Stability of slopes in Fissured Stiff Clay 5

[AlaaElnahas]

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

 

[jdmm]

you appear to have a good handle on theoretical soil mechanics. From a practical perspective, for long term slopes the surface stability is generally governed by the weathered strength of the soil. This means that the cohesion generally goes to zero and there is often a swelling or bulking of the natural clay. For overconsolidated clays that are fissured, the surface material often becomes nuggety and represents the lowest strength that will occur in the long term for the material. I once (30 years ago) carried out an analysis of slopes in bentonitic clayshales (heavily overconsolidated). I plotted slope height versus stable slope angle and determined a gentle curve. If you are sure that the stable long term slope is 2:1 (about 27 degrees) then this represents the long term bulk strength of the slope including fissures. In comparison to your friction angle from critical state analysis this slope is consistent with the phi for a material where the cohesion has gone to zero. The only caveat is that groundwater conditions, difficult to determine in clays, may seriously affect your analysis. Look for possible groundwater discharge areas and artesian pressures that might result from underlying sand layers.

good luck

 

[AlaaElnahas]

Thanks JDMM for your reply. In the UK, the limit state is used in the design of foundations and earth retaining structures. However, in slope stability a partial safety factor is still used (FS=1.3) to make sure that the stress levels in the ground are permissible and wouldn't violate serviceability or stability requirements. In the limit state design, you have to assume a minimum surcharge of 10kPa, if its existence is not favourable for your structure's stability or serviceability.

I would like to ask you whether you have to use a minimum surcharge in the slope stability analysis or not?

 

[BigH]

I never have done so - only if there is the possibility that we would have sustained live loading - a truck traveling past is not a sustained live loading - a train of some length at the crest might be a bit of a different story - this is where judgment comes in.

 

[jdmm]

BigH always gets it right. If all you are asking is if you need to include truck loading, I would say that this is the least of your concerns, as does BigH. However, in order to get this right, you have to look at the long term stability of the slope that really is not affected by short term loading.

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

 

[Mccoy]

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.

 

[BigH]

I recently found my copy of Skempton's 4th Rankine Lecture - "Long-Term Stability of Clay Slopes", given in Geotechnique (Year, I forgot to note). Might be worth a read - especially as you are in the UK. An excellent article that I recommend to all.