historic caisson construction methods

huangyhg

historic caisson construction methods
i am currently working on a project in chicago in which we are trying to determine the capacity the existing foundations.  the 10-story building was built in 1911 and one was on of the first reinf concrete buildings.  it was discovered that the foundations are concrete caissons.  it was also determined that the caissons do not go down to bedrock, but rsst on clay 60 feet below grade.
given the caisson shaft diameter at the basement level, it appears the diameter is not wide enough to support the building.  my question is........in 1911, were they physically able to form bells or some sort of stepped foundation at the base of the caisson to spread the load on the clay?  or did the shaft get wider, as it went deeper?
are there any historic resources that would discuss caisson construction from this era?  your help would be much appreciated. thanks.
terry

i worked for a structural firm in south texas some years ago - the founder had helped develop drilled and belled piers with the assistance of terzahgi - they even had an autographed terzahgi book in their library -
but i remember looking through some old boxes of photos and i re  
    i would check out tschebotarioff's 1951 edition.  he explains the "chicago method".  "the chicao caisson is used in clays of at least medium stiffness so that the wall of the cylindrical excagftion shaft will stand up vertically to a distance l,which is usually made equal to 5 to 6 ft.  the caisson is made of sufficient width to permit a man to work in it.  the walls of the shaft section are then lined with vertical planks, a few inches shorter than l, and are supported laterally by two pairs of steel half rings wedged apart and thus pressed against the timber lining.  in this manner, the caisson can be carried down step by step to hardpan or to rock."  hardpan is the glaical clayey silt till.  it is thus likely your method of construction and also the fact that the piles are in the stiff clay to hardpan.  he suggests you see dunham's book on foundations of structures.
    also see other "old" books - krynine and judd's book (1957).  also see tomlinson's book - especially the earlier editions.  he has great chapters on well and caisson foundations (box bridge ones).  i would also do checking in the older engineering news-records - they have had a lot of good info when i was researching a project from 1916.
i might add, you might call dr. j. osterberg who is professor emeritus from northwestern.  he would know all you would ever need to know.
thanks for the input.  after further testing, the caissons go down to bedrock and we have plenty of capacity.  
after exposing the top of the caisson in the basement, the original vertical wood plank forms were found, as was described above in the "chicago method."  very interesting stuff!