Good info Chris! I had heard alot of that watching 'Modern Marvels' awhile back. I just don't understand why they would build these huge bridges attached to bedrock in siesmic areas (CA) but the code says you 'can't' do this with a house??

Chris,

Pretty close.  When the Brooklyn Bridge was built, it was something of an engineering marvel of the day.  And they had to go through a lot of muck to get down to the bedrock, just as you've noted.  So the engineer put caissons in place where the piers would go -- essentially a large-diameter vertical pipe with the top sealed.  Pressurize the caisson so it won't admit water.  The work crews would airlock inside and dig.  As they went down, so did the caisson.  When they got to the rock, they stopped digging and the caisson stopped sinking; it's now a form for the pier.  (This recitation from memory)  I recall from the reading the digging job was not the safest occupation in the world.  Or even New York. 

Not a dumb answer!  Every structure must deal with some horizontal loads weather it is from wind, seismic or other dynamic forces.   Keying it into bedrock is certainly one way to do it and is probably the case in the bridge example (although in seismic areas large buildings are also likely to be built on rubber or sliding dampeners  to control motion). 

On massive structures like bridges and tall buildings, the bearing capacity of the underlying material is very important.  When building on rock (especially limestone), the upper interface with the overlying soil is likely to be 'weathered' (i.e., cracked, fissured, with veins of soil).  This weathered material must usually be removed to get to sound rock and is probably the most usual reason for rock excavation.  The cores (stairs, elevators, restrooms, etc.) of tall buildings typically have higher foundation loads than the perimeter and care must be taken to minimize differential settlements, so sound bearing is important.

Just as an aside, the bends (nitrogen bubbles in the blood) was also known as "caisson disease" because the poor souls working in pressurized caissons frequently suffered from this problem.  They were definitely not nice places to work.

Bruce

Do you have any pictures of projects using this method?

I do not know of any recent projects that have used pressurized caissons. I suspect if you google on pressurized caissons, I suspect you will find some diagrams or pix.

Bruce

In case this helps, i remembered seeing these photographs http://www.quadlock.info/photos/thumbnails.php?album=182

This is the foundation system we use for ICF houses, it eliminates the Cold Bridge where the wall sits on the ground and the material costs are much less. The structural EPS directly under the footing can take very high loadings.