×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Stability of slopes in Fissured Stiff Clay
5

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

RE: Stability of slopes in Fissured Stiff Clay

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

RE: Stability of slopes in Fissured Stiff Clay

(OP)
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?

RE: Stability of slopes in Fissured Stiff Clay

2
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.

RE: Stability of slopes in Fissured Stiff Clay

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

RE: Stability of slopes in Fissured Stiff Clay

2

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

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.
cheers

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close