effective normal stress造句

It has also been observed that the effective normal stress is decreased.
Normally, effective normal stress is calculated by subtracting the pore water pressure from the normal stress, in natural conditions.
This effect reduces the effective normal stress in the slab and allow slip to occur on pre-existing fault planes at significantly greater depths that would normally be possible.
However, why is it that in the untrained triaxial compression test, the pore water pressure is negative, meaning it is added to the normal stress to obtain the effective normal stress?
It's difficult to find effective normal stress in a sentence. 用effective normal stress造句挺难的