Shot-Crete on Soldier Piles for Basement Walls

[bf343]

I have a project where we are placing a precast concrete building with a basement directly next to an existing building. The basement excavation depth is 18 feet. The owner and contractor want to look at using auger-cast soldier piles with a shot-crete wall for the basement excavation shoring and load bearing element. My design assumes all the new building loads are resisted by the soldier piles. I have also assumed the entire excavation shoring is resisted by the soldier piles. Therefore, structurally, it seems the shot-crete wall is not a structural element, but is only the interior "cladding" (probably not the correct term, but you get the idea). However, it is still an element of the building I am specifying. My issue is that I am not very familiar with shot-crete and am not exactly sure how to detail it.

- How does the shot-crete wall "pin" or "tie" to the soldier piles? Epoxied #4 hooked bars at x" on center? What is the force used to design these tie elements? Or do I solely rely on the bond of the shot-crete to "stick" the shot-crete wall to the soldier piles?

- My basement is fairly long (+/- 300 feet) so does the shot-crete wall need construction joints in it? If so, do I need to detail stopping some of the reinforcing at the joints?

I have tried to push as much of the shot-crete specifications onto the shot-crete contractor (unknown at this time) as I feel comfortable doing, but some of this seems like it is in my court as the EOR.

I appreciate any input and shared experience you all can give.
Thanks

 

[JAE]

On a project I did some years ago we used 24" diameter concrete cast soldier piers spaced at about 4 ft. o.c.
This left a bit of a gap between the piers - the earth between the piers was excavated back a bit such that a reinforced concrete wall cladding was placed in front of the piers.
The concrete between the piers was tied into the interior wall reinforcing with hooked "U" bars which tied in the wall face with a slight bulge of concrete between and just a bit behind the piers.
You can do all this infill concrete with shotcrete as well.
One thing we did (which isn't shown in the very rough sketch below) is that we also placed drainboard with filter fabric between the earth and concrete between the piers and drained it
down to the bottom of the wall and under the wall to sumps. Not necessarily needed but simply an added assurance that water had a path to go.

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[oldestguy]

Maybe I'm too nosy, but do you mean "verified" instead of "assumed"
?

My design assumes all the new building loads are resisted by the soldier piles. I have also assumed the entire excavation shoring is resisted by the soldier piles

 

[jdgengineer]

Maybe not related to your question, but I hope at 18' depth you have a tieback in the soldier beam. That's pretty tall for a straight cantilever. I'd be very concerned with deflection of the shoring system.

 

[bf343]

Thank you all for the responses!

JAE-
I like the way you were able to "key" your concrete wall in with the piles. We are probably not going to be able to have that option as spacing the piles that far apart will result in too much moment in the piles during excavation, resulting in a pretty non-economical pile. I have included our detail of the pile wall and shot-crete below. We did a similar wall drain as you described, to help give the water a path. More of a feel good factor than something we completely relied on for the wall design. We still designed the soldier piles for the saturated soil lateral pressures.

Retrograde-
Thanks for the detail. It mentions scabbling the face of the pier in contact with the concrete wall. However, it appears that the dirt side of the pile is the surface to be scabbled. Does this mean that the soil against the concrete wall is actual back fill? Not necessarily my situation, but very interesting.

oldestguy-
No you are not being too nosy, I came to you all with the questions! You are correct. I have not assumed the piles ability to resist the lateral pressures from the excavation, I have verified this as part of the design.


On the detail we have below, currently there is no tie between the shot-crete and the soldier piles. The pile contractor is already on board for the project and took my preliminary pile sizes and selected the HP reinforcing as the most economical, which is why we show that in our detail. We can provide some #4 bars with epoxy embedment into the face of the piles and hooked into the shot-crete wall as the red line below indicates. However, is there any standard for what force this tie bar would need to resist? The pile will resist the entire lateral pressure of the soil, I feel that is pretty clear by the geometry of the detail. However, the hydrostatic pressure of a saturated condition may put pressure on the soil side of the shot-crete wall. In essence, will the water in the soil behind the wall want to push the shot-crete off the piles? Am I way in the weeds on this? If I am off in the weeds, then what do I use to determine my epoxy bar spacing?

 

[bf343]

jdgengineer-
Sadly no, this 18' excavation does not have any tiebacks. So yes the deflection of the cantilevered soldier pile is a very real concern we have looked at.

A little more back story, we did a very similar building with this owner not too long ago. For that basement excavation (18' as well) we used steel sheet pile with a single whaler and tiebacks. Then had a one sided formed concrete wall against this sheet piling. The concrete wall was on a pile foundation right up against the sheet piles. The same owner and contractor want to look at the option of a cantilevered soldier pile wall for the excavation and building structure to shorten the construction schedule and hopefully lower costs. I am a little skeptical on the lower cost option of what we are pursuing, but I guess that will be for them to make the final decision on...

 

[JAE]

I would think that your piers are too close together to create any real positive soil pressure against your shotcrete.
Perhaps using 62.4 psf/ft pure water pressure on the shotcrete to determine the amount/strength of ties into the piers.
And that pressure would be only on the small width between piers I'd think - not the full width - so factored down a bit due to that effect.

Also look at a uniform 5 psf lateral on the interior face - seismic, wind pressure type loading?


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Retrograde-
Thanks for the detail. It mentions scabbling the face of the pier in contact with the concrete wall. However, it appears that the dirt side of the pile is the surface to be scabbled. Does this mean that the soil against the concrete wall is actual back fill? Not necessarily my situation, but very interesting.

I agree that the scabbling requirement is a bit strange. This is not a detail from one of my jobs but I am certain that the scabbling did not get done. Drill and epoxy starters into the pile is very common in my part of the world.

 

[oldestguy]

OG again: Your spaced perforated drain pipes is a good idea, but I think spacing should depend on soils conditions. Is there any way you can start off with say every third opening with a pipe, and observe results? I'd think that the more cohesive, silty, etc. as compared to sand and gravel should have the closer spacing, but with porous soils the wider may be OK considering the better permeability. Trial early on with long spacing in one smaller part of the job, if possible, probably will help to confirm what is best.