how can determine subgrade modulus 3

[rebwar69]

if I have Beareaing Capacity ( 10 Tn/m² ) , how I can determine The Value of ( Subgrade modulus ) for this soil

 

[Ron]

Do a plate load test.

 

[BigH]

. . . and then the fun begins . . .

 

[Ron]

BigH...I forgot to mention you then have to figure out what it means!

 

[Robbiee]

BigH,
Please rescue me if I slip.
The allowable soil bearing pressure is given for a max. settlment of 25mm (1"), so, conservativle, we can assume that the modulus of subgrade reaction is 40x ABP (in metric units) or equal to ABP (in US units). sounds basic so far right?
my question is: is factor of safelty used for the allowable bearing pressure incororated before the 1" settlement or after. In other words, what caused the 1" settlement: the ABP or ABPx SF?

 

[JoshPlumSE]

Since I'm exclusively a STRUCTUAL guy, my tendency is to always ask a Geotech guy about Subgrade modulus.... By the way, it is often difficult to get them to commit to a value (probably because is soil is never really elastic). That's why my old company always made sure that providing this was part of the contract.

Josh

 

[JStephen]

I would think the elastic modulus would be a good bit stiffer than indicated by the maximum settlement figures.

 

[BigH]

Ailmar - the allowable bearing pressure is, in my view a serviceability issue (viz., settlement). There is no "factor of safety" on it other than conservatism in estimating the settlements under the bearing pressures. Allowable bearing capacity is based on shear and, of course there is a safety factor involved or load reduction factors (coming at it from the other end).
The net allowable bearing pressure to get 25 mm settlement is really size dependent as well. As the footing gets larger, with the same bearing pressure (and assuming consistent soil behaviour below), the settlements will obviously increase. This is why you will many times see a curve of allowable bearing pressure (for service limit) vs width of footing.
You can determine the "operative" subgrade modulus at your bearing pressure from knowing the settlement vs pressure curve for each size of footing.
Hope this makes sense? I've got sliding and seepage on the mind today . . .

 

[BigH]

CarlB: - but you blew it by apologising!!

 

[rebwar69]

Thank you (Ailmar) & Please Tel me What you Men ( ABP ) ?

 

[BigH]

Allowable bearing pressure (ABP)

 

[youngstructural]

Rebwar69: What country are you in? If you didn't know ABP meant Allowable Bearing Pressure, are you in a fully rationalised Metric country which applied Limit States Design? I wonder if you are more familiar with UBP (Ultimate Bearing Pressure) and SBP (Sericeability Bearing Pressure). Be careful when getting advice on these forum... Our American friends are fine engineers, but their factors and approaches are different, and sometimes require careful conversions.

Also, you'll find they tend to be highly prejudicial against limit states... This is not meant to be insulting, it is just that WSD (working stress) and ASD (allowable stress) design are only now being replaced in the US.

The funiest comment I have yet seen was that, and I paraphrase, "You want to know the stress, not some multiple of the stress"; As if the stress calculated analytically were ever close to the true stress in the structure! lol.

Okay, I'll get off my soapbox, but I just wanted to mention in case there was a potential trip hazard in your way...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[BigH]

perhaps from India? - skip if you must!!!

sadly we are seeing the US and Canada switching to LFRD. We will spend hours determining the damn bearing capacity (oops, yes, for only one independent footing) and spend very little time on the issue that counts - what you rightly call the SBP.
I, for one, do not see any advantage in the LFRD - as a strutural engineer, you are dealing with materials with covariance values of 10% or less; with soils, our coefficients of variability range from 25 to 40% for most properties and even more. We deal with sites that just are not uniform (see some of the beautifully drawn cross sections in Krynine and Judd, in Tschebotarioff, in Terzaghi and Peck) Big difference. Structures, do not dealing with joint planes in a rock mass - or joints infilled with clay; you are not dealing with materials, once you pass the yield value see their strengths drop 5, 10, 100 times. (steel, if I remember right, holds fairly evenly until it again starts to rise to ultimate) - you reach the plastic hinge concept. You don't deal with thixotropic materials. You don't deal with materials with water, solids and air all rolled into one mass.
It is, in my view, riduculous that we are seeing the "new paradigm" having to calibrate the LFRD load factors to "match" the design methods that many are ready to take out to the scrap bin as they are putatively outdated or inferior. How does the LFRD model take into account contractors disturbing the foundation material that they are to build on - having to remediate it to a state that is "different" from that assumed in design and yet the "design" doesn't change? How does the LFRD design take into account a contractor's screw-up in grout strength for a rock or soil anchor? I have yet to see where the LFRD designs take into account adjacent footings as to the Ultimate state or service limit - other than a paragraph thrown in here or there. Foundations are seldom singular.
We don't live in a design utopia - construction reality is often seriously different. I think, to get off my that we all have a tendency of believing computer printouts (even when we don't know what the input terms really mean), of believing methods of analysis (by the way, does the Ultimate limit designs take into account that there are 5 or 6 very different formulations for the end bearing factor of a driven pile . . .? Naw.) and we forget that "dirt" is a dirty animal that just doesn't always behave to fit the neat little box we try to put it in.
(sorry all - its been a stressful few weeks and I am now off to Bali for a little R&R). It's Friday soon, a brew would be nice!

 

[msucog]

i'm glad to see someone else that thinks LRFD for soils is a little ridiculous. i understand it for things like concrete and steel where the materials are essentially "controlled"...but dirt is wild--crazy wild. it varies foot by foot in all three dimensions (x,y.z) where i'm at.

and if anybody deserves a pint, i'd vote for bigh...hell have two!

 

[hokie66]

Right on, BigH. All I really want from a dirt engineer is a good estimate of how much my footings are likely to settle (or heave). It's all more art than science anyway, so I really don't understand forcing a new methodology on geotechnical engineers. The best advice is always based on long term knowledge of local conditions and performance, not on testing and theory.