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?

 

[RFreund]

I suppose 55' seems a little long for the 1200plf load. 2" doesn't seem bad and for sure the pile is just 'plowing' thru the soil. 4" seems a little high though.
I also just realized that you're not going to have a footing below this wall. The piles are just going to be embedded in the wall. I don't think we will come up with a solution that doesn't raise eyebrows with the loads you have. I would expect something on the order of 300plf to 500plf. If you're stuck with 1200plf I think I would go with a specifying some sort of slab and blame it on the geotechnical report. Or potentially embed the wall further to get more passive pressure. Or have them run some trenches that act in compression between the walls. But really at that point seems like any of the options previously discussed are available.

 

[HouseBoy]

KK
I'm having trouble seeing how you are getting 1200 plf at the base of the wall.
Did you answer this and I missed it (or is that where you wrote "just trust me")?

Someone posted that you are not using a footing at the bottom of the wall but I thought your first post shows one.
I def think a footing is appropriate.

When I came across this situation recently I asked the soils engineer if I needed to place piers in pairs (due to my previous experience with auger cast pile needing to be that way). They said they were not aware oof a need for that (not super comforting...)

Anyway, at 55 ft long, I wonder if you could add a couple of perpendicular walls maybe 4 or 6 ft long and with a pier at the end as a way to provide substantial redundancy for stability ( I THINK that's a term). Def need a footing though and a 2" mud slab will help contain the soil that the footing is pressing against. Maybe hard to put a real number to it but....just call it an "Integrity Slab". (Like integrity steel - you just want SOMETHING there to help out.

Just some thoughts.

 

[RFreund]

Someone posted that you are not using a footing at the bottom of the wall but I thought your first post shows one.
I def think a footing is appropriate.

You use a footing with helical pier supported foundation? Not saying it's wrong, I just don't see that often. I usually just see a grade beam (aka foundation wall) spanning between piles. I don't see the need for the footing. Or are you saying that you would use one in this case because of the crawl space condition / large lateral load?

 

[BAretired]

I am not clear on the seismic forces acting on the foundation. Should 1200 plf read 1200 psf?

In any case, a 2" mudslab cannot be considered suitable to resist seismic forces. The footing could be reinforced as a horizontal beam, but for a straight wall of 55' length, tie beams at midpoint or even third points might be considered.

BA

 

[SJBombero]

Unless you have an overly conservative geotech directing you to apply seismic earth pressure, I would not consider it a real force. The studies are pretty clear that for walls up to 12', seismic earth pressure does not affect the design. That said, slab friction can rarely restrain any retaining wall.

 

[haynewp]

IBC requires seismic loads to be provided by the geotech in SDC D to F for walls with 6 ft or more backfill.

 

[HouseBoy]

RF
The helical pier manufacturers all seem to indicate that there will normally be some sort of footing (under a wall and into which the helical cap will reside) but when I spoke to the manufacturers engineering dept (recently) they just said that they have no design info for that - it is whatever the EOR says it should be.
With that said, I agree that the footing is not going to do MUCH work (certainly not much vertical load work). The wall the is above it will be vastly more stiff and THAT is what I figure is spanning from pier to pier. I do like the footing for what it might do in the horizontal direction.
I just HOPE they can install the piers in a very straight line and dead nuts in line with the center of the wall above. 55 ft straight line wall does not seem like a good fit without some other stabilizing features.

 

[SJBombero]

About half the geotechs here comply with the letter of the IBC and specify that seismic earth pressure may be neglected. As long as they considered it and recognized that for walls under 12' it is not a reasonable concern then they have complied. The rest just cause the clients to dumps tens of thousands of dollars of concrete into the ground unecessarily.

 

[haynewp]

About half the geotechs here comply with the letter of the IBC and specify that seismic earth pressure may be neglected. As long as they considered it and recognized that for walls under 12' it is not a reasonable concern then they have complied. The rest just cause the clients to dumps tens of thousands of dollars of concrete into the ground unecessarily.

So do the geotechs in your area also take responsibility for whatever structure may be sitting on top of that wall that is also experiencing seismic movement that may be imparted onto the wall at the same time? I don't see how them not providing it to the structural engineer that is designing the wall is within the letter of the IBC. We are responsible for the seismic detailing of loads with overstrength as I noted above.

 

[bones206]

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.

I just meant that for your everyday static earth pressures, hopefully your wall design as a vertically spanning would suffice, and only in the earthquake case would you rely on the horizontal span mechanism.

 

[bones206]

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.

