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Low Strength Concrete in Permanent Shoring Piles

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KootK

KootK

Structural
Oct 16, 2001
18,292
For a project with a seven story basement, I'm evaluating a proposal to use the secant pile excavation shoring as a permanent load resisting element (hydrostatic / earth pressure loads). The shoring tie backs would be assumed to become ineffective over time and the secant piles themselves would span between floor diaphragms. Every second concrete shoring pile will encase a steel pile. Composite action will be assumed.

Our primary secant piles are 10 MPa concrete and our filer piles (every second one) will be 4 MPa concrete. I have two concerns regarding the low concrete strengths proposed:

1) What is the minimum f'c for durability?
2) What is the minimum f'c for composite action?

Reference to any pertinent standards would be appreciated. Our geotechnical consultant will be signing off on this system. That being the case, I still need to ensure that I'm comfortable with it conceptually.

Thanks,

KootK
 
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Where we are any concrete that is permanently exposed to sulphates needs to be 32MPa. We've circumvented this a few times by having soil testing done and having a engineer confirm that there are no sulphates (or the chance of sulphates) present in the area.

And seriously 10MPa and 4MPa are they mixing this stuff by hand? That's terrible strength concrete.
 
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Thanks Jayrod.

The same geotech that will be responsible for the shoring design will be giving us the green light for sulphates.

The goal of the 4 MPa concrete is for it to be weak. Every second pile has to be drilled through it's neighbors.
 
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Temporary work is one thing, but permanent basement walls are another. There is no reason for using 10 MPa concrete in the "male" pile, the one with the steel beam. I would want structural concrete, 32 MPa or stronger, in that one, for protection of the steel if for no other reason. The "female" or soft pile, the one which is drilled, can be of much less strength, but 10 MPa would be my target for those. I assume the temporary ground anchors are installed through the soft pile, and I don't think 4 MPa concrete is good enough.
 
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When we last did this we had a mix that was a slow strength gain, at 4 mPa we drilled and they finally ended at about 11 mPa.

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."
 
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Thanks Rowing. At the least, it's reassuring to know that someone's done this before.
 
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We have done a couple of secant pier walls for similar type of use. We call for 500-1000psi on the females (non-structural), and 4500psi for the males (structural piers). We use rebar cages in the male piers instead of steel sections.
 
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