Lateral Pressure of Flowable Fill concrete against a basement wall

[Ron247]

I have a situation where a house transitions from a crawl space to a basement. The crawl space area immediately adjacent to the basement wall was not backfilled and the first row of interior piers for the crawl space are very close to the 8'+ drop-off. There has been soil sloughing off over time near the piers.

I told the Owner of the possible remedies, and they decided they want to fill the area with flowable fill. I have suggested doing it in 3-4 lifts and letting each lift set up before installing the next lift.

I have questions:

1. Am I correct that once the concrete cures, the lateral form pressure from that pour is no longer present against the concrete basement wall other than any lateral soil pressure from the embankment that may have "shifted" during the pour?
2. I recommended placing plastic across the bottom and up each side at least equal to the total pour height to help prevent liquid/flowable fill from seeping into the finished basement area due to the pressure. Is that reasonable to expect? It looks like the concrete wall sets on stone, not a concrete foundation.
3. I did not design the concrete wall but it was a manufactured product that was installed by the supplier. I am having owner's get confirmation from supplier it is designed for lateral soil loads of the expected height. Is there more I should have them confirm?
4. The finished floor above is wood. I have recommended well ventilating the crawl space during curing to minimize effects from the heat and moisture from the drying process of the flowable fill. Is that a reasonable expectation.
5. Anyone ever do this before, and if so, what problems did you encounter?

Any advice is greatly appreciated.

 

[XR250]

I have run into this before but have never had to come up with a fix. I assume flowable fill because access is limited?

 

[Ron247]

Yes, not easy to bring in soil or stone. Owner likes the "cleaner look" of concrete. I filled a large area before with flowable fill but it was a large crawl space some guy excavated to make a basement. He died before he ever finished. Years later his wife was trying to sell the house. Difference was, I was pouring against a wall of dirt, not a concrete wall and the floors were carpet or ceramic tile above. We still ventilated the area.

 

[TRAK.Structural]

1. I would think that the pressure from the wet concrete would still be there after curing. Maybe over time it could lessen due to shrinkage but seems like there is no physical mechanism to relieve the pressure if everything is still in contact. Multiple lifts is a good idea and will lessen the overall lateral pressure due to not having the full height liquid all at once.
3. Is there a way to figure out how the concrete wall was attached to the foundation?
5. I've done this before, but in a commercial application. I specified a maximum unit weight for the mix when liquid, not sure how that would go in residential.

 

[Ron247]

The wall does not look like it is attached. In fact, it appears to me that without the backfill, the wall is free to move laterally towards the embankment. Below is a sketch of a typical precast basement wall like this project.

 

[jhnblgr]

So best option pour a footing for the CLSM to keep the wall base pinned. Likely the precast wall was designed with soil loads on that side and relies on the floor slab and flooring to provide bracing. Investigate light weight fill that is a sand cement mix with a foam agent.

Calculate the pressure created by the wet fill and compare against typical soil pressure. When it hardens it will have unconfined strength. Use foam board to lessen the residual stress on the existing wall. If really concerned about lateral pressure between lifts use ICF as a bond break and after each lift cut the ties out. This in the end will leave a gap between the fill and existing wall.

 

[Greenalleycat]

1. Am I correct that once the concrete cures, the lateral form pressure from that pour is no longer present against the concrete basement wall other than any lateral soil pressure from the embankment that may have "shifted" during the pour?

I agree with this. I would be propping the basement wall until the grout is set as I think the pressure in the wet phase will be substantial

1. I recommended placing plastic across the bottom and up each side at least equal to the total pour height to help prevent liquid/flowable fill from seeping into the finished basement area due to the pressure. Is that reasonable to expect? It looks like the concrete wall sets on stone, not a concrete foundation.

100%. This is a must-do IMO. Right now that area is well ventilated. You are going to be providing an easy path for moisture to get through into the back of the basement wall. There is a serious risk IMO that this fix creates a significant issue that leads to dampness and mold.

1. I did not design the concrete wall but it was a manufactured product that was installed by the supplier. I am having owner's get confirmation from supplier it is designed for lateral soil loads of the expected height. Is there more I should have them confirm?

