Geotechnical Properties of Weak Excavated and Relocated Rock

[TimesNewRoman]

Hi,

We have this massive layer of weathered and altered rock (cohesion around 85 kPa and friction angle of 22 degrees) found on a hill.

Now part of the hill is being excavated, and pieces of this rock layer are being dumped in a downhill. Now If i want to calculate the slope stability of the downhill, what cohesion and friction angle should i use for this layer (considering its not in the same condition anymore)? We know the cohesion of friction angle of other layers.

Do you have any experience with such a case? Should we just treat it as granular material and take cohesion 0 kPa?

Help is very much appreciated.

 

[fattdad]

Seemingly impossible scenario?

Punky rock that's excavated may include some, "Rock pieces." These are of some value in the short term, but. . .

Freezing-thawing, wetting-drying and the ravages of time will reveal continued loss of strength. Such softening reduces the strength of the soil mass to its, "Fully-softened" value.

The best way to gauge fully-softened strength is to take the rock and fully disaggregate the rock pieces. Push the sample through the No. 40 sieve and then run strength tests. These strength tests are done after bringing the disaggregated material up to its liquid limit and consolidating the sample in the direct shear machine. Just have to wait for the end of primary consolidation and shear. Do that for three or four confinements and you'll get a fully-softened shear strength. (interpret the stress-strain on the basis of peak resistance.)

So, there's a strength value that's better than residual, not as high as some compacted value and a strength that's clearly above the critical void ratio.

f-d



ípapß gordo ainÆt no madre flaca!

 

[Mad Mike]

Yeah, there's a bit of a problem here. When you're "dumping" the rock on a downhill, the material will initially stand at a steeper repose than what can ultimately be achieved, for all the reasons Fattdad stated above.

It needs a bit of research, but ultimately, you won't be able to produce a stable pile by "dumping". If there is any significant soil beneath the dumped rock, this could also contribute significantly to large-scale instability.

I would look at the site carefully and see if there isn't a better (flatter) place the materials could be economically stockpiled, failing which you will be dealing with a series of gradual failures of your rock dump.

All the best,
Mike

 

[oldestguy]

I differ from Prof. fatdad. You will have loose material and so compacted material tests don't apply. Since the material isto be there forever, consider it will settle and the ultimate slope will be less than early on. It also will be less than the temporary dumped condition. I'd measure the slope after it is dumped from a loaded vehicle. Then, consider the ultimate slope as at least 5 percent less than that temporary dumped slope.

 

[fattdad]

(I never suggested compacting anything.)

f-d

ípapß gordo ainÆt no madre flaca!

 

[EireChch]

I think it would definitely have to be considered as a phi only and no cohesion. As you likely know cohesion is from interparticle forces and bonds and cementation. You are destroying this with excavation and dumping.

If you were dumping sand and assessing its long term stability, sand would typically sit at its friction angle. You could adopt 30 degress as your phi value. For your situation i think you could adopt 30 degrees as above, or as OG recommends reduce it with a small FoS. I think anything you do isnt going to be an exact science.

Also - How come your friction angle is so low? 22 degrees is equivalent to a low to medium strength clay.

 

[oldestguy]

OG again. I'd sure like to have a description on how you run direct shear tests on loose material that doesn't have any load compressing on the sample, which is a loose condition. Quote from above. "consolidating the sample in the direct shear machine"

 

[fattdad]

I may not have the time to get into the conventional approach to determining the fully-softened strength of shale fill. I'm not citing junk. You take the shale fill, you disaggregate the shale (or other punky rock type), you increase the moisture content to the liquid limit and you normally consolidate the sample in the DDS box. So, you have a nearly-saturated sample, you evoke the consolidation process in the DDS and when pore pressures have dissipated, you shear the sample. You can measure the dissipation of pore pressure in the DDS, just as you can in the odometer.

Now if you have a different approach, that's fine with me. I just don't care. If the OP is curious what I'd do, then I claimed an approach. This is all published; however. I'll refer you to the work of professors Duncan, Brandon, Stark, Mitchell, etc. CGPR (Virginia Tech) actually held a symposium on the very topic and published their notes.

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

It all sounds great. My bet is the OP has very limited acres to a simple lab, if any. My method takes no lab, no calcs.