contact with ground or river-bed at downstream side.
• Spillway: It is the arrangement made (kind of passage) near the top of structure for the passage of surplus/ excessive water from the reservoir.
• Abutments: The valley slopes on either side of the dam wall to which the left & right end of dam are fixed to.
• Gallery: Level or gently sloping tunnel like passage (small room like space) at transverse or longitudinal within the dam with drain on floor for seepage water. These are generally provided for having space for drilling grout holes and drainage holes. These may also be used to accommodate the instrumentation for studying the performance of dam.
• Sluice way: Opening in the structure near the base, provided to clear the silt accumulation in the reservoir.
  
  

  Illustration of dam-parts in a typical cross section (click the image to view it clearly)
• Free board: The space between the highest level of water in the reservoir and the top of the structure.
• Dead Storage level: Level of permanent storage below which the water will not be withdrawn.
• Diversion Tunnel: Tunnel constructed to divert or change the direction of water to bypass the dam construction site. The hydraulic structures are built while the river flows through the diversion tunnel.

CLASSIFICATION OF DAMS

Dams can be classified in number of ways. But most usual ways of classification i.e. types of dams are mentioned below:

Based on the functions of dams, it can be classified as follows:

1. Storage dams: They are constructed to store water during the rainy season when there is a large flow in the river. Many small dams impound the spring runoff for later use in dry summers. Storage dams may also provide a water supply, or improved habitat for fish and wildlife. They may store water for hydroelectric power generation, irrigation or for a flood control project. Storage dams are the most common type of dams and in general the dam means a storage dam unless qualified otherwise.
2. Diversion dams: A diversion dam is constructed for the purpose of diverting water of the river into an off-taking canal (or a conduit). They provide sufficient pressure for pushing water into ditches, canals, or other conveyance systems. Such shorter dams are used for irrigation, and for diversion from a stream to a distant storage reservoir. It is usually of low height and has a small storage reservoir on its upstream. The diversion dam is a sort of storage weir which also diverts water and has a small storage. Sometimes, the terms weirs and diversion dams are used synonymously.
3. Detention dams: Detention dams are constructed for flood control. A detention dam retards the flow in the river on its downstream during floods by storing some flood water. Thus the effect of sudden floods is reduced to some extent. The water retained in the reservoir is later released gradually at a controlled rate according to the carrying capacity of the channel downstream of the detention dam. Thus the area downstream of the dam is protected against flood.
4. Debris dams: A debris dam is constructed to retain debris such as sand, gravel, and drift wood flowing in the river with water. The water after passing over a debris dam is relatively clear.
5. Coffer dams: It is an enclosure constructed around the construction site to exclude water so that the construction can be done in dry. A coffer dam is thus a temporary dam constructed for facilitating construction. These structure are usually constructed on the upstream of the main dam to divert water into a diversion tunnel (or channel) during the construction of the dam. When the flow in the river during construction of hydraulic structures is not much, the site is usually enclosed by the coffer dam and pumped dry. Sometimes a coffer dam on the downstream of the dam is also required.

Based on structure and design, dams can be classified as follows:

1. Gravity Dams: A gravity dam is a massive sized dam fabricated from concrete or stone masonry. They are designed to hold back large volumes of water. By using concrete, the weight of the dam is actually able to resist the horizontal thrust of water pushing against it. This is why it is called a gravity dam. Gravity essentially holds the dam down to the ground, stopping water from toppling it over.
   
   

   Types of dam
   
   Gravity dams are well suited for blocking rivers in wide valleys or narrow gorge ways. Since gravity dams must rely on their own weight to hold back water, it is necessary that they are built on a solid foundation of bedrock.
   Examples of Gravity dam: Grand Coulee Dam (USA), Nagarjuna Sagar (India) and Itaipu  Dam (It lies Between Brazil and Paraguay and is the largest in the world).
2. Earth Dams: An earth dam is made of earth (or soil) built up by compacting successive layers of earth, using the most impervious materials to form a core and placing more permeable substances on the upstream and downstream sides. A facing of crushed stone prevents erosion by wind or rain, and an ample spillway, usually of concrete, protects against catastrophic washout should the water overtop the dam. Earth dam resists the forces exerted upon it mainly due to shear strength of the soil. Although the weight of the this structure also helps in resisting the forces, the structural behavior of an earth dam is entirely different from that of a gravity dam. The earth dams are usually built in wide valleys having flat slopes at flanks (abutments).The foundation requirements are less stringent than those of gravity dams, and hence they can be built at the sites where the foundations are less strong. They can be built on all types of foundations. However, the height of the dam will depend upon the strength of the foundation material.
   Examples of earthfill dam: Rongunsky dam (Russia) and New Cornelia Dam (USA).
3. Rockfill Dams: A rockfill dam is built of rock fragments and boulders of large size. An impervious membrane is placed on the rockfill on the upstream side to reduce the seepage through the dam. The membrane is usually made of cement concrete or asphaltic concrete.
   
