pier design

huangyhg

pier design
i am trying to design a small pier (4' x 6') to support a fish ladder in a river. we have already designed the pier itself. we cant figure out the foundation. the tough part is the dam the fish ladder is connected to is under a bridge. also, under the bottom of there pier will be solid rock. there is no soil at all. we are not sure how to form the foundation of the pier. the water is relatively shallow, <10', and flows are not too high. there is no head room for a cofferdam because of low bridge clearance.
any ideas?
you probably want to use precast concrete structure than you lower into the water.  make this structure higher than the top of the water.  then build the pier above the water embeded into the heavy foundation.  you can either make it with solid concrete, or hollow conrete filled with rocks.
never, but never question engineer's judgement
i assume the pier is to be placed under water. here are some thoughts:
1. select the location with relatively flat rock surface to place the pier.
2. sizing the footing pad dimension, and forming underwater.
3. place steel casing, just above the water and a distance above the bed rock, and place the concrete.
if sliding is a concern, you can key the footing into the rock by excavation, or install epoxy drill in dowells to fasten it.
an experienced under water construction contractor is essential in this case. also, the season/timing of construction is quite important, to which affects water deepth and velocity of flow, if you have control over it.  
use precast manhole sections as stay in place forms for placing concrete in the center of the sections. first excavate down to the stable soil elevation you wish to bear on, place first section with cmu blocks at base to support the section 8" above the soil, place concrete into the center of the mh section with tremie or pump by keeping the end of the tremie 2' below the concrete level.  fill slowly, less than two vertical feet per hour and use accelerators in the mix design and low slump. use as many vertical sections as needed to until above the water level.  discard the top 6" of concrete which will be diluted by the water.