Triaxial CU test why we apply horizontal and not vertical

[ketanco]

when doing the consolidated undrained test, first we apply the chamber pressure and let the pore pressure develop in the specimen to dissipate, so it drains. and THEN we close the drainage line at the top and bottom and apply axial load, so the name "consolidated undrained". I understand this overall. but for the steps in between i have questions...

my question is, why do we do the drainage by the chamber (horizontal) pressure and not vertical pressure? the consolidation is a vertical thing, so i am confused why we apply the horizontal to do drainage first.

and how do we know how much to apply the radial pressure to make the drainage, and how do we know at which point to stop when it drains (pore water dissipates, i mean how do we know how much it should dissipate to stop drainage process)?

after that we apply the axial load in undrained condition. but for the steps before this, as i wrote, i have those questions.

 

[Erdbau]

You are mistaken. The water pressure in the chamber acts equally in all directions (radially and vertically). It acts on the top and bottom caps just the same as it does on the sides of the sample. Consolidation is not just a "vertical thing"; it's horizontal as well. When you consolidate, you will get vertical consolidation (the specimen gets shorter) and vertical consolidation (the diameter reduces).

The confining pressure mimics the confining pressure in the ground. The pressure you choose for testing depends on the in situ stress state, and the proposed (after construction) stress state.

When running the test, you keep track of the volume of water coming out of the sample. When there is no more water coming out of the soil (or the rate is sufficiently reduced) then primary consolidation is complete, which means that the excess pore pressures have dissipated.

 

[ketanco]

ok i understand.

and another question for the reality implications of the test.

so, after the pore pressure dissipates, which depends on the in situ and proposed conditions as you said, we close the drainage. in reality this closing of the drainage means what then? because the sample had consolidated as we wanted now. when we close the drainage now, in reality what does it relate to? i mean if the confining pressure when drainage was permitted reflects our proposed loading too, such as our new building on top, then after we finish our building, there will be no more loading so everything is finished. so for our sample after that consolidation by chamber pressure is done, what remains in reality that we still want to know an undrained strength?

 

[fattdad]

with the drainage closed off you can then measure the change in pore pressure as shearing develops. Using these pore pressure data, you can then solve for effective stress and get both drained and undrained properties of the soil at the original consolidation pressure.

f-d

¡papá gordo ain’t no madre flaca!