Cantilevered Retaining Wall in Limestone

[Christopher C]

Happy Sunday everyone,
Kind of a 3 part question here...

I am engineering an 11ft retaining wall for a hillside swimming pool. The geotech report predicts limestone about 3 ft below grade. The sub-contractor has requested getting rid of the heel and the toe of this wall, and keying it 2-3 ft into the limestone. At first glance I don't see a problem with designing it this way as long as the rock can be exposed and confirmed. Do any of you have any experience with this sort of detail?... Do you recommend it?... How do I make sure this wall doesn't rotate out of it's key?... Is there anything I should watch out for?

Part 2:
The sub-contractor also told me he was planning on using gunite to fill these forms. This one does not seem like a good idea to me. I understand gunite is used for the shell forming all the time, but I just don't see how they are going to spray gunite inside a form and ensure complete coverage and a consistent density for a 12in+ thick wall. Is this possible with gunite?

Part 3:
The sub-contractor has also requested using sand bags instead of select fill. My thoughts on this are that the bag material should be expected to deteriorate, and thus all the voids between the bags should be filled with fill anyway, and each layer of bags + fill should be compacted. I feel like this would be more work than just laying down fill. Any thoughts?

I believe in being receptive to input from sub-contractors, and I also believe in saying no. I just like to make sure that my "no" comes from a place of reason, and not fear of the unfamiliar. Thank you in advance for your input.

 

[dik]

Sometimes the upper surface of limestone can be badly weathered and may not be sound.

Dik

 

[BAretired]

1. Wait and see how the limestone looks. I have never seen this condition. Your geotech may have some thoughts about this. You may want to check to see if this has been done in the neighborhood and how it has performed.

2. Gunite (or pneumatically placed concrete) is not used to fill between forms. It has often been used to build the walls of odd shaped and difficult to form swimming pools where it may be sprayed against an earth form. It has been used against a single plywood form with mesh attached to form the cylindrical wall of a water reservoir or dome shaped roof.

3. I agree with you on the sand bag idea for the reasons you gave. It doesn't sound like a good permanent solution.

Edit: Perhaps you should inspect similar projects of the sub-contractor to see how his ideas worked out.

BA

 

[Mjkkb2]

I would not be ok with keying into the limestone as you would be relying on the very upper layer to resist high stresses to provide the fixity.
I think that the failure mechanism for this Detail would be very hard to model and evaluate without some significant experience in rock mechanics.

The sandbag idea it’s also not very good.
Ask your geotechnical engineer for opinion but I believe he wouldn’t recommend using bags.

I am skeptical of the Gunite idea. Are you designing the wall with only one layer of reinforcement? Wouldn’t you need per code to have two layers of rebar for 12 inch thickness?
Unless the wall is a freeform shape I would rather have it be formed and poured in a traditional manner.

 

[dhengr]

Christopher C:
Why not a free standing, battered back, reinforced Gunite wall. You claim 3' of soil and about 8' of limestone, which should be battered back as it is excavated, including some sort of a base key trench down into the limestone and a water drainage system. Then install a rebar gage and the pneumatically placed concrete.

 

[BridgeSmith]

An 11 ft exposed height with 3 ft of embedment into ANY natural material is a failure in the making. Rock formations of any kind typically have internal fracturing that is impossible to identify, must less predict. Limestone that near the surface is likely to be heavily weathered, meaning it will likely have more fracturing than the underlying unweathered rock. Keying footings into bedrock is commonly done to prevent sliding, but not for resisting moment in a non-gravity cantilevered retaining wall. For that, you would need core samples of the rock tested and the overall strength of the rock mass estimated using one of the standard methods that accounts for the internal fracturing. This is generally undertaken by a geotechnical engineer. Depending on the regulations in your state, you may be able to go through the process in the AASHTO LRFD Bridge Design spec. or another procedure, but doing it right if you're not familiar with the method can be very time-consuming. Once you have the cohesive strength of the rock mass and the loading due to soil pressure (water pressure?) and applicable surcharges, you can use the design method in the AASHTO LRFD or another source to calculate the required embedment and bending moment on the wall. From there, it's just a cantilevered slab design.

Even with hard rock with minimal fracturing retaining typical soil backfill, my initial guess would be the required embedment would be at least 4 ft. With weathered bedrock, it will probably be closer to 6 ft.

In any case, rock that is hard enough to support an 11 ft high wall with only 3 ft of embedment would be too hard to excavate with anything less than a rock saw. My suggestion would be to consider a soldier pile wall consisting of augered holes with grouted steel piles, or reinforced drilled shafts.

 

[Christopher C]

Hello everyone,
We ended up designing a buttress system that broke the retaining wall up into 12' long pieces that I was able to design as 3 side supported slabs. The whole system is keyed into limestone and will be much more easily stabilized against turnover than a cantilevered wall... and it's way more cost efficient. Anyway, I just wanted to say thank you for your input!

Here's a cool little phasing slideshow I created for my client:

https://docs.google.com/presentatio...mYw5/pub?start=false&loop=false&delayms=10000