Mud Slab Over Top of Drilled Pier

[mrengineer]

I am working on a project where concrete walls will span between drilled piers. Will not have grade beams, maybe just a mud slab to set wall forms on. There are fairly heavily loaded pilasters at the piers, supporting concrete columns above. Contractor says for this condition they like to hold the pier down a few inches and run the mud slab over the top of the pier, in order to have a more finished surface to set the forms on, since top of drilled piers are typically very rough. Has anyone seen this done before and/or have any precautions?

 

[csd72]

Are the walls precast? If so you will have eccentric loads on the piers when there is wall on only 1 side.

Also check that the corner is not going to break off.

csd

 

[UcfSE]

So are your column and wall connections interupted by the mud slab if you run the slab over the top of the pier? Can the mud slab transfer the forces from the members into the piers in this case? I'd guess no.

 

[mrengineer]

Walls are c.i.p. The mud slab would need to have strength adequate to transfer compression from pilaster to pier.

 

[hokie66]

Sounds like you have a thinking contractor. There is nothing wrong with his approach as long as the concrete in the "mud slab" will transfer the load from the walls and pilasters to the pier, and the reinforcement laps occur above the "mud slab". Need to consider also any lateral loads you have.

 

[Zambo]

Is the "mud slab" blinding concrete with a low compressive strength? in this case it cannot usually be used in place of structural concrete.

I assume the "drilled piers" are what I would call bored piles. Aren't they constructed using either bentonite or polymer excavation support? If this is the case they need to be cast high and then trimmed to the correct level.

Otherwise why not just cast to the correct level and then spend some time getting a good enough surface to set the forms on.

 

[mrengineer]

The piers are 4' to 6' diameter.

 

[jike]

The contractor is trying to save the cost of extra formwork at the caisson. I would have him stop the mudslab short of the caisson and provide the extra formwork and waler at the bottom across the width of the caisson.

 

[Zambo]

jike, didn't understand that.

Is my decription correct. The drilled piers (4' to 6'in diameter)will have cast in-situ walls between them. Directly above them the piers will have a rectangular column, which is in effect a thickening of the wall (a pilaster)

The pilaster will need formwork, the contractor intends to cast bliding concrete (mud slab) on which all this formwork will sit.

What we don't know is the dimension of the pilaster compared to the drilled pier. Assuming the pilaster is larger than the drilled pier the mud slab can surround the pier but not run over the top of it.

If the pilaster is of smaller dimension than the pier and the forms somehow have to sit on the pier then structural concrete wll have to be used.

 

[jike]

The formwork for the walls and pilaster is essentially all part of the same except the pilaster is bumped out on one or both sides. I am assuming that the bottom of the mudslab is even with the top of the caisson then the top of the mud slab is higher and does not extend over it, then there will be a some area below the form that needs to be fill in with more formwork.

 

[mrengineer]

The overall footprint of the pilaster (including wall thickness) is 20" x 36", which is smaller than the minimum drilled pier size (48") we will be using.

 

[hokie66]

jike,

You didn't read the OP properly. The contractor wants to run the "mud slab" through over the piers to simplify his formwork. Nothing wrong with that, in fact it shows good common sense. Mrengineer has stated that the "mud slab" concrete has to be strong enough to transfer the pilaster load.