concrete weir design, like a retaining wall, but only water 1

[bhauth]

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.

 

[SlideRuleEra]

From your description, it sounds reasonable to assume that the soil on both sides of the weir will be saturated. With that being the case, the lateral earth pressures do cancel each other.

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[bhauth]

Yes, the soil on both sides of the weir will be saturated. I just didn't know if one side was passive and the other side used active earth pressure or what was safe to assume. So the only overturning force I will have will be the force of the water?

 

[msquared48]

And any overturning from the dead load of the retaining wall due to the stem wall weight and the relationship of the toe and heel lengths. If the toe is short and the heel is long, it will add to the overturning of the water. The opposite subtracts. If the toe and heel are equal, there is no difference.

Mike McCann
MMC Engineering

 

[LCruiser]

You will also have to consider erosion on the downstream side. Depending on the downstream conditions and flow velocity (supercritical vs. subcritical. distance to next and ultimate grade control, penalty for failure, etc.) you may want to treat it as a submerged retaining wall with nothing on the downstream side.

 

[cvg]

saturated soil both upstream and downstream may provide little resistance against lateral movement. also consider that buoyancy will reduce the effective weight of the footing. Seepage under the footing needs to be considered also For this reason, flood walls / weir walls generally have very large and deep footings to provide stability and prevent seepage.

 

[VoyageofDiscovery]

Ice loading is generally the governing case.

HTH

 

[LCruiser]

Good Answers on this forum!

 

[miecz]

If the water level is up to the top of the weir on one side, what is the minimum water level or water table on the other side?

 

[bhauth]

The design case for water level on the other side would be at the bottom of the ditch, basically the top of the ground level.

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?

 

[fattdad]

I'm just posting a question on this topic for some further discussion: Will this weir always convey flow? If this is a ditch and there is some measured flow (as governed by the weir) only during storms, the question pertaining to the "water table" level seems irrelavent - at the time of flow the surface water would be perched and likely not connected to the phreatic surface. Now if the flow lasted for a matter of days or weeks, there may be some infiltration to "mound" the phreatic surface and there may become some bouyant affects, but this may or may not be the case. This is the problem with transient flow problems.

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!

 

[bhauth]

The weir will have a gate which can be closed in the case of flooding downstream to "back up" the water behind it. 99% of the time, this gate will be open and water will be allowed to flow freely. The gate will only be closed during a flooding situation.

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?

 

[miecz]

I was thinking that, in case the flow over the weir wasn't continuous, if the water table on the downside was below the ground level, that would effect the "balance" of soil pressure.

 

[SlideRuleEra]

bhauth - You have not included the weight of the water on the footing (consider it a surcharge). The weight would be uniformly distributed above your "No. 4".

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[miecz]

I don't think you can say that your soil pressures "balance". In your sketch, it looks as though your only lateral pressure is [γ]H, where H is 9 feet. I think you need to calculate your pressures on each side of the wall independently, and include the water level on the left side of the wall.

 

[bhauth]

meicz - The flow through the gate IS continuous. So I think it is safe to assume the soil is saturated at all times on both sides.

 

[bhauth]

Do any of you have an equation for "Surcharge as an equivalent soil height"? My unit weight of soil is 120 pcf. Water is 62.4 pcf.

 

[miecz]

bhouth-

In your sketch, your resultant lateral pressure is [γ]H, where H is 14 feet. The resultant would be applied at Elevation 1049.67 feet, which is below the ground level. You showed [γ]H above the ground level.

 

[SlideRuleEra]

Sure, from your sketch:
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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[bhauth]

Yeah, that's what I thought the Equivalent Surcharge would be. The program that I'm using is just making things more confusing than they really are. I will just keep things separate and do my stability analysis by hand. Technology is useful if I can fully understand it.