Using Mud Slab to Resist Basement Wall Thrust 2

[KootK]

My situation shown below. This is making me pretty unpopular with the helical pile supplier and the only "outs" that I can think of are to:

1) Use the 2" mud slab like a regular SOG to resist lateral loads.

2) Use the backfill around the base of the wall to resist lateral loads.

Thus far, I've not been willing to do either of these things owing to concerns over of quality and permanence. How do others feel about this? Am I being too much of a hard ass?

 

[phamENG]

I have to ask: why do you have a 4' differential between crawl space and exterior? I understand there's probably frost considerations (snow drift on a deck?)...but why not salvage your spoils and back fill after your foundation is set? Set requirements for geotech to inspect lifts for compaction in the back fill, and you can get your lateral resistance requirement waaaay down.

There may be a good reason and my experience is colored by the fact that, in my area, FEMA suggests a minimum of 1" differential in the other direction to minimize ponding of flood waters in crawl spaces...

 

[KootK]

Unless you need to tie the piles in two directions for a code reason, can you not simple get them to size the pile for the lateral load?

I can try but I'm not confident that I'll be successful in that since, as mentioned above, it seems like lateral on a bare pile is good for about 1k. On an encased pile, about 5k. I've got 16K coming in per pile from basement wall pressures alone. That doesn't even consider the three, lone shear walls that are holding this glass monstrosity up. I'll be reaching out to the pile supplier this afternoon to see what they think of all this.

You even if you need a buttress at each pile it would not surprise me to find that was still cost effective.

Really?? I figured just the formwork alone for twenty of these thing would cause the builder to go nuts. Screw it, I'll run it by them to see what they think. Part of the problem is that much of the surrounding area has pretty good soils somehow. It's just the top of this damn hill that apparently has these nightmarish, collapsible soils that are causing lawsuits. This discrepancy of perception is part of the problem for sure. The builder just told me that the original owner of the side decided to abandon his construction plans and sell the lot. They speculate that the cost of the foundations may have sunk that venture.

I have to ask: why do you have a 4' differential between crawl space and exterior?

The 4' is just the permanent, unbalanced, seismically retained soil behind the walls of the crawlspace. I could lower the walls, kind of like sheet piles but, then again, that's more $$$.

...FEMA suggests a minimum of 1" differential in the other direction to minimize ponding of flood waters in crawl spaces...

Wait... so crawl spaces are actually above grade in your area?

 

[phamENG]

Yep. That's what happens when you live in a place called "The Dismal Swamp" (official name...not kidding...). With the possible exception of coastal Louisiana, we're the US poster children for the negative effects of sea level rise. Something about an eon's old meteor strike forming the Chesapeake Bay that is resulting in ongoing subsidence of the underlying landmass AND rising sea levels leaving us with one of the worst flooding situations on the continent. Pretty nifty.

So yeah...there are several houses with crawl spaces a bit below grade...but when I inspect them I have to wear a wet suit (I'm only sort of kidding). So yes, new construction is almost exclusively slab on grade elevated a few feet above adjacent grade OR above DFE or on a crawl several feet above ground (above DFE) with the crawl space grade an inch or so above the lowest adjacent grade.

 

[Enable]

If this is a known issue can I ask why you think you'll get such resistance to some of the ideas here? I mean, if literally everyone on the hill has a lawsuit pending due to soil conditions, you shouldn't need a lot of ammo to convince people that maybe we might want to do something different on this one.

How big of an area are we talking about KootK? As a contractor all these solutions, many of which are fine / will work, still seem to be arguably so much more headache inducing than simply increasing mud slab thickness + upgrade the concrete to make it a true SOG.

Like I said, pretty much the only difference in cost would be the concrete because finisher / pump would already be on-site. Use a difference of $130 CND / M3 for increased cost over the mud slab. What is the number we are talking about? Even at a 5000ft2 footprint with 4" SOG that's $6500 CND. Is that really a killer here?

 

[Brad805]

1k was a 2 7/8" diameter pile. I have never used a tiny pile like that, and unlikely would ever agree to unless I know it is being installed plumb. 90% of the piles we use get installed with an excavator. They can easily be out of alignment by 4" with even the best operators. Can you use something larger? We use 4"/6"/8"/10/12" piles frequently with single, double or triple helixes. It all depends on who you have for pile suppliers. I am in oil country.

Yes, I agree buttress take forming, but unless you have a simple solution for the battered pile it seems it will need some forming. Maybe not.

 

[dauwerda]

If helical piles weren't involved and this were just a typical crawlspace that were being built per the IRC (or other local code) how would this lateral force be getting resolved? I feel that it is typically largely ignored and/or handwaved away with the friction interaction between the soil and the bottom of the footing. Am I off base on this?

