Mixed Deep and Shallow Foundations for same building

[GeotechD]

Need your thoughts, advantages, and disadvantages of utlizing deep foundations (Auger cast piles) under the core of a 12 story building and supporting the rest of the building on shallow isolated and combined footings.

Should the structural frame be designed to withstand the anticipated differential settlement?




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Amr Sallam, Ph.D.
Senior Geotechnical Engineer
Nodarse & Associates, Inc.
1675 Lee Rd, Winter Park, FL 32789
Phone: 407-740-6110 (112)
Fax : 407-740-6112
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[jike]

I would require an expansion joint between the sections that are supported on different foundation systems.

 

[BigH]

This is/was done quite often in Atlantic City for the new Casino structures. The tall highrise (20 stories plus) were founded on expanded base piles and the lower 2 storey parts on spread footings. What they did was construct the central core first before the lower parts and then actually have the outside foundations of each be near each other - with connecting slab that could take any remnant differential movement.

 

[eric1037]

How about if you design the deep foundations to settle the same (or within acceptable tolerances) as the shallow foundations?

I know it sounds good on paper and may not be practical, but may be worth a try.

 

[eric1037]

Or, conversely, limit settlement of shallow foundations.

 

[fndn]

As long as the interior shallow footings are not in overapping stress zones, you would be fine.

 

[GeotechD]

BigH

What if the structure frame is continuous and it can take differential settlement up to the expected level (let say 1.5 inches)? I think that should be ok. what do you think?

 

[GeotechD]

fnfn

Can you clarify your point?

 

[fndn]

Since the core is on piles, the neighboring shallow foundations are sitting up higher in elevation. There should be enough horizontal distance so that there is a lateral support for the shallow footings AND that the soil to the inside of the shallow footings is not being loaded by both shallow and deep footings. This is similar to footings near slopes.

You could draw a 1:1 projections from the top of the shallow footings near the core in the direction of the deeper footings. If you have clearance to a depth of 2B before intersecting a deep footing, you would be in good shape.