Retaining wall reinforcement

[BH6]

Hello everyone,
I have a retaining wall 50 feet in length and around 7' high that will be going up against an existing retaining wall. It will be now holding back water that can be on either side. I will not be using the existing wall to take any of the loads but it will not be removed. Therefore, the retaining wall will be L shaped. It is also in seismic zone D so I am checking ACI 350.3 for rectangular tank design even though this will work more like a retaining wall until you get to the corners. Also, I have shown the typical corner detail--any thoughts on what else I need to consider for either layout? I am putting it into a program with the bottom of the wall fixed and comparing the sides fixed with the sides pinned to see what loads might develop there. Any thoughts on what else I need to be aware of would be appreciated. Thanks for any help.

 

[dik]

Is the existing wall deficient? Can you reinforce with shotcrete?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BH6]

They are adding another "pond" next to the existing one so the existing retaining wall was your typical retaining wall with soil behind it. Now it will be essentially a wall that separates the water on either side. (Can be completely filled on one side and empty on the other) Also, we don't know anything about the existing wall so we are not using it to take any of the load to be conservative.

 

[dik]

Thanks... that explains the reinforcing for a 7' wall...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JedClampett]

You're going to have a problem with your waterstop. Either lower the top mat of reinforcing so there's about 3 1/2 inches of cover or use a starter wall detail.

 

[BH6]

Thanks, I should be able to lower the top mat just fine. I have not seen a starter wall on a retaining wall. Just curious if that creates any problems or other things to consider.

 

[JedClampett]

The starter walls create much whining, wailing and moaning from the contractor. If it's only 50 ft., it's better to just depress the reinforcing. It does change the depth and moment capacity of the slab, but slab on grade concrete is cheap, so just make it thicker.

 

[KootK]

Probably not. So, reluctantly, I reprise my role of concrete retaining wall detailing maven once again...

 

[hokie66]

As shown in KootK's sketch, U-bars are more effective than any other type of opening joint reinforcement. I wonder why there is resistance to making them standard practice. The diagonal bar is also good, but 'belts and braces' in most applications.

 

[BH6]

Thank you!

1. Why don't we do that on a normal retaining wall more often? Is it just cheaper/easier to do the L shaped bars?

2. And just so I understand --based on the moment I can get the tension in the bar by dividing the distance between the two bars on the U bar. But what about the four horizontal bars? I'm trying to think through how those would be designed.

3. For the diagonal bar --will the waterstop cause a problem for that? Have you seen that done with a waterstop?

4. Would you use this same detail in the corner?

Thanks again for your help.

 

[dsdt]

I always have this question.. What are these four horizontal bars shown in blue dots for?

 

[hokie66]

I would use the same detail at the corner intersection between walls. If you want to learn more about efficiency of corner joints, Arthur Nilson is the man. Lots of research into this topic, which is referenced in his books.

The four bars shown as blue dots serve to tie the U-bars correctly, and also as anchorage.

Problems with waterstop...yes, if you use that type waterstop.

 

[mchen96]

1. Why don't we do that on a normal retaining wall more often? Is it just cheaper/easier to do the L shaped bars?

It's easier on the contractor, particularly when bending the rebar by hand, since the tolerance for L-shaped bars is not as restrictive as for U shaped bars. When bending a U bar, the bottom part must have the exact width required, or there will be cover issues (if the bottom part is too wide) or capacity issues (if the U is too narrow).

On retaining walls with toes, the L-shaped bar works fine (with the leg pointing towards the toe). That being said, the L-shaped detail is not acceptable for toe-less retaining walls (water on the left), so you'll have to use the U-shaped bar.

Out of curiosity, how is the sliding of the wall check? That's usually a problem on heel-less walls.

 

[hokie66]

Bending rebar by hand? Who does that, and for what size bars?

 

[Settingsun]

Nilson is the man

Or go straight to the source: Nilsson.

In addition to the U-bar detail, keeping the bar size in proportion to the stem/base thickness is important so that the bars are developed within the corner joint. Nilsson's tests were on 10 & 12mm bars with Fy=390 MPa nominal. Most places are using higher strength bars now, substantially more in many cases. Combined with larger bars sizes, the conclusions need to be tempered. There was later research using this U-bar detail (but without the diagonal) that concluded bond failure is the likely culprit based on reanalysis of the old test data plus new tests. Smaller bars improved results (better bond), and more of the same size bar (greater reinf percentage without scarificing bond) had some benefit maybe due to confinement. Jackson in The Structural Engineer, July 1995.

In summary, large bars are the enemy.

 

[BH6]

Is there a specific book or paper that you would recommend I get? I can find several under either name. Just wondering which is best to buy.
I am thinking of this one

Out of curiosity, how is the sliding of the wall check? That's usually a problem on heel-less walls.

The slab in the bottom of the "pond" will restrain it and handle the sliding.

The four bars shown as blue dots serve to tie the U-bars correctly, and also as anchorage.

I will order his book and this may be in there, but how do you design these 4 bars? Or do you just match the horizontal reinforcement and then it's more a matter of just placing them there?

The diagonal bar is also good, but 'belts and braces' in most applications.

Does the book from Nilson or Nilsson contain when this is required and how to design for it? Or is this more of a place it there to help the joint behave like we would like and use it as an added factor of safety?

 

[bones206]

This might be a good application for an external pvc waterstop. Attach it to the existing wall, then place your new concrete up against it.

You could also use swelling or non-swelling strip type waterstops in the center of the wall. If preventing water seepage is critical, you can do both.

 

[BH6]

Problems with waterstop...yes, if you use that type waterstop.

I don't know of a better waterstop to use. I think this is what I need in this situation. Do you have a different suggestion? It needs to be completely waterproof.

 

[Settingsun]

That Nilson book is worth $3.99 (if that's really the price). I wouldn't recommend it at $100 if you only want it for this particular question.

The Nilsson article is available here:

https://www.scribd.com/document/460...-Subjected-to-Bending-Moments-Nilsson-Losberg

The bars in the corners of the U-bars are the same size as the U-bars or larger.

The diagonal bar helps to control cracking so would be good in this situation if you can fit it. Can you haunch the inside corner and push the diagonal bar into the haunch so it doesn't interfere with the waterstop?

 

[BH6]

You could also use swelling or non-swelling strip type waterstops in the center of the wall. If preventing water seepage is critical, you can do both.

I thought the swelling and non-swelling deteriorated over time. I also read that they can weaken the concrete around them. It is critical to stop all water seepage so that's why I didn't consider them. It will also be exposed to salt. I'll look at the external waterstop to see if that might work but I may still need the waterstop in the wall as well.