concrete weir design, like a retaining wall, but only water
concrete weir design, like a retaining wall, but only water
(OP)
I am designing a concrete weir to be placed in a ditch for a project I'm working on. The basic layout is that of a cantilever retaining wall, except the soil height is the same on both sides of it. It is designed to hold water back up to the height of the top of the wall. Do the lateral earth pressures cancel each other out since the soil level is the same height on both sides? Then would I just have the lateral force of the water to deal with for overturning and sliding stability? Please let me know as soon as possible because I'm not very experienced in these designs. Thanks.





RE: concrete weir design, like a retaining wall, but only water
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RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water
Mike McCann
MMC Engineering
RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water
HTH
RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water
I've designed it for the bottom of the foundation to be 5' below the surface, (for frost depth). This is basically quite a small structure compared with what you are all probably imagining. The top of the wall will be approximately 9 feet out of the ground, with a 5 ft deep foundation. The weir will also be about 30 ft in length from one ditch bank to the other.
Will bouyancy be an issue because the structure is not actually displacing any water?
RE: concrete weir design, like a retaining wall, but only water
I agree that the absence of unbalanced earth pressures means that the overturning forces are water pressure (both static and dynamic) and (good point) ice. This problem requires a free body diagram showing all external forces and then sum the forces and moments to see just how much of the passive earth pressure is mobilized. The way the questions have responded so far, the sense is that you would look at this as a plane strain problem. Heck if the weir is only a few feet square, you may also want to look a the forces along the sidewalls too - don't have these details.
Regarding seepage forces, cutoff walls, etc. The chance of blowing out the weir is pretty great if you don't account for internal drainage along (around) the edges of all concrete. There are conditions where gravel can be your friend as it's not prone to piping failure. Without some design sketches, however I could be way off track. . .
Good luck on your project.
f-d
¡papá gordo ain't no madre flaca!
RE: concrete weir design, like a retaining wall, but only water
I've included a quick sketch of what it will look like. This drawing is not to scale but I've included the dimensions and elevations. Do I have the basic FBD correct or are there additional forces that I haven't accounted for?
RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water
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RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water
In your sketch, your resultant lateral pressure is γH, where H is 14 feet. The resultant would be applied at Elevation 1049.67 feet, which is below the ground level. You showed γH above the ground level.
RE: concrete weir design, like a retaining wall, but only water
Saturated Soil Equivalent Height for 9 Ft. of Water =
62.4 pcf (water) / 120 pcf (saturated soil) x 9 ft.
But the free body diagram would be easier to understand if you keep the weight of the water separate (say a "No. 5" item added to your sketch).
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RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water
I'd worry about piping at either abutment however. . .
f-d
¡papá gordo ain't no madre flaca!
RE: concrete weir design, like a retaining wall, but only water
1.) if the soil is clay, use the 120 psf and water to elevation 1050.
2.) if the soil is sand, the eefective unit weight of the soil will be about 65psf and the net water pressue will reach a maximum at 1050 and the decrease to 0 at 1045.
Sum moments about the toe. Soil masses 3&4 will have same wt, but different moment arms, so they need to be included in calculations. FS should =1.5 Check sliding & Bearing also
RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water
I'll retract part of my earlier statement as the calculation of base shear will require unit weight to get the "N" of the NtanDelta. Just use bouyant unit weight.
O.K., I'll go to bed now. . .
f-d
¡papá gordo ain't no madre flaca!
RE: concrete weir design, like a retaining wall, but only water
I'll use the lateral water pressure as my counterclockwise moment placed at 1/3 of the height of the water.
This being said, without doing the calculations, my factor of safety should be sufficient for overturning.
RE: concrete weir design, like a retaining wall, but only water
There will be water pressure upwards against the bottom of the footing, and if there is seepage it will be greater under the upstream edge that the downstream edge. A conservative (usually) assumption is that the water head at the upstream edge is the full headwater elevation and the head at the downstream edge is the tailwater elevation (in your sketch, the ground surface). In any case, the uplift creates an additional overturning moment.
Hydrostatic uplift is ignored in most retaining wall design, but never in dam design. This is a dam.
RE: concrete weir design, like a retaining wall, but only water
Also, the soil pressures would cancel out. In fact, I would model active earth pressures on the water side and passive earth pressures on the ditch side.
RE: concrete weir design, like a retaining wall, but only water
Look up flood mitigation for more info.
RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water
RE: concrete weir design, like a retaining wall, but only water