I believe the IRC has a footnote that states when walls are supporting more than 4ft of unbalanced fill than the wall must be laterally supported at the top and bottom before backfilling (I don't think it states how that needs to be done or what requirements need to be met). So, if the typical crawlspace has less than 4ft of unbalanced fill then as far as I can tell the IRC does not explicitly require any type of lateral support at the bottom (I'm not saying that it shouldn't). In fact, the typical details in the IRC tend to show the interior grade at the level of the bottom of the footing for concrete footings and stem walls.

I am not aware of this lack of lateral restraint being a cause of failures in crawl space foundations. Is it?

But, perhaps even if that is typical design when helical piles aren't involved you could still get into trouble as you would essentially have zero friction force between your footing and soil as the whole purpose of the pile is to take the vertical normal force, preventing it from going into the soil at the surface.

For residential construction I would be inclined to either increase the stem wall height or decrease the interior crawlspace height to try to resolve the force with passive pressure of soil on the inside. Or just throw a few grade beams across to the opposite wall and design the footing to span laterally between grade beams.

 

[KootK]

If this is a known issue can I ask why you think you'll get such resistance to some of the ideas here? I mean, if literally everyone on the hill has a lawsuit pending due to soil conditions, you shouldn't need a lot of ammo to convince people that maybe we might want to do something different on this one.

You'll just have to trust that I've got a pretty good handle on what the design and construction team's expectations are here. The lawsuits make for good ammo, and I've used it. But, still, if nobody else near by has built what I'm proposing to build, it tends to look shocking even if there have been problems in the past.

How big of an area are we talking about KootK? As a contractor all these solutions, many of which are fine / will work, still seem to be arguably so much more headache inducing than simply increasing mud slab thickness + upgrade the concrete to make it a true SOG.

The crawl space is about 5,200 SF. It is a very common experience here on Eng-Tips for:

1) Engineers here to think that the costs of things are no big deal and;

2) The contractors that we deal with going nuts over the costs of these things anyhow. On a project like this, contractors will come in with very finely tuned budgets and pretty small profit margins.

Besides, the slab on grade only fixes the shear problem perpendicular to the wall. There's still the parallel to wall issue to deal with. And, again because of the crap soils, the geotech doesn't want any slabs on grade attached to the foundation walls. I don't love that resistance mechanism anyhow although my current desperation may lead me to compromise on that.

1k was a 2 7/8" diameter pile.

And that's what's planned here too. The geotechnical report mentioned it and the pile supplier has run with it. In my first conversation with the pile supplier, he mentioned "yeah, we're really hoping that most of these can be just compression piles". By that, he meant no tension and nominal shear. What I'm doing is a pretty far cry from that I'm afraid. Logically, I don't really see how even a 6" pile shaft would be that much better than an encased shaft of any diameter. Maybe there are larger moments below the encasement than I'm imagining. I'm in oil country too.

Yes, I agree buttress take forming, but unless you have a simple solution for the battered pile it seems it will need some forming. Maybe not.

I mean to do no pile battering if possible. I actually think that it might be prohibited in high siesmic applications. If anyone is able to speak this aspect of things that would be appreciated.

 

[KootK]

I feel that it is typically largely ignored and/or handwaved away with the friction interaction between the soil and the bottom of the footing. Am I off base on this?

You are not off base with that.

I am not aware of this lack of lateral restraint being a cause of failures in crawl space foundations. Is it?

I have no personal experience with any problems of that sort. This is my first ever crawlspace with helical piles however.

For residential construction I would be inclined to either increase the stem wall height or decrease the interior crawlspace height to try to resolve the force with passive pressure of soil on the inside. Or just throw a few grade beams across to the opposite wall and design the footing to span laterally between grade beams.

Those are my first and second choices as well. In one direction, the grade beams would cross a lidless basement which would require some attention in order to take the loads across.

 

[KootK]

Just to dial things back a bit, the crux of my question is really just "Is crawlspace on helical piles weird? Am I nuts to not want to use a mudslab for lateral restraint?". I've done helical piles with no basement or crawlspace. And I've done helical piles with a full basement and slab on grade. Just never this particular combination. And I don't want someone stumbling across this thing and saying "OMG! Crawlspace with screw piles! Those walls are just going to roll over, what were you thinking???". If this is actually normal, I'll roll with the punches.

 

[haynewp]

You get penalized for using battered piles in high seismic. This is from IBC 2015 for SDC D to F.

 

[haynewp]

I think a crawl space on helical piers is a little odd and I would try to use a key to get the passive needed. I wouldn’t rely on a 2” mud slab as a compression member.

 

[KootK]

Thank you much. Does nominal strengh of the pile acting as a short column equate to the [As x Fy] capacity of the pipe do you think?? Might need those buttresses after all.

 

[haynewp]

Yes, I would take it as that.

 

[RFreund]

A few thoughts here:
The first is that the lateral load seems high for a 4' crawl space. Is 1200plf correct? I mean at 120pcf or so (might even be lighter) your overburden at 4' is 480psf.