   

   Mohale Dam, Lesotho, Africa
   In early rockfill dams, steel and timber membrane were also used, but now they are obsolete. A dry rubble cushion is placed between the rockfill and the membrane for the distribution of water load and for providing a support to the membrane. Sometimes, the rockfill dams have an impervious earth core in the middle to check the seepage instead of an impervious upstream membrane. The earth core is placed against a dumped rockfill. It is necessary to provide adequate filters between the earth core and the rockfill on the upstream and downstream sides of the core so that the soil particles are not carried by water and piping does not occur. The side slopes of rockfill are usually kept equal to the angle of repose of rock, which is usually taken as 1.4:1 (or 1.3:1). Rockfill dams require foundation stronger than those for earth dams.
   Examples of rockfill dam: Mica Dam (Canada) and Chicoasen Dam (Mexico).
4. Arch Dams: An arch dam is curved in plan, with its convexity towards the upstream side. They transfers the water pressure and other forces mainly to the abutments by arch action.
   
   

   Hoover dam (USA)
   An arch dam is quite suitable for narrow canyons with strong flanks which are capable of resisting the thrust produced by the arch action.The section of an arch dam is approximately triangular like a gravity dam but the section is comparatively thinner. The arch dam may have a single curvature or double curvature in the vertical plane. Generally, the arch dams of double curvature are more economical and are used in practice.
   Examples of Arch dam: Hoover Dam (USA) and Idukki Dam (India).
5. Buttress Dams: Buttress dams are of three types : (i) Deck type, (ii) Multiple-arch type, and (iii) Massive-head type. A deck type buttress dam consists of a sloping deck supported by buttresses. Buttresses are triangular concrete walls which transmit the water pressure from the deck slab to the foundation. Buttresses are compression members. Buttresses are typically spaced across the dam site every 6 to 30 metre, depending upon the size and design of the dam. Buttress dams are sometimes called hollow dams because the buttresses do not form a solid wall stretching across a river valley.The deck is usually a reinforced concrete slab supported between the buttresses, which are usually equally spaced. In a multiple-arch type buttress dam the deck slab is replaced by horizontal arches supported by buttresses. The arches are usually of small span and made of concrete. In a massive-head type buttress dam, there is no deck slab. Instead of the deck, the upstream edges of the buttresses are flared to form massive heads which span the distance between the buttresses. The buttress dams require less concrete than gravity dams. But they are not necessarily cheaper than the gravity dams because of extra cost of form work, reinforcement and more skilled labor. The foundation requirements of a buttress are usually less stringent than those in a gravity dam.
   Examples of Buttress type: Bartlett dam (USA) and The Daniel-Johnson Dam (Canada).
6. Steel Dams: Dams: A steel dam consists of a steel framework, with a steel skin plate on its upstream face. Steel dams are generally of two types: (i) Direct-strutted, and (ii) Cantilever type . In direct strutted steel dams, the water pressure is transmitted directly to the foundation through inclined struts. In a cantilever type steel dam, there is a bent supporting the upper part of the deck, which is formed into a cantilever truss. This arrangement introduces a tensile force in the deck girder which can be taken care of by anchoring it into the foundation at the upstream toe. Hovey suggested that tension at the upstream toe may be reduced by flattening the slopes of the lower struts in the bent. However, it would require heavier sections for struts. Another alternative to reduce tension is to frame together the entire bent rigidly so that the moment due to the weight of the water on the lower part of the deck is utilised to offset the moment induced in the cantilever. This arrangement would, however, require bracing and this will increase the cost. These are quite costly and are subjected to corrosion. These dams are almost obsolete. Steel dams are sometimes used as temporary coffer dams during the construction of the permanent one. Steel coffer dams are supplemented with timber or earthfill on the inner side to make them water tight. The area between the coffer dams is dewatered so that the construction may be done in dry for the permanent dam.
   Examples of Steel type: Redridge Steel Dam (USA) and Ashfork-Bainbridge Steel Dam (USA).
7. Timber Dams: Main load-carrying structural elements of timber dam are made of wood, primarily coniferous varieties such as pine and fir. Timber dams are made for small heads (2-4 m or, rarely, 4-8 m) and usually have sluices; according to the design of the apron they are divided into pile, crib, pile-crib, and buttressed dams.
   
