Foundation of Bedrock 2

[akvargas]

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

 

[fattdad]

What could you possibly be building that would need to fully mobilize the strength of the bassalt? What sort of column loads are you dealing with?

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!

 

[akvargas]

Thanks fattdad for the reply.. The type of foundation is a mat foundation and the "exposed" bedrock covers all the proposed powerhouse with design bearing pressure of 150 KPa. I do have the Peck et. al. graph as a reference which gives the qa is per the RQD...but if you can provide another solution then it would be better. Thanks

 

[BigH]

150 kPa only on sound basalt? You don't have a problem with allowable bearing capacity - I'd use that on a stiff clay or compact sand. Doing the computation just to say you did one? Assuming that the mat is not going to be on the edge of a cliff (plunge pool - if hydrodam) and the rock doesn't have seriously negative fault/joint planes. As for settlement, rock will settle elastically as the loads go on - but doubt you will notice.

 

[akvargas]

Mr. BigH the "design pressure" was 150KPa based on the Design load of the structure not the qa. The calculated Allowable Bearing Capacity (qa)based on Peck. et al graph is 2700 KPa..The area is located on a flat to undulating topography with slope ranging from 5-10 degrees. Can you suggest any other alternative in the calculation of the allowable bearing capacity for a bedrock mat foundation?

 

[BigH]

Again, the design bearing pressure is so low compared to what the allowable bearing pressure on the rock would be, the issue is non sequitur. I doubt I, or most geotechs, would ever even try to "calculate" the allowable bearing capacity (which would be very high). It is like proving that steel is "hard."
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.

 

[hedcor]

mr. bigh me with akvargas are using the attached chart of peck which estimates qa as per the RQD...do you have any suggestions or alternatives in calculating the qa for the bedrock...we are currently working for a proposed 16.4MW hydro electric power plant.. thanks

 

[BigH]

Again, I ask, you have an applied bearing pressure of 150 kPa. Your chart is for jointed rock; even in you have "poor" rock (RQD of 20%), the allowable bearing pressure is 2500 kPa >>> 150 kPa. So why are you so intent on "computing" an allowable bearing pressure? When I had/have problems where the actual bearing pressures are so much lower than what would be allowable, I simply say that.

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.

 

[Mccoy]

BigH, I too sometimes have to provide a bearing capacity on rock simply because the regulations require a bearing capacity to be calculated.

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.

 

[BigH]

Mccoy - the chart given from Peck is for jointed rock mass. If you have to give "computed" values for a mat foundation where the mat is up to hundreds times wider than the jointing/fracture pattern - and they are on flat land - something is out of kilter with the regulators. Give the structurals the "big number" and let them sort it out. It is sort of like proving that a 0.2 m high 0.3 m wide concrete curb on 2 m of compacted (to 98% MDD Modified Proctor)crushed stone granular engineered fill is not going to settle 25 mm. Maybe we should all review, again, the obituary for common sense published in the Indianapolis Star about 11 years ago.

 

[fattdad]

I'm with BigH on this one. It goes back to my first question, what's being built that needs to fully mobilize the bearing capacity of bassalt? You could start with an unconfined compressive strength test on an intact core sample and then consider rock mass reductions for jointing and other discontinuities.

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!

 

[Mccoy]

BigH,
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.

 

[BigH]

So Mccoy - how do you do it? Assume a small footing on the fractured bedrock - apply the philosophy of Wyllie; and say it is a lower bound? or is Peck's graph enough? If you have to show a calculation, Tomlinson's book gives some details for a "calculation". I don't think one is dismissing the case based on "qualitative" considerations. It is based on published data by experts, engineering judgment, and, yes, common sense. You have quoted a good point about the architect putting foundations half on rock and half on rock fill without telling you - well, that would obviate any computation you did. And, don't forget, that the general bearing capacity equation is based on Prandtl's work - which if my memory serves me right is based on metals. (

http://www.eng.fsu.edu/~tawfiq/ceg4111/ShallowFoundation.html)

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!! What I'd like them to do is "prove" that global warming is really happening!

 

[Mccoy]

BigH,
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://livesite.newcastle.edu.au/si...sing the generalised hoek-brown criterion.pdf

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.

 

[hedcor]

Thanks for the help mccoy..next time again.

 

[DRC1]

I have had to do this on occasion, agree that 90% of the time it is only an exercise. What I have done is reasoned intact rock is beter than crushed rock, assumed the rock was crushed with a high phi, say 50 degrees, and computed bearing capacity as if it were on crushed stone, using terzaghi bearing equations. Generally this gives me what I need and allows me to get on with my life in about 10 minutes.

 

[csd72]

This is a good example of things that get crazy when non technical people start dictating things to engineers.

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.

 

[Mccoy]

csd72,
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.

 

[csd72]

McCoy,

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.

 

[Mccoy]

csd72,
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.