Determining Kp by testing 1

[allisch]

How would one go about determining Kp for a clayey soil in the lab?

I have old soils lab textbooks (Das) that show how to do a direct shear test on sand? This test (and unit weight)could be done to get the internal angle of friction and from that, you can compute the theoretical Ko, Ka, and Kp.

But that test really doesn't apply to clays? Any suggestions?

I realize that it could be conservative to use Kp=1.0 and the usual range for cohesive soils is between 1 and 2. My question is in how to proove it by testing?

 

[DarthSoilsGuy]

triaxial test of the sample for a phi angle, or have the geotech/cmt people that would have ran the triaxial give you an approxiamate phi angle for a sample.

 

[Panars]

Measuring the passive earth pressure coefficient directly is not commonly done. Usually, you measure the internal friction angle and calculate Kp, as DarthSoilsGuy alludes to.

I suppose you could test for Kp by pushing a large plate against the side of an even larger volume of soil, but I don't think it would be practical.

 

[allisch]

Thanks fellas- That solved my question about getting internal angle of friction for clays.

 

[fattdad]

Just for the record: pushing a large plate against the side of an even larger volume of soil may not directly give you the Kp value. Kp is for the plane strain condition and the large plate would have end effects, which with arching would give you a larger-than-calculated value.

f-d

¡papá gordo ain’t no madre flaca!

 

[DRC1]

For clays, the undrained and drained condition should be considered. For normally to slightly over consolidated soils, the undrained strength will generally control. For overconsolidated soils, the drained strength may control. Drained strength will be a function of phi.
Undrained strength is a function of cohesion c and unlike granular soil, the passive capacity is finite and a function of the depth of the cut.