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What would you do? - Drilled Piers

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I am working on adding office space on the roof of an existing building. 
I have some of the original design drawings from the 1965 construction. 
The steel columns have sufficient capacity fro the additional office
loading.  The foundation is a problem.  The general notes state that the
drilled piers were designed for a gross allowable soil bearing pressure
of 4,000 psf.  The ends are belled.  The soil is probably expansive clay.
 No soil borings available and the original design engineer is gone. 

The existing drilled pier is 4 ft. dia.  For the added load I need 5 ft.
dia. for the soil bearing value.  If I use the 4 ft. dia. the soil
bearing pressure is about 6,000 psf.

What would you do?  I can't reasonably take borings because it is inside
a nursing home.  It sits up on the peak of a  hill so taking borings
outside probably wouldn't be difficult and is significantly away from the
area of the proposed addition.

Appreciate any insight you might have.


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