Coefficient of cohesion of soils and angle of friction relation using the Rankine method 1

[Iasonasx]

I have ran into this problem of solving for a safety factor for a retaining wall using the Rankine formula, but instead of having the angle of friction "φ" I had only a coefficient of cohesion Ks. I am unaware of the relationship between the two. Please help!

 

[IDS]

Please don't take this the wrong way, but if you need to ask the question, you really need a geotechnical engineer to advise you on the project. Cohesion and friction angle are separate properties of a soil, and designing a retaining wall for a cohesive soil is not something to be undertaken by someone without a good understanding of soil behaviour.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Iasonasx]

No I don't take it wrong by any means, this is not my specialty and I definitely find your response correct. Nevertheless, I wanted to set myself on the right track before engaging with a geotech engineer, ...just to make sure I did my own homework and not look like I did not care and I just drop it there without doing my part.

 

[jnich]

Iasonasx - how is the soil classified that you are trying to retain? If you're retaining a 100% fine-grained (clay or silt) soil, then theoretically you will not have a friction angle (φ = 0 or φ' = 0). Usually the critical parameter for fine-grained soils is the cohesion (c or c'). Friction angles only apply to granular soils (gravels, sands, etc), in which case the cohesion value of these is usually neglected (c = 0 or c' =0).

 

[Iasonasx]

Thanks jnich. It is a mainly clayey soil with a noteworthy amount of gravel. I was under the impression that the Ks of Sandy soil is 30% and that of Clayey soil is between 35% and 40%, with fully saturated soil being 100%. I fear that I am talking about a different thing here if you refer to values below 30%, in which case I have to apologize for the confusion I am causing. I find this quality described also as "Equivalent fluid pressure coefficient" and "Lateral Pressure coefficient".

 

[Iasonasx]

Y'all thanks, I just managed to find it in my notes. Thanks for the help though. I uploaded it on my site just to clarify what i was looking for.

http://www.avant-garde-engineering.com/K-to-phi.png

 

[RFreund]

I believe the Ks your referring to is the lateral earth pressure coefficient for soil. phi and Ks can be related by a number of equations depending on the conditions and which formula you a using. The lateral earth pressure coefficient is gives you an 'equivalent fluid pressure' when you multiply it by the soil weight. A few examples:

Ks for the active soil pressure maybe related to the effective friction angle of the soil 'phi' by Rankine or Coulumns formulas. This is typical for earth retention however you may also need the 'at-rest' or passive pressure as well which also relate K to phi which other geometric variables.

Usually you are given the 'phi' of a soil, wall batter and backslope and your able to find your Ks by using coulumb equation or rankine. However if you already have Ks I'm not sure why you are interested in phi. For a clayey-sandy soil I could see a phi of 26-28 as possible. Which would give Ka (active soil pressure) around 0.35 for a flat backslope and vertical wall with no friction between the wall and soil.

EIT

http://www.HowToEngineer.com

 

[Iasonasx]

RFreund, you are right. I guess getting overwhelmed with work and tired causes us to think backwards when we already have the data needed. Why should I search for the "phi" if I have the K? I think what I need now is from myself to rest a bit. LOL.