heres one. we are just starting w

huangyhg

here's one. we are just starting w
here's one.  we are just starting work on a retail development that has the columns in the back wall of the building extending into the future retention pond for the site (see caveman sketch below).  right now the future pond is dry and we don't have a soils report yet. we plan to thoroughly discuss settlement issues with the geotechnical engineer(especially differential between those footings in the water and those over the rest of the building).  
we don't really have a question here, just wanted to get some input from the community on potential pitfalls.  ones we see are:
1. settlement and differential settlement
2. watertightness of concrete cols and retaining wall
3. maintenance of framed parking due to moisture
              !--------------roof
              !
              !
              !------------- bldg floor
    cols-->!
              !------------- parking underneath
              !           !!     
              !           !!
  water    !           !!<--retaining wall
--------------------!!
              !           !!
              !           !!
            -----        ---------
            -----        --------- bearing
any insights would be geatly appreciated.
pc bob
check out our whitepaper library.
retention ponds (detention ponds?) are generally dry unless you get a real heavy rainy season.  differential settlement would be a big concern of mine if the footings were shallow, spread-type footings vs. piling or piers that extend to a deeper strata untouched by the intermittent water in the pond.
watertightness:  is there a basement behind the retaining wall?  
settlement - you are most likely ending up piling the extension foundation to get a good support or erecting a pad foundation that cannot match the settlement characteristics of the existing building.  thus it may be prudent to design the bay over the water as simply supported so that the extension can tolerate a bit of rotation if it happens.
watertightness on concrete column and retaining wall - the watertightness can be improved by additives and partial replacement of cement.  there is no need to make the column watertight but watertightness helps to prolong the life of the reinforcement which should have a geneous cover of say 75mm (50 min. and more cover if there is chloride around).  if you want a bomb-proof solution then simply cast a impervious liner (pvc or grp pipe as a permanent formwork) over the section of the column subjected to water.  for retaining wall one can design it as a water-retaining structure standard with more reinforecement to lower the steel stress within 100 to 150 mpa range.
maintenance of frame due to moisture - needless to say you will need the underside well ventilated.  a high quality coating will help also.
good luck
there is not a basement behind the wall, it is retaining dirt.  
i was told (by the architect, mind you, not the ce) that the re/detention pond was going to be full regardless of rainfall.
i like the idea of using permanent impervious forms.  thanks for the suggestions.
pcb
  
move the building. make your life easier and sleeping better.
depending on loads and soil conditions, this would likely be a good application for augered concrete piles to hold the column.  the cap could then be placed above the high water lines and made "architecturally" compatible to hold your columns.  this would negate such issues as bearing capacity failure of a shallow footing, potential scour and undermining, and allow appropriate waterproofing of the concrete in the critical "splash" zone of the pile/cap.  it would make the settlement issue less prominent as well.
in my area of the country, most of the retention/detention ponds are wet and designed so.  the crud (a highly technical term) that accumulates in the bottom can contain anything and has the potential to attack concrete footings (easier to protect the pile face than a footing).  further, depending on the outfall conditions, the pond might be alternately wet and dry, thus increasing the potential for changes in the amount of overburden for the footings, thus changing the bearing capacity of the soil below the footing.