Slope stability 1

[ismailH]

I was just wondering if any 1 can help me with this, I'm doing final year project on this title "cohesive and cohesion less soils and their stability performance in slopes; an analysis and comparison". I just wanted to know how would you guys go about doing this project, what variable would vary etc

 

[fattdad]

for long term performance, it's all about friction angle. Question is, "How do you go about getting friction angle." Now, impuluse is to rely on cohesion for the fine-grained soils. That's an oppoutnity for failure. Do yourself a favor and read up on, "Fully softening benavoir" of fine-grained soils. For the case of levees and highway slopes (i.e., 75 to 100 year design life, we have to consider the near-surface effects of freezing and thawing, wetting and drying, etc. These processes will degrade cohesion over time. Many case studies of slopes that failed decades after construction from this softening effect.

Good luck in your project.

f-d

ípapß gordo ainÆt no madre flaca!

 

[ismailH]

well my supervisor thinks I should look at the effect of cohesion. He wants me to do a parametric study by varying cohesion and carry out analysis on short term and long term stability. Looking at my title, I think I should also look at the effect of varying friction angle. what do you think about this, I have looked around and can't seem to find any1 who has done something like this before ?

 

[cvg]

try Duncan, I believe he has done it

 

[ismailH]

Michael Duncan ? I have his book, he did a parametric study on undrained shear strength but nothing on effective cohesion and friction angle. The problem I'm having is that I don't know what value to use for effective cohesion when I'm varying my friction and vice versa ? cos i cant really do a parametric study on a purely cohesive and non cohesive because in real life soils have both components. Do you have references on Duncan i can look at ?

 

[Compositepro]

IsmailH welcome to eng-tips. Please do not use text-speak here. This is a forum for professionals.

 

[DougHole]

Without knowing the details of your project, i would try the following combination of parameters...

silt: use phi'= 27,29,31,33 deg combined with c' of 1, 2, 5, 10 and 15 kPa.

clay: use phi' of 24,26,28 and c' of 1,2,5, and 10 kPa

You will find the c' makes a big difference to the FoS.

 

[killswitchengage]

your project is very broad , i mean that slope failure happens for many reasons

I highly recommend that you read "an introduction to geotechnical engineering by Holtz et al" , also read Terzaghi's book if you can
Slope instabilities occur because of either : decrease in shear strength by environmental or human activity or increase in driving forces also by the same causes .
Granular loose materials fail under UU conditions by liquefaction , to make it simple they fail when pore water pressure is equal or greater than effective stresses . Dense granular materials however can behave quiet well , but they do fail under CD conditions or when erosion remove part of the materials .

Cohesive materials are most troublesome , since they can fail after considerable time or spontaneously w/o warning . Normally consolidated clays fail under UU condition by compression or lateral active spreading ( think erosion , chemical alteration , PWP ) . OC clays fail only under CD conditions , but they are a hard bit : intact unfissured clays can be good , highly fissured clays are not . The latter tend to suffer creep and progressive failure by reduction of shearing strength because mostly of water , the shearing strength lowers down to a residual value or fully softened one . Therefore , shales or claystone can be tricky rocks .

But bear in mind ,organic soils or fill materials are highly hazardous , in fact one slope i am working on failed catastrophically because of this reason.

If you just want to study mathematically how variation of the parameters affect slope stability well , its already done watch for the basic analytical methods such as Bishop's or Fellenius methods . If you want to try and quantify erosion and or other processes leading to failure then good luck i will see if i can help