Why does shear strength decrease as pore water pressure dissipates?

[tzx9633]

Homework Statement

In the notes , I can't understand that why as the pore water pressure dissipated , the shear strength of soil will decrease
secondly , why the excess pore water pressure is negative when unloading occur ?

Homework Equations

The Attempt at a Solution

I think it's wrong , as the pore water pressure dissipated , the shear strength of soil should increase , am i right ? This is because total stress = effective stress + pore water pressure stress , i assume shear stress = effective stress here . Correct me if i am wrong ...

 

[tzx9633]

The pore pressure dropped to negative in the beginning of the cut , does it related to zone of capillary rise ? If yes ,. i can't relate it to the case of cut slope , can someone help ?