Rectangular Foundation - Zone of Influence
Rectangular Foundation - Zone of Influence
(OP)
I am trying to get a rough estimate on settlements beneath a 4' x 6' slab loaded with 35 kips (non-eccentric). The bearing material is predominantly sands (SC). Am I being over conservative with a 12' settlement seat? I am using schmertmann's method to give approximations. Thanks in advance.
J
J





RE: Rectangular Foundation - Zone of Influence
RE: Rectangular Foundation - Zone of Influence
The load is at -1' below grade and I am assuming the 2B to the 10%P depth using boussinesq. Thats were I'm getting the 12' settlement seat from.
RE: Rectangular Foundation - Zone of Influence
I'd agree that the seat of settlement is the depth where delta sigma v is greater than 10 percent sigma v (i.e., below that depth the settlements become moot).
Your non-eccentric load is 1.5 ksf. If we assume a trapezoidal change in stress between bearing depth and 12 ft, that'd return an integrated change in stress of about 10,000 #/'. If we divide that by some modulus value - let's say 100 tsf (200,000 #/ft^2), that'd return 0.6 inches of total settlement. My methods are based on Hook's law.
Seems to me there's not likely to be too much a problem. Then again, I just made up numbers!
f-d
ípapß gordo ainÆt no madre flaca!
RE: Rectangular Foundation - Zone of Influence
RE: Rectangular Foundation - Zone of Influence
Earth Structures, by Gregory P. Tschebotarioff show methods that are generally used for machine foundations. Chances are a more bulky foundation may be all that is needed. I'd guess settlement by weight is not as important as effect of vibrations.
RE: Rectangular Foundation - Zone of Influence
RE: Rectangular Foundation - Zone of Influence
BigH, do you know of any decent reference material for computing settlements induced by a raft foundation? Maybe westergaard/bousinessq/hough? Any preferred method?
Thanks in advance everybody
RE: Rectangular Foundation - Zone of Influence
Regarding dynamic analyses, I'd hope the structural engineer is making the slab thick enough to offset the vibrations.
I'd be very reluctant to go with a 3-ft thick engineered subbase, even with grid layers. Sure, it'll minimize stress concentrations associated with differential settlement, but there'd still remain a dozen feet (or more) of compressible soil and you'd be vulnerable to performance problems.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Rectangular Foundation - Zone of Influence
RE: Rectangular Foundation - Zone of Influence
My current practice is statewide in Virginia. In the Coastal Plain, we avoid hollow stem augers, 'cause auguring below the water table often leads to heaving sands and unreliable N-values. We use mud-rotary in the Coastal Plain to (mostly) eliminate the problem. We are also more prone to cone or dilatometer soundings in the Coastal Plain.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Rectangular Foundation - Zone of Influence
Lastly, we would have to sub for a dilatometer and cpt, this is pretty time critical. So I feel that I am in the position to stay conservative with the info I have/can provide or lack there of.
RE: Rectangular Foundation - Zone of Influence
RE: Rectangular Foundation - Zone of Influence
You can do more investigation, but if you have to go with the information you have, you must assume it is correct. It says you have potentially treacherous soils. The structure is expensive, sensitive, and highly visible. You have good material available at 20 feet. Give it a highly reliable foundation. Don't mess with numbers until they lull you into doing otherwise.
RE: Rectangular Foundation - Zone of Influence
Refer back to BigH's comment. You are in, or on the very edge of a moderate seismic zone (USGS 16% to 32% g).
www.SlideRuleEra.net
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RE: Rectangular Foundation - Zone of Influence
slideruleera, yeah so I am taking this as an additional reason for deep foundations since the site is set up for potential liquefaction.
I really appreciate everyone's advice. This forum has been so helpful (in general).
RE: Rectangular Foundation - Zone of Influence
www.SlideRuleEra.net
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RE: Rectangular Foundation - Zone of Influence
RE: Rectangular Foundation - Zone of Influence
Dik
RE: Rectangular Foundation - Zone of Influence
Dik, thanks for that. Flattering to spark the post but tbh I've seen some fascinating topics on this forum. There is definitely some material on here that makes me seem real green as a Geo.
RE: Rectangular Foundation - Zone of Influence
RE: Rectangular Foundation - Zone of Influence
RE: Rectangular Foundation - Zone of Influence
f-d
ípapß gordo ainÆt no madre flaca!
RE: Rectangular Foundation - Zone of Influence
RE: Rectangular Foundation - Zone of Influence
f-d
ípapß gordo ainÆt no madre flaca!