Foundation of Bedrock
Foundation of Bedrock
(OP)
Can someone help me how to calculate the allowable bearing capacity of a bedrock of basalt? The bedrock is slightly fractured, slightly weathered and moderately strong (12.5-50MPa).I do appreciate if you can provide an excel spreadsheet...Thank you





RE: Foundation of Bedrock
This link will get you to my thesis, which provides some information on bassalt strength and modulus values. Also some information on how to consider the affect of discontinuities.
http://home.comcast.net/~fatt-dad/thesis.pdf
f-d
¡papá gordo ain't no madre flaca!
RE: Foundation of Bedrock
RE: Foundation of Bedrock
RE: Foundation of Bedrock
RE: Foundation of Bedrock
By the way, Peck's chart of N values to allowable bearing pressure for size of footings and 25 mm settlement does not apply to bedrock.
RE: Foundation of Bedrock
RE: Foundation of Bedrock
If you really want to delve into this, you might check out Tomlinson's Foundation Design and Construction - 6th Edition (you can get low price Asian edition) - Section 2.3.6 which covers shallow foundations on rock - but these are more geared towards normal sized footings or piling on rock than for a large mat foundation. He quotes much from Wyllie, Foundations in Rock, Spon Publishers, London, 1991.
Tomlinson also gives presumptive allowble bearing pressures for strip footings (B/L<10) for various rock types and "strength". The "worst case" listed is 250 kPa - and this is for weak mudstones with steep foliations and discontinuities about 200 to 600 mm. Most typical is no less than 1000 kP.
RE: Foundation of Bedrock
Sometimes numbers are so far above the loadings order of magnitude that structurals have an hard time to grasp them, once a structural just didn't grasp them, after what could be defined a grotesque phone call with myself.
Hedcor, akvargas, do you have only cores from soundings or do you have also exposed rock faces to survey?
Besides, the TP method you attached is suitable for an isotropic rock mass, otherwise failure parameters at the rockjoints would govern.
Since it is a large foundation, the rock mass is likely to be isotropic at the foundation's spatial scale.
RE: Foundation of Bedrock
RE: Foundation of Bedrock
Bottom line, unless this turns into some shear stress problem on a fracture trend (ala slope stability concern), you have plenty of bearing for 3,000 psf. I too would use that value on stiff clay or medium dense sand.
For rock, heck that's nothing, you probably have 10 times that value.
f-d
¡papá gordo ain't no madre flaca!
RE: Foundation of Bedrock
I very much agree on the common sense, 150 kPa on a mat on fractured basalts is laughable since you might easily build a 100-floors structure on it.
I'm pretty much into bureaucreacy and formalism though, so even if you have to give a proof of sheer evidence, you have to put it on paper.
Since I'll have to put it on paper which will be delivered to the building authorities, I cannot dismiss the issue with qualitative considerations.
Obviously, bearing capacity with such loads is never going to be a priority.
I worked on a project with residential buildings on pretty good limestone with bearing capacities way over concrete resistance.
I later discovered though that the architect in charge of the project founded the houses half on rock mass, half on rock fill, without warning me. There went all the rock hi-resistance
Sometimes one ounce of good communication is worth several pounds of technical calculations.
RE: Foundation of Bedrock
I suppose that the only true way you can confirm is by a finite elements model showing stresses at any point within the mass if <<<< critical stress. As,though, with any models, there are inherent inaccuracies, modeling assumptions, etc. (good discussion on this point - Mccoy - I'll hire you if I have to ever "prove" a condition like this; you seem to have a knack of dealing with inane regulators!!
RE: Foundation of Bedrock
one easy way of doing it sure would be thru geomechanical classification of rock mass (RMR, GSI),which yields mohr-coulomb parameters to be used in one of the variations of Prandtl's equation. That is, you treat the isotropic rock mass as a soil (or as a metal as you imply).
Another way would be the lower bound mehtods you cite, T-P is probably one of them.
This paper by Meryfield, Lyamin Sloan is very good in that it uses rigorous FEM to assess the Mohr Coulomb parameters obtained by Hioek-Brown. They turn out to be under conservative.
Probably the best paper on fractured isotropic rock mass, You'll like it, many plots, it's freely available for individual use:
http://liv
If the rock mass is anisotropic, then things are going to be more complex.
In the case of the fractured basalt, it would be enough though to provide the resistance of the few governing joint sets and show the available strength exceeds by far the mobilized strength.
All this for a mat with a limited loading?
Sure it sounds crazy, but if the rock is isotropic it would only take about 15 minutes to completely prove the point quantitatively. I would only need a clear picture of the rock mass.
It takes longer if the rock is not isotropic.
RE: Foundation of Bedrock
RE: Foundation of Bedrock
RE: Foundation of Bedrock
Not everything we do can have equations put to it, some things we just know from exerience, either individually or as a profession. In almost any other country but the US this would be allowed to be put down to engineering judgement.
RE: Foundation of Bedrock
please do not overestimate other countries.
Situation is like that in Europe as well.
Fact is that 'engineering judgment' may be criticized if something goes wrong.
A lawyer in court might say in that case (i.e.: too much settlement damages building): "your engineering judgment was evidently faulty".
Whereas if I apply my engineering judgment AND comply with the law, the same lawyer will have a hard time trying to convince the court that the law is faulty.
RE: Foundation of Bedrock
Yes but thats not really applicable in this case, it is obvious to any reasonably experienced engineer that virtually any bedrock could take 150kPa.
You make reasonable judgements all the time in design, otherwise it would take me 2 months to design the average house if I checked every single nail and bearing.
RE: Foundation of Bedrock
we all know lawyers can make a fractured basalt sound like soft clay.
As far as nothing happens it's all right.
Since likelyhood of a fractured basalt giving rise to failure or excessive sttlements is so low in the above conditions, then we are all right with that.
That's intuitively applied Bayesian probability estimate.
RE: Foundation of Bedrock
RE: Foundation of Bedrock
RE: Foundation of Bedrock
Could someone blame you if you can show you followed rigorously the law? If something goes wrong then it is the unfavourable fate, tough luck, Murphy's law, you name it. Of course you have the obligation to be updated on the state of the art and nobody will object if, by sound technical reasoning, you'll use a more conservative approach. A more conservative approach means higher construction costs so you should always justify that.
What parameter the 30% COV refers to?
RE: Foundation of Bedrock
As Examples:
1. CBR: 17 - 58%
2. Angle of Friction in clays - 12 - 56%
3. Cohesion (undrained sands - 25 - 30%
4. Consolidation Coefficient - 25 - 100%
5. Liquid limit - 2 - 48%
6. Moisture Content - 6 - 63%
7. Permeability - 200 - 300 %
8. Undrained Shear Strength Clay Triaxial - 5 - 20%
9. Su (index Su) 10 - 35%
10. OCR - 7 - 30%
11. Ratio of Su/p' 5 - 15% (a good one)
RE: Foundation of Bedrock
The Eurocode 7 has developed a way to use cautious representative values of the geotechnical parameters, the so called 'characteristic values'.
These are a function of the size of foundation, the size of the dataset and the variability or standard deviation or COV.
The EC7 tells you which is the appropriate point-estiamte value for the whole distribution; with a 30% COV and a small dataset and small foundations it may become pretty low and cautious. You use that, you're pretty safe, dude.
If anything goes wrong then something was wrong in sampling, or correlation, or testing, or in adopting an initial value which wasn't representative.
Engineering judgment keeps being king.