Unknown Walkout Retaining Wall

[CANeng11]

We have been asked to provide a repair detail for a walkout retaining wall (see attached picture). We don't have details on the reinforcement or footings, but the wall has a bow to it and it's about 10 years old. It's about 30 feet long, 6 feet high and 8 inch thick concrete (icf). The contractor would like to pour a slab in the bottom to restrain the bottom of the wall, but he is unsure how to reinforce the top of the wall. I'm wondering if anyone has any ideas?

 

[Craig_H]

Looks like a bit of a dead space at the moment. Perhaps a counterfort wall and footing could be added at the wall's midpoint, and that wall could be used as a mounting spot for a built-in BBQ or other amenity? Could also add a stair access to/from the depressed area, and use the side walls of the stair to add stiffness.

 

[oldestguy]

It might not take any repair, if the site drainage is not good. In oher words, if there is poor drainage, that may increase pressure beyond ideal. I'd check that first before doing any repair.

 

[CANeng11]

Good ideas Craig. We thought about the counterfort on the inside idea. We don't know the reinforcing of the wall, so we wouldn't be able to confirm that the wall could span comfortably to the counterfort. Do you think that's an issue? I also like the stair idea, the side walls would be like counterforts I guess?

Oldestguy, the drainage is definitely an issue. Without knowing the design of the wall reinforcement and footing, do you think it's enough to just fix the drainage issue?

 

[jayrod12]

Perhaps you just need to do some destructive investigation.

Dig down to the footing to determine what's there. Dig out the outside face of wall and cut the insulation away. Expose the reinforcing in a few locations to determine the size, spacing and location.

You could, in my opinion, give the owner the option of improving the drainage and see if that cures the issues. However as you already indicated the wall already has a substantial bow to it and therefore there are probably large cracks formed beneath the IFC insulation on the inside face. I wouldn't be surprised if you could see the vertical crack in the top of the wall around mid-length.

 

[JLNJ]

Is it a cantilever wall? Is it supposed to be for basement egress?

I’m guessing that it wasn’t engineered at all and a “typical” wall design which is intended to span vertically is now spanning horizontally.

Tread lightly (and conservatively) or you will have the liability for a collapsed wall on your hands.

 

[jayrod12]

I wouldn't be surprised if they reinforced it on the incorrect face either, given that most of the people using ICF blocks for formwork are used to pouring basements only.

 

[oldestguy]

It isn't caving in. So a slight bend. Pressures can more than double if saturation or near so happens. If you want to waterproof the area, I have done this treatment for reducing and eliminating basement wet conditions. I waterproof the ground surface outside and if possible give more slope to the surface. Briefly: Strip sod to WELL outside the infiltration area. In this case could be 15 feet or so. Work in POWDERED bentonite at a rate of 3 lbs per sq foot, about 3 inches. Use a garden roto-tiller. Replace sod. Do a test area first. If you only go 2 inches, reduce to 2 #/sf Ask for Driller's mud.

 

[Craig_H]

Yeah, as others have pointed out, there's no real way of knowing whether the wall can indeed span to a counterfort or other means of providing support to the wall without first figuring out where the wall's internal reinforcement exists. That's concern #1 obviously, figuring out what you're working with and then determining what is needed in order for it to be safe and serviceable.

I think you'll need to follow the suggestions of others in doing destructive testing. I've recently gotten a couple of quotes for GPR for smaller structures, and the cost of that service routinely gets close to the actual cost of replacing the structure altogether.

 

[JoshPlumSE]

I'd get someone out there to scan the wall for reinforcement. That seems like the most logical step. What's the cost of that $2000 per day. That's not too bad if it gets you the information you need.

 

[r13]

I think two concrete counterforts will work. The existing wall shall have reinforcement, the question is the bar size and spacing. You can chip out the concrete at where the counterfort and the wall meets to reveal the reinforcing detail. Do improve the drainage as indicated by OG. If the project is in cold region, it might pay for deeper excavation near the wall and replace with coarser fill.

