Rectangular Foundation - Zone of Influence

[geojosh84]

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

 

[oldestguy]

A rule thumb is to compute settlements on the basis of a depth to where the added pressure is 10% of existing. Also the unloading due to basement depth needs figuring.

 

[geojosh84]

oldestguy, thanks for the response.

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.

 

[fattdad]

In what manner have you determined soil modulus? I'm assuming that the basis for your settlement calculation is compression (i.e., you are not using 1-d consolidation theory).

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!

 

[geojosh84]

fatdad, much appreciated. I was using values from a NAVFAC manual to correlate an elastic modulus based on a blow count. I am in a situation were I have 2 blow material (SC intermittent OL) extending to depths of 19.5' below before a very dense kaolin sand mix. I was using Es values on the order of 20 ksf for the real loose material. Just fyi, the owner is not happy with the cased micropile recommendation we gave in a report and wants to undercut 3' and be done with it. The structure is a 6' diameter solid granite kugel ball fountain assembly (you know like the rotating ball and pedestal kind). The underlying soils are saturated and I am very concerned with long term differential consolidation and the moment applied by the rotation of the ball. This structure has a very strict settlement tolerance as y'all probably assumed.

 

[oldestguy]

This may be of base, but a dynamic analysis may tell something. It has been years since I did any, but texts such as "Foundations, Retaining and
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.

 

[BigH]

Cutting out 3 ft gives a "raft" like approach but you still have quite a bit of very loose soil beneath. While settlements might be computed in static case to be okay - as OG says, this is a machine foundation so there will be some dynamic settlement as well. On top of this if you are in an earthquake area liquefaction might be (probably will be) an issue. A number of texts discuss the concept of whether a soil is prone to liquefaction.

 

[geojosh84]

oldestguy and BigH, thanks for the info. We're a D site class (avg N approx. 20) and we are 130 miles from the coast. I am not familiar with a dynamic approach and plan on doing further research on this topic.

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

 

[fattdad]

for N=2 OL, I'd question whether a Hook's law approach to settlement is reasonable. It's also likely that you need consolidation parameters. For OL soil I'd also want to calculate secondary compression. Do you have Atterberg limits (air- and oven-dried Liquid limits)? Can you share the natural water content and air dried liquid limit values? If the natural water content is close to the liquid limit, you may be close to normally consolidated.

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!

 

[geojosh84]

fatdad, thanks man. I absolutely agree that hooks law in fine grained soils is not applicable. The OL soil is thin relative to the rest of the soil profile in the ZOI. I am in predominantly fine sands with fine contents ranging from approximately from 15% to 25%. On a visual basis water contents are near if not surpassing LL's. thanks in advance.

 

[fattdad]

So, there is an additional outstanding matter. . . Any way the drilling method allowed for disturbance of the fine-sand density? I mean, if you could do a cone penetration test or dilatometer sounding, do you think the resulting strength and behavior would be much different than correlations to N-value?

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!

 

[geojosh84]

fatdad thanks again. that is absolutely a possibility for submerged cleaner sands. Though, wouldn't the fine contents (25%) be high enough to prevent/reduce the possibility of quick conditions? Just a thought. For further info, I am in the upper coastal plain near the fall line west of the piedmont in SC also this area use to be a floodway to a creek that was piped and covered back in the 30's.

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.

 

[geojosh84]

east of the fall line.... man its friday lol

 

[aeoliantexan]

Friendly advice:
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.

 

[SlideRuleEra]

I am in the upper coastal plain near the fall line east of the piedmont in SC...

Refer back to BigH's comment. You are in, or on the very edge of a moderate seismic zone (USGS 16% to 32% g).

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[geojosh84]

aeoliantexan,thanks for the advice. the last thing I want is a monument to my name a mile down the road in my home city. if the client doesn't like it then he can get another recommendation from another engineer.

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).

 

[SlideRuleEra]

Good decision. I'll look forward to stopping by the "Plaza" to see the results of your work.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

Yup, lots of info and client might think twice about going light, especially likely later, when it sits tipping sideways and won't motivate any more.

 

[dik]

This was a fascinating posting... one of the best.

Dik

 

[geojosh84]

Once again thanks everyone for the thoughts and encouragement.

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.