settlemen

huangyhg

settlement
we have a project that has a differential settlement limit of <15mm. now the founds will be a mixture of piled unpiled. the si is pretty poor, with only boreholes supplied. regardles of the poor si would you be comfortable working to tolerances of 15mm? i would think that this is to tight a margin to be able to determine.  
is the "si" the subsurface investigation?
for the geotechnical side of things, that differential tolerance is fairly tight depending on the location and loads i suppose. however, it's not totally unreasonable. i usually see 1 - 1 1/2 inches total settlement with about 1/2 of that for differential for light to moderate loads. these estimations are based on consideration of the boring data, lab data, experience in the geology, traditional relationships between loading and deformation of soils, foundations type, building type, etc and so the variance of these results due to the variability of subsurface conditions below the structure's foundation should be kept in perspective. in other words, if the specified differential tolerance is say 1/2", let's hope that the building won't collapse if one portion of the building experiences say 3/4" differential. after construction, odds are that this movement will not even be able to be seen depending on the span that the movement occurs in. in my personal opinion, for larger loads in my area of the u.s., i think 1/2" differential is unreasonable without an extensive exploration and testing prgram.
as with any subusurface exploration, there are project/site specific considerations to account for (foundation type, geology, foundation depth, loads, settlement tolerances of the structure as supplied by the eor, etc.). keep in mind that settlement analysis are only as "good" as the subsurface exploration program. if the owner wants to go cheap, the geotech may be able to perform a bare-bones analysis using more assumptions and minimal lab tests...this results in a broad assessment of what may happen but may not have a high confidence in the results. if the owner is willing to spend the necessary money, a thorough investigation can be performed with closely spaced borings, lots of lab tests and more calculations...and thus the confidence in these results should be much higher. one should keep in mind that the geotech does not have x-ray vision so a thorough testing/evaluation program should be performed during the construction phase to help identify potential conditions that differ from those encountered during the exploration. so if the owner went "cheap" during the exploration, there is a much larger possibility that undersirable conditions may be encountered during construction taht would require redesign, delays to schedules, change orders, etc.
i hear what your saying this however is a gas pipeline. the construction is going ahead, we have only just been given this toleance. as it is a gas pipeline 'visable' settlement doesn't come into it. it is required to stop pipeline breaking at joints i assume. the si we have is what we will have and thats it. there will be no more analysis by the geotechs. we need to make a decision on what we have.  
any idea what sort of span the tolerance applies to? 15mm in 10' is tough. 15mm in 100 feet is nothing.
also, is the 15mm the spec or the estimated differential settlement (for some distance)?
does this project involve grading with fills more than a few feet thick? and is the pipeline above or below ground (buried or not)?
to be frank, depending on the exploration and boring spacing, there may not be enough information to provide anything more than generalized information. at some point, someone will need to make some assumptions based on known information and more or less develop a model to run with. if there's not enough info to feel comfortable with the model, then i'd suggest obtaining more information. the geotech you mention may have been asked to simply provide boring information and general comments for a set (maybe low) price...so that may be what you got.
build the foundations with no connection between them.  after completed, shoot the top or support elevations and adjust for correctness with a threaded support bolt.  if settlement occurs after installation of the pipe, come back and shoot the elevations and adjust the support bolts to re-establish the proper elevations.  do this adjusting every six months until adjustments are neglible.
i have never seen a detail like this, and there may be much, much better posters on this matter, however i think civilperson has the solution here.  it maintains your existing found design and solves the problem.
good one!  i'll be keen to see other peoples opinions on that solution...
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
yes civils person solution has been discussed. i however am concerned that the settlement will not be noticed until to late for the pipe. the settlement tolerance is diferential between bases. i assume that they have taken into account the distance between supports in arriving at this figure although probably just making conservative estimate. the bases are to far apart to tie. i am not particarly asking for a 'design' solution to limit settlement as i believe we have that in hand, the question i am asking is, can we be resonably expected to work to this tolerance.?  
'can we be resonably expected to work to this tolerance.? '
it depends on the soil; soft clays etc no, dense gravels yes.
but as a general answer i would say probably not, it will be difficult to ensure compliance with a mixture of piled and unpiled. perhaps it should be all piled.
it's hard to say for sure whether the estimate is reasonable or not without having all the project specific information. for your scenario, i personally would be hesitant to say that <15mm of settlement would occur unless a preloading and settlement monitoring program was implemented.
who is giving the settlement limit?
if it's the pipe installation that is limiting it, i'd say that's too tight to accomodate without significant soils evaluations or some form of adjustability in the supports. my expectation is that no amount of soils investigation is really going to prove that settlements won't occur. i'd say it will be necessary to design some form of adjustability into the pipe support system.