Overpouring Drilled Shafts

[STrctPono]

For as long as I've been working for my current company, the standard of practice for CIP drilled shafts has been to have the Contractor overpour the shaft equal to the diameter of the shaft. We then have them come back later and demolish the shaft down to the finish top of shaft elevation. The theory being that the upper 1.0 * diameter of the shaft is usually junk concrete due to segregation. We've done it for every job and every shaft. I had a recent project where I had a chance to see some core strength and density results for the in-situ material in the upper 3ft of a 5'-0" diameter shaft and for the most part it did not seem as if the theory of segregation in the top of the shaft was true. Has the more prevalent use of SCC's in deep foundations made this standard of practice obsolete? Do any other firms do this?

 

[jayrod12]

We definitely do not mandate this. WE tell the contractor top of pile elevation, and the strength we need. It is up to them to provide what we've asked for. I don't think I've ever seen them intentionally poured high just so they could ensure appropriate consolidation at the final pile elevation.

 

[BridgeSmith]

Ditto what jayrod said. I'll add, our standard specs require that they consolidate (vibrate) the concrete for a depth of 2 shaft diameters from the top.

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

 

[MrHershey]

Thirded on what jayrod said except I have seen them pour high and chip down occasionally. Usually not nearly as much as your are, but I've seen them over pour a little bit.

May also be prudent to do that on sites were piles want to settle a bit. A lot easier to chip down a pile that's too high than to extend a pile that's too low.

 

[dik]

a little crud won't hurt... just strike it off... usually the stress level is pretty low. What you might want to do is vibrate the top 10' or 15'. With 40' caissons, I've seen the concrete drop by a foot or so with vibration.

Dik

 

[JAE]

My mentors from Texas (part of the clan that invented drilled piers with Terzaghi), used to require that the contractor overfill the pier by 2-3 inches. Then they'd have them chip off that amount after the concrete had hardened.

The idea was that there could be segregated concrete at the top, or the top could have dirt or debris mixed into it (many piers are topped out near ground level).
Also - there is some level of "bleed water" that comes up, similar to slabs-on-grade, that weakens the top levels of the concrete.

ACI 336.3R states: "Segregated concrete and laitance should be removed before proceeding with construction of the cap."....after a pier is constructed.

Even with a slurry method of placement (vs. free-fall dropping of the concrete) you would still probably have poor concrete with dirt or laitance in the top.

Over-placing concrete a distance of the full diameter seems a bit much to me, though.

 

[BridgeSmith]

JAE's post reminds me of another requirement in our standard specs - the the tremie or hose has to stay a minimum of 1 ft below the level of the concrete pour at all times (except obviously the first few feet at the bottom).

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

 

[Retrograde]

It my experience it depends a lot on the ground conditions and water table.

 

[hokie66]

I have never heard of removing so much from the top of a pile. You just need to clean it off, including laitance, mud, whatever.

 

[STrctPono]

I sort of figured I'd be alone on this but thought that some of you may have at least seen this done in the past.... I'm a bit surprised. Thanks.

The story I heard for why our office enacted this practice is supposedly 20 or so years ago my boss was working on a large viaduct project pouring some 8ft diameter shafts and he said that after the concrete had set, the top 5 ft or so was terrible. Just sand and cement, not a rock to be found in the matrix. Again, my belief is that the more wide spread use of SCC for drilled shafts has more of less fixed this issue to some degree.