footing missing under an existing foundation

huangyhg

footing missing under an existing foundation
i have a 4" slab on grade foundation. 8" cinderblocks, filled, 24" below greade, 36" above grade. eventhough my blueprints state that there should be an *" x 20" footing under my cinderblocks i was unhappy to find no footing what-so-ever. my foundation has not sunken or anything like that and i am not sure if i like the idea of underpinning. are you allowed to escavate and just poor a footing under the cindeblocks in a similar style as underpinning?
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how long has this been the situation?  was the house built originally for you?  is there any signs of distress - cracking, doors or windows not shutting properly, etc even though you say that the foundation wall hasn't sunken?  is there any environmental change (water seepage, dry or wet spells, etc) that has occurred recently or is the general envioronment similar for the last so many years?  is the builder still in business?  did the city inspect the foundations at the time of construction?  these are some of the questions that when answered may help to provide you the best way to proceed.  
while there is probably a good argument to be made for if it isn’t broke don’t fix it, it is possible to fix this by pouring a foundation.  we had a very similar case except that our client’s footing was just way under sized.  the contractor excavated down the side of the outside of the existing footing and then excavated half way underneath.  dowels were drilled and placed with epoxy and concrete was placed.  the process was then repeated on the inside of the existing footing.  this process resulted in a footing twice as big as that originally place.  it was painstaking work for the contractor.
in your case, you have to take steps to not undermine your block wall.
if the wall is your own and has been in place for a while, and remains servicable, i wouldn't risk undercutting and installing a footing.
is 20" the width or depth of your missing footing?  as it is a 4" slab is not much of a slab and this must be a residential property, for any kind of commercial building project i would expect a 6" slab minimum. i guess 20" is the width (500mm) and that this is more of a slab thickening (perhaps 200mm deep) than any designed footing. as there is no apparent settlement or cracking i think everything is ok and that there would be more likely to be damage caused than improvement by attempting to rectify the situation.
the home was built the early to mid fifties. i have owned the house for almost 10 years now. the foundation is showing no significant cracks or leaks and all door and windows function properly. we are planning on building up the soil around the house 6-12" sloping away from the house in order to ensure water does not sit next to the foundation. the 4" reinforced concrete slab (14' 6" x 10') is hanging over the cinderblock foundation by about 2-3 inches on each side. my gut feeling and re  
for a house built in the early 50s - that means it has functioned fine on the "foundation" for +50 years.  you've indicated that the underlying soil is rocky, etc.  i see no reason you can't add the 6 to 12 inches as you've planned and enjoy the next nfl game!
the first question is, what is the bearing capacity of the subgrade soils?  if the soils are sufficiently firm/stiff and the applied loads are light, is any more bearing than an 8" wide block required?
i know this sounds like heresy but, there many soils/loading conditions for which the footing is only required as a leveling pad to place the superstructure on. this may be one of those cases.