Is crawlspace on helical piles weird?

No, seems reasonable

Am I nuts to not want to use a mudslab for lateral restraint?

Slab on grade say 4" thick, No. Mud slab, Cringe, tough call. I'd really want to specify something I could rely on. However, see this calc for some wild way (feel free to comment) to compute capacity and make it seem like there is actually a fair amount of capacity.

My first choice would be to span the footing laterally (not too worried about deflection and also knowing there is a mud slab).
I would think that the helical contractor would rather put in a few diagonal piles instead of a grouting the top. At 1200 plf, even grouting the top won't get you much b/c if the soil is bad enough to need helical piles in the first place, then you won't get much lateral resistance.
If the lateral earth pressure is so high due to the seismic forces, then the contractor will understand some extra cost for the SOG or whatever you decide with. I'm guessing there is some heavier shear wall stuff going on above, so it shouldn't seem that crazy.


EIT

http://www.HowToEngineer.com

 

[Brad805]

Kootk, I only mention my general location since I know you work all over. I also have access to two or three companies that put screw piles under large compressor skids where they have load tested piles. That is helpful since many of the stock geotech reports are general in nature so you need someone that can drill down on the subject. Some pile companies have geotechnical engineers on staff that completed their advanced degrees studying only screw piles. Go figure? That said, with you having only 2 7/8" piles in your bag of tricks, I think that discussion is moot.

I think you have two options unless you almost double the pile count:
1. re-visit the lateral loads. If this is a code req't wherever the project is or was defined by the geotech, then I get it. If in a region of Canada where it is a known high seismic region, I also understand. If you are working around home, then I fear the reality is the vast majority of foundations under 6,000sqft are designed for Part 9 lateral loads and that is all. I recently completed a house project outside my normal work area, and I had quite a few comments about the conservatism in the design.
2. Mechanically stabilize the earth (MSE) component. Tensar Grid (Link) is not expensive and you could use that to tie back the foundation into your excavation. That might be a downdrag problem, but they have a basket system as well that does not tie to the wall.

I would not use the mudslab to resist the lateral load. I have crawled on far too many that were clearly and afterthought. I doubt anyone will pay you to come out and complete a field review of that.

I see nothing out of the ordinary with piles and a crawlspace. We have done it many times, but we have frost problems and a lot of CH clay.

 

[Teguci]

I've done a retaining wall like this (on a line of drilled piers though). I used the slab on grade to resist lateral sliding but required that it be reinforced to ACI 318 for structural concrete. And, I used the passive pressure of soils at the toe.

Because of the safety factor for overturning, the wall, once soils settle, will want to tilt into the backfill and needs to be reinforced accordingly. In my case, the wall was cantilevered (slide bearing at the top).

 

[bones206]

I agree with RFreund and would try to span the wall horizontally out to the orthogonal walls. You can use the footing as a horizontal beam at the base where the pressure is greatest. It would be straightforward and not-too-eyebrow-raising to make the footing wider if a deeper beam section is needed.

 

[KootK]

I agree with RFreund and would try to span the wall horizontally out to the orthogonal walls.

1) I agree with RFreund in that I suspect that there's been an error in the geotech report and the loads will wind up smaller than I've presented above. We have a conference call shortly to go over that and some other stuff.

2) I'm skeptical of the horizontal spanning business. Let's say I have a 55' wall run with maybe five piles along the way. Whether it's the wall or the footing/pile cap thing doing the spanning, that can be expected to see, what, 2" - 4" of lateral deflection at midspan even with a 3' wide footing? So that would represent an imposed displacement of that same amount at the top of the piles along the way? I'm concerned that would be enough to shear the piles from the footing and, thus, compromise their stability and ability to contribute to shear in the more important, along the wall direction. Respect for stiffness in all things. Do you and RFreund not share this concern?

 

[bones206]

In an earthquake I probably wouldn't mind the 2"-4" wall deflection but would definitely check the shear connection of the piles. Although I can envision the narrow pile stems in reality would plow through the earth a few inches, reducing the shear at the connection.

If we were talking about a static earth pressure case, then yeah the horizontal span of 55 ft might be a stretch, but hopefully the earth pressures would be low enough to design as a vertical span in that case.

 

[KootK]

In an earthquake I probably wouldn't mind the 2"-4" wall deflection but would definitely check the shear connection of the piles.

By the time that you've moved the bottom of the grade beam 4" relative to the top, I feel like you're bound to just create a rolling instability in the grade beam, particularly where largely loaded posts touch down. Perhaps this is an "overthinking" concern.

...but hopefully the earth pressures would be low enough to design as a vertical span in that case.

Can you elaborate on the vertical spanning? In my mind, it's already spanning vertically from the low grade level to the first floor diaphragm. Then, in addition to that, maybe we have the footing + wall bottom span horizontally to the piles.