Switching to Micropiles foundation 2

[DoubleStud]

So I have a new house project with 6 ft tall crawlspace. The plan was to build the tall crawlspace wall on footers on top of bedrock. I made the heel of the footers wider and design the crawlspace wall to be able to take some load vertically. That is how the drawing went to permit. Now once they start digging, they found out it is not feasible to go to bed rock because they are much deeper than what everyone anticipated. Now they want to do micropiles. My questions is, can I stick with my old design and just put micropiles right below the walls and make sure my walls can span from piles to piles? Do I need to put piles on my heel? By doing this I can make sure my walls can still span vertically (because of the large heel), and I will have big enough dead load above heel to reduce sliding load. If I redesign the foundation to grade beam (with minimal or no footers) then I would think I need to introduce series of counterforts. Also I am not sure how much lateral load micropiles can take to resist sliding along the tall crawlspace. Any advice?

 

[KootK]

I'd be inclined to either:

1) Put piles under the heel or;

2) Cantilever the walls from the piles (likely less viable).

I feel that it's weird to rely on piles for vertical load but then revert back to shallow footing design principles for the retaining wall overturning.

I think that you'll be fine with the lateral on the micropiles but I'd try to connect with a local supplier to get a feel for available capacities and tolerance for potential battering (which I doubt you'll need).

 

[BridgeSmith]

Did you design the wall and footing as a cantilever retaining wall, or does the wall only take vertical loads? Is the heel outside, held down by soil, or is it inside in the crawl space?

I don't understand why you'd have a need for counterforts for a 6' retained height. Much taller cantilever retaining walls are commonly designed without them.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[DoubleStud]

KootK:
If my load on my wall is in the middle 1/3 and the piles will be in this middle 3rd, what capacity and spacing would I need for the piles I put on the heel? I am not sure what capacity I would need. My understanding is that these piles cant take that much lateral/bending. Only good for axial loads.

BridgeSmith:
I designed it as cantilever retaining wall. The heel is outside. The toe is inside the crawspace and very short. I do not need counterfort, but if eliminate the heel and just have a grade beam with piles, I think I will need some kind of counterforts. Can't rely on the piles to take the moment, I don't think.

 

[KootK]

If my load on my wall is in the middle 1/3 and the piles will be in this middle 3rd, what capacity and spacing would I need for the piles I put on the heel?

No choice but to let the numbers be your guide I'm afraid. I would normally have thought that the soil lateral load and the wind shear coming in from above would dominate the OT demand such that hitting the kern with the axial loads would be relatively unimportant.

From a detailing perspective, I'd be inclined to go heel-less as shown below. This way you can get a robust attachment to your tension piles without having to add a lot of unnecessary thickness to the footing element. Usually the piles are capable enough axially that it's not a big deal saying sayonara to the backfill OT resistance. This might also reduce the extent of excavation although I doubt that's much of a benefit in this situation.

 

[KootK]

If there's any way to set this up such that you've got lateral restraint at the top of the wall, that would be ideal. Since you were contemplating a cantilevered retaining wall setup all along, I've assumed that's not possible here for some reason.

 

[Celt83]

any reason not to maintain the heel and batter the "front" pile for sliding resistance?

 

[KootK]

any reason not to maintain the heel and batter the "front" pile for sliding resistance?

This way you can get a robust attachment to your tension piles without having to add a lot of unnecessary thickness to the footing element.

Not a deal breaker but something to consider. I am a bit jaded in that I have often seen that joint detailed atrociously.

 

[Celt83]

ah good point, was thinking at 6' most of the extra concrete for the foundation may come from the cover requirements for the pile head embedment.

 

[DoubleStud]

I feel that it's weird to rely on piles for vertical load but then revert back to shallow footing design principles for the retaining wall overturning.

What do you think would happen if we do this? I know it is weird but I feel like it will work? Are you worried about the retaining wall rotating into the soil side?

 

[WARose]

Also I am not sure how much lateral load micropiles can take to resist sliding along the tall crawlspace.

Don't mean to paint with a broad brush but (IIRC) Micropiles have very little resistance to a horizontal load applied to the tip (when vertical; in a flexible strata). You'd probably need a batter pile (and looks like someone has suggested that).

 

[XR250]

Why are you designing the walls as cantilevered? Can you not use the floor system to brace them?

 

[PEinc]

Why did this residence need to have footings bearing on rock? Is the house so big that the foundation loads are too much for bearing on soil or is the soil that bad?

http://www.PeirceEngineering.com

 

[DoubleStud]

Why are you designing the walls as cantilevered? Can you not use the floor system to brace them?

I have been ignoring them. But I do make them build the floor system first before backfilling the wall. Extra factor of safety. How much force are we talking about here especially where the joists run parallel to the wall and you do only blocking in the end of joists space every 16" o.c.?

Why did this residence need to have footings bearing on rock? Is the house so big that the foundation loads are too much for bearing on soil or is the soil that bad?

We were following the geotech recommendation. It is on a steep hill and we have a lot of fill down hill.

 

[PEinc]

If you are excavating down 6 feet to create a crawl space, you may be removing more soil weight from the hill than the house weighs. 6' x 120 pcf = 720 psf. The crawl space plus 2 floors and a roof might weigh as little as 4 levels x about 60 psf average (DL + LL) = 240 psf. There might be a net reduction in load to the hillside. Check bearing and slope stability?

http://www.PeirceEngineering.com

 

[KootK]

ah good point, was thinking at 6' most of the extra concrete for the foundation may come from the cover requirements for the pile head embedment.

That's just it. If the footing is thickened to accommodate the pile head embedment and to create a decent connection at the tension piles, all good. In my area, practice seems to be to leave the footing thickness the same as it would have been if it were a no-pile design, shove the pile head up 3" into that, and then just toss on some studs or DBA "designed" as [As x Fy] of the studs or DBA.