Michael (and others who responded to this post),
Thanks for responding, I have been slaying other dragons of late and am now
coming back to this topic.
First, allow me to lay down some more info:
A large portion of our work consists of underpinning residential structures
experiencing distress through foundation settlement. In many cases we are
dealing with hillside conditions in which soil creep is a factor (combined
axial and bending).
Our underpinning designs typically consist of new concrete grade beams cast
adjacent to and attached to existing foundations, supported by drilled, cast
in place concrete caissons. Caissons are usually 24", 30" or 36" dia.
depending on conditions.
Section 1808.2.2 of the 97 UBC requires limiting concrete compressive stress
to .33f'c and steel compressive stress to .34fy or 25500 psi. My primary
question is, where does this requirement come from and what is its' basis.
I have had many discussions with many colleagues and found no clear
reasoning. Some have suggested it is a "left over" from allowable stress
design. Others have reasoned code authors want to limit stresses in shaft
due to uncertainties associated with construction quality of caissons.
Chapter 21 of ACI 318-99 has requirements for dcip piles, some of which are
similar to UBC but no limitation of compressive stresses.
As you can imagine (and as Michael hit on in his response) foundation repair
is expensive. Caissons are expensive, in our case beyond even what Michael
likely imagined since he was thinking new construction when he responded.
Caissons are also THE BEST SOLUTION to these type of subsidence problems.
There are lots of other solutions out there. We have on multiple occasions
gone back onto jobs after someone elses non caisson solution did not perform
and installed caissons (doing it the way it should have been done in the
Due to expense we try to limit the quantity of caissons on a given job. This
results in maximizing the benefit of axial and bending capacities of 24, 30
or 36" dia. caissons. Limiting compressive stresses per 1808.2.2 simply
results in more caissons (add more caissons, decrease individual caisson
load) and the expense soars. Expense can very quickly get to the point where
the repair is not economically feasible.
Now, to respond to Michaels post (I'm not picking on Michael)
Caisson construction is continuously inspected by a registered deputy. No,
he can't watch the flow of concrete around the cage but he does ensure shaft
depth, concrete mix, delivered steel etc.
Concrete to the bottom: Concrete is delivered into the shaft via a tremie
extending to the bottom of the hole, no free fall.
Reinf. cover: Flowability is a must and mix designs specified to ensure it.
Caving: Depends on soil strata, shafts can be cased or drilling fluid used.
Soil content: These projects simply don't happen without competent soils
engineers, geologists and engineering geologists involvement. Sulfates is
just one of many things these professionals evaluate.
Unbraced column length/base fixity: An enormous amount of research has been
conducted by Dr. Lymon Reese and many others on the non linear response of
soil to lateral loading. You would be amazed at how accurately pile
deflections, point of maximum moment/shear can be calculated.
Point bearing vs. skin friction: 99% of our projects utilize skin friction
for developing axial loads. This always includes the effects of downdrag
from the "loonshit" (I like that term and henceforth will use it often:)).
In the end, my problem with 1808.2.2 is that it can kill a project.
Considering the type of caisson work we are talking about (repair) the
expense associated with it, the level of quality control, extent of
preliminary soil exploration, all the things mentioned above... why has the
compressive stress been limited? Is it because of the fear that caissons get
put in the ground with none of the quality control measures or design care
stated above or something else?
I must quit, I have said more than intended when I started.
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