Monolithic concrete pour with turned down edges?

[tocoadog]

Contractor wants to build wharehouse with a monolithic pour with turned down edges. I understand this to be that the perimeter of the slab consists of turn down footings. Okay.

For the geotech report, I based my allowable bearing pressure on spread foundations bearing on 48 inches of engineered fill due to the potential for hydrocompaction here in New Mexico. The contractor has asked me to revise my recommendations to include turned down edges and has asked me if I could lessen my engineered fill thickness? Can I do this? What is the mechanic of the turned-down slab? Does the turned down slab act as a continuous footing without any pier locations?

Any insight would be appreciated.

tocoadog

 

[tocoadog]

BTW, I did a search on the site for the topic and found a post for the structural perspective but nothing for the geotechnical perspective.

Looking forward to any of your replies.

 

[JAE]

I've used turn-down footings before - mostly in expansive clay regions. This was usually a slab with perimeter and interior gradebeams that were placed monolithic with the slab. The grade beams were usually spaced between 10 feet and 16 feet apart, depending on the severety of the expansive clay.

From a strucural perspective, we've used the allowable bearing given and then used whatever width of grade beam would provide us with a pressure less than the allowable. So it works essentially like a continuous spread footing, its just reinforced and has more stiffness than say a traditional continuous footing with a block wall on it.

At columns, we just expanded the grade beams to form a large spread footing centered on the column. I don't see how using this system would affect a recommendation for engineered fill as this system works essentially the same. Perhaps with a rigid grid of grade beams the slab/beam system can span across weak zones and thus reduce the fill thickness required?

 

[tocoadog]

JAE, thanks for the quick response.

Re: limiting the engineered fill thickness, I was thinking along the same lines as you in that the turned-down edges are rigidly connected to the slab thus able to distribute the load to a larger area...essentially a mat foundation. Larger area of load distribution, same bearing capacity, less engineering fill thickness...?

We ran consolidations test on 2 samples and had about 4 to 6 percent hydrocompaction. Blow counts on the order of 5 to 10 in the upper bearing levels. Natural densities on the order of 100 to 105 pcf.

Looking forward to more responses.