There's design, then there's construction. How confident are you that both have been executed the same way? Also, it may be designed for those lateral loads, but normally the lateral loading would be applied before other structure is built around it i.e. install wall -> backfill (applies pressure) -> build framing above etc. You're now loading it last which means that any forces & displacements from the backfilling will be transmitting into the rest of the structure. Can it handle that? Again, I'd recommend propping the wall to do your best to mitigate any loading/movement of the existing structure

1. The finished floor above is wood. I have recommended well ventilating the crawl space during curing to minimize effects from the heat and moisture from the drying process of the flowable fill. Is that a reasonable expectation.

I think you'll need to mechanically vent the subfloor for a while (I couldn't put a time on it. Maybe Google can give you some drying curves). In general, is the subfloor well ventilated?

1. Anyone ever do this before, and if so, what problems did you encounter

I have encountered a surprising number of subfloors that look basically exactly like your scenario there, and sometimes they have had previous (or current) moisture issues to be dealt with. However, they are normally left open (your current state), built a timber pole retaining wall, or have standard filter cloth/drainage chip/drainage pipe behind the wall. I have not encountered one with your proposed solution of mass concrete. I am concerned it's not the right solution. My thinking is: your issue is moisture in the soil. Your solution should not focus on stabilising the face, it should focus on getting the moisture out. Then stabilise the slope at the end, if you need to - that soil will probably stand vertical if dry.

Where is the moisture coming from, and can you control that? If not, I think your grout solution runs the risk of worsening the problem, not making it better.

 

[TheDW]

Have you considered any potential settlement risk of existing footings associated with the fill placement?

The bulb of influence of the new fill would extend to below at least the soils below the first line of piers. If there are any compressible layers seeing new stress from your hunk of concrete you could have some minor movements.

I'm in the middle of a project where the want to do this for a substantially larger footprint (~6' of fill in a ~20' x 20' square). I requested they get a soils guy to comment on any potential settlement risk on the existing foundations due to the fill placement, and I'm still waiting to hear back. Your application is much smaller so maybe not as much of a concern.

 

[Tomfh]

The pressure is still there after curing but it’s not there once the wall moves 0.1mm in response.

 

[Greenalleycat]

The pressure is still there after curing but it’s not there once the wall moves 0.1mm in response.

I think the pressure itself won't be there, right? But if the wall has deflected under the fluid pressure then it will be unable to rebound once the grout dries so it will effectively be locked in to the deformed state as if the pressure did exist...Maybe that's getting too technical and hair-splitty

 

[Ron247]

I agree with this. I would be propping the basement wall until the grout is set as I think the pressure in the wet phase will be substantial

I am pouring in lifts to minimize pressure during the pour and while curing of each lift. If the pressure subsides with curing, then it should not be that large.

You're now loading it last which means that any forces & displacements from the backfilling will be transmitting into the rest of the structure. Can it handle that? Again, I'd recommend propping the wall to do your best to mitigate any loading/movement of the existing structure

I had not thought much on the added loads at this stage. That is why I want to confirm concrete lateral pressure subsides with curing. I cannot prop the wall because inside is finished space.

Where is the moisture coming from, and can you control that? If not, I think your grout solution runs the risk of worsening the problem, not making it better.

I do not have a moisture intrusion problem that I know of. The sloughing appears to be the exposed soil drying out with age and getting "crumbly".

Have you considered any potential settlement risk of existing footings associated with the fill placement?

I have not thought of settlement of the soil near the embankment or at the stone foundation of the precast concrete basement wall. I have mentioned any time you add loads, how it can affect other areas, but I need to directly mention this aspect rather than using a "catch phrase". A soil guy would be helpful but even they are limited since it is existing construction.

Looking at the standard detail, I don't see a great way to build the system during construction. Have to install the wood floor, for the top of the wall to be stable unless you install temporary wall braces. Can't put in the backfill in before the top is stable. Can't backfill with the wood floor in the way.

Basically, the floor framing needed to be designed around the presence of the embankment. Put a row of beams and tall piers in the unbackfilled area as close to the embankment as practical. Then place another row of beams at the correct lateral distance from the embankment to meet the required offset.

