Stability of slopes in Fissured Stiff Clay
Stability of slopes in Fissured Stiff Clay
(OP)
We are designing a widening of a road to become a motorway. The cut sections for widening have heights of 6-8 m. The existing slopes have been stable for about 30 years with a 2:1 slope, and the cut for widening will create the same side slope. the clay has cu=84kPa, c' =21 kPa & Phi'=22 Degrees. I estimated Phi' critical state from the q-P plot and found it to be about 27.5 degrees. The minimum Factor of Safety against slope failure is 1.3, and this cannot be fulfilled, if we use Phi'_critical state, without assuming a small adhesion of c'=2kPa. My questions are:
1- Is assuming c'=2kPa acceptable so as I use the soil critical state shear strength parameters (Critical State Soil Mechanics assume c'=0 at critical state), especialy the existing slopes haven't failed and have been there for 30 years?
2- How to take into consideration the fissures in the clay (and the laminations in a laminated clay, if any)?
3- Do, I have to asume a minimum surcharge, as we do in the limit state design of reinforced slopes, retaining walls, ...etc?, or with the Partial safety factor of 1.3, such a surcharge is not needed?
Thank you
1- Is assuming c'=2kPa acceptable so as I use the soil critical state shear strength parameters (Critical State Soil Mechanics assume c'=0 at critical state), especialy the existing slopes haven't failed and have been there for 30 years?
2- How to take into consideration the fissures in the clay (and the laminations in a laminated clay, if any)?
3- Do, I have to asume a minimum surcharge, as we do in the limit state design of reinforced slopes, retaining walls, ...etc?, or with the Partial safety factor of 1.3, such a surcharge is not needed?
Thank you





RE: Stability of slopes in Fissured Stiff Clay
good luck
RE: Stability of slopes in Fissured Stiff Clay
I would like to ask you whether you have to use a minimum surcharge in the slope stability analysis or not?
RE: Stability of slopes in Fissured Stiff Clay
RE: Stability of slopes in Fissured Stiff Clay
In Canada we use both limit states and servicability limit states for provincial and federal projects. Don't ask me why, except that some people like to have a foot in each to protect each of their feet.
jim
RE: Stability of slopes in Fissured Stiff Clay
Fissured clays?
After a recent review of Fang's Handbook, I found a couple of references, which reccomend to reduce Su by 75% or 50% if clay is fissured.
Laminations? I'm not aware of any suggestions, other than common sense: lamination= anisotropy, so if the visible laminations correspond to weaker silty and very thin layers, you might have a preferencial direction for the failure surface. And for drainage.
Do you use EC7 characteristic values in UK for slope + foundations design? (conservative estimates of resistance parameters?)
Do you follow EC8 guidelines for limit state slope stability
analysis?
One thing which is worrying everyone here in Italy is that, in strongly seismic areas and by limit state design in pseudo-static conditions, slopes will almost inevitably turn out to be unstable.
It's a known fact that pseudo-static is conservative, but coupled with characteristic values and strong inertial forces analysis are becoming atrocious.
I see only 2 ways out:
1) Fully probabilistic analysis
2) Newmark method
I never tried the latter. As far as I knmow, you have a displacement as an output. That's an added difficulty, because if the block displaces, it's not stable, at least in terms of regulatoy semantics.
No, out here we never apply a surcharge. Neither EC8 reccomends such practice.
RE: Stability of slopes in Fissured Stiff Clay