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?

 

[IceNine]

How about using a shear key to allow for additional passive pressure resistance?

 

[Enable]

My Answers to 1/2

1) If you specify 2" you'll likely get 1" or less in places and if those happen to be near the wall I'd think buckling would be of some concern. I'd not rely on it.

2) Doubtful compaction will get great there. They are going to excavate a couple feet away to put the footing formwork into place. Maybe it might be okay but I'd bet on it being trash backfill.

Other Options (I feel you must have considered these but worth throwing out there)

A) Can we simply increase the SOG depth? The material cost cannot be that much. If they already have the finisher, and pump, a couple extra trucks is nothing.

B) Design the footing / wall stem as a distributing element and run grade beams from side to side

C) Increase footing / wall size to develop the required friction (not sure what size this would be or if reliable depending on wash-out / water)

 

[KootK]

How about using a shear key to allow for additional passive pressure resistance?

Definitely. That, our enough doweling for shear transfer. That path then takes me back to consideration of the mud slab / backfill though.

I feel you must have considered these but worth throwing out there

Thanks, the more ideas the better:

a) I did consider that but feel that it would be an egregious eyebrow raiser.

b) I did consider that and would likely go this way over SOG. Still, it's a cost eyebrow raiser. That, especially since the code specifically gives you an out to avoid having to do this when your piles are in a "wall" thing. That said, the load gots to go someplace...

c) I didn't consider more weight to get more friction. I just roughed that out, though, and feel it would add a prohibitive amount of concrete.

 

[Celt83]

counter-fort the crawl space side of the the wall and place the footing interior surface to act like a heel block.

(probably another path with a prohibitive amount of concrete though)

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[HouseBoy]

If it makes you feel any better - I'm doing the exact same thing (2" mud slab) however, my wall sections are not terribly long before they turn 90* (in plan).
Still, I want SOMETHING to hold the wall in place. How long and straight is YOUR wall?
I'm not thrilled with the possible eccentricity of the vertical load either. With other small diameter piles (like auger cast) it is recommended to place them in pairs.
I suppose the loads might be much greater.
Helical pier maker says just align the piers with the center of the wall "as best you can"!

 

[haynewp]

1. Helicals can only take around 1 kip of lateral in my experience if they are installed straight as shown and with no encasement.

2. I did a project last year where we encased the top 2 ft of the helicals in 18" dia of concrete below the footing to get additional passive pressure. I would consider doing an encasement of some diameter that is needed in combination with the passive on the footing/cap itself, if it is alright with the geotech and you didn't want to use a key.

3. I chose #2 above instead of battering on the previous project, even though we got a quote for a pier with up to 80 kips ultimate vertical capacity (so decent horiz battered component) and was less than $1,000 to install each of those piers on that site. But I don't normally have battered piles without a vertical pile under the same footing, or at least have opposing battered piles to resist gravity loads during construction. I was able to encase them and get the lateral I needed with a cheaper 40k pile and not deal with battering.

4. I've never used a mud slab for anything structural and I don't think the compressive strength qualifies as structural concrete by ACI, which is what I use. 2" is pretty thin and I expect the mud slab may shrink more than usual from the face of the wall depending on the size. It may be questioned if it has to be permitted and it is called a mud slab.

 

[KootK]

How long and straight is YOUR wall?

I've got 54' and a couple of runs about half that. At 1200 PLF, even the wall/footing probably couldn't handle that in bending. And the deflection involved would be pushing on the intermediate piles pretty hard.

I would consider doing an encasement of some diameter that is needed in combination with the passive on the footing/cap itself, if it is alright with the geotech and you didn't want to use a key.

I'd not considered that although I do believe that I've seen it in details before. I've got a hunch that may be the way things go. Did your encasements get any rebar in them or just a blob of concrete around the pipe? Did you specify it or did you leave it to the pile supplier?

 

[KootK]

Also, what kind of numbers did you get with the encasement?

