secant pile wall - shear capacity in concrete "lagging" 1

[mikeCTE]

I am tasked with trying to make a secant pile wall work for a contractor.

The contractor wishes to use 36" diameter piles. The primary piles will be concrete mix with f'c = 1000. The secondary piles will have f'c = 4000 psi with a large W section embedded. Like all contractors, he wants to pinch pennies, so he's trying to push the center to center spacing as large as possible. Due to driving tolerances, this could result in very little overlap between primary and secondary piles.

Everything I have read and researches talks about the primary pile acting as lagging, which I can see. But, I have yet to come across information that specifies how to design it or check its capacity. ACI 318 (2002) has some language in section R22.5.4 talking about shear flow in plain concrete which I think could apply to this situation. However, there seems to be a lot of room for interpretation.

So, I wanted to check with the forum to see if anyone had experience with this and how best to handle it.

Thanks.

 

[hokie66]

The spacing in secant pile walls is normally about 0.8 to 0.85 of the pile diameter. For 36" diameter piles, I would expect the spacing to be 30" +/-.

 

[FixedEarth]

See attached drawing which confirms hokie66 statement. You can design for a spacing of 54" like in this example. The middle pier is treated as lagging. The secant pile if cantilevered, is designed the same way as soldier beam with drilled piers with a W or HP or round HSS pile section. If the shored height is minimal, you can even design it with a rebar cage in alternate piers instead of W section.

http://www.soilstructure.com/

 

[hokie66]

That geometry looks wrong, FE. Instead of 24" piles, I think you mean 36" as per the OP. So your suggested spacing is 27", or 0.75 x pile diameter. Fair enough, but it sounds like the OP's client wants to cheat a bit on that arrangement.

 

[FixedEarth]

It is just an example. He can use 36 inch and overlap 12 inch. I wanted to point out that the spacing is measured from the reinforced piers. Sometimes, we want it water tight and in which case we can use smaller diameter pier and increase the overlap dimension.

http://www.soilstructure.com/

 

[hokie66]

Sure. I was just pointing out that the arithmetic in your sketch is wrong.

 

[FixedEarth]

There is a proverb that comes to mind- "Fikratak Ta'ban". It means your thinking is flawed.

http://www.soilstructure.com/

 

[hokie66]

That is often the case, but 54-24=30. If you put a 24" pile in that space, they don't overlap.

 

[ztengguy]

lol

 

[hokie66]

Back to the OP's question. I believe the appropriate way to check the 'soft' or 'female' pile is to assume that the part behind the centreline acts as a circular arch, spanning the clear distance between the reinforced piles. So you need enough overlap, allowing for tolerance in installation, to ensure adequate thickness of the arch.

 

[mikeCTE]

thanks for the input so far.

the biggest question i have is how to handle the shear through the "lagging". if we check the shear through the "beam" as: phi*(4/3)*gamma*f'c*bw*d, what do i use for "d"?

in the sketch below, i show two different assumptions for "d". one assumes it's half the depth of the overlap area, whereas the other assumes it's the full depth of the overlap area. since the primary pile is installed first, it seems to me that i should only count half the depth because of the way the concrete will bear on the secondary pile. if the process were reversed, i think i could justify using the full depth.

thoughts?

 

[dcarr82775]

Why are the overlaps different each end, is that the design or just drafting error?

I think you have an arch instead of a beam, but if it was a shear failure it would be a little piece that would have to extend back to the back face of the pier. The failure surface shapes of Appendix D come to mind.

The soil will largely arch between the male piers anyway.

 

[mikeCTE]

the overlap shown is the worst case overlap based on plumb and plan view installation tolerances. nominally, the piles would all be equally spaced, but in terms of design, we cannot assume that to be accurate.

which Appendix D are you referring to?

 

[dcarr82775]

Appendix D of ACI 318 deals with anchorage to concrete so it doesn't apply, but it shows various failure planes and that image was in my head. Take the left side of your pic. I envision a failure plane starting at about the apex of the female pier and extending up and to the right until it daylights at the back of the pier. Similar surface up and to the left on the other side

What is the purpose of the secant wall? Does it need to act as a cutoff wall against water? If not, and the soils are suitable, you could get rid of the female piers and just use shotcrete as the lagging similar to a soil nail wall.

 

[mikeCTE]

yes, the wall is to cutoff water.

the simple solution is to increase the pile size or decrease the pile spacing so that i can get the numbers to pencil out. of course, that means more $ for the contractor which he will not like.

 

[dcarr82775]

Graphically at least, for a cutoff wall I don't think you have enough overlap. I don't know of any specific reference, but I try and keep about 10"-12" of overlap after tolerances. Even then the wall leaks, but I have had what I think it pretty good success using that rule which typically give me the spacing at 80-85% of the diameter.

If the contractor wants to stretch things out, make sure you are not on the hook if it leaks worse than expected.

 

[hokie66]

As indicated in my previous post, there is not a shear issue here. The 'female' or 'soft' piles bear on the reinforced piles as circular arches, so the stress is compression, and the thickness of the arch is controlled by the amount of overlap. The part of the 'soft' pile which is on the inside of the arch is superfluous in the final structure.

 

[dcarr82775]

Hokie,

In general I agree it is an arch, but I think there comes a point where the overlap gets small enough that shear becomes an issue.

 

[hokie66]

Small arch spans only require thin arches. You have to allow for tolerances in installation, but I can't see beam shear coming into it.

 

[mikeCTE]

i examined the overlap assuming an arch. it was a shallow arch, but it had adequate capacity to handle the axial compression and shear forces. so, i think i'm all good. thanks for your help!

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