determining concrete cast-in-pile depth

huangyhg

determining concrete cast-in-pile depth
okay, i'm looking for suggestions.  our city is having difficult with residential foundation contractors not installing concrete cast-in-place piles to the specified depth.  there are no inspections during the installation of the piles by the city.
are there any instruments or methods of determining pile depth after casting?  a good example is a 10"dia x 15ft deep or 18 ft deep pile being commonly installed to a depth of 10-12ft.  we just dugg up a "18ft deep pile" that had failed which turned out to only be 6ft deep. go figure.  
other than being everywhere at once, and standing beside everyhole in the city with a tape measure.  any thoughts?   
an affordable, non-technical approach that may help, but not solve the problem, is to require the contractors to provide proof of concrete delivery to the specific jobsite on the day that piling are cast. quantity of concrete delivered would have to equal or exceed theoretical volume of the piling.
there are lots of ways for a contractor to cheat, but it is a deterrent and provides a "paper trail" if legal action against the contractor is taken later.
good idea!   
what about requiring a 1/2 inch pvc pipe to be installed through the center of it?  with that, you can drop a 20' 1/4" steel rod down the pipe.  
they might be able to defeat the inspection by ramming the tube down to 18' and pouring concrete down to only 6.  if that's a concern, then have them install 2 pvc pipes with two 90 degree elbows on the bottom next to the column.  then you do two tests -- 1 to check for depth and another to check for continuity between the two pipes.  at that point, it's possible for them to cheat, but it would probably be more effort to cheat than it would be to do it right.  
crazy?  hare-brained?  maybe.  but it's cheap and the good thing about being in the government is that you get to write the rules.   
engineering is not the science behind building things.  it is the science behind not building things.   
why not just follow the special inspection requirements (at for commercial...don't do residential so not sure what it says about piles) and have someone there watching the installation? who's checking reinforcement? the owner should be required to hire the geotechnical firm to watch the installation.
i wouldn't trust batch tickets...tickets can be fabricated with no problem and i know that it happens regularly. and there are non-destructive methods to estimate the length and integrity but they are relative expensive, time consuming and in this case, not appropriate.
but given that it is after the fact and apparent that the contractor is cheating...i would make the contractor (at their cost) pay to have the a geotechnical engineering firm drill/core a hole through the center of the pile and determine the length. if more than a few came up short, then i think the entire bunch should be rejected at the contractor's expense.
and i'd like to see how someone in this situation came to the conclusion that piles were necessary. and as far as keeping it cheap, i wonder how much it will cost the unknowing home owners once they have a problem. their insurance will likely not be willing to pay and they will have an uphill fight with the developer in court. not to be preachy here, but it sort of defeats our obligations to look out for the public when this sort of thing happens.
this should be police matter.  it is fraud, and if a contractor is charged, tried, and found guilty, it would go a long way to stopping this type conduct in your area.  but i agree with msucog--there should be inspection.
try calling grl.  i don't know if they go to canada but they can test for the length of concrete piles.
this grl web page talks about pile integrity testing, including length testing.
be cautious with such ndt methods. there are other ones that would work better or along with pit testing. it is "estimated" or "inferred" length. while i personally have confidence in using it, the quality of the results depend in part to the quality of the pile, the surrounding soil conditions, and few other factors. i would not use it as means to check the issue at hand on this thread. i also would not use other "indirect" methods to check something of this nature.
here's link that does pretty good at describing the different methods.