cutting an area out of tunnel floor

huangyhg

cutting an area out of tunnel floor
i have a large tunnel that i need to put a spread footing in.  the tunnel is 100 years old and the existing floor cannot support the load new column load.  i am proposing cutting a section of the floor out to install a new spread footing.  
the box section that represents the tunnel obviously has compressive forces acting through the floor ....i am thinking/hoping that those forces redistribute around the new opening and the floor handles those forces like a diaphram.....what kinds of things can i check to make sure this works as planed

sounds like a recipe for trouble. perhaps you can avoid having to cut a large hole and possibly creating a big problem. maybe several spaced smaller holes with micropiles installed thru each...
design and install a moment frame by installing piece by piece with minimal disturbance and in-plane with the floor slab so as to take the loads.  then remove the enclosed floor slab.  perhap some studs on the frame at the cold joints.  some field welding will be required for the moment connections.
vod
if the floor is just carring compression loads between the bases of the walls of the tunnels, is it possible to use shoring to transfer the the thrust at the bottom of one wall to the bottom of the other wall. much like trench shoring boxes transfer from one side to the other.
i can picture a beam along, and parrellel to, the base of each wall of the tunnel with a compression   
one more thing, you will have to confirm if this is possible by checking if the modified box section has capacity throughout and if the resulting bearing pressures are acceptable with the reduced bearing area.
vod
the hole will be 20 feet or so from both tunnel walls.  wouldn't the slab (3' thick) act sort of like a 20' deep beam spanning across the length of the excavation?  the slab has reinforcement in both directions.....it seems like that slab would have to bend transversly to cause a problem...