methods of dewatering piers

huangyhg

methods of dewatering piers
we have a project with piers, when the pier contractor start drilling the piers they found water, the depth of the piers are about 45 feet deep, the water in the hole is about 10' from the bottom of the hole.  to dewater the hole the contractor is placing the pump hose all the way to the bottom of the pier and start pumping the concrete, once the concrete lifted the water to about 10 feet below the top of the pier they start pumping the water out.  the specs allow dewatering the hole by this method, is this a good way to do it, or which method is reccomended.
thanks
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that is a good and practical way to place concrete in drilled shafts.  if a long tube was used to convey the concrete to the bottom of the pier and the bottom of the tube remained in the mass of concrete, then this method is referred to as tremie concrete.   
regards,
qshake
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your contractor's method will work, concrete can be placed successfully under water using that tremie technique.
imho, it is preferable to remove the water before placing the concrete (unless the water inflow rate is to high to manage). to do this, submersible pumps with an electric motor located more or less at the pump intake may be used. there is no practical limit to how high a head the pump can produce - 45 feet would be no problem. here is a link to generic information on submersible pumps:
opps, sorry qshake for the repeat info.
concrete placed by the tremie method will displace the water and no dewatering is required prior to placement.  the top foot of contaminated concrete is usually wasted when completed.
concrete can also be pumped underwater into fabriform bags sized to fit the holes. which will give you a more consistant mix with less chance of contamination.in any event concrete pumped into water should always be pumped from the bottom and not dropped.the end of the hose should always be immersed in the fresh concrete.   
intrusion prepakt /marineconcrete.com

civilperson brought up an important point - the tremie method is the way to go as others have indicated.  but as the concrete rises, and the water is expelled from the hole, there should be some form of directional ditch formed to transfer the water, and the first amounts of concrete, away from the pier top.  some contractors will use sonotube forms for the top several feet and this is ok as long as they set the top of the sonotube a foot or so higher than the pier cutoff.
after the concrete has initially set (still within hours of the pour) the top foot or so of concrete should be chipped away as it usually has a very high water cement ratio.  the chipping of the top concrete is ok to do early as this chipping will be less likely to damage the top concrete of the pier if the concrete is still relatively green.

your geotechnical engineer should be involved in the decision of how to place the concrete.  below the water table, collapse of granular layers can be a problem in unlined pier holes.  with liners, this can be controlled, and in this situation, i think it is more common to pump rather than to tremie.  with stable conditions below the water table, tremie is the way to go.  
even if you have granular soils below the water line giving way to collapsing, it is often necessary to use bentonite to hold the walls of the excavation intact until the tremie concrete is pumped and displaces the bentonite.
other times a temporary casing is used and is typically pulled when the tremie concrete is above the bottom of the tremie by a foot or so.
regards,
qshake
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