Cracking to Bearing Pile due to Heave - Statement in Design Report 1

[EireChch]

I am reviewing a design report and came across the statement below. I am not sure what to make of it.

The development comprises a 6m deep basement, 450 and 600mm dia contig pile wall. Ground conditions comprise 2.5m of gravel then Clay. It is over consolidated clay so due to the stress relief there will be heave. The internal columns, walls are supported on pile caps.

As the bearing piles are in a basement, there will be a degree of basement heave. Hence, there will be a degree of pile cracking, which usually closes up as the building load is applied

How will the pile crack? due to the shaft friction and vertical heave causing a horizontal type crack? Is it acceptable to say it will crack and then close up. Can the reinforcement not be spec'd to minimise the crack width? Interested to hear peoples thoughts.

Cheers

 

[humanengr]

Sounds like they are saying heave (basement uplift) will result in axial tension on the piles.
If that is the case, the piles should be designed for the tensile force to prevent "cracking".
It does not sound like this is acceptable cracking. This is not normal shrinkage / temp. cracking.
It sounds like they are talking about actual structural tensile failure of the pile. If the basement
is "pushed" back down by loading, that does not fix a structural failure. Will there be any de-loading and
re-cracking?
What kind of piles are these (reinforced concrete?)

 

[JAE]

Similar to what I used to see in south Texas with expansive clays - the drilled piers we used (with belled bottoms) would have additional vertical reinforcement used in response to anticipated levels of tension forces in each pier due to clays swelling upward.

The geotech reports usually provided a formula for the tension steel but as I recall, it usually ended up in the range of 0.75% to 1.0% of the gross pier area.

The reinforcement doesn't prevent cracking - only minimizes the crack widths.

Once loads are applied to the piers (piling) you counter the soil-induced tension with axial compression and the cracking is reversed as much as it can be due to possible material infiltration into the cracks.

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