Retaining Wall Retaining Water 3

[kvillebasser]

I have been asked to design a cast-in-place concrete retaining wall that will retain water, not soil. As it turns out, the client wants to use this wall to act as the dam for a small retention pond. The wall can retain as much as 10' of water.

Typically, in retaining wall design, I would count on the soils on the heel of the retaining wall footing to help provide stability to the wall, but when the wall is retaining water, it seems to me that one cannot count on the water to aid in stability, and in fact makes matters worse, as the heel must now be calculated to have a buoyant weight instead of its normal dead weight, which makes the wall even more unstable (more likely to overturn).

Seems to me, the dead weight of the wall and the dead weight of the toe, and the buoyant weight of the heel are all that can be counted upon to provide stability. Do you guys agree?

 

[JedClampett]

You can use the weight of the water over the heel. We do this in reservoir design all the time. If you don't you are going to end up with one huge retaining wall.
Don't forget that the factor of safety should be at least 1.5 vs sliding.

 

[SlideRuleEra]

Concur with JedClampett - If that's what the client wants, cast-in-place concrete will work just fine. Depending on the soil conditions (maybe rock?) sheet pile would not be practical anyway.

 

[PEStructural]

Also, make sure to include a neoprene waterstop at the wall/footing connection so water can't leak under the wall.

 

[kvillebasser]

Seems to me that in a standard, soil retaining, retaining wall, the weight of the soil is used in the calculations to help provide stability because it acts as a "rigid" structure....in other words, if the heel raises up, the full weight of the soil is still engaged on the heel.

But when retaining water only, it seems as though if the heel moves slightly (rotates due to lateral water load) and water can get beneath the heel, the water pressure equalizes (top of footing heel verses bottom of footing heel) and only the buoyant weight of the concrete can be counted upon for stability.

Thank of it this way.......lets say I had a 10' x 10' plate buried 10' in the ground. Imagine the plate had a cable hooked to it at the center. If I tried to pull the plate out of the ground, I'd have to raise the plate, plus the weight of a block of soil 10' x 10' x 10'.

but if that same plate was in a 10' deep pool of water, and I went to raise it with a crane, I'd only have to pick up the buoyant weight of the plate.

 

[letrab]

Hi kvillebasser,

Another way to look at your problem is to consider its function. It should retain the water and not let it flow under the wall to allow uplift pressures under the foundation.
The way I understand it is that you must ensure that the retaining structure is the most watertight element and everything downstream is more permeable. Consequently water passing by or through this element must be able to drain faster than it can enter.
If this is not the case the retaining wall does not retain the water, you get full uplift under the wall and it falls over. You would therefore require some watertight element like a concrete floor upstream of the wall with the joints to the wall foundation sealed.
However, if the structure is designed correctly the water pressure under the foundation is effectively drained and the full weight of the water can be used for stability.
Agrees further with JedClampett regarding sliding

 

[kvillebasser]

Hi letrab!

I'm wondering if assuming no water, or a drained condition, is practical in my case. The particular wall I've been asked to design will double as a dam for retention pond. The civil client is determining the "key" needed at the end of the heel that will provide cut-off of water that would otherwise flow thru the soils beneath my wall.

I think that in reality, there's going to be water beneath this wall.....I dont think there is any way around it. Water can not be cut completely off from the underside of this wall, in my opinion. I think that this wall's stability must be provided by the buoyant weight of the foundation only.......because I have no way to absoutely prevent water from getting under it.....also, retaining walls move. Soil consolidation over time, lateral loading of the wall, and many other items, contribute to movement of retaining walls. If I knew this wall would not move, then I'd be more likely to lean your way, but I've done this so long, I know that:

1. Water will get beneath this footing.
2. This wall will rotate, at least to some degree.

Thats just my gut feeling.

 

[haynewp]

I would talk to the geotech about all of this. I thought the drained assumption was dependent on the type of soil and how fast it is loaded.

Does the guy working on the key depth know enough about the soil permeability in order to determine the key depth required?

 

[jheidt2543]

I agree with JedClampett as long as flow beneath the "dam" is prevented. How to do that is a soils flow net problem that your Soils Engineer should be involved in. The cut-off key can be concrete or steel sheet piling and it's length is determined by the flow net analysis. The more permeable the subsoil the deeper the cut-off key (wall) must be. But once flow under the dam is prevented, then the weight of the 10' of retained water should be able to be used in the stability calculation.

 

[letrab]

Hi kvillebasser,

The way I understand your problem is that the wall will not have an impermeable "blanket" upstream of the wall. Consequently you will get full uplift under your foundation.
In my opinion it should be considered as a small dam and you should look seriously at the flow under the dam to assess water loss, piping and the pressure gradient under the foundation.
The position of your cut off will also influence the stabilty and pressure distribution of you wall.
Mass concrete or an earth embankment may be the way to go.
Another item you may consider is a spillway to safely discharge larger floods and ensure the excess energy is destroyed in a stilling basin.

 

[BigH]

There are more types of concrete walls than a cantilever which appears to the be main wall of discussion. Use a gravity wall as an alternative - yes, it will be more concrete but it is a common type wall - 10 ft. is not all "that" much of a head. You didn't mention what kind of soils you are founding on. You could also use the cantilever wall but use tie down soil/rock anchors on the heel to provide the "lost soil weight" so that you have more resistance to overturning. Could also use an inclined soil/rock anchor for help in resisting sliding.
There is no question that you have to consider uplift forces on the structure over the long term even if you do provide clayey blanket at base of wall extending out into the retention area and "think" that you are negating the pressure beneath the wall. Water will eventually reach its equilibrium flow and you need to take into account the various uplifting pressures. You don't need to have a small "uplift" so that water seeps into the "space". See Terzaghi and Peck on the basics of flow nets and pressures.