   

   Timber dam
   The openings of timber dams are restricted by abutments; where the sluice is very long it is divided into several openings by intermediate supports: piers, buttresses, and posts. The openings are covered by wooden shields, usually several in a row one above the other. Simple hoists—permanent or mobile winches—are used to raise and lower the shields.
8. Rubber Dams: A symbol of sophistication and simple and efficient design, this most recent type of dam uses huge cylindrical shells made of special synthetic rubber and inflated by either compressed air or pressurized water. Rubber dams offer ease of construction, operation and decommissioning in tight schedules. These can be deflated when pressure is released and hence, even the crest level can be controlled to some extent. Surplus waters would simply overflow the inflated shell. They need extreme care in design and erection and are limited to small projects.
   Example of Rubber type: Janjhavathi Rubber Dam (India).

Roofing Systems to Environment and Sustainability

 

General Overview

Sika has developed solutions which can  help ensure that the roof creates minimal impact on the environment whilst meeting the functional requirements of clients, specifiers, contractors and nature. Roofing membranes/ products are not only manufactured in an ISO 14001:2000  accredited production facility they have a low embodied energy and long life expectancies.

The following important environmental and  sustainability aspects are always taken into account in Sika roofing systems:

• Recycling
• Embodied Energy
• Durability
• Sun light reflectivity

Recycling

Sika has proactively recycled factory waste  back into production since 1960. Wherever  possible, higher quantities of production and  post-consumer membrane are recycled into  new products, such as Roof Protection Sheets and Walkway Pads.  Today walkway pads, manufactured in Europe  since 2000, provide tough, durable solution  for pedestrian access on exposed roofs and at the same time they are produced from almost 100% recycled material. The only non-recycled product being less than 1% carbon black that is added for colour consistency. Sika recyclate is also sourced from existing membrane roofs when they are removed  to enable the client to thermally upgrade the building, a common practice in Western Europe. This process is managed through the company’s involvement with the Roof Collect  scheme operated by Vynyl 2010.

Embodied Energy

Embodied energy is the measurement of the  amount of energy required to produce a tonne or square metre of a product, it can also be used to measure the carbon embodied within a product. Generally the lower the embodied energy and carbon levels, the better a product is for the environment. All Sika roofing membranes have low embodied energy, contributing less to global warming than many alternative roofing technologies.

Sun Reflectivity

The benefits of solar reflective materials and colours are well known and understood in warm climates around the world. With urban density increasing, the “heat island” (Albedo) affect is impacting on cities at an ever-increasing rate. A significant contribution to reducing the  Albedo affect can be made by simply replacing dark roof surfaces with a lighter colour, ideally white – Sika roofing systems include solutions which allow the reflection of  up to 83% of the heat in sunlight.

Ronnen Levinson & Hashem Akbari’s December 2007 report “Potential Energy Savings and Environmental Benefits of Cool Roofs on Commercial Buildings” demonstrated that by changing from a relatively low solar reflectivity light grey membrane, to a higher reflectivity white membrane, large energy cost savings could be made. Therefore significantly reducing emissions of carbon dioxide (CO2), nitrogen dioxide (NO2), sulphur dioxide (SO2) and mercury (Hg).

Durability

Long service life is a key element of sustainability, the longer something lasts the less damage it should do to the environment in use. Sika roofing systems has been tested for aging and life expectancy by different institutes and organizations with outstanding results. As an example the British Board of Agrément (BBA) certifies a standard 1.2mm thick Sarnafil G/S membrane to have a life expectancy “in excess of 35 years”.

Cost effective and favourable technology solutions for high-head Francis Turbines

For high head turbines there is often a choice between Francis and Pelton. The main advantages of Francis turbines are that the dimensions will be smaller and the speed higher. Due to this the cost will be lower, both for the electro-mechanical equipment and for the powerhouse civil works. By further utilizing the most efficient solutions available, the construction of modern high-head Francis turbines give a more easy to use and less maintenance demanding result than previously.

This presented technology has been used in a several of the world’s high-head Francis turbines. It traces its roots back to the 1950s, when already Francis turbines with heads above 400 m were routinely delivered. One example is Holen 3 in Norway with a rated net head of 610 m and a maximum gross head of 660 m. In Asia, it has recently been used in more than a dozen high head Francis turbine projects in China. Recently, it was also being used for the Nam Theun 2 project in the Republic of Laos, which will have 4 of 247 MW Francis turbines at a rated head of 350 m. This paper gives an introduction to several important solutions of the high head Francis turbine design. It includes some of the important design details on runners, guide bearings, shaft seal, shaft couplings, guide vane friction device and the principle of equalized loads utilized on stationary part embedment. Implementation of these design features will have positive impact on construction cost, maintenance program and the general operation of the turbine. Following are descriptions of each design solution with its listed advantages for making the turbine more reliable, stabile and flexible to operate.

