Half and half foundation-Piles & Shallow footings

[JoeBaseplate]

So this is somewhat a theoretical question. A new proposed building 330'x200', 3 stories, in schematic design. Nearly half the borings show that the building will be OK @ shallow footings, while the west half has either to be excavated and refilled, comapacted to 8' or we can go to deep foundations.

The arch asked me if it was OK to have half the building on shallow ftgs and half on H piles. My response was that there will be significant differntial settlement since piles are somewhat rigid support compared to footings which will settle. Any thoughts?

 

[msquared48]

Go to all pile or make two structures. Don't mix it.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Lion06]

I agree with Mike.

 

[BAretired]

JoeBaseplate,

My response was that there will be significant differntial settlement since piles are somewhat rigid support compared to footings which will settle.

You are absolutely right. Use one or the other but not both.

BA

 

[JAE]

Generally I'd agree with Mike and Lion above but I once had the exact condition and we mixed them.

What we did was use drilled concrete piers on one half (soft, expansive clays with shale about 30 feet deep) and on the other half used spread footings.

Now we knew that the spread footings would tend to have more settlement than the deeper, "fixed" piers so as the spread footings got closer to the piers, we started diminishing the allowable soil pressure....i.e. using larger and larger spread footings...until we got to the fault line where the footing type changed to piers and suspended grade beams.

At that point we added a building expansion joint as near to the fault line as possible.

This was in 1985 or so - no problems since.

 

[hokie66]

I agree with the others, but in some cases, it's not that simple. If the bearing strata is rock, then a combination of footings in the high rock and piles in the lower rock can be a satisfactory solution. Conversely, if part of the building bears on natural ground and part on engineered fill, there can still be differential settlement issues.

 

[csd72]

Agree with JAE and hokie, in certain situations this is feasible.

Firstly, I would discuss this in detail with the geotech (if there is not one then get one!).

If you are bearing on the same strata with similar bearing stresses then you should be okay. But this will depend on what lies beneath i.e. thickness of this strata, depth to water table e.t.c.

 

[kieran1]

I agree with the above posts that it is feasible, as i've done it many times. I usually provide a movement joint though to account for differential settlement.

Kieran

 

[Ron]

In general, you can't mix the two simply because of the incompatible strains; however, if you have two distinct parts of a structure, you can found them on two different systems, provided you know the differences and join them accordingly.

 

[csd72]

Ron,

Why do the strains necesarily need to be incompatible? Surely for many situations it is possible to control the stress in order to negate the difference in strains. Hard work, yes, but impossible, no.

In some strata, I would say that the worst thing you could do for strains would be to locate everything on shallow footings.