Cantilevered wall retaining water 3

[DenverStruct]

Lets say you have an L shaped retaining wall that is buried around 3 ft and stick out of the ground about 10 ft. Can you count on the weight of the water above the heel to resist the load? I do not think I can, can't I? Worst case, water will fill up to the top of the wall.

 

[WARose]

Are you talking about the water table level? If so, it's likely going to hurt you because you'd have to consider buoyancy and hydrostatic pressure. (While only getting the "help" of a saturated soil weight.)

 

[rb1957]

you mean like the water is an elastic support for the wall ?

I've thought that the water would be applying a lateral load to the wall, adding to it's other loads.

another day in paradise, or is paradise one day closer ?

 

[DenverStruct]

Basically worst case is there liquid all the way to the top of the wall. Can I use the weight of water on my resistant moment calculation?

 

[WARose]

Basically worst case is there liquid all the way to the top of the wall. Can I use the weight of water on my resistant moment calculation?

Yes....as long as you consider any buoyancy effects and hydrostatic pressure (as well).

 

[DaveAtkins]

I agree with WARose.

If you consider buoyancy and lateral hydrostatic pressure as loads on the system, then it is fair to assume the overburden on the toe and heel of the wall is saturated soil (saturated, not submerged, as long as you apply the uplift due to buoyancy below the footing).

DaveAtkins

 

[DenverStruct]

So just subtract 62.4 pcf to the dead weight of the soil and the footing? You do not think there will be about 870 psf (14 ft deep) of hydrostatic pressure pushing the bottom of the footing up? What if the soil is saturated?

 

[JedClampett]

Most Geotechnical Reports encourage you to drain the back of retaining walls. The soil loads get very large (90-110 pcf for saturated soils)and then you still might have to add in the fluid pressure. Is there a reason you're not draining the wall?
This doesn't change the answers above, but I'm curious.

 

[DenverStruct]

This is not a retaining wall. It is a cantilevered wall to hold liquid. It is a secondary containment system for a large chemical tank. It will only hold chemical if there is a failure.

 

[WARose]

[blue](DenverStruct)[/blue]

So just subtract 62.4 pcf to the dead weight of the soil and the footing? You do not think there will be about 870 psf (14 ft deep) of hydrostatic pressure pushing the bottom of the footing up? What if the soil is saturated?

I'd just figure all the weights like normal....but subtract the submerged volume of the foundation (multiplied by water density).

And you'd also have a net (lateral) hydrostatic pressure on the wall.

[blue](JedClampett)[/blue]

Is there a reason you're not draining the wall?

I never count on that. The drains get plugged pretty often (in my experience).

 

[WARose]

[blue](DenverStruct)[/blue]

This is not a retaining wall. It is a cantilevered wall to hold liquid. It is a secondary containment system for a large chemical tank. It will only hold chemical if there is a failure.

Getting that info (or a pic) from the outset would have been good.

If the liquid is contained in the concrete structure (and the foundation will never be below the water table level (according to the geotechnical report)), there should be no uplift.

 

[MotorCity]

I think we need a sketch or photo to make sure we are all talking about the same thing.

How does an L-shaped wall retain liquid?

Whats the difference between a retaining wall and a cantilever wall holding liquid?

 

[Guest262653]

I call it a bund wall. For large containments of dense liquids, these walls can be quite big and do not have the same Height/Base ratio as normal retaining walls.
If an impervious ground slab that prevents seepage under the foundation is connected to the wall or to the foundation, I would consider the positive effect of the liquid vertical pressure on top of the foundation and no uplift. If you can't avoid seepage under the wall, I'd consider a toe drain and assume an additional uplift triangular pressure diagram under the foundation, going from 0 at the toe to the maximum pressure value at the heel tip.

 

[WARose]

[blue](avscorreia)[/blue]

I call it a bund wall. For large containments of dense liquids, these walls can be quite big and do not have the same Height/Base ratio as normal retaining walls.

Interesting. But like MotorCity said: you couldn't retain fluid with a L-shaped wall......that is, unless the top of footing slopes (down) to the corner.

 

[Guest262653]

It's possible, if you use four walls and a ground slab to connect them, thus forming a large tank. They are usually part of the spillage control system around large liquid storange tanks (usually chemicals).
I was once involved in the preliminary design of one of such systems for two 10000m3 NaOH tanks. We specified one of these walls along with a ground slab that covered the entire 4200m2 area around the tanks, connected with water stop joints to to the walls and tank foundations, thus avoiding seepage and uplift issues. I enclose a sketch for clarity.

 

[ATSE]

Either you have a robust seepage control system, or you don't.
If you do, then the hydro down pressure provides a resisting moment.
If seepage pressure exist below the footing = (water unit weight) * (wall height + ftg thickness), then you just have the dead load of the footing providing a resisting moment.
As noted above, a sketch of complete explanation of the boundary conditions will help this conversation.
Not sure if this is what you have, but open floor structures can be problematic for overturning and sliding.
For the environment structures I work with (mostly water & wastewater), they all have continuous thick concrete floors.

 

[DenverStruct]

It is exactly like avscorrie's drawing except it does not have a slab. The L shape is just buried down 3 ft with just soil. No concrete slab on the finished grade. The project is also in South Texas where we assume ground water is at 0.

 

[MotorCity]

So, you DO have a retaining wall (i.e. L-shaped cantilevered wall). To answer your original question: yes, you can count on the weight of the water above the heel to resist overturning, as long as you also consider buoyancy. If the water is there to provide a lateral load, it is also there to provide a vertical load.

On a side note, since there is no "bottom" for this containment area (other than where the heel is located) aren't you concerned about soil contamination if/when there is a chemical spill/overflow?

 

[BridgeSmith]

"...you can count on the weight of the water above the heel to resist overturning..."

If I'm understanding the configuration we're talking about, without a slab or some other means of preventing infiltration of the water into the backfill and under the footing, you cannot count on the weight of water. The water pressure will equalize and the only resistance is the net weight of the footing and soil backfill on top of the footing (density of soil and footing minus density of water). The slab with a waterstop is required to be able to use the weight of water for resistance. The soil around the footing can be saturated, but not submerged.

If the wall is circular, you could design it essentially as a tank, using the "hoop strength" to resist the water pressure.

 

[MotorCity]

HotRod10 - Agreed. The weight of the water being retained will counteract the effects of buoyancy. Also, if the height of water being retained above grade is greater than the depth of the wall below grade, it wall also counteract overturning.