1. Hi Guest
    Pls Attention! Kazirhut Accepts Only Benglali (বাংলা) & English Language On this board. If u write something with other language, you will be direct banned!

    আপনার জন্য kazirhut.com এর বিশেষ উপহার :

    যেকোন সফটওয়্যারের ফুল ভার্সনের জন্য Software Request Center এ রিকোয়েস্ট করুন।

    Discover Your Ebook From Our Online Library E-Books | বাংলা ইবুক (Bengali Ebook)

Educational Basic Civil Engineering

Discussion in 'Education' started by Tazul Islam, Jun 15, 2016. Replies: 62 | Views: 4305

  1. Tazul Islam
    Offline

    Tazul Islam Kazirhut Lover Member

    Joined:
    Apr 20, 2016
    Messages:
    23,777
    Likes Received:
    543
    Gender:
    Male
    Location:
    Dhaka
    Reputation:
    142
    Country:
    Bangladesh Bangladesh
    Structures Resisting Hydrostatic Pressure

    Hydrostatic Pressure is the force resisted by structures against some hydraulic forces. Hydrostatic pressure plays a very important role in the field of Hydrology and Hydraulics and in most of the cases the stability and reliability of hydraulics structures depends upon the ability of the concerned structures to resist the externally applied hydrostatic pressure.

    Following are the few examples of the structures resisting the hydrostatic pressure.

    • Retaining wall
    • Aqueducts
    • Pressure tunnels
    • Penstocks
    • Pipes etc
    Civil engineering projects often require the construction of systems that retain earth materials. An excavation support system for a cut-and-cover trench for utilities installation is an example of a temporary retaining structure. A reinforced concrete retaining wall utilized in a highway project to accommodate a change in elevation over a limited distance is an example of a permanent retaining structure. Numerous earth retention systems have been developed over the years and a few systems.

    The design of retaining structures requires an evaluation of the loads likely to act on the system during its design life and the strength, load-deformation, and volume-change response of the materials to the imposed loads. Lateral pressures develop on retaining structures as a result of the adjacent earth mass, surcharge, water, and equipment. The development of lateral earth pressures and the transfer of these pressures to the retaining system are inherently governed by soil-structure interaction considerations.

    Hence, the analytical procedure should consider the relative rigidity/flexibility of the earth retention system. In this chapter, retaining structures will be broadly classified as either rigid or flexible Before applicable design procedures are discussed, lateral earth pressure concepts will be reviewed.

    1. Retaining Wall
    A retaining wall is a structure designed and constructed to resist the lateral pressure of soil/water when there is a desired change in ground elevation that exceeds the angle of repose of the soil. The active pressure increases on the retaining wall proportionally from zero at the upper grade level to a maximum value at the lowest depth of the wall. The total pressure or thrust may be assumed to be acting through the centroid of the triangular distribution pattern, one-third above the base of the wall.

    2. Aqueducts
    Aqueducts are an artificial channel for conveying water, usually by gravitation. The term (from the Latin aqua, water; and ducere, to lead) is most commonly understood to mean a bridge of stone, iron, or wood, for allowing the passage of water across a valley. But a pipe, open channel, or a tunnel through a mountain is equally an aqueduct, if its purpose is to convey water from one place to another. All great aqueducts have been constructed for the purpose of conducting water from some more or less distant source to large towns or cities. The term is also applied to a bridge carrying a canal for the purposes of navigation.

    Features
    1. Aqueducts are man-made tunnels that carry water from one place to another. They are used so that dry areas can get water from areas that have more water.
    2. Aqueducts have been used for water supply or for transportation from one place to another.
    3. Aqueducts my be covered or uncovered form the top.
     
  2. Tazul Islam
    Offline

    Tazul Islam Kazirhut Lover Member

    Joined:
    Apr 20, 2016
    Messages:
    23,777
    Likes Received:
    543
    Gender:
    Male
    Location:
    Dhaka
    Reputation:
    142
    Country:
    Bangladesh Bangladesh
    3. Pressure Tunnels.
    Pressure tunnels work on the principal of Pressure flow and the water flows through the tunnel under the pressure.

    Pressure tunnels can be used in a verities of situations such as

    • In dams as penstock.
    • In Irrigation for water supply
    • In water tanks for supply of water to community.
    Pressure tunnels can be made from different materials such as

    • Concrete
    • Steel etc
    Depending upon the situations and the pressure demand but in most of the cases such as in the construction of powar house or dam pressure tunnels are made of concrete.