Yea, I mean everything is going to be rocking and rolling in random directions. The inertia of the superstructure could be going one way, while the basement wall is tilting the other way. As long as you have resilient connections between superstructure and foundation, I would think that some differential lateral movement could be tolerated.

 

[KootK]

Quick update after having a Friday afternoon pow-wow with the geotech, pile supplier, and conctractor:

1) I am using a "footing" on this project as it seems to be the expectation for where this project is located. That may well change as things unfold. In my home location of Alberta, I never see the footing, just the grade beam / wall. But, then, I've also never seen it with open space on one side which changes the picture a bit.

2) The 1200 plf was indeed an error. The load perpendicular to the walls will be much smaller than that and manageable either as a bare pile or an encased pile. This being the case, my concern for this connection really becomes more about gravity load stability than retained earth.

3) A 4" slab on grade is a non-starter. So is utilizing passive soil pressure. Apparently the 2" mudslab has already been value engineered away and the the footing thing will not be keyed into the supporting soil.

4) I don't think that it will be necessary but the piling contractor offered up battering piles perpendicular to the wall for dealing with those kinds of forces. That sounds kind of great for overall stability really. That said, it makes me worry a little more for tolerances etc and what I can reasonably get done within a 12"H x 22"W footing/cap. I seem to be excessively paranoid about the tolerances though. The piling contractor isn't worried at all.

5) The piling contractor has never heard of anybody in residential having to comply with that IBC, capacity design at the connection provision that haynewp tables. They're trying to figure out how to make it go away for me.

6) I enquired if it was possible to arrange a pair of battered piles, in the plane of the wall, like a chevron such that the punching forces cancel. These guys said that they've seen that done in the real world one time and it's possible. It might just require a local grade beam thickening to encase the things needing encasing.

 

[KootK]

So coming back to the provision below.

1) Would anyone be willing to say that provision doesn't apply to my situation? Considering some examples:

a) Clearly, for a moment frame column foundation, it applies.

b) For a shear wall system on a pile cap where the piles are not under the walls, I would certainly hope that it appies.

c) For my shear wall on grade beams system, does it apply?

2) If I have to apply this provision, how far do I take it? What qualifies as the "connection"? If it's just concrete bearing at the piles, no problem. I'd planned to extend this to include punching shear through the grade beams which is a bit trickier to deal with. Should I stop there? Consider some flexural stuff for the grad beams?

 

[haynewp]

Can you use the IRC? That connection requirement may not be in it.

 

[KootK]

It would be great if I could use IRC if that's possible. I've honestly struggled to figure out where I can and can't use IRC on larger scale residential work. The floors of this project have 3" concrete toppings and the weight of that kicks me out of IRC I think. But, then, can I still use IRC for the parts of the building that are not in violation of the limits?

 

[EZBuilding]

KootK,

If your concern now is in regards to stability; IBC 2021 1810.2.2 Exception #2 applies and alleviates your situation.

Take a look at the second paragraph of that section as well - in regards to utilizing staggered piles. If the exception provides a code exception, you could consider staggering the helical piles as much as possible along the length of the wall to provide some form of mechanism against the roll-over concern. With a 22 inch wide footing it seems you can reasonably stagger those helical piles. You would then have a torsion effect as your stabilizing mechanism, which you could reinforce with ties if need be.

 

[RFreund]

Kootk -> Thanks for following up on this.
Is your question "If I use battered piles, can I get out of the IBC connection provision?"
My question to your question would be - what are you using the battered piles for? To resist overall sliding of the structure due to base shear or to resist out of plane loads due to lateral earth pressure? or Both?

Are these piles battered in the plane of the wall? If so, I'd think that getting the connection to develop the strength of the short column would be fairly simple as it is just bearing on the concrete at that point. If there is tension/uplift then that might be more difficult. If the piles are battered perpendicular to the wall, then that's a different story.

I think what EZBuilding is referring to is the stability of the pile itself (i.e. braced vs flagpole condition).

 

[EZBuilding]

RFreund - the code section I reference is in regards to stability of the entire foundation assembly - not just of the actual deep foundation element.

 

[WARose]

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?

#1: Hell no (if it is unreinforced). (Agree with you there.)

#2: Yes (if we are talking it being properly placed where it can develop normal passive pressure).

 

[Teguci]

As I remember, the reason for the strength overdesign in high seismic is to make sure that the inelastic flexure happens in some other member before the connection strength is exceeded. Actual seismic forces experienced are higher than the elastic design. In your case, I don't see where you can argue that you have that ductility near the battered pile to pile cap connection.