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.
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concrete weir design, like a retaining wall, but only water 1
- Thread starter bhauth
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If you sum the moments from the bottom of the vertical concrete (i.e., at the bearing layer) you get 20.24 k-ft counterclockwise (the weights of the concrete and the soil prisms atop the concrete null out). If you consider the bearing surface (assuming that you'll find at least 3 ksf), it's likely that you need no lateral earth pressure for your stability. 3 klf over 2/3rds of the footing provides 16 kips and a moment arm of 1.33 ft, which is 21.3 k-ft. You can apply more finesse if you want to, but I'm just working on the back of an envelope - ha.
I'd worry about piping at either abutment however. . .
f-d
¡papá gordo ain’t no madre flaca!
I'd worry about piping at either abutment however. . .
f-d
¡papá gordo ain’t no madre flaca!
I see 2 cases:
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
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
Unless the forces require the mobilization of lateral earth pressures, the unit weight of the soil doesn't appear too relavent. You have the same prism of soil on either side of the vertical wall, so these weights do not contribute to overturning stability unless a portion of the bearing goes into tension. Again, look at the geometry of the sketch. If built as designed, there is sufficient bearing to offset the overturning of the water pressure. Summing the horizontal forces only requires either enough friction or passive pressure to offset the unbalanced hydrostatic force, which shouldn't be much of a problem.
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!
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!
- Thread starter
- #25
Ok, so on my FBD, I'll use the unit weights of the saturated soil on top of my footing, the weight of the concrete, and the weight of the water on top of the upstream side of the footing as my clockwise moments.
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.
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.
aeoliantexan
Geotechnical
The lateral water pressure is a triangle from the top of the wall to the bottom of the footing and acts at 1/3rd of that total height, as meicz said. If the downstream soil is always saturated from continuous flow, there will be a similar resisting triangle of water pressure against the downstream side from the ground surface to the bottom of the footing. Work with total heights of water, not differential heights.
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.
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.
My two cents (for whatever it is worth): Construct a flow net to help determine seepage forces, particularly along the bottom of the footing. In addition, the hydraulic gradient can be estimated to determine the factor of safety against piping failure.
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.
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.
The maximum height of the water on the upward side can be higher than the top of the dam. In order for the water to flow over the dam its has to be some depth higher than the top of dam. This depth depends on the actual flood water flow and needs to be calculated.
Look up flood mitigation for more info.
Look up flood mitigation for more info.
civilperson
Structural
Weirs to function correctly, (measure amoint of flow by measuring height of fluid passing over), need a SHARP edge for the fluid to pass over. Perhaps the top of your weir could have a metal plate with a beveled edge to produce a sharp crested weir. Otherwise you have built a check dam to produce a constant elevation of fluid behind the dam.
- Thread starter
- #30
None of the last 2 comments have any merit for the scope and context of this structure. Its not technically a "weir," It is basically a dam at a set elevation that will allow water to divert down a ditch if the water surface gets high enough. This diversion will lead INTO our Impoundment, which will have a dam at an elevation higher than that of the "weir."
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