Subgrade at Tunnel Construction

[pantro1977]

We are building a tunnel that is about 40 feet deep. Ground water has saturated the fat clay subgrade and now the subgrade will not meet the 2000 psf required per the drawings. One option would be to bridge the subgrade by undercutting 12" (This is the max we can undercut, since anything more would undermine the soldier piles) placing geogrid, and 12" of #2 stone. However, there was a guy representing the owner at the meeting we had this morning that said "we really don't have to attain the 2000 psf, the tunnel is sitting a mat foundation, we only needed to have a stable subgrade"

Is this a true statement? I still think that if don't get the 2000 psf, we should undercut and bridge it. Any thoughts?

 

[hokie66]

Not enough information for an informed answer. A cross-section of the tunnel would help. Also a better description of the material under and around the tunnel.