triaxial loading and unloading

using a triaxial cell, can a shear stress be "applied" by holding the confining stress constant and reducing the verticle stress? if so, how does this show up on a plot of the stress history on a p-q plot?

 

[Ron]

This can be done assuming you have pre-loaded the soil. Your result will be the unloading cycle of a cyclic modulus test.

 

thanks. will the stress path reach the K-f line, denoting failure? My thinking is that eventually the soil will fail because the confining stress will be greated than the verticle stress but i can't get the graph to work out.

 

[Ron]

Failure point won't necessarily be defined when the confining stress exceeds the vertical stress. That depends upon the soil character. Might be so in clean sands, but wouldn't necessarily work out that way in cohesive soils or even in silty/clayey sands.

 

[irangeotech]

hi alikav;
we have 2 different kind of triaxial testing:
1- Compactional Testing(C.T)
2- Extentional Testing(E.T)
in this case that u mentioned,it is extentional testing
and this kind of testing shows extentinal behaviour of soil
(tentional behaviour);
but Compactional method(that is custom)shows compactional
behaviour of soil.
the shear strength of soil is much lower in E.T method because
as u know soil is weak in tention.
BUT:loading an unloading and reloading is somthing else.it is
usually used in C.T method.
OFCOURSE we have other methods exept C.T &E.T method to see other
parameters like G(shear modulus).

 

[irangeotech]

I correct my prev. edition:
C.T method =Compressional Test not compactional...
so we have compressional and extentional test.

 

[Focht3]

You have to have special triaxial equipment to perform extension tests; the rod has to be fixed to the top platen on the soil. Check with your triaxial cell's manufacturer to see if you have "the right stuff."