 

[haynewp]

Blob of concrete. The supplier I was working with helped to get the size of the encasement as I remember, based on his number, layout, and size of piers.

I gave a specified encasement as the basis along with the lateral loads that are required to be met by the mfr, in case that another mfr won the project and they wanted to use a larger amount of smaller diameter piers (for example) that may qualify for a different encasement amount.

I will have to go back and look to see what capacity we got from them.

 

[dhengr]

KootK:
Similar to Haynewp’a encasement idea, cast a 2 to 4’ deep grade beam around the helicals, compact the fill around this grade beam, and then use it to distrib. the lateral loading. This grade beam might actually be your ftg. or the bot. foot or so of your found. wall with the proper rebar in both faces across the horiz. joint. With a real grade beam you might even increase the helicals spacing and save a few of them.

 

[Brad805]

Haynewp, what size of helical piles were those?

 

[haynewp]

It was these and it looks like I was given up to 5 kips of lateral capacity after encasing the top which seems like a lot now. I am going to call them again tomorrow. I assume the value I was given was L-Pile based and they likely assumed some fixity of the head because they were required to be embedded a certain amount into the caps.

I found this which gives some test values for different top encasements which seems to kind of justify what I was given by some comparison of the SPTs.

https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=2661&context=icrageesd

"This research demonstrates that lateral capacities of helical piles increased substantially with the implementation of an LRD, which can be addressed early in a site investigation with correlation to blow count data and laboratory testing programs."

 

[Brad805]

Ahh, 2 7/8" shaft. That makes sense. We never use anything smaller than 4" and most times I prefer 6" because of how badly these wander during install. Thanks Haynewp.

 

[Aesur]

I will admit I didn't read all the comments yet, but have you considered soil nails or sloped helicals to counter the horizontal forces?

 

[XR250]

Instead of sloped helicals, why not horizontal ones at the base of the wall? I too am a little concerned about the wall teetering on a single line of helicals. I typically use two.

 

[KootK]

Thanks for your input Aesur:

1) I hadn't considered soil nails. I'm not sure how practical that would be for a silly, 4' crawlspace wall though, driving through backfill.

2) I had considered battered piles and continue to. Man does that seem nuts to me though. I'd basically have to do that the entire building perimeter at every pile. I'd have to think that sinking the foundation deep and trying for passive pressure or something would be preferable.

3) I'll have to do some research but I'm also concerned that battered piles might be prohibited in high seismic applications. That business about it tending to punch through whatever it's attached to under load. For the load perpendicular to the wall, celt's counterfort with a batter under it would be mechanically awesome if utterly nutty in terms of the cost of building a house. For load parallel to the walls, maybe I could A-Frame it like shown below. I'm not sure if that's a thing...

 

[KootK]

Instead of sloped helicals, why not horizontal ones at the base of the wall?

Is that possible with the drilling rigs? The problem here may well just be my lack of helical pile knowledge.

I too am a little concerned about the wall teetering on a single line of helicals. I typically use two.

You mean two staggered rows, right? Problems with that for me:

1) Code allows you to not do it for wallish things. Sure, I could force it, but that won't do anything for my popularity.

2) Much of the load on this building comes down in localized spots. Not so wallish. I'm not sure that the small offset would help much as the offset piles would be far from the points of heavy load application. Maybe that in itself is a good reason to use multiple piles where loads come down. But then I'm down to triangular pile caps over three pile groups all over the place. That's starting to look pretty commercial.

 

[XR250]

s that possible with the drilling rigs? The problem here may well just be my lack of helical pile knowledge.

Around here, they use small, mobile hydraulic equipment for the horizontal ones as they normally have to fit it thru a basement door for retrofits.

Code allows you to not do it for wallish things. Sure, I could force it, but that won't do anything for my popularity.

Probably won't win any popularity contests on this one anyway. Hopefully you can at least get on the podium!

 

[Brad805]

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? When we do this type of design we have contacts we can reach out to that have the ability to provide an engineered design. You even if you need a buttress at each pile it would not surprise me to find that was still cost effective.