Basement Wall at Attached Garage

[StrucDesignPE]

I am working on the design of a custom house, nothing fancy, but the client's layout has an attached garage at the rear basement wall.

My design approach:
1) Designed the wall as a cantilevered retaining wall so that the client can have the basement backfilled with controlled, compacted fill prior to placing the floor framing to provide restraint to the top of the wall.
2) I then design for the restrained condition wherein the reinforcing is on the opposite face as the cantilevered condition.
3) My surcharge load on the retained soils is 150. I assumed 75 psf for a 6ish inch thick slab and then another 75 psf just in case heavier vehicles are parked in the garage. This is greater than the 40 or 50psf that I have seen in building code.
4) I don't know much at this point about the soils on site, so I designed for 45 active and 60 at-rest. My client said they are okay with designing it conservatively.
5) The total wall height is 9'-4". There may be a larger step in the location of garage slab in relation to the floor framing. I am waiting to hear back from the client for the definitive answer.

My concerns:
1) I am always concerned about compacting soils next to the foundation wall, especially with CMU. However, my client wants to keep the CMU. What concerns do any of you have with compacting in small lifts with a small plate tamper against basement walls. Mind you, this will be done when the wall is in the cantilevered configuration and not restrained at the top of the wall.
2) I am also concerned about potential for the garage slab to settle and crack if there happens to be settlement in the compacted soil. I don't anticipate that, but I think it is possible given the right conditions.
3) I don't want to detail the slab to sit on the foundation wall and be reinforced to bridge over the excavated and loosely backfilled soil wedge. I personally think that is a messy detail and depending on the skill level of the contractors, may not be done correctly or may be overly costly.

Pasted below, you will find my detail for this condition as I have designed it so far. I am asking all of this because I have not personally designed a condition like this to date and like the idea of second opinions. I know this is commonly done here in my region of the US, but I have not personally dealt with one such as this.

 

[milkshakelake]

What concerns do any of you have with compacting in small lifts with a small plate tamper against basement walls. Mind you, this will be done when the wall is in the cantilevered configuration and not restrained at the top of the wall.

I'm wondering why the framing can't be installed first and then the wall be backfilled and compacted (bonus point for not needing to design footing for retaining wall). Anyway, I think you can easily account for compacting by incorporating some surcharge when calculating the active pressure. Something like 100 psf to 300 psf, more on the side of 100 psf. Meaning you take that surcharge, multiply by Ka, and add it to the soil active pressure.

I am also concerned about potential for the garage slab to settle and crack if there happens to be settlement in the compacted soil.

You're right to be concerned. There will pretty much always be slab settlement. You just use an expansion joint at the slab-foundation interface and put some control joints in the slab, and add WWF to the top for good measure. Also add a course of gravel under the slab for water to flow and reduce the potential for settlement. It's very commonly done. I believe it will be physically impossible to have zero cracks over the lifetime of the slab, but this will help. Another option, which doesn't look too good, is to use 4'x4' squares of slab like in a sidewalk pavement, which can tolerate a ton of settlement without cracking.

I don't want to detail the slab to sit on the foundation wall and be reinforced to bridge over the excavated and loosely backfilled soil wedge. I personally think that is a messy detail and depending on the skill level of the contractors, may not be done correctly or may be overly costly.

I wouldn't do that. Doesn't make sense to me, I don't really see the reasoning. So I agree about not doing it. I only do reinforced slab-on-grade over a backfilled area if it's bridging a 4' gap between two foundation walls or something (like a ramp or walkway).

 

[DoubleStud]

Your footer will be huge if you try to design it like a cantilevered retaining wall and keep the deflection on top to a minimum. How long is this wall? You may want to utilize some kind of counterfort to stiffen the wall up.

 

[JAE]

All residential projects like that I'm aware of do what milkshakelake suggests - require the floor framing and diaphragm to be fully constructed prior to backfilling.

Your backfill, even if plate compacted, will settle - to what degree is unknown but I'd expect at least 1/2" and at most...well, we've had garage slabs removed in the past to reveal a 24" gaping void below the slab. To this day I've never figured out how the slab remained essentially in place with car wheel loads on it and the void below.

 

[StrucDesignPE]

Thanks milkshake, you are echoing a lot of the thoughts that I was having on this. I am going to go back to my client and see if we can get the framing in place and design this solely as a restrained wall.

Doublestud, that is exactly what I was hoping to avoid, a huge footing and an unusual detail such as a counterfort for a residence. Just seems like overkill for something that can be remedied in an easier way.

JAE, thanks for your input. I think the consensus is to just really press to get this designed as a restrained wall and avoid this type of detail. I know I am much more comfortable with that simpler design and construction. Less chance for cost overruns and errors.

Regarding the backfill and compaction, I have seen some folks specify backfilling with a clean or washed stone, such as #57 stone (3/4" dia). This could be placed in lifts and still hit with a plate tamper to consolidate (because it won't compact). This would also allow for good drainage down to the foundation drain tile. Thoughts on this?

 

[XR250]

I don't want to detail the slab to sit on the foundation wall and be reinforced to bridge over the excavated and loosely backfilled soil wedge. I personally think that is a messy detail and depending on the skill level of the contractors, may not be done correctly or may be overly costly.

This is how I do it. Does not take much rebar and detailing. Keeps the customer happy as they hate when the slab settles and they have a lip to drive over. I have fixed many of these over the years that settled.

 

[StrucPatholgst]

Why is the footing so big. 8 feet wide?

 

[jhnblgr]

Why a CMU wall? My experience is the blocks rot out in 20 years. A 10” concrete wall with a 4” brick ledge on top and pin the slab into it. Also if you use clean crushed stone compaction should not be an issue.

