Precast footings construction concerns 2

[skewl]

The contractor is proposing to replace cast-in-place footings with precast footings during construction. The original design was to have the cast-in-place footings casted on top of a "leveled" mud slab. Our concern is the precast footing would not have uniform bearing with the mud slab. My questions are:

1) Are gaps between the precast footing and the mud slab even a concern? Under loading I expect the mud slab will crack and loads will be redistributed.
2) Is it better to replace the mud slab with a compacted layer of granular? Having said that the work zone is in a creek so we can expect frequent water runoffs.
3) Is it feasible to provide grout injection points along the footing so a flowable grout can be injected to fill the voids between the footing and the mud slab?

 

[SlideRuleEra]

Our concern is the precast footing would not have uniform bearing with the mud slab.

1) Are gaps between the precast footing and the mud slab even a concern?
Under loading I expect the mud slab will crack and loads will be redistributed.

2) Is it better to replace the mud slab with a compacted layer of granular?
...the work zone is in a creek so we can expect frequent water runoffs.

3) Is it feasible to provide grout injection points along the footing so a flowable grout can be injected to fill the voids between the footing and the mud slab?

Your concerns are well founded for the stated reason... plus other reasons.

1) Yes, gaps are a concern.
Yes, the mud slab may crack... but it may not crack immediately when the precast slab is placed on it. Cracking (and potential differential settlement) may happen after pier construction is underway... knocking the footing and the pier on top of it out of alignment.

2) No granular material. You stated the reason "...the work zone is in a creek". A mud slab is used to provide a reasonably stable, approximately level working surface to so that footing rebar can be accurately placed, supported, and expected to stay where it should be during footing concrete placement. Also a mud slab provides a definite boundary between the soil and structural concrete.
Gravel in a (soft, muddy?) creek cannot be relied on to do this.

3) No, it is not feasible. Grout is used to fill gaps between two structurally sound surfaces. A mud slab is not "structurally" sound. A mud slab is a "temporary" solution (see #2, above, for reasons). A cast-in-place footing conforms to the shaped of the mud slab. From that time on, think of the now covered mud slab as only "really good backfill" immediately below the footing.

The Contractor is trying to take advantage of you and the Owner. The only "winner" will be the Contractor... if "something" goes wrong (see above), guess what, it is your fault for allowing the change. And the Owner may have to pay (for remedial work).
A mud slab / cast-in-place footing are used for good reasons when field conditions are difficult... like a bridge footing in a creek.

 

[hokie66]

Agree with everything SRE said, and he said it better than I could.