    Pressure tunnels can be:

    • Construction without pre stressing; and,
    • Construction with pre stressing.


    [​IMG]

    Construction without pre stressing
    When the rock can respond elastically to all the loading conditions which occur and if, furthermore, it contains few fissures and is not very permeable, a carefully constructed un-reinforced concrete lining of medium thickness is sufficient for a pressure tunnel. It is essential, however, that the voids which always occur between the concrete and the rock in the roof region be sealed by grouting. Complete water tightness cannot be achieved with a simple concrete lining. Attempts to approach this ideal.

    Construction with pre-stressing.
    Basically, both unreinforced and reinforced concrete linings are quite unsuitable for one of the most important tasks, namely the prevention of water losses because of their low tensile strength. Recognition of this fact led soon to the development of forms of construction comprising a so called passive prestressing, where the pre-stressing is produced by support from the surrounding rock. The best known of these methods include the core ring lining by Kieser' and the gap injection method.
     
  3. Tazul Islam
    Offline

    Tazul Islam Kazirhut Lover Member

    Joined:
    Apr 20, 2016
    Messages:
    23,777
    Likes Received:
    543
    Gender:
    Male
    Location:
    Dhaka
    Reputation:
    142
    Country:
    Bangladesh Bangladesh
    4. Penstocks.
    A penstock is a sluice or gate or intake structure that controls water flow, or an enclosed pipe that delivers water to hydraulic turbines and sewerage systems.

    The modern penstock is designed to cater for a wide variety of duties from low seating to high off-seating heads in sizes from 150mm to 5000mm square. Selection of the correct penstock to suit the duty is important to satisfy the design criteria and provide the most cost effective solution. Operation of the penstock is governed generally by factors outside the control of the penstock

    manufacturer. However, the range of available operating equipment is extensive: from simple direct operation by handwheel to complex control systems for electrical, pneumatic or hydraulic actuation. The range of penstocks together with its associated operating equipment will cater for the most demanding specification and application.

    Types
    1. Stainless Steel Fabricated Penstock
    2. Coplastix® Series 50 - 40
    3. Cast Iron Weir Penstocks
    4. Cast IronSeries 90 - 6 0
    [​IMG]

    Stainless Steel Fabricated Penstock
    The Fabricated Stainless Steel Penstock is a rectangular faced penstock designed to meet the ever changing fluid handling demands of customers, combining the latest in penstock design and parametrics software.The generic penstock design, which is tailored using parametrics software, develops and individualizes the penstock to suit specific duty and aperture requirements.

    Coplastix® Series 50 - 40
    The Coplastix® Series 50-40 is a rectangular faced pen stock designed and manufactured to suit modern industrial and domestic effluent environments. Utilization of the latest synthetic materials for both sealing mechanisms and door construction combined with the use of steel or stainless steel in the construction of the penstock frames ranges.

    Cast Iron Weir Penstocks
    Weir Penstocks are utilized in water and sewage plants for accurate regulation or measurement of flow. Operation of the Weir Pen-stock is opposite to that of a conventional pen-stock, with the door opening downwards.

    Cast IronSeries 90 - 6 0
    The Series 90-60 is a rectangular metal faced pen stock suitable for wall and thimble mounting, with on-seating heads up to 9 metres and off-seating heads up to 6 metres.
     
  4. Tazul Islam
    Offline

    Tazul Islam Kazirhut Lover Member

    Joined:
    Apr 20, 2016
    Messages:
    23,777
    Likes Received:
    543
    Gender:
    Male
    Location:
    Dhaka
    Reputation:
    142
    Country:
    Bangladesh Bangladesh
    Components of a Barrage
    Advertisements
    Weir (Barrage) may be masonry; rockfill or cement concrete. All weirs normally have the following components:

    1. body wall usually with shutters;
    2. upstream rough stone or boulder pitching;
    3. upstream curtain wall; iv) upstream impervious apron;
    4. crest shutters; vi) downstream impervious apron;
    5. downstream curtain wall;
    6. downstream apron for channel bed.
    Components of barrage
    Main barrage portion:
    1. Main body of the barrage, normal RCC slab which supports the steel gate. In the X-Section it consists of :
    2. Upstream concrete floor, to lengthen the path of seepage and to project the middle portion where the pier, gates and bridge are located.
    3. A crest at the required height above the floor on which the gates rest in their closed position.
    4. Upstream glacis of suitable slope and shape. This joins the crest to the downstream floor level. The hydraulic jump forms on the glacis since it is more stable than on the horizontal floor, this reduces length of concrete work on downstream side.
    5. Downstream floor is built of concrete and is constructed so as to contain the hydraulic jump. Thus it takes care of turbulence which would otherwise cause erosion. It is also provided with friction blocks of suitable shape and at a distance determined through the hydraulic model experiment in order to increase friction and destroy the residual kinetic energy.
    Divide Wall
    • A wall constructed at right angle to the axis of the weir separating the weir proper from the under sluices (to keep heavy turbulence at the nose of the wall, well away from upstream protection of the sluices)
    • It extends upstream beyond the beginning of canal HR. Downstream it extends up to the end of loose protection of under sluices launching apron)
    • This is to cover the hydraulic jump and the resulting turbulence.
    • The divide walls are costly structures
    • These walls are likely to be subjected to maximum differential pressure when the full discharge of the river is passing through the weir, (there will be difference in water level on the two sides ).
    • Also there may exit difference in silt pressure on the two side . The values of differential pressure are taken arbitrarily say 1.0 m for water heads and about 2.0 m for silt pressure
     
  5. Tazul Islam
    Offline

    Tazul Islam Kazirhut Lover Member

    Joined:
    Apr 20, 2016
    Messages:
    23,777
    Likes Received:
    543
    Gender:
    Male
    Location:
    Dhaka
    Reputation:
    142
    Country:
    Bangladesh Bangladesh
    The fish ladder:
    [​IMG]
    Fish ladder or fish passes are generally provided to enable the fish to ascend the head waters of the river and thus reach their spawning grounds for propagation or to follow their migratory habits in search of food.

    • For movement of fish (negotiate the artificial barrier in either direction)
    • Difference of level on the upstream and downstream sides on the weir is split up into water steps by means of baffle walls constructed across the inclined chute of fish ladder.
    • Velocity in chute must not be more than 3 m/s
    • Grooved gate at upstream and downstream - for effective control.
    • Optimum velocity 6-8 ft/s
    • Plenty of light should be admitted in the fish-way
    • The water supply should be ample at all times.

    Sheet piles:

    Made of mild steel, each portion being 1/2' to 2' in width and 1/2" thick and of the required length, having groove to link with other sheet piles.

    Upstream piles:
    Situated at the upstream end of the upstream concrete floor driven into the soil beyond the maximum possible scour that may occur.

    Functions:
    1. Protect barrage structure from scour
    2. Reduce uplift pressure on barrage
    3. To hold the sand compacted and densified between two sheet piles in order to increase the bearing capacity when barrage floor is designed as raft.
    Intermediate sheet piles:
    • Situated at the end of upstream and downstream glacis. Protection to the main structure of barrage (pier carrying the gates, road bridge and the service bridge) in the event of the upstream and downstream sheet piles collapsing due to advancing scour or undermining. They also help lengthen the seepage path and reduce uplift pressure.
    • Downstream sheet piles: Placed at the end of downstream concrete floor. Their main funtion is to check the exit gradient. Their depth should be greater than the possible scour.
    Inverted filter:
    • Provided between the downstream sheet piles and the flexible protection. Typically 6" sand, 9" coarse sand and 9" gravel. Filter may vary with size of particles forming the river bed. It is protected by placing over it concrete blocks of sufficient weight and size. Slits are left between the blocks to allow the water to escape.
    • Length should be 2 x downstream depth of sheet.
    Functions:
    • Check the escape of fine soil particles in the seepage water.
     
  6. Tazul Islam
    Offline

    Tazul Islam Kazirhut Lover Member

    Joined:
    Apr 20, 2016
    Messages:
    23,777
    Likes Received:
    543
    Gender:
    Male
    Location:
    Dhaka
    Reputation:
    142
    Country:
    Bangladesh Bangladesh
    Flexible apron:
    • Placed downstream of the filter
    • Consists of boulder large enough not to be washed away by the highest likely velocity
    • The protection provided is enough as to cover the slope of scour of 1 1/2 x depth of scour as the upstream side of 2 x depth of scour on the downstream side at the slope of 3.
    The under sluices: scouring sluices
    Maintaining a deep channel in front of the Head regulator on the downstream side.

    Functions:
    1. As the bed of under sluice is not lower level than rest of the weir, most of the day, whether flow unit will flow toward this pocket => easy diversion to channel through Head regulator
    2. Control sil entry into channel
    3. Scour the silt (silt excavated and removed)
    4. High velocity currents due to high differential head.
    5. Pass the low floods without dropping
    6. The shutter of the main weir, the raising of which entails good deal of labor and time.
    7. Capacity of under sluices:
    8. For sufficient scouring capacity, its discharging capacity should be at least double the canal discharge.
    9. Should be able to pass the dry weather flow and low flood, without dropping the weir shutter.
    10. Capable of discharging 10 to 15% of high flood discharge
    River training works
    To ensure smooth and axial flow of water, to prevent the river from out ------ the works due to change in its course.

    Guide banks:
    Earthen embankments => stone pitching

    Force the river into restricted channel, to ensure almost axial flow near the weir site. (embankments in continuation of G-Banks. To contain flood within flood plains)

    Marginal Bunds:
    Provided on the upstream in order to protect the area from submergence due to rise in HFL, caused by afflux.

    Groans or spurs:
    • Embankment type structures constructed transverse to river flood, extending from the banks into the river (also transverse dykes)
    • Protect the bank from which they are extended by deflecting the current away from the bank.
     
  7. Tazul Islam
    Offline

    Tazul Islam Kazirhut Lover Member

    Joined:
    Apr 20, 2016
    Messages:
    23,777
    Likes Received:
    543
    Gender:
    Male
    Location:
    Dhaka
    Reputation:
    142
    Country:
    Bangladesh Bangladesh
    Bligh's Creep Theory
    Advertisements
    Design of impervious floor for sub surface flow: Iit is directly depended on the possibilities of percolation in porous soil on which the floor (apron) is built. Water from upstream percolates and creeps (or travel) slowly through weir base and the subsoil below it. The head lostby the creeping water is proportional to the distance it travels (creep length)along the base of the weir profile. The creep length must be made as big as possible so as to prevent the piping action. This can be achieved by providing deep vertical cut-offs or sheet piles.

    According to Bligh’s theory, the total creep length for first drawing: L = B and for second drawing: L = B + 2(d1 + d2 + d3)

    If H is the total loss of head, then the loss of head per unit length of the creep shall be

    [​IMG]

    Bligh called the loss of head per unit length of creep as Percolation coefficient. The reciprocal, (L/H) of the percolation coefficient is known as the coefficient of creep C.

    Assumptions
    • Hydraulic slope or gradient is constant throughout the impervious length of the apron.
    • The percolating water creep along the contact of the base profile of the apron with the sub soil losing head enroute, proportional to length of its travel. The length is called creep length. It is the sum of horizontal and vertical creep.
    • Stoppage of percolation by cut off (pile) possible only if it extends up to impermeable soil strata.
    [​IMG]
     
  8. Tazul Islam
    Offline

    Tazul Islam Kazirhut Lover Member

    Joined:
    Apr 20, 2016
    Messages:
    23,777
    Likes Received:
    543
    Gender:
    Male
    Location:
    Dhaka
    Reputation:
    142
    Country:
    Bangladesh Bangladesh
    Design criteria:
    Safety against piping:
    The length of creep should be sufficient to provide safe hydraulic gradient according to the type of soil.

    Safe creep length = L = CH, C = 1/c

    Safety against uplift pressure:
    Let h' = uplift pressure head at any point of apron (Hydraulic gradient line above the bottom of floor)

    The uplift pressure = wh' where w = density of water. If t = thickness of floor at the point, l = specific gravity for floor material. Then, downward force per area (resisting force) = t.w.e or wh' = t.w.e

    For portion of floor upstream of barrier only nominal thickness need to be provided since the weight of water will counterbalance the uplift pressure.

    A certain minimum length of impervious floor is always necessary to the downstream of the barrier (thickness of downstream floor for worst condition)

    Limitations of Bligh's theory
    1. This theory made no distinction between horizontal and vertical creep.

