Shear strength of weakly cemented sand?

[howardoark]

I've been asked to opine on the long-term stability of a 0.5H to 1V cliff, 30-m high. From pictures, it looks to be formed out of weakly-cemented sand.

I'm told there are even higher vertical faces in the area, so I'm not too concerned. But other than carving block samples out of the face for laboratory testing and back calculations based on the higher vertical faces, does anyone have any notion of how else I could get material properties for slope stability calculations?

I'm going to suggest anchoring some sort of matting to the surface of the slope to try and establish vegetation and that they try to route surface water away from the face.

Thanks

 

[fndn]

We could take in-situ unit weights and a bulk sample and run a remolded direct shear test at abouth 85% of the in-situ wet unit weight. The plotted RESIDUAL strength will give us a long-term C and Phi values that are realistic.

You describe a very steep and tall slope. Are there any surficial failures? In the long term, maybe the top half may be cut back to give us a shallower two slope hill?

 

[howardoark]

Thanks for getting back to me. I'm going to go look at the slope this morning.

From the pictures, it's a pure sand slope standing now on a 63 degree slope. So I imagine the residual c is zero. So I don't think doing shear strength tests on a remolded sample will help.

I'm thinking of using a pocket torvane on the free face, but I'd hate to extrapolate that strength back to the potential failure surfaces.

Cutting back the slope isn't an option because of the property line (not that I don't think the top of the slope won't eventually be well beyond the property line).

 

[fndn]

Ok good. I agree with Cohesion being zero, but how would you approximate phi and unit weight of the soils for the slope stability analysis?

Also check with engineering geologist opinion on the slope and let us know how the taller slopes are performing.

 

[casimmons]

You could probably use the pressuremeter to get some shear strength values.

 

[howardoark]

That's a good idea. Do you have any thoughts on whether the pressuremeter or the Iowa borehole shear test would give more useable shear strength values?

Thanks

 

[dmoler]

You might want to do a few static cone penetrometer test full length runs to determine the weakest layer/area to evaluate with additional testing rather than blindly selecting test locations.

 

[VAD]

The observational approach and determining what would happen if failures were to occur would be the approach I would take with this issue. Slope stability analysis in the traditional sense would not be very meaningful. At 30 m and standing with other higher cliffs says a lot. Weathering and thin slab failures would appear to be what can be expected. Your ideas of matting the slope face etc might be the best practical approach if you are sure that any type of failure would not have disastrous consequences. Hence if there are to be buildings next to slope etc then this could be a concern.

In Hong Kong they use chunam a wek concrete mix sort of to place on the slope faces to avoid deterioration due to weathering re residual soils. This may be expensive.

I do not agree that cohesion intercept residual or not would be zero for the material in question.

The understanding of the site geology, historic evidence and observations are probably the key to making a decision on the long term. Review some past aiphotos etc if available. Remember not all geotech answers have to be borne out by numbers. Convincing answers can come from logic as well.

 

[howardoark]

It turns out that the excavation has lengthened the slope by about 10 meters (from 20 meters to 30 meters).

Other than by doing some sort of borehole test (I went out to the site and the cemented gravel is up to 3 cm in diameter) I don't think I'm ever going to get an idea of the cohesive intercept. The client doesn't appear to want to do any testing.

My understanding of the site is that it's currently at about a 90 degree slope (and stable at 30 meters). The client is going to cut it back to 0.5H to 1V. Hence, it probably will have a pretty good factor of safety. I've recommended stabilizing the surface with a Multimat (a plastic erosion mat with some thickness that we can get some hydroseeding (mulching) into, which might stop the weakest strata from raveling. And, I've told them that the slope wants to be at about 1H:1V (based on the slope of the screed at the bottom of the slopes) and will eventually get there despite their best efforts.

My only real concern is that since the slope requires quite a bit of cohesive strength, a failure could be quite sudden. But, even so, what I envision as a worst case failure is the slope giving way and dropping about 15 m in 30 seconds or so. This would leave it at about 1 on 1. It would scare the crap out of anyone who happened to be on the top of the cliff, but I can't imagine it being a threat to safety. There's no structures at the top of the cliff and none would ever be built there (or at the bottom of the cliff).

Am I missing something?

Thanks