Settlement calculation - layer thicknesses
Settlement calculation - layer thicknesses
(OP)
hello
this is rather basic but i have seen several variations and was wondering what you have to say...
for consolidation settlement calculations, assuming a homogeneous soil, square ftg, how many layers and what thicknesses are appropriate?
examples ive seen:
3 layers - 0.5B, B, 2B seems conservative
2 layers - B, B
every half foot until %reduction is 5%. conservative
thanks
this is rather basic but i have seen several variations and was wondering what you have to say...
for consolidation settlement calculations, assuming a homogeneous soil, square ftg, how many layers and what thicknesses are appropriate?
examples ive seen:
3 layers - 0.5B, B, 2B seems conservative
2 layers - B, B
every half foot until %reduction is 5%. conservative
thanks





RE: Settlement calculation - layer thicknesses
f-d
¡papá gordo ain't no madre flaca!
RE: Settlement calculation - layer thicknesses
for homogeneous soil, i believe it says something like .5B, B, 1.5B,2B
for layered system, maybe .25B,.5B,.75B,B,1.5B,2B
obvious caveats would be to use changes in strata, consistency, etc as well as your judgment. and for consolidation settlement calcs, i'm not sure what the true definition of "conservative" would be unless you've got lots of field data versus calcs to back it up.
RE: Settlement calculation - layer thicknesses
On the other hand for a footing placed on a compacted fill, it gets more complicated, since the rules you state could not do so well. Timing of the fill is important also.
Another situation that I like to play with is a wall footing for a basement, with fill placed later alongside the wall.
I guess I didn't help much, but keep that 10 percent rule in mind on these and it takes care of many situations.
Oh, use what ever layer thickness you want, recognizing the approximations involved with the whole process.
RE: Settlement calculation - layer thicknesses
"geotechnical engineering is an art"--very true statement
RE: Settlement calculation - layer thicknesses
im guessing the second...
thanks
RE: Settlement calculation - layer thicknesses
Using this approach, you can also take into account what depth you need to calculate to when you are figuring what happens when a surcharge is placed. In that case the decrease of added pressure with depth may not decrease much with depth and so you are then using many layers down below that original ground surface.
RE: Settlement calculation - layer thicknesses
some codes like the European geotechnical code specify 0.2 sigma_v, that is you can stop at the depth where delta_P <= 0.2 sigma_v, a condition a little less conservative than the one cited by oldestguy.
As to the layer thickness, I'd choose it as little as practically possible, of course according to the type of tests which have been carried out.
RE: Settlement calculation - layer thicknesses
seems like it is very deep before it gets there.
RE: Settlement calculation - layer thicknesses
I have spreadsheet programs set up for each kind. There are probably a zillion softwares out there set up for these comps, but a spreadsheet gives you the opportunity to really play with the variables that a dedicated software program may not allow. If you feel like it, let the spreadsheet go deeper than 10 percent, but notice the minor effect it has on the result
The main thought I'd like to leave is where you can actually monitor the performance of a site under loads, it is much more comfoftable than trying to predict settlements by use of computations alone. Think about how you would do this for a footing.....not easy, but doable at times. What does a fully loaded scraper weigh?
One more item: Take a site where you wish to preload the site before placing spread footings. Some overload of the shallow soils (beyond the surcharge loading) from the spread footings seems justifiable, since the depth of "not fully compressed soil" below that footings is quite shallow. This can be avoided pretty well by undercut and compacted fill placement if it appears important.
RE: Settlement calculation - layer thicknesses
RE: Settlement calculation - layer thicknesses
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RE: Settlement calculation - layer thicknesses
Eurocode 7 and probably BS 8004 recommend that you stop settlement calculations at the point that the net applied vertival pressure equals the vertical overburden pressure at that point.
RE: Settlement calculation - layer thicknesses
Further, one cannot know unit weights to an accuracy as would be implied - and the variation of unit weights in a particular stratum (this is one reason that statistics are coming into
Pressure distributions, settlement calculations are still, in my book, an art (and luck) and not a science. How does one calculate pressure increases due to footing load anyway? Elastic theory - as load on a semi-infinite half space? . . . or on the theory of particulate matter? or using plasto-elastic strains . . . Given that actual pressure increases are inaccurate (used loosely), how can one increase accuracy of settlement computations by varying layer thicknesses. If same soil, doesn't seem necessary - unless it is to minimize difference of curved effects of pressure increase rather than in linear fashion.
RE: Settlement calculation - layer thicknesses
RE: Settlement calculation - layer thicknesses
iandid explains it well.