Most recent, several  features are being incorporated in the Nam Theun 2, High-head Francis turbines. It uses a X-Blade runner but without splitter vanes, vertical shaft guide bearing with fixed pads, non-contact seal and guide vane friction device. And the bottom cover is together with the draft tube cone not embedded, making them available to be dismantled downwards and sideways with good access for runner inspection and

Effect of Silica Fume to Water Permeability of Concrete

 

Water permeability of concrete can be reduced by up to 100 times through the use of Silica fume.

Test by some independence authorities indicates that so called waterproofing admixtures had no effect on water permeability of concrete. Rather that supervision and improved mix designs associated with such systems can lead to a more impermeable concrete. It should be noted that permeability of concrete is not a simple function of its porosity but depends also on size, distribution and continuity of the pores.

Pores in concrete are reduced significantly with the inclusion of silica fume and illaries reduced through reduced bleeding and production of denser paste. The permeability is lower the higher the cement content of the paste i.e. the lower the water/cement ratio. Values obtained for pastes in which 93% of cement has hydrated. It is the permeability of the paste that has the greatest effect on the permeability of the concrete. Water vapor transmission of concrete is reduced by a decrease in the water cement ratio. A decrease in water cement ratio from 0.8 to 0.4 decreases  the  vapour permeability by 50-65%.

Vapour permeability, however, decreases as the relative humidity decreases, an increase in relative humidity decreases the airfilled pore space available for diffusion. It follows the that if the moist side is for instance saturated an increase in the relative humidity of the  dry side reduces the vapor permeability.

The permeability of concrete is reduced by the use of silica fume in combination with superplasticizers which reduce the water cement ratio up to 30%.

Home Building Information – Learn from very basic things

 

Do you love to stay in your home? Many people do, because they think that their home is perfect. In order to have this feeling, they actually spent a lot of time and effort to create a quality house indeed. This article is just a brush up about home building, not really an advice, just some important notes for your consideration.

You would find that there are plenty of people who are experienced with home building nowadays. Since there are more and more people who have a house, they would spend the time to read books and study information about home building. Gradually, they would have the techniques for this and they would be experienced by working on their own house.

If you want to learn home building, you should first try to learn about how to match the color of the furniture and the wall. Usually, some people would not know this. If you want to have a simple style in your house, you can consider using white wall and purchasing wooden furniture. Black furniture can also work but black leather furniture would be preferred to the plastic ones.

During the work of home building, you should also remember that the best home would be the home which can be built within budget, instead of the home which is built with millions of dollars. As long as you find it comfortable to stay in the house, you would regard it as the best house and there is not a need for you to make everything grand in your house indeed.

You can try to check the license of the contractor and also the bond status. You would be able to ask the experience of some customers who have tried the service before. They would be able to give their personal experience as the objective review of the quality of services from those contractors. Therefore, you can consider trusting them.

In conclusion, home building can be something tiring if you do not have the experience before. You would find that it is not something easy at all for you to get the right direction of home building if you are new to this. Therefore, you should try to consult someone who is experienced or professional. If you do not want to hire someone because of the budget concerns, you can try to search online because there are people who are experienced with home building and they would possibly give you suitable pieces of information about home building which can give you insights.

What is Civil Engineering?

This definition of Civil Engineering is found at Answer.com. The tasks of civil engineering now come larger and larger.

Civil Engineering is a branch of engineering that encompasses the conception, design, construction, and management of residential and commercial buildings and structures, water supply facilities, and transportation systems for goods and people, as well as control of the environment for the maintenance and improvement of the quality of life. Civil engineering includes planning and design professionals in both the public and private sectors, contractors, builders, educators, and researchers.

The civil engineer holds the safety, health, and welfare of the public paramount. Civil engineering projects and systems should conform to governmental regulations and statutes; should be built economically to function properly with a minimum of maintenance and repair while withstanding anticipated usage and weather; and should conserve energy and allow hazard-free construction while providing healthful, safe, and environmentally sound utilization by society.

Civil engineers play a major role in developing workable solutions to construct, renovate, repair, maintain, and upgrade infrastructure. The infrastructure includes roads, mass transit, railroads, bridges, airports, storage buildings, terminals, communication and control towers, water supply and treatment systems, storm water control systems, wastewater collection, treatment and disposal systems, as well as living and working areas, recreational buildings, and ancillary structures for civil and civic needs. Without a well-maintained and functioning infrastructure, the urban area cannot stay healthy, grow, and prosper.

Because the desired objectives are so broad and encompass an orderly progression of interrelated components and information to arrive at the visually pleasing, environmentally satisfactory, and energy-frugal end point, civil engineering projects are actually systems requiring the skills and inputs of many diverse technical specialties, all of which are subsets of the overall civil engineering profession.

Some of the subsets that civil engineers can specialize in include photogrammetry, surveying, mapping, community and urban planning, and waste management and risk assessment. Various engineering areas that civil engineers can specialize in include geotechnical, construction, structural, environmental, water resources, and transportation engineering.