The sketch you attached has been seen here before...from another engineer, presumably the design engineer on your project.  Why not ask him?

Stability of the material directly under the pile cap should not matter, as the vertical load is supported by the piles and the lateral load is braced by the slab.  However, I don't understand why the undercut was made.  What does a space problem have to do with it?  I trust this was for one column only.

hokie66,

tnx to your reply, greatly appreciated.Our foundation is clustered piles in different pile caps, it is located the edge of building with adjacent existing structure,on one side and a deeper pile cap on the other so overcut was made. In this case, I want to place boulders first with concrete binder up to the level of water, then backfil with selected fill materials. However, because of deep overcut, I doubt if stability of subgrade level below the cap might be affected. Hope you can explain further.tnx a lot.

• http://files.engineering.com/getfile.aspx?folder=90929a4a-fb78-4516-a364-ac

If you are concerned with stability of the adjacent existing structure, you should have built a retaining wall to prevent movement of the subgrade into your overexcavation.

 

Why "boulders with concrete binder"? Sounds like a prime situation for flowable fill (controlled, low-strength concrete).  However, using large rubble or rocks with no intermediate size aggregates sound like a recipe for expensive disaster.  If you are looking to conserve on the amount of fill by adding very large rocks, the difficulty will be in getting consolidation so water does not accumulate in voids and promote collapse of the fill above into the voids.  If you decide to go with your idea, be sure the concrete used to fill the voids is modified (HRWA, viscosity modifier) so that you get the result you need.

I agree with the flowable fill recommendation.  Boulder fill under an addition to an existing hosptial building sounds like a bad idea.  Usually expensive medical equipment is sensative to settlement.  The only one who wins will be the contractor and/or owner that pocket some money while you take on unnecessary risk.