Rock Classification Slope Design Cohesion Value 1

[cap4000]

I am looking for the typical cohesion values used in rock slope design. The RMR or GSI value is around 42. My cliff is 50 feet high high and inclined at 87 degrees. Does anyone have the typical cohesion value used in fractured sandstone. The uniaxial intact compressive strength is around 10,000psi. The RQD value is 43. Thanks in advance.

 

[Backfill]

cap4000,

Do you need substance(intact rock) or fracture cohesion values? Both is usually the right answer.

Failure is so often in the degree, pun intended. What can you live with, what can you operate with? If you're looking at a hospital above or below the cliff, or it's a bridge or highway abutment, I would test it for bankable (not geometry, think insurance) friction angle and cohesion values. If it's a short term opening, you may have latitude. If you are subject to water or freeze-thaw cycles, testing again becomes prudent.

If this is an order of magnitude thing, try the literature as in Kennedy: Surface Mining (1990), Hoek & Bray: Rock Slope Engineering (1977 (at least mine is...)) and lots of others. You'll get a range. Corps of Engineers have numbers as well, and their stuff is generally current with literature. The slope folks at NIOSH (former Bureau of Mines hands) have some slick stuff which may be applicable.

Being as you've done the intact uniaxial work and have RMR values, do a sensitivity check on what you can live with. If it looks reasonable, run with it. If there are concerns, especially with any weathering, pop a couple of tests. They're not bad in price, and you can either use cores or block samples which you may choose to core.

BTW, since you have the RQD and RMR, do you have fracture stats on the slope? Dip directions and dips (not a political thing, honest) can be really handy, and for a permanent structure, really significant. NIOSH redux.

Hope these ideas are useful.



Ralph R. Sacrison
Sacrison Engineering

http://www.sacrison-engineering.com

 

[cap4000]

Thanks for the info. I ended using the cohesion value 2,000 psf and #9 threaded long rock anchors at 12ft centers.