Suction and Undrained Shear Strength

[ebenozen]

In researching field conditions that would warrant an undrained total stress analysis, I’ve come across an example that perplexes me. This quote is taken from a technical report (verbatim):

“When the sediments are unloaded, such as occurs when a mine slope face is excavated, the reduction in overburden stress causes the pore pressures to become negative, resulting in an increase in apparent strength approaching the “undrained shear strength” values. With time, these negative pore pressures dissipate, causing the strength to decline to the “drained strength” values.”

I understand the concept of inducing negative pore pressures with the presumption that excavation/unloading results in some elastic rebound (volume expansion) that induces suction pressure in a low permeability, partially saturated soil. However, I don’t understand how this condition approaches the undrained strength.

With suction (negative U), the shear strength term (sigmaN-U)*tan(Phi) increases.

Whereas, with undrained conditions (hi positive U), the term (sigmaN-U)*tan(Phi) goes to zero.

How then could a soil experiencing negative pore pressure approach the undrained strength?

 

[fattdad]

I think there is a fundamental truth to the statement. I mean if there is transient soil suction that will attenuate with time, that's a result of "drainage." Such undrained strength is not exactly what we are used to for engineering strength over time though.

f-d

¡papá gordo ain’t no madre flaca!

 

[ebenozen]

I'm with you fatdad, but I still have a hard time, in terms of Mohr-Coulomb as described in the post, how a soil experiencing negative suction pressure approaches the undrained strength. Maybe the quote above is simply incorrect. Thoughts??