HELP meeeeeeee GEOTECH...

[Blu1913]

Can someone explain cohesions in stupid mans terms to me...I've read the CERMS def and I still dont get it........internal frictions and stuff....

Can someone put it in easy terms.... :brick:

 

[DVINNY]

They sound like something that needs a BAND-AID?

 

[GTScott]

No prob...are you having trouble with the concept or calculations around cohesion?

Cohesion is found in fine grained materials (clays etc.) and is the force by which particles appear to adhere to one another. This is caused by various particle forces as a particle distance changes (Van der Wals, diffused double layer, etc. depending on how far apart particles are). On the other hand, sand is a frictional material meaning that the frictional forces are very large in comparison to these other smaller forces.

Cohesion and undrained shear strength are directly related for most cases you would analyze. Cohesion is typically taken as half of the undrained shear strength for most calculations that I have run across in studying.

If you have some specific questions, shoot. I can pull some references if you need more info.

 

[Blu1913]

So............... sand has friction therefore it has high cohesion. It the concept i cant grasp...why doesnt clay have cohesion? Because the particles are too small to have any forces?

If thats true, i though clay was great stuff to build on..............................lost..... :brick:

 

[JasonT33]

Clay is great stuff to build on? How so?

 

[Hill William]

I may be mistaken because I havent studied in six months. But, I thought that sand was a cohesionless soil, phi=0

Try thinking of cohesion as "stickiness" for lack of a better word.

 

[EL Nica PE]

So............... sand has friction therefore it has high cohesion.  It the concept i cant grasp...why doesnt clay have cohesion?  Because the particles are too small to have any  forces? 
If thats true, i though clay was great stuff to build on..............................lost..... :brick:

No. Sand strength is measure in phi angles.

Technically clay will turn its strength to angle (will be come sand).

This is what is call long term analysis mostly done for slopes.

you can get a phi angle from clay by doing a triaxel test. This will give an very low

cohesion and the phi angle.

The equation being T=c + tan(phi)

T=shear strength (hence what GTScott said)

c= cohesion

typical this is taken as T=c when phi =0 (clay for short term analysis)

Basic concept is that if you have CLAY (fine material) they will give you cohesion or shear strength.

For Sand (granular material) they will give you phi angle.

Hope this helps. Man I love Firefox 2.0 it has spell check built in. Awesome!!!

 

[GTScott]

So............... sand has friction therefore it has high cohesion. It the concept i cant grasp...why doesnt clay have cohesion? Because the particles are too small to have any forces?
If thats true, i though clay was great stuff to build on..............................lost..... :brick:

You really dont need to know this but ....

Sand does not have cohesion in the classic sense because the forces relating to the individual particle weigt are so much greater than the forces that result from the surface attraction that you get in clays. The total surface area in a clay is MUCH larger than that of a sand and thus you get a larger area by which to contribute these forces.

You rarely see phi reported for a cohesive material, however, it can be calculated and is sometimes needed when doing excavation or similar analysis.

 

[GTScott]

While not directly realted to your initial question, I am a strong supporter of doing ANY triax test analysis using the p-q stress paths. This works for ANY condition. It may take a few more steps than some of the other methods that work just for one or two cases, but it will always save you in the end.

 

[McEngr]

While not directly realted to your initial question, I am a strong supporter of doing ANY triax test analysis using the p-q stress paths.  This works for ANY condition.  It may take a few more steps than some of the other methods that work just for one or two cases, but it will always save you in the end.

If sand has theoretically zero cohesion, it would seem that it would not be a good material for bearing capacity if it were saturated? Am I right on this?

 

[Blu1913]

HAHAHA i didnt even think this post was still alive..

No i was saturated and was confused. Clay sucks to built on.

The rest, ie cohesion, i dont give a crap about at this point......

:???: