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Note for Civil Engineering Materials and Construction - CEMC by Pratik Acharya

  • Civil Engineering Materials and Construction - CEMC
  • Note
  • Biju Patnaik University of Technology BPUT - BPUT
  • Civil Engineering
  • B.Tech
  • 6 Topics
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POND LEVEL GATE PONDING BY GATE P2 POND LEVEL TOTAL PONDING BY GATE CREST LEVEL P1 <<<< P2 PONDING BY CREST P1 = 0 P2 = P (BARRAGE) P1 (c) Fig: Barrage with a small raised crest (d) Fig: Barrage without any raised crest 5. Weir If the major part or the entire ponding of water is achieved by a raised crest and a smaller part or nil part of it is achieved by the shutters, then this barrier is known as a weir. 6. Fig: A typical cross-section of a modern concrete weir 1. Gravity and Non-gravity weirs: When the weight of the weir (i.e. its body and floor) balances the uplift pressure caused by the head of the water seeping below the weir, it is called a gravity weir. On the other hand, if the weir floor is designed continuous with the divide piers as reinforced structure, such that the weight of concrete slab together with the weight of divide piers keep the structure safe against the uplift then the structure may be called as a non-gravity weir. o In the latter case, RCC is to be used in place of brick piers o Considerable savings may be obtained, as the weight of the floor can be much less than what is required in gravity weir. (a) 2. Types of weirs Masonry weirs with vertical drop (b) Rock-fill weirs with sloping aprons (c) Concrete weirs with sloping glacis 3. Masonry weirs with vertical drop Masonry weir wall is constructed over the impervious floor. Cut-off walls are provided at both ends of the floor. Sheet piles are provided below the cut off walls. The crest shutters are provided to raise the water level, if required. The 2

Text from page-3

shutters are dropped down during flood. The masonry weir wall may be vertical on both face or sloping on both face or vertical on downstream face and sloping in upstream face. 4. Rock-fill weirs with sloping aprons It consists of masonry breast wall which is provided with adjustable crest shutter. The upstream rock-fill portion is constructed with boulders forming a slope of 1 in 4. The boulders are grouted with cement mortar. The downstream sloping apron consists of core walls. The intermediate spaces between the core walls are filled up with boulders maintaining a slope of 1 in 20. The boulders are grouted properly with cement mortar. 5. Concrete weir Now-a-days, the weir is constructed with reinforced cement concrete. The impervious floor and the weir are made monolithic. The cut off walls are provided at the upstream and downstream end of the floor and at the toe of the weir. Sheet piles are provided below the cut-off walls. The crest shutters are also provided which hare dropped down during the flood. 3

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1. Barrage  If most of the ponding is done by gates and a smaller or nil part of it is done by the raised crest, then the barrier is known as a barrage or a river regulator. 2. Fig: A typical cross-section of a barrage Afflux:  The rise in the highest flood level (HFL) upstream of the weir due to construction of the weir across the river is called.  In case of weir, the afflux caused during high floods is quite high. But in case of a barrage, the gates can be opened during high floods and the afflux will be nil or minimum. 3. Choice between a weir and a barrage The choice between a weir and a barrage is largely governed by cost and convenience in working. A shuttered weir will be relatively cheaper but will lack the effective control possible in the case of a barrage. A barrage type construction can be easily supplemented with a roadway across the river at a small additional cost. Barrages are almost invariably constructed now-a-days on all important rivers. 4. Difference between Barrage and Weir SL (a) Barrage Weir Low set crest High set crest 4

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(b) Ponding is done by means of gates Ponding is done against the raised crest or partly against crest and partly by shutters (c) Gated over entire length Shutters in part length (d) Gates are of greater height Shutters are of smaller height, 2 m (e) Gates are raised clear off the high floods to pass floods Shutters are dropped to pass floods (f) Perfect control on river flow No control of river in low floods (g) Gates convenient to operate Operation of shutters is slow, involve labour and time (h) High floods can be passed with minimum afflux Excessive afflux in high floods (i) Less silting upstream due to low set crest Raised crest causes silting upstream (j) Longer construction period Shorter construction period (k) Silt removal is done through under sluices No means for silt disposal (l) Road and/or rail bridge can be constructed at low cost Not possible to provide road-rail bridge Costly structure Relatively cheaper structure (m) 3. Layout of a Diversion Head Works and its components A typical layout of a canal head-works is shown in figure below. Such a head-works consists of: Weir proper  Under-sluices  Divide wall  River Training works  Fish Ladder  Canal Head Regulator  River Training Works e.g. Guide bank, Marginal bunds, spur and groyne etc.  Shutters and Gates  Silt Regulation Works  Marginal bund o 5

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