Steep Rock Slopes 8

[voi]

Hello,
I need an input in how to analyze steep rock slopes. I particularly are interested in software that can assist me in the analysis,
thanks
Voi

 

[appot]

Are you looking for a computer program, or is your question more general?

Rocscience has a program called swedge. Essentially you enter the major rock joints (dip and dip direction) and it will tell you the factor of safety of the resulting wedge.

If you want to get fancier, you could try flac, or flac-3d by Itasca. Itasca also has software that would help with analyzing land-slides (probably not of much interest for your application of rock slopes).

You could also use a general FEM program.

 

[voi]

Hi Swiver,

Thank you for responding to my thread. I have a case where we are excavating four basements down ( 25 m below ground). The area is urban and the terrain is all rock. I know SLOPE/W implements the Generalized Hoek-Brown but it is very expensive. So I am looking for any other ideas or recommendation of analyzing this slope next to the deep excavation.
voi

 

[miningman]

I have no idea how expensive or useful SLOPE/W is but I do know how expensive it will be if someone does not properly identify and mitigate against failure of a 25 metre excavation in an urban area . I can see not one, but several buildings falling into the hole. Get help quickly, I suspect you are out of your professional depth..... the fact that you are worried about the cost of a piece of software strongly indicates you have your priorities all wrong.

 

[voi]

I know it is difficult for one with mining background to understand the complexities of slope stability. I am from the local council department and the consultants ( by the way the project is being handled) by a consortium of Geo-technical firms with international experience They have submitted their drawings and calculations and backed up with SLOPE/W . The reason I asked about another software is because they used SLOPE/W which the local approving authority does not have and is not going to spend $4500 so as to approve one project. This forum is for technical discussion not by re-sellers of SLOPE/W like miningman coming with meaningless comments.

 

[miningman]

Glad I was able to help. You're welcome

 

[BigH]

Why would the "local council department" think they need to do the calculations themselves? I would suggest that you farm out a review of the design to another "respected" geotechnical/rock mechanic specialist and have them give you a thumbs up or down. Unless you have such a person "on staff" even if you found other software (and Swedge is one) to do the trick, do you or one in your council have the technical expertise? If it were I in your position, farm it out to a specialist for their opinion on your behalf. Make sure that you advise (ethics) that you are reviewing their design. Miningman was not wrong in his comment; no need to get testy.

 

[miningman]

Thanks BigH. And for the benefit of the OP, I have probably seen more physical slope failures than you have ever dreamed about . We call them hanging wall failures and 40,000 tonnes at a time is not unknown. The big difference between us and typical geotechnical engineers is that we cannot afford factors of safety in the 3.0- 5.0 range. Typically we use 1.15 as an F of S, so yeah sometimes we get it wrong, but so long as it is only a financial loss, and no one gets hurt, we chalk it up to another learning experience and carry on.

 

[voi]

Hi Bigh, In my initial note I just wanted to know where I can obtain software on steep slopes. Swiver mentioned rockscience and swedge. That is the input i was seeking.

 

[oldestguy]

At these Engineering Tips forums we get mamy questions.

Yours: "I need an input in how to analyze steep rock slopes. I particularly are interested in software that can assist me in the analysis,
thanks"

Then when someone pipes up to help you, his "tip" is criticized. One hell of a way to expect any other members will come forth.

 

[RobPE]

The majority of my practice is in rock slopes and I feel that the tone of this thread is far off track. The issue at hand should be "What kind of failure modes should I expect?" rather than "What kind of software should I be using?". To ask the latter first is putting the cart before the horse. The mode of failure will drive the analysis method and hence choice of software if any software is needed.

For a 25m HARD rock slope, the primary mode of failure would be structural ie along fractures, joints, or other discontinuities in the rock. If the excavation is truly in hard rock, then the strength is on the order of 100MPa which would stand at any angle for a 25m height ignoring the jointing. The weak link is therefore sliding along joints. To analyze this, you first need geologic data. I assume a site reconnaissance and investigation has been completed. If so, the report should include mention of the orientation and character of joints (nearby outcrops have hopefully been mapped). The joints should be analyzed relative to the orientation of the excavation. A simple stereonet analysis would be a good start. You can then do a kinematic analysis of potential wedges and planar failures using hand calcs or SWEDGE as swiver mentioned. The slope should be designed based on a criteria that limits the potential for daylighting wedges and potential planar failures.

If the slope will be unsupported, then you have additional considerations including sizing benches based on the geotechnical limitations as described above, and the size of the equipment you'll be using to drill and blast. If you want a steep slope without benches, then you'll likely have to do a cable bolt design. This involves an additional level of complexity that I won't comment on for now.