    2. Did not explain the idea of exit gradient - safety against undermining cannot simply be obtained by considering a flat average gradient but by keeping this gradient will be low critical.

    3. No distinction between outer and inner faces of sheet piles or the intermediate sheet piles, whereas from investigation it is clear, that the outer faces of the end sheet piles are much more effective than inner ones.

    4. Losses of head does not take place in the same proportions as the creep length. Also the uplift pressure distribution is not linear but follow a sine curve.

    5. In case of two piles the width between should be greater than twice the head or the piles are not effective.
     
  9. Tazul Islam
    Offline

    Tazul Islam Kazirhut Lover Member

    Joined:
    Apr 20, 2016
    Messages:
    23,777
    Likes Received:
    543
    Gender:
    Male
    Location:
    Dhaka
    Reputation:
    142
    Country:
    Bangladesh Bangladesh
    Khosla's theory

    After studying a lot of dam failure constructed based on Bligh’s theory, Khosla came out with the following;

    Following are some of the main points from Khosla's Theory

    • From observation of Siphons designed on Bligh's theory, by actual measurement of pressure, with the help of pipes inserted in the floor of two of the siphons،

    • Does not show any relationship with pressure calculated on Bligh's theory. This led to the following provisional conclusions:

    • Outer faces of end sheet piles were much more effective than the inner ones and the horizontal length of the floor.

    • Intermediated piles of smaller length were ineffective except for local redistribution of pressure.

    • Undermining of floor started from tail end.
    • It was absolutely essential to have a reasonably deep vertical cut off at the downstream end to prevent undermining.

    • Khosla and his associates took into account the flow pattern below the impermeable base of hydraulic structure to calculate uplift pressure and exit gradient.

    • Starting with a simple case of horizontal flow with negligibly small thickness. (various cases were analyzed mathematically.)

    • Seeping water below a hydraulic structure does not follow the bottom profile of the impervious floor as stated by Bligh but each particle traces its path along a series of streamlines.
     
  10. Tazul Islam
    Offline

    Tazul Islam Kazirhut Lover Member

    Joined:
    Apr 20, 2016
    Messages:
    23,777
    Likes Received:
    543
    Gender:
    Male
    Location:
    Dhaka
    Reputation:
    142
    Country:
    Bangladesh Bangladesh
    For case of two dimensional flows under a straight floor where:
    Thus for the first flow line AB which touches the outline of the floor, the pressure can be obtained by putting different values of x in equation. Fig shows the pressure distribution diameter both by equation 4 as well as Bligh's Theory.

    From the fig the following conclusions can be drawn:

    Slope of Pressure diagram: At A and B in infinite, hence the floor at A will be theoretically infinite acting downward and that at B will also be infinite acting upward. This will cause sand boiling and hence the floor should be depressed or cut off should be provided at the downstream end.

    Composite profile:
    The following specific causes of general form were considered.

    • Straight horizontal flow of negligible thickness with pile at either end, upstream or at downstream end.
    • Straight horizontal floor of negligible thickness with pile at some intermediate point.
    • Straight horizontal floor, depressed below the bed, but with no cut off.
    Method of independent variable:
    • Most designs do not confirm to elementary profiles (specific cases). In actual cases we may have a number of piles at upstream level, downstream level and intermediate points and the floor also has some thickness.
    • Khosla solved the actual problem by an empirical method known as method of independent variables.
    • This method consists of breaking up a complex profile into a number of simple profiles, each of which is independently amiable to mathematical treatment. Then apply corrections due to thickness of slope of floor.
    • As an example the complex profile shown in fig is broken up to the following simple profile and the pressure at Key Points obtained.
    • Straight floor of negligible thickness with pile at upstream ends.
    • Straight floor of negligible thickness with pile at downstream end.
    • Straight floor of negligible thickness with pile at intermediate points.
    • The pressure is obtained at the key points by considering the simple profile.
    For the determination of seepage below the foundation of hydraulic structure developed the method of independent variable.
    In this method, the actual profile of a weir which is complex, is divided into a number simple profiles, each of which cab be solved mathematically without much difficulty. The most useful profile considered are:

    A straight horizontal floor of negligible thickness provided with a sheet pile at the upstream end or a sheet pile at the downstream end.
     

Pls Share This Page:

Users Viewing Thread (Users: 0, Guests: 0)