Analysis of New Slab over Existing Foundation Elements

[failuremode]

I am working on a transformer replacement project. The new transformer (~330kip) will be located at the same location as the existing transformer so they want to reuse the existing foundation elements as much as possible. The existing foundation is a 3ft 'T' shaped pile cap on (9) 18" dia precast piles - circa 1960s. The new slab poured over the top of the existing foundation will be 20ftx14ftx~2ft thick with an additional (4) 3ft diameter piers in the corners to avoid overstressing the extreme existing piles.

Any thoughts on how to analyze this configuration?

In the image - Blue is existing. Magenta is new.

 

[Lomarandil]

Oof. That's a tricky analysis alright.

What are the lengths of piles? And what are they embedded in? End bearing on bedrock or on compressible soil will change your analysis assumptions, particularly if one set of piles is in different strata than the other.

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just call me Lo.

 

[HTURKAK]

-There are (9) 18" dia precast piles , so the new load will be in the range of 38-40 kips.. for the 18" precast driven pile ( if driven to refusal ) too small ..
- The piles and cap constructed in 1960s..so 60 yrs old concrete..

If i were in your shoes, i will go ;


- Try to find pile records, geotechnics for the site, soil properties,
- Demolish the existing cap
- Excavate an inspection pit to see the condition of piles,
- Perform static test and pile integrity test

If the piles reusable, construct the new pile cap with proposed dimensions..if not, abandon the exisitng piles and drive new piles and construct new pile cap..

 

[failuremode]

I will add a wrinkle to this - an identical project was done at the site doing exactly as shown above (already installed), but the engineer who did the design had no calculations and is no longer at their company.

The existing piles bear somewhere between 10-20ft below grade based on the original pile plan stating to bear in 'damp hard clay' and the 1960s soil borings labelled as 'damp hard clay' in that 10-20ft range. The new drilled piers would bear at about 20ft below grade based on the previous installed condition which is a silty/sandy layer based on the 1960s soil boring. Bedrock is much further below.

I have requested a geotechnical recommendation to design the new drilled piers.

My thought would be to consider the new 2ft slab as flexible and distribute the loads based on tributary to each different foundation element. Treating the existing pile cap/piles as a single rigid element. I think the new large diameter drilled piers bearing in a stiffer soil (silty/sand) will have good stiffness compared to the existing cap element which has hopefully completed settling in the hard clay.