ks value for rocks - need info

[georam]

I wonder if there is a table which provide some information about Modulus Subgrade Reaction (ks) for various type and quality of rocks. In particular, I am looing for ks value for a dolomitic limestone bedrock.

Is there any relationshipp between ks and Young's modulus of the rock, or between ks and unconfined compressive strengths. Or any relationship with other rock properties.

I recall that ks can be determined using plate load tests, however, there has been some discussion on the size of the plate to do the test (which means the ks value may depends on the size of foundation). I am specialized in geotech, not civil engineering. It appears that some civil design software requires input on ks of foundation bedrock to design some retaining walls.

Thanks for info.
PS
There are ks values available for various types of soil and fill, but I never see one on bedrock.

 

[fattdad]

What's the end-game (i.e., what type of project do you plan on using the value of ks)? Just today, I had a colleague ask me if I could provide a value of ks for a project that they are working on. I said, "sure", but after talking with the structural engineer realized that they wanted to use ks as a means to evaluate whether an industrial floor slab could handle bulk paper storage. Well, the bigger issue may really be the 1-d consolidation characteristics or the soil modulus of the soils at depth. Once I realized where the use of ks was headed, I begged off in favor of getting more information.

f-d

 

[Ron]

Why do you need this parameter for rock? If it's for pavement design, the value would be so high it wouldn't make any difference. If for foundation mat design, if the rock is contiguous, then shear will probably control, not bending.

 

[Wtong]

georam

Inciddentally I am looking for the same information for a power plant proejct. Have you have any success so far. There must be something that be co-related based on UCS and/or Point Load Index.

 

[jdonville]

Wtong,

You miss the point. ks assumes that the response is elastic. However, when you're dealing with large loads or loads distributed over large areas, the response of the rock will not necessarily be elastic. Therefore, determining ks is not your real problem.

Jeff

 

[EvanGr]

jdonville,

So you say that for large mat foundations with heavy loads on top we cannot use the Winkler approach to model the soil-structure interaction? Does this apply only for rock or for any kind of soil?

Vangelis

 

[jdonville]

Hmmmm,

May not have thought that one through all the way.

Let's put it another way: the elastic response of the rock mass is difficult to determine outside of actually performing foundation load testing due to the difference between the intact rock properties (which are strightforward to measure using discrete sampling and lab testing) and the rock mass properties.

This is complicated by the fact that guidance for subgrade modulus on rock is practically nonexistent. What data there are tend to be for specific sites.

Finally, the degree of conservatism inherent in assuming moduli and bearing capacities based on presumptive values is unknown. An actual modulus that is stiffer may result in
damage to shear wall elements and an actual modulus that is softer may result in damage to building frame.

There used to be a PDF version of a MACTEC PowerPoint presentation to an ASCE chapter in California floating around on the chapter's web site (don't know which chapter or branch), however I cannot find the URL (although I have a personal copy of the file). The presentation showed the results of footing load tests performed on 2.5 x 2.5, 4 x 4 and 6 ft x 6 ft full-scale footings for the new Stanley Hall at UC Berkeley. They used a (big-ass) PLT machine to apply cyclic loadings of up to 800 kips to develop site-specific load-deformation curves and simplified design guidance for various-sized footings on rock.

It's a good presentation (even without the speaker) and if anyone knows where it is, please post the URL.

Jeff