settlement
settlement
(OP)
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.






RE: settlement
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.
RE: settlement
RE: settlement
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.
RE: settlement
RE: settlement
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...
RE: settlement
RE: settlement
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.
RE: settlement
RE: settlement
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.