 

[Greenalleycat]

Honestly, I'd be getting some gophers in there with a shovel and a posthole borer
Put some postholes down in the open void, drop some poles in, lay some boards, geotextile/drainage coil/drainage chip, done job

 

[Ron247]

Honestly, I'd be getting some gophers in there with a shovel and a posthole borer
Put some postholes down in the open void, drop some poles in, lay some boards, geotextile/drainage coil/drainage chip, done job

I did a preliminary design a few years ago for a framing system to support the floor in the event of complete failure of the row of piers. This was for a budget estimate. Lowering the drain lines would provide for a better framing system than my preliminary and it appears they can be lowered. the water lines have to be lowered also but they are not an issue since they are not gravity fed.

The Owner has finally decided to do something because of the recent sloughing. Even the framing support can have issues since the potential sloughing shifts the load from old to new but that can create cracks/issues also.

 

[Greenalleycat]

I'm sure you know better than me as you've actually been to site, but that soil doesn't look dry in the photos to me - it looks damp
I'm really dubious about dry soil sloughing - I wouldn't expect dry stuff that's been there vertically for what looks like a very long time to suddenly start crumbling
Either there's a change in moisture or a change in loading

As an intermediary grout option, could you just create a ~150mm wide shutter, stick some mesh in it, and pour it as a protective layer
Maybe dig a wee footing on the base to key it in
It would be a non-structural solution that's just designed to prevent fettering of the slope to allow it to stand vertically

 

[Ron247]

I'm sure you know better than me as you've actually been to site, but that soil doesn't look dry in the photos to me - it looks damp
I'm really dubious about dry soil sloughing - I wouldn't expect dry stuff that's been there vertically for what looks like a very long time to suddenly start crumbling
Either there's a change in moisture or a change in loading

That picture is from 3 years ago when I first saw the property. I was back 2 weeks ago. It has more sloughing now and does appear drier It may be moist in areas but there was not any water on the ground that I saw.

 

[Greenalleycat]

Wetting and drying doesn't have to mean visible water, it can just be greater than normal swings in moisture
I would be seriously looking at that first and getting a handle on any pipes, uphill groundwater flows, etc before designing a repair

I have 2 comparable jobs on the go at the moment
One of them there is a leaking pipe right next to the foundation which has caused undermining
Another was built in ~1990s and started leaking in the ~2010s which necessitated a new timber retaining wall in 2015
For both of these, the natural clay loess faces stood vertically and fine UNTIL the moisture was added - then they had issues

 

[Tomfh]

But if the wall has deflected under the fluid pressure then it will be unable to rebound once the grout dries so it will effectively be locked in to the deformed state as if the pressure did exist

A small permanent deflection is of no concern once the originating load is restrained and incapable of performing further work.

 

[Greenalleycat]

A small permanent deflection is of no concern once the originating load is restrained and incapable of performing further work.

Yes I agree, I was responding to you saying that the pressure still exists once it's dried

That said, you have to be sure that it IS a -small- deflection
If you're relying on a weak cantilever system then it may not be small - that's why I've said multiple times that I would be propping this
Or, as Ron has suggested, doing it in multiple lifts to mininmise pressures

Or ideally, I wouldn't be doing this system at all and would be putting in a retaining wall in front of the cut face after figuring out why this slope is actually fettering, which I'd put money on it is related to water

 

[TRAK.Structural]

I think the pressure itself won't be there, right? But if the wall has deflected under the fluid pressure then it will be unable to rebound once the grout dries so it will effectively be locked in to the deformed state as if the pressure did exist...Maybe that's getting too technical and hair-splitty

I'm not totally convinced either way on this, but still struggling to see how the pressure is somehow relieved after curing. Especially if it is in a deformed state, load has to be present to cause the deformation.

 

[Tomfh]

Yes I agree, I was responding to you saying that the pressure still exists once it's dried

Yep, until something moves an infinitesimal amount, at which point the load vanishes, so it's effectively irrelevant once the concrete is set. I.e. you needn't design for it which is what OP seems to be enquiring about. We seem to agree on that.

I typically specify these repairs in ~600mm lifts, which most contractors seem happy with. Sometimes prop too. It depends. It depends on the mix too. If they can use a stiffer concrete it's not as critical as a highly fluid self levelling mix.