 

[StrucDesignPE]

XR250, I'd be interested to know your thoughts on the potential outcome of using washed stone for backfill, especially since you have had experience in repairing settlement from such problems.

StrucPatholgst, the footing in my detail is so large due to a combination of overturning in the cantilevered wall condition and bearing being assumed at 1500psf. Reference my original post for the details.

Jhnblgr, the client has their heart set on CMU. I don't know if it is a cost thing or if they had bad prior experiences, but that is what they want and are not willing to consider CIP concrete.

Also, not sure what you mean by the CMU rotting. Not seen that personally, even in the extreme foundation failures I have investigated.

I was able to have a short exchange with my client today and they are willing to follow my recommendation and place the floor framing prior to backfilling, so at this point, I plan to design as a restrained basement wall and am strongly considering using washed gravel for backfill. Again, just not crazy about the idea of bearing the slab on a wall ledge and reinforcing it to span.

 

[JAE]

CMU and placed (not poured) concrete are both made of ..... concrete. No rotting involved technically speaking.

Perhaps CMU is more susceptible to freeze-thaw damage - mostly in the mortar joints.

 

[XR250]

Why a CMU wall? My experience is the blocks rot out in 20 years. A 10” concrete wall with a 4” brick ledge on top and pin the slab into it. Also if you use clean crushed stone compaction should not be an issue.

I actually saw this recently with some 60 year old CMU that was not damproofed. Have not ever seen it before though.

I'd be interested to know your thoughts on the potential outcome of using washed stone for backfill, especially since you have had experience in repairing settlement from such problems.

It def, helps but I usually still see settlement. If they vibrated it in 8" lifts, it might work but no one does that.

 

[StrucPatholgst]

The idea of backfilling a foundation before the fist floor framing is in place strikes me as pretty odd. It's good you talked them out of it. But it reminds me of the means and methods trap. In my area, contractors are all booked up, and property owners are starting to drag the bottom for anyone to do their projects. Since the start of the year, I've averaged 2-3 clients per month that are trying to do the work themselves, or hiring folks that changed careers and want to be contractors, or simply hiring laborers and buying them tools. Once a month now, I have to have a conversation where I'm highlighting a bad shortcut, or a gross overkill, or the dreaded "have you ever done this before." I even get calls from people wanting to know how to do wall removals or vault their ceilings. I am not Mr. Means and Methods. I am not a job superintendent. I am not a teacher. I am the engineer/draftsman of the as-built condition. If your beam falls on your leg because you've never done this before, I will not be involved, nor will I be caught dead documenting suggestions or procedures on how you should do your build. That is liability with a capital "L."

This is pure speculation, but I suspect your client is trying to get the foundation done and the crews out of the way before he himself starts framing.

 

[StrucDesignPE]

This is pure speculation, but I suspect your client is trying to get the foundation done and the crews out of the way before he himself starts framing.

I have a feeling you are hitting the nail pretty close to the head.

 

[DoubleStud]

I do many projects in the mountain where we build on an extreme slope so the basement wall can be 20 ft tall (2 levels). I try to design the foundation to be free standing with the help of counterforts or buttresses. But we do put notes to frame the floor first. But last time I went to a job visit for a job like I mentioned, they backfilled it without framing the floor first. My spec notes give me extra protection if the wall does deflect too much, but I am glad I designed the foundation that way. When I look at the job site, it is somewhat impossible not to backfill it first. You wont be able to drive anything close to the house if you don't. That would be very hard to drop off your lumber to frame the floor first.

 

[XR250]

I would not count on framing bracing a 20 ft tall wall anyway.

 

[Eng16080]

I’ve used counterforts for similar designs as this (maybe slightly larger). I didn’t think it was that outrageous of a solution. The counterfort will be large but should offset the massive 8 ft wide footing for the retaining wall design in this case. Another option is a full length concrete tie beam (shallow wall) running front to back which is located at mid span of the back wall (where the counterfort would presumably be).

With that said, in this case I would probably try to rely on the floor framing for restraint as well as possibly a connection between garage slab and wall.

I feel like the typical connection between top of concrete wall and floor framing is often inadequate based on the design loads from lateral soil pressure. Of course, I’ve never seen this fail. This does seem a little higher risk at a garage though as opposed to being an exterior wall.

 

[Tomfh]

Similar experiences to XR250.

We’re remediating an 25 year old building at present, where the adjoining pavement has settled 50mm due to settlement of the backfill.

The fines washed out over time, reducing volume, making it worse. Settled pavements. Broken services.

In my opinion it’s risky to count on the backfill not to compact over time. We typically bridge across from the building as discussed above.

 

[Eng16080]

If you're relying strictly on the floor framing to resist the lateral wall pressure, I think you'll find that you need anchor bolts spaced about 6" o.c. to transfer the load from the wall to the sill plate.

 

[HouseBoy]

With the counter-fort approach I find that the footing width at the counter-fort is fairly wide (lots of to pressure). Still, it beats an 8 ft wide footing (especially over-digging on the soil side so much).

Regarding the resistance to high lateral pressures at the top of the wall, I have used Simpson Foundation Wall Angles to help this condition.

Often I see rebar dowels at the edge of the slab, into the foundation wall. Sometimes I see a small ledge or "groove" met into the foundation wall where the slab will be located along with the dowels.

Others are right in that "nobody" places the backfill in lifts with connection. Usually it is placed with a "Slinger".

 

[DoubleStud]

If you're relying strictly on the floor framing to resist the lateral wall pressure, I think you'll find that you need anchor bolts spaced about 6" o.c. to transfer the load from the wall to the sill plate.

What if the joists hang from the wall? The joists will be in compression and not rely on the sill plate? Or if the joists run perpendicular, you do blockings the other way for several feet and rely on the nailing to the plywood?