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This width may be adopted. The weir will have a trapezoidal profile as shown in figure. Abutments, wings, and returns The top width of abutments, wings, and returns will all be uniformly 0. Length of the wing walls must be enough to completely encase the tank bund as shown in figure. Abutments Portion AB is called the abutment. It has its top level same as that of the tank bund at The bottom width required is about 4.
The wall BD is called upstream wing wall. The section of the wing wall at B is same as the section of the abutment. This wing wall start sloping down from B till it reaches about 30cms. Above MWL i. So , the portion of wing wll BC will be having its top sloping down from Level wing and return Since the level wing and return i. Upstream side transition In order to give an easy approach , the upstream side wing wall may be splayed as shown.
Downstream side wings and returns. As the water after passing over the weir goes down rapidly to normal MFL in the water course, the wings and returns need not be high as those on the upstream side. The wing wall from A to F will slope down till the top reaches the ground level at F.
The section of wing wall at A will be the same as that of the abutments. The top wing wall at F may be fixed at Downstream transition. The downstream side wings are given a splay of 1 in 5 as shown in figure 4.
Generally no apreons are required on the upstream side of the weir. Howeverit is desirable to provide puddle apron as shown in figure 4.
It is also sometimes provided with nominal rough stone apron 30cms. Thick packed well on puddle clay apron. In case where the head of percolation is great, in order to reduce the length of aprons on the downstream side of the weir, it is necessary to provide upstream side solid apron.
This apron is not subject to any uplifts and hence can be norminal thickness. However this acts in considerably reducing the creep length and consequently reduces th elenghts and thickness of aprons, downstream of the weir. Downstream aprons. Since the ground level is falling down to The aprons may be designed for a hydraulic gradient of 1 in 5 so that the residual gradient at the exit of aprons can be limited to 1 in 5 which is safe enough and will not start undermining the structure.
Maximum uplift pressure are experienced on the down stream aprons when the water level in the tank is upto Top of dam stone level i. However assume that the downstream water elevation is ta The upstream water has to percolate under the foundations of the weir, if it has to establish any uplifts under the aprons.
The possible path of percolation is shown in the figure. Assuming the puddle apron formed on the upstream of the weir to be not impervoius , the water will start percolating from A at a level of From here, it will follow the least path D to E under the end cut-off and then appear at F.
The total lengh to of solid apron from the body wall as provided in the drawing is 8 meters and this will be enough.
These can be reduced if the upstream side puddle clay apron is really impervoius. To ensure safety, the whole upstream side apron can be packed with stone and well grouted with cement concrete. At the end of the second apron retaining wall of the downstream side apron , a nominal 3 to 5 meter length of talus with a thickness of 50cms.
May be provided as a safety device. Thickness of solid apron The maximum uplift on the apron floor is felt immediately above point D in the sketch. Assuming a thickness of 80cms of apron the bottom level of apron is Total creep length from point A on the upstream side upto the point above D under the solid apron is 1.
Since the bottom of apron is above the assumed tail water elevation , the weight of concrete fully takes care of the uplift, as there is no loss of weight in concrete due to buoyancy. Each meter depth of concrete can withstand a head of 2.
Allowing an extra 20percent thickness to withstand any variations, the thickness of apron required is 1. The second apron can be similarly checked and a thickness of 50cms. Related Interests.
Design of surplus weir