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Phi and C of Diabase material by Hoek Brown Failure Criterion

Phi and C of Diabase material by Hoek Brown Failure Criterion

Phi and C of Diabase material by Hoek Brown Failure Criterion

First Time Poster here!  Please bear with me.

I am evaluating a small roadway re-alignment project involving a rock cut slope in predominantly diabase material (and ophiolitic material).  Length of cut is approximately 200 feet, max depth of cut will be ~8 ft horizontally, with a max slope height of 25-30 feet.  I am proposing a 1/2H:1V or 3/4H:1V cut slope with a catchment area below for any rock fall.  Due to the hardness of the rock, I am going to propose blasting.  

Small trenching investigation performed, and some large block samples were taken to the lab and cored.  These were tested for Unconfined Compression.
Unconfined compression averaged 7000 psi.

Material is decribed as massive, with random fractures (partially to fully healed with calcite).  Very hard upon removing the weathered material.  

Using the Hoek-Brown Failure Criterion, I came up with a Phi of 48 degrees and a Cohesion of 2300 psf to input into my slope stability model.

Do these values seem reasonable for modeling hard rock?  

The 48 degree phi angle makes sense.  I do not have a feel for cohesion of rock.


RE: Phi and C of Diabase material by Hoek Brown Failure Criterion

Thanks, Fattdaddy.

I took a look at your dissertation, when you originally responded to my post.  Sorry for the delayed response.  It was very useful to look and see that the phi and C values I was getting were in the range of previously analyzed materials.  I am going forward with my analysis using a phi of 48 degrees and a cohesion of 1000 psf.  The cohesion was reduced to less than half of the given value to give me a bit of conservatism in the analysis, as this is my first time using this analysis on hard rock.

Bombs away!


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