High Cemented Slope

[BigH]

howardoark has asked question, under lab testing forum (thread 261-139412), about stability of high "lightly cemented" sand bluff. VAD and dmoler have commented - good comments. Sometimes, these kinds of bluffs are hard to theoretically quantify a reserve of safety - they stand and shouldn't (or you think they shouldn't).

The one point - he indicated that he wants to cut it back to 0.5H:1V from it's present near vertical slope. Do you think this is a good idea? Sometimes, you go messing about with a stable slope like this and it backfires.

This is a concern I have. Anyone else?

 

[Mccoy]

BigH,
as far as I can say, without the help of visual details, I have such a slope just behind home. It's lower pleistocenic beach sand, weakly cemented, the type you can bore tunnels into because it's soft but the cementation is just enough to make it self-supporting. Why you say it shouldn't stand? evidently, if there is no grass cover, it should have some kind of cohesion of its own, barring suction forces (30 m sound pretty high).
Vertical faces ("mine" is 12 m) will weather progressively, with accumulation of debris at the toe, sometimes a small slide suddenly will occur, if a few lithified layers are present, small blocks will fall. 2-3 years after excavation (it was an illegal quarry) it hasn't yet reached equilibrium, and surface is irregular. Vad decribes it well as "slag" an superficial weathering.
If you speak of even softer sand, in a "metastable" cemented state, it does not seem possible for such a bluff to stay vertical for a long time. Maybe if water percolates on the surface and carbonates precipitate and make up a plaster-like layer?
Also, Howardoark, after a site investigation, discovered "cemented gravel", and that would make the rock a conglomeratic sandstone, whose stability is usually not bad.

 

[BigH]

I didn't say it wouldn't stand - I've seen a fair number os situations where I have to scratch my head as to why it is behaving suchly. I said that, in analysis typically done, it is not always easy to compute that it would stand - you would have to know the amount of cementation, etc. I have seen fairly high slopes at near 70deg standing but not cemented to any extent. It has more to do, I believe, with the vegetation and with suction pressures. Wet it to near saturation and it will slough. It is like loess that stands vertically until it gets a significant amount of saturation. It would be interesting, too, to see his 'cemented' gravel - is it caliche from SW USA?? He's described it as a cemented soil so it would be putting one out on a limb to call it a sandstone - perhaps a friable sandstone-like.

 

[Mccoy]

To your knowledge, do some of those slopes (with face steepnes apparently too high for lithologic and boundary conditions) remain stable for a long time? And how long ? Apparent cohesion should disappear in saturated conditions concomitant to concentrated rain.
Thick vegetation of the right kind may be very effective, if continuos enough.
In those cases, it would appear that human intervention (even excavating to obtain a theoretically more stable angle) may disrupt the factors contributing to a delicate state of equilibrium, I fully agree on that.

 

[GeoPaveTraffic]

I would be concerned about letting water into the slope by cutting the slope to a flatter angle. Don't know if this has ever been a problem, but seems like it could be. I agree with BigH, if it is working, I wouldn't change it unless I really had to.

 

[Mccoy]

One more point, mulling over the subject:
if apparent cohesion really were a significant factor, do you think such a slope could be a closed system as far as soil saturation is concerned?
In other terms, I would expect over the time changes in water content; it would appear impossible that conditions favourable to mantain capillarity may persist over the long term.
Maybe the soil is very "unwilling" to release capillary water, with the presence of strong superficial bonds, and that may create a metastable condition whose time scale is years or decades.
Just theoretical speculation.