 

[CANeng11]

Thanks for the input everyone. The contractor also had the idea of adding a concrete beam to the top that extends towards the outside like a sidewalk. My concern would be how would you take the reaction at the ends into the side walls. Any thoughts on if this is viable? I think I'm leaning towards counterforts either way.

 

[r13]

IMO, uncertainty of the sidewalk idea is high.

 

[STrctPono]

Here's another thought. Instead of approaching it by trying to make the wall strong enough to resist the load why don't you remove the load? Excavate out the back of the wall and either pour some CLSM or place a GRS backfill? This will also give you the chance to properly take care of the drainage. We've done this on many walls that were experiencing issues like you mention.

 

[PEinc]

I am with STrctPono on this one. Backfilling the wall with angular, crushed stone would reduce the earth pressure and improve the drainage. Add some new weep holes or clean out the existing weep holes. If you backfill with low strength flowable fill, it should be done in lifts to prevent full height liquid head. That will increase the cost. I would consider crushed stone.

http://www.PeirceEngineering.com

 

[CANeng11]

Would you still scan the wall to check reinforcement and then check the wall with the reduced loading from the lighter weight backfill?

 

[KootK]

The contractor also had the idea of adding a concrete beam to the top that extends towards the outside like a sidewalk. My concern would be how would you take the reaction at the ends into the side walls. Any thoughts on if this is viable?

It's viable as one of the options available to your. I've done it behind stairwells on a few buildings, in Alberta no less. It requires some attention to detail:

1) You may want to take some measurs to ensure that frost and or clay swelling don't exert an upwards pressure on the beam. Frost cushion, fill replacement with not frost susceptible material... whatever works for you and your contractor.

2) I like this as a beam but not a sidewalk. I worry there wouldn't be enough moment support of the walking surface at the joint with the wall at those proportions if you'll be lacking dirt support below the beam as I expect.

3) As you've rightly identified, proper detailing to get the load into the side walls is important. Important but straight forward. See my suggestion below.

4) You might circumvent the frost / heave issue by actually constructing the beam on the inside of the wall. If the owner would have been okay with counterforts, surely they'd be okay with this. I'd chip down a bearing pocket in the sidewall to cast the beam into.

 

[PEinc]

Would you still scan the wall to check reinforcement and then check the wall with the reduced loading from the lighter weight backfill?

I personally would not waste time and money scanning for resteel. In my experience, marginal (at best) information is obtained. If you excavate and backfill behind the wall with concrete or flowable fill (in lifts), the wall becomes a mass gravity wall and reinforcing steel is not needed. If you excavate and backfill with angular crushed stone and improve the wall drainage, you will have reduced the lateral load on the wall, probably significantly. Lightweight backfill would reduce the load on the wall as long as the backfill slopes up and away from the base of the wall at a slope of about 1.5H:1V. Installing a thin, vertical amount of lightweight backfill would not reduce the lateral load significantly. If you try to make the wall into a gravity wall, you don't want lightweight backfill. Lightweight backfill defeats the purpose of a gravity wall.

http://www.PeirceEngineering.com

 

[BAretired]

I like the idea of excavating outside the wall and backfilling with drain rock, provided the water can be drained away to a positive drainage system. Otherwise, the walkout area will be a lake after a good rainfall.

BA

 

[STrctPono]

1. If you use flowable fill (CLSM), PEinc is correct... this becomes a large gravity wall and you not longer have to worry about the rebar in the wall since you have a giant mass of solid material.

2. If you use GRS, you could also theoretically relieve almost the entirety of the load if detailed properly but you will need to familiarize yourself with how to design GRS. Not terribly difficult but will require some research.

3. If you use crushed angular backfill then yes, you will still have some load on the wall and you will still be left with questioning whether or not the wall is still sufficient.

I personally prefer option 2 but understand that you may not want to design the GRS or maybe the Contractor doesn't have confidence to construct it properly. Option 1 is good too but will need you to pay attention to drainage. You shouldn't backfill the CLSM to the finish grade. I would hold it back maybe 1 ft from the surface and then provide surface drainage in the area.

Side Note: With any of the 3 options, the homeowners should know that this pretty much precludes them from planting anything with a deep root system on the backside of the wall (which they shouldn't be doing anyway).