Economics of shallow spread footings vs. piles
Economics of shallow spread footings vs. piles
(OP)
Hey everyone,
So I just got pulled into this project pretty late and there's some drama going on with the foundations (drama + foundations = uh oh)
The geotech report states there's a bunch of fat clays, silty soils and generally expansive and unwieldy soil at our site, and recommended one of two foundation systems.
The first is the very conventional shallow spread footings, but doing so would require a gratuitous amount of over-excavation and backfill from soil not found on site.
The second option is to use 18' piles.
We requested which would be more cost effective for our client, and the geotech claimed they would not perform any rough estimate because it's outside of their scope and would be more along the responsibilities of the GC, which is understandable.
My question, is when you are provided an option between foundation systems, what are your decisions on a specific foundation system based on?
Thanks!
So I just got pulled into this project pretty late and there's some drama going on with the foundations (drama + foundations = uh oh)
The geotech report states there's a bunch of fat clays, silty soils and generally expansive and unwieldy soil at our site, and recommended one of two foundation systems.
The first is the very conventional shallow spread footings, but doing so would require a gratuitous amount of over-excavation and backfill from soil not found on site.
The second option is to use 18' piles.
We requested which would be more cost effective for our client, and the geotech claimed they would not perform any rough estimate because it's outside of their scope and would be more along the responsibilities of the GC, which is understandable.
My question, is when you are provided an option between foundation systems, what are your decisions on a specific foundation system based on?
Thanks!






RE: Economics of shallow spread footings vs. piles
RE: Economics of shallow spread footings vs. piles
Where is the water table? If "high", forget the excavation / backfill to avoid extensive dewatering. Go with piling.
What is the schedule? Piling are "fast" and installation is much less weather-dependent than excavation / backfill operations.
Will vibration from pile driving be a potential problem for nearby activities and structures?
Is differential settlement a concern? With piling, much easier to minimize the probability.
What is the off-site source of the backfill?
How is the unsuitable soil disposed?
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RE: Economics of shallow spread footings vs. piles
On the piling alternative, what carries the lower floor? Chances are it has to be structurally supported on the piles. Is there any unknown about the action of the expansive soil that may affect plumbing (Carry it from the floor?) or possibly lift the piles with expansion due to the friction between soil and piles?
What about entry to the building? Will walks or buried utilities move up and down and create hazards.
Answering the "What is best" requires plenty of checking many things.
RE: Economics of shallow spread footings vs. piles
With shallow spread footings, do you excavate to below the soil suction depth (based on climate and soil profile), found/build your footing and back fill? With the pile option, what stops the expansive clay gripping your pile and then heaving vertically causing differential movement in the superstructure?
In my area of practice (South Australia) soil heave of +80mm-120mm is not uncommon (soil suction depths of 4.0m) and we design shallow footings in the form of a stiffened raft slab
RE: Economics of shallow spread footings vs. piles
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Economics of shallow spread footings vs. piles
--- Is concrete slabs-on-grade performance (interior & exterior) a big concern? The overdig may solve a lot of those problems.
--- Is site drainage a big concern, with no good solution? The deep foundation may solve a lot of those problems.
--- Is deep wetting of the geologic/bearing formation a concern? The overdig may solve a lot of those problems.
--- Is the Foundation/Structure Engineer very uncomfortable with anything except steel & concrete? The deep foundation may solve a lot of those problems.
--- Are future, substantial building additions possible on a difficult site? The deep foundation may solve a lot of those problems.
RE: Economics of shallow spread footings vs. piles