Comparing strength values on either side of a gulley.

ORUSA · Aug 15, 2014

I have a dipping siltstone (soft, jointed, bedded rock, R1) which dips into the slope on one side of a creek and out of the slope on the other side. I have back-calculated strength parameters for the natural stable slope on the in-slope dipping side and I am wondering which values would change for the out-slope dipping analysis. Should phi and c both change to weaker values? Which strength parameters should be affected by a change in dip direction?

The one side of the creek is stable and the other side has several failures which shows differences in stability of the natural slopes. Of course one can choose various combinations of phi and c to arrive at whatever FOS is assumed, but in the same formation shouldn't one of the values remain the same as on the other side?

I am modeling initial, pre-failure values and have assumed the stable slope to be at a FOS of 1.1 which I am hoping will give me "minimum" strength values to apply elsewhere. I want to do the same for the out-slope dipping side.

I began by using RocLab to estimate phi and c to apply to a limit equilib. analysis...

Ideas?

GeoPaveTraffic · Aug 17, 2014

It sounds like you are trying to model a rock slope, however weak it sounds like the siltstone is still rock and not soil, using a soil stability model. The likelihood of that working is very small.

I suggest you treat this as a rock slope and analyze it as such. If you don't have experience analyzing rock slopes, get with someone who does and have them teach you.

Good Luck.

Mike Lambert

ORUSA · Aug 19, 2014

I backed in phi and c using Hoek's method but obviously the bedding planes (as a discontinuity) are not the same as the other joint sets since they definitely control failure on the one side of the creek. There is not one plane that is the controlling plane but a whole section of 1" to 6" beds.........

Would you use a structural analysis to do this problem?

To address your other comment, I don't have this specific experience and neither does my peer whom I work with because we don't normally delve into this area. We are surrounded by foresters!

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Comparing strength values on either side of a gulley.

ORUSA

Geotechnical

GeoPaveTraffic

Geotechnical

ORUSA

Geotechnical

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