I caution that use of the Hoek-Brown method of rock mass strength estimation has no applicability for short slopes in high strength rock. Unless the slope will be in weak/highly altered rock, use of this method for this case is meaningless. Beware of consultant reports that suggest the contrary.

Although, spending money on the SLOPEW software isn't a magic box that is going to get you the right answer after spending a few bucks, I appreciate miningman's encouragement to do it right.

 

[BigH]

@miningman - I've seen quite a bit of slope failures too - during construction on a road project in a SE Asian country - we had about 150 of them in the first monsoon season! Many were of a 10m3 variety although a few involved quite a lot of material. Same with several other projects. I remember the discussion of SF way back in my university days - and we looked at slope SF for mining - and the cost savings involved if even to increase the slope angle by a 1/2 degree - typically, I would think that 1.1 would be realistic. RobPE brings up good points - the point I was making is that regardless of software - or not using, why would a council be putting themselves on the hook when, given the many many nuances as RobPE pointed out - it would seem that hiring in an expert would be a far more efficient and risk averse way to go about it.

 

[voi]

The comments by RobPE is great and highly appreciated it. I am not sure why he feels the Hoek Brown has no applicability for Rock slopes. Are you suggesting that theories such as Mohr - Coulomb are better.

 

[fattdad]

are you analyzing the slope just for the transient condition of construction? Are you striving to develop horizontal earth pressures that will act on the basement walls?

Will the drilling data include a televiewer so you can map the rock structure? What method will give you the strike and dip of foliation, discontinuities, bedding, etc.?

I mean fundamentally, you need to assign intact rock strength, temper that strength by the shear strength of the discontinuities and the presence of water pressure. At the macro scale there will be some overall rock mass property, which you could use for horizontal earth pressures. There may (will) also be some liklihood of stress concentration associated with a random wedge that the basement wall will have to shoulder. That will be a bit differnt then you may calculate using Rankine-type earth pressures.

I don't know about FEM solutions, other than to know they exist. Good luck getting hyperbolic modulus parameters though!

At least you don't have to worry about rock fall containment!

f-d

ípapß gordo ainÆt no madre flaca!

 

[RobPE]

Clarification of my statement regarding the Hoek-Brown criterion - this is a method of rock mass shear strength estimation with rock mass being defined as whole rock inclusive of joints and intact rock. The "rock mass" concept is used to downgrade the shear strength from intact rock free of joints in an attempt to account for weakening due to jointing. Note the following shear strength relationship: intact rock >> rock mass > joint strength.

In other words if you do a uniax test on a core of solid rock, then you'd get a very high strength on the order of 100MPa. However, because of the effect of joints (not necessarily daylighted) it is impossible to mobilise strengths of this magnitude. Hence, the rock mass concept. Using the HB method, the intact rock strength is downgraded empirically using GSI which is an index of rock quality. There are other factors as well. The method was developed for analysis of high stress environments primarily in mining where excavations are large and deep. It is not the only method of rock mass shear strength estimation by the way.

For a 25m excavation, stresses are relatively low in comparison with typical rock mass strengths. Another way of saying this is that rock mass failure for a 25m slope is very unlikely to happen unless the rock is altered and very soft. At 25m, the max normal stress is about 0.5MPa - a pittance in comparison with typical rock strength! Sliding along joints (discontinuities, foliation, bedding, faults, etc.) is therefore the mode of failure that is important.

A good start would be to map the joints, measure the orientations, test the shear strength, and do a stereonet analysis.

 

[miningman]

I suspect all this emphasis on the necessity of geological mapping, coring samples, joints and discontinuities, must be starting to give the OP a migraine. All he was... and probably still is... looking for is some software to give him the answers. Too bad its not that simple

 

[voi]

No Miningam. The comments by RobPE are very educative and exciting. What he has brought to my attention is before I think about software or analysis of the slope I should look at the rock mass in its entirety. He points out that at 25 m deep excavation is very unlikely to happen far from the cry that you mentioned that "I can see not one, but several buildings falling into the hole". Thank you RobPE

 

[fattdad]

if you can see the rock face, then the mapping suggested by RobPE makes sense. Often you can't. I recommended that you consider using the optical televiewer as a basis to "map" the core hole - i.e., get structural orientations. The televiewer also has the interface to bring the data into the computer - i.e., develop the steronet. Ultimately, from core holes you can then get the rock samples for unconfined compressive strength and to visually classify the morphology of the discontinuities.

The end game's the same - to get the rock mass properties.

f-d

ípapß gordo ainÆt no madre flaca!