Combination Foundation
Combination Foundation
(OP)
We have a project where we have oversight reponsibility. The allowable contact pressure is 2000 psf. Since some of the equipment has dynamic loads (a debarker for instance), the design engineer has piling under large mat. In this case (95)-60k piling under a 32' wide a 116'6 long 2'3" thk mat. The piling are Auger cast in-place 16" dia to 40' BG. The total load super-imposed on the piling is about 4200k (including the mat weight). The piling arrangement grid is 7'6 spacing except near the 4 piers where the spacing goes to 4'6.
My view of the piling efficency (66%) is that the layout needs to be revised. I also think that since the adjacent equipment is on shallow foundations that the design for this piece should utilize piling below the piers only and let the mat carry the other loading (small conveyors, shrouds at either end, and of course the mat weight).
Again, the soil at this area has heavy shallow foundations (mats).
Any comments? I've had other engineers speak both pro and con. I had one tell me that most A&Es put all loads on piling regardless as they are the much stiffer carrying elements then the soil. It just strikes me as wasteful.
My view of the piling efficency (66%) is that the layout needs to be revised. I also think that since the adjacent equipment is on shallow foundations that the design for this piece should utilize piling below the piers only and let the mat carry the other loading (small conveyors, shrouds at either end, and of course the mat weight).
Again, the soil at this area has heavy shallow foundations (mats).
Any comments? I've had other engineers speak both pro and con. I had one tell me that most A&Es put all loads on piling regardless as they are the much stiffer carrying elements then the soil. It just strikes me as wasteful.





RE: Combination Foundation
i'd rely on the project geotech and discuss the hows/whys with them to see if there are other approaches that can be utilized. by the way, on a side note i seldom use the term "wasteful" as this can create unnecessary and unjustified headaches within a project team...it's either more/less conservative. i have thrown out an "overly conservative" on rare occassion when a lowball geotech got the job because they sold a no-brain approach to the exploration instead of actually engineering the solution (is much easier/cheaper to default to deep foundations instead of engineering say shallow foundations on a tough site for example).
RE: Combination Foundation
I wouldn't want a footing part supported on piles and part supported on ground with only 2000 psf capacity.
RE: Combination Foundation
RE: Combination Foundation
Based on some of my experience I would not be surprised that this is a conservative approach. However, we must give the designer some credence. As mudman says, the best is to have some dialogue with the designer. I wish that structurals and geotechs can have this type of discussion. however, i presume from the practice that this is astructural engs design with some recommendations from the geotech but little or no cross discussion. This is part of today's on going practice - The silo effect.
Interseting topic.