Economics of shallow spread footings vs. piles 1

[AnimusVox]

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!

 

[Guastavino]

Cost and potential issues (ie future potential costs). Most geotechs I deal with give one option unless you press them. If they say 18' piles work, I might would go with that. It's a great foundation and you don't have to worry about backfill, compaction tests, and unknown quantities. With piles, the client knows the cost up front. With over-excavation, it's like Vegas. I'm dealing with that now on a project where the Geotech did estimate, and they were low. Luckily unit cost, but still.

 

[SlideRuleEra]

Other concerns:

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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[oldestguy]

With footing foundations on replaced material, what is the history of the area as to depth of moisture changes and thus expansion and shrinkage? If not deep enough will the compacted fill even out these movements?. I assume the floor is placed on the fill.

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.

 

[jwilki]

Just a general question (addressed to OP or anyone willing to comment). How are expansive clay soils handled in your neck of the woods?

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

 

[KootK]

I do a lot of work in expansive clay country too. it's usually piles, grade beams, and void formed structural slabs at grade. Piles are prevented from heaving by founding them well below the active zone where seasonal moisture changes are taking place. Texas has this problem too a seems to have invested the most in researching it: Link

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.

 

[emmgjld]

As a geotech, I am in an area where this choice is very common. Both systems are used & I personally have preferred the over-excavate/replacement with shallow foundation option (for over 40 years). As I tell my clients, there are a lot of factors;
--- 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.

 

[nonplussed]

What are people's views on over-excavation in clay soils in relation to creating a sort of pool filled with water under the building? The water will not be able to escape through the clay and will just pond. Any opinions?