Rock UCS with Young's Modulus and Poissons Ratio

[LibraTech]

Good Day All,

I have some test results on Mudstone materials (Generally 3-10MPa UCS) where the Secant and Tangent E-value aswell as the Secant and Tangent Poissons Ratios were determined with the specific failure load and associated UCS of the rock core specimen.

I am looking for some correlations / empirical methods to infer the typical cohesion and phi angle of this rock material from these results.

Any recommendations / suggestions / pdfs of papers welcome.




*** Thanks for the excellent forum ***

 

[mudman54]

good start might be hoek & brown 1980 appendix 5; if you add a few parameters like the rock mass classification you can get c and phi.
are you looking for the laboratory c and phi?

 

[ebenozen]

I would encourage you to stay away from generic published values as it sounds like you have core samples and access to a laboratory. You can do better.

To determine rock C and Phi you can do triaxial testing using a Hoek cell to apply confinement stress. This gets you the "intact" rock shear strength envelope defined in terms of linear Mohr-Coulomb or a power relationship. From the power relationship, you can calculate the mi regression parameter and use this parameter and other estimated empirical parameters to determine the Hoek-Brown rock mass shear strength.

The "intact" shear strength is the upper bound strength of rock excluding the contribution of fractures, whereas the rock mass shear strength is inclusive of the impact of fractures on strength and is therefore less than the intact shear strength, but greater than the strength along fractures. Rock mass shear strength is what you'd use in your analysis unless you're considering sliding along joints, which is the most important mode of failure when analyzing jointed rock!!

Another option to calculate rock mass shear strength is by weighting the intact rock shear strength (from Hoek cell triaxial compression testing) with the fracture shear strength (determined from direct-shear testing of rock joints). A good method to do this is published in: Call, R. D., Cicchini, P. F., Ryan, T. M., and Barkley, R. C. 2000. Managing and Analyzing Overall Pit Slopes. In Slope Stability in Surface Mining, Hustrulid, McCarter, and Van Zyl, eds. Littleton, Colorado: SME, p. 39-46.

Considering the low strength of your "rock" samples, you may want to consider doing intact direct-shear tests on slugs of core (slug height=0.5* core diam). This is a technique used to test soft rock that may or may not be appropriate for the mudstones you're working with. You may also want to consider CU triax testing.

Best of luck.

 

[LibraTech]

Thanks to all the contributors.

To clarify, yes I am looking for the "lab" or intact rock c and phi values inferred via empirical relationships from the UCS and associated Secamt and Tangent E-value and poisson's ratios.

I do not currently have the ability to conduct the triaxial lab testing, this would be done in future as a confirmatory excercise.

I am looking for an initial estimate in order to do some prelim design work with and to conduct a sensitivity analysis on the parameters, c, phi and rock mass rating.

Any information appreciated.

Regards

 

[asp1230]

Depends on your purpose but you can look at AASHTO LRFD Bridge Design specifications or Hoeks corner at the following link:

http://www.rocscience.com/education/hoeks_corner.

 

[fattdad]

you are welcome to review my M.Eng. thesis and see if that helps.

http://home.comcast.net/~fatt-dad/thesis.pdf

f-d

¡papá gordo